Muscarinic cholinergic inhibition of adenylate cyclase in airway smooth muscle

The Relationship Between Asthma and Food Allergies in Children

Asthma and food allergy are two complex allergic diseases with an increasing prevalence in childhood. They share risk factors, including atopic family history, atopic dermatitis, allergen sensitization, and T2 inflammatory pathways. Several studies have shown that in children with a food allergy, the risk of developing asthma, particularly in early childhood, is high. Food allergen intake or the inhalation of aerosolized allergens can induce respiratory symptoms such as bronchospasm. Patients with both conditions have an increased risk of severe asthma exacerbations, hospitalization, and mortality. The current management of clinical food hypersensitivity primarily involves the dietary avoidance of food allergens and the use of self-injectable adrenaline for severe reactions. Poorly controlled asthma limits the prescription of oral immunotherapy to foods, which has emerged as an alternative therapy for managing food allergies. Biological therapies that are effective in severe asthma have been explored for treating food allergies. Omalizumab improves asthma control and, either alone or in combination with oral immunotherapy, increases the threshold of allergen tolerance. Understanding the interplay between asthma and food allergy is crucial for developing successful treatment approaches and ameliorating patient results.

Standard medical therapy with vs. without nebulised magnesium for children with asthma decompensation

Pediatric asthma is a common condition, and its exacerbations can be associated with significant morbidity and mortality. The role of nebulised magnesium as adjunct therapy for children with asthma exacerbations is still unclear. To compare clinical and functional outcomes for children with asthma exacerbation taking either nebulised magnesium sulfate added to standard medical therapy (SMT) versus SMT alone. PubMed, Embase, and Cochrane Library were systematically searched for randomised clinical trials (RCT) comparing the use of SMT with vs. without nebulised magnesium. The outcomes were respiratory rate, heart rate, % predicted peak expiratory flow rate (PEFR), % predicted forced expiratory volume (FEV1), peripheral O2 saturation, asthma severity scores, and need for intravenous (IV) bronchodilator use. Twelve RCTs and 2484 children were included. Mean age was 5.6 (range 2-17) years old, mean baseline % predicted FEV1 was 69.6%, and 28.66% patients were male. Children treated with magnesium had a significantly higher % predicted PEFR (mean difference [MD] 5.33%; 95% confidence interval [CI] 4.75 to 5.90%; p < 0.01). Respiratory rate was significantly lower in the magnesium group (MD -0.70 respirations per minute; 95% CI -1.24 to -0.15; p < 0.01). Need for IV bronchodilators, % predicted FEV1, heart rate, asthma severity scores, and O2 saturation were not significantly different between groups.

CONCLUSION

In children with asthma exacerbation, treatment with nebulised magnesium and SMT was associated with a statistically significant, but small improvement in predicted PEFR and respiratory rate, as compared with SMT alone.

WHAT IS KNOWN

• Magnesium sulfate has bronchodilating properties and aids in the treatment of asthma exacerbation when administered intravenously. • There is no significant evidence of benefit of nebulised magnesium as an adjunct therapy to the standard medical treatment for children with asthma exacerbations.

WHAT IS NEW

• Our study suggests nebulised magnesium sulfate may have a statistically significant, but small benefit in respiratory rate and peak expiratory flow rate. The addition of nebulised magnesium does not seem to increase adverse events.

Bronchial Cryo-Denervation for Severe Asthma: A Pilot Study

INTRODUCTION

Targeting the parasympathetic nervous system innervating the airway with pharmacologic products has been proved to improve the clinical outcomes of severe asthma. Bronchial cryo-denervation (BCD) is a novel non-pharmacologic treatment for severe asthma using an endobronchial cryo-balloon administered via bronchoscopy to denervate parasympathetic pulmonary nerves. Preclinical studies have demonstrated that BCD significantly disrupted vagal innervation in the lung.

METHODS

A total of 15 patients with severe asthma were enrolled in this prospective, single-center pilot study. Patients underwent bifurcated BCD treatment at a 30-day interval after baseline assessment. Follow-up through 12 months included assessment of adverse events, technical feasibility, and changes in pulmonary function; asthma control questionnaire-7 (ACQ-7); and asthma control test (ACT).

RESULTS

BCD was performed on all 15 severe asthma patients, with technical feasibility of 96.7%. There were no device-related and 2 procedure-related serious adverse events through 12 months, which resolved without sequelae. The most frequent nonserious procedure-related adverse event was increased cough in 60% (9 of 15) patients. Pulmonary function remained unchanged, and significant improvements from baseline ACQ-7 (mean, -1.19, p = 0.0032) and ACT (mean, 3.18, p = 0.0011) scores were observed since the first month's follow-up after a single lung airway treatment, with similar trends till the end of the 12-month follow-up.

CONCLUSION

This study provides the first clinical evidence of the safety, feasibility, and initial efficacy of BCD in patients with severe asthma.

An underlying mechanism behind interventional pulmonology techniques for refractory asthma treatment: Neuro-immunity crosstalk

Asthma is a common disease that seriously threatens public health. With significant developments in bronchoscopy, different interventional pulmonology techniques for refractory asthma treatment have been developed. These technologies achieve therapeutic purposes by targeting diverse aspects of asthma pathophysiology. However, even though these newer techniques have shown appreciable clinical effects, their differences in mechanisms and mutual commonalities still deserve to be carefully explored. Therefore, in this review, we summarized the potential mechanisms of bronchial thermoplasty, targeted lung denervation, and cryoablation, and analyzed the relationship between these different methods. Based on available evidence, we speculated that the main pathway of chronic airway inflammation and other pathophysiologic processes in asthma is sensory nerve-related neurotransmitter release that forms a "neuro-immunity crosstalk" and amplifies airway neurogenic inflammation. The mechanism of completely blocking neuro-immunity crosstalk through dual-ablation of both efferent and afferent fibers may have a leading role in the clinical efficacy of interventional pulmonology in the treatment of asthma and deserves further investigation.

A microphysiological system combining electrospun fibers and electrical stimulation for the maturation of highly anisotropic cardiac tissue

The creation of cardiac tissue models for preclinical testing is still a non-solved problem in drug discovery, due to the limitations related to thein vitroreplication of cardiac tissue complexity. Among these limitations, the difficulty of mimicking the functional properties of the myocardium due to the immaturity of the used cells hampers the obtention of reliable results that could be translated into human patients.In vivomodels are the current gold standard to test new treatments, although it is widely acknowledged that the used animals are unable to fully recapitulate human physiology, which often leads to failures during clinical trials. In the present work, we present a microfluidic platform that aims to provide a range of signaling cues to immature cardiac cells to drive them towards an adult phenotype. The device combines topographical electrospun nanofibers with electrical stimulation in a microfabricated system. We validated our platform using a co-culture of neonatal mouse cardiomyocytes and cardiac fibroblasts, showing that it allows us to control the degree of anisotropy of the cardiac tissue inside the microdevice in a cost-effective way. Moreover, a 3D computational model of the electrical field was created and validated to demonstrate that our platform is able to closely match the distribution obtained with the gold standard (planar electrode technology) using inexpensive rod-shaped biocompatible stainless-steel electrodes. The functionality of the electrical stimulation was shown to induce a higher expression of the tight junction protein Cx-43, as well as the upregulation of several key genes involved in conductive and structural cardiac properties. These results validate our platform as a powerful tool for the tissue engineering community due to its low cost, high imaging compatibility, versatility, and high-throughput configuration capabilities.

Monophasic and biphasic electrical stimulation induces a precardiac differentiation in progenitor cells isolated from human heart

Electrical stimulation (ES) of cells has been shown to induce a variety of responses, such as cytoskeleton rearrangements, migration, proliferation, and differentiation. In this study, we have investigated whether monophasic and biphasic pulsed ES could exert any effect on the proliferation and differentiation of human cardiac progenitor cells (hCPCs) isolated from human heart fragments. Cells were cultured under continuous exposure to monophasic or biphasic ES with fixed cycles for 1 or 3 days. Results indicate that neither stimulation protocol affected cell viability, while the cell shape became more elongated and reoriented more perpendicular to the electric field direction. Moreover, the biphasic ES clearly induced the upregulation of early cardiac transcription factors, MEF2D, GATA-4, and Nkx2.5, as well as the de novo expression of the late cardiac sarcomeric proteins, troponin T, cardiac alpha actinin, and SERCA 2a. Both treatments increased the expression of connexin 43 and its relocation to the cell membrane, but biphasic ES was faster and more effective. Finally, when hCPCs were exposed to both monophasic and biphasic ES, they expressed de novo the mRNA of the voltage-dependent calcium channel Cav 3.1(α1G) subunit, which is peculiar of the developing heart. Taken together, these results show that ES alone is able to set the conditions for early differentiation of adult hCPCs toward a cardiac phenotype.

Tyrosine urea muscarinic acetylcholine receptor antagonists: achiral quaternary ammonium groups

Tyrosine ureas had been identified as potent muscarinic receptor antagonists with promising in vivo activity. Controlling the stereochemistry of the chiral quaternary ammonium center had proved to be a serious issue for this series, however. Herein we describe the preparation and SAR of tyrosine urea antagonists containing achiral quaternary ammonium centers. The most successful such moiety was the 2-methylimidazo[2,1-b][1,3]thiazol-7-ium group which yielded highly potent antagonists with long duration of action in an inhaled animal model of bronchoconstriction.

Models to understand contractile function in the airways

Although the role of contractile function in the airways is controversial, there is general consensus on the importance of airway smooth muscle (ASM) as a therapeutic target for diseases characterized by airway obstruction, such as asthma or chronic obstructive pulmonary disease. Indeed, the use of bronchodilators to relax ASM is the most common and effective practice to treat airflow obstruction. Excessive pathologic bronchoconstriction may originate from primary alterations of ASM mechanical function and/or from the effects exerted on ASM function by disease processes, such as inflammation and remodeling. An in depth knowledge of the potentially multiple mechanisms that distinctively regulate primary and secondary alterations in ASM contractile function would be essential for the development of new therapeutic approaches aimed at preventing the occurrence or reducing the severity of bronchoconstriction. The present review discusses studies that have addressed the mechanisms of altered ASM contractile function in models of airway hyperresponsiveness. Although not comprehensively, in the present review, animal models of intrinsic airway hyperresponsiveness, normal ontogenesis, and allergic sensitization are analyzed in the attempt to summarize the current knowledge on regulatory mechanisms of ASM contractile function in health and disease. Studies in human ASM and the need for additional models to understand contractile function in the airways are also discussed.

The effect of asthma therapeutics on signalling and transcriptional regulation of airway smooth muscle function

SCOPE OF THE REVIEW

Our knowledge of the multifunctional nature of airway smooth muscle (ASM) has expanded rapidly in the last decade, but the underlying molecular mechanisms and how current therapies for obstructive airway diseases, such as asthma and chronic obstructive pulmonary disease (COPD), affect these are still being elucidated. Our current knowledge has built on the pharmacology of human ASM contraction and relaxation established prior to that and which is reviewed in detail elsewhere in this issue. The advent of methods to isolate and culture ASM cells, especially human ASM cells, has made it possible to study how they may contribute to airway remodelling through their synthetic, proliferative, and migratory capacities. Now the underlying molecular mechanisms of ASM growth factor secretion, extracellular matrix (ECM) production, proliferation and migration, as well as contraction and relaxation, are being determined. A complex network of signalling pathways leading to gene transcription in ASM cells permits this functional plasticity in healthy and diseased airways. This review is an overview of the effects of current therapies, and some of those in development, on key signalling pathways and transcription factors involved in these ASM functions.

Biphasic electrical field stimulation aids in tissue engineering of multicell-type cardiac organoids

The main objectives of current work were (1) to compare the effects of monophasic or biphasic electrical field stimulation on structure and function of engineered cardiac organoids based on enriched cardiomyocytes (CM) and (2) to determine if electrical field stimulation will enhance electrical excitability of cardiac organoids based on multiple cell types. Organoids resembling cardiac myofibers were cultivated in Matrigel-coated microchannels fabricated of poly(ethylene glycol)-diacrylate. We found that field stimulation using symmetric biphasic square pulses at 2.5 V/cm, 1 Hz, 1 ms (per pulse phase) was an improved stimulation protocol, as compared to no stimulation and stimulation using monophasic square pulses of identical total amplitude and duration (5 V/cm, 1 Hz, 2 ms). This was supported by the highest success rate for synchronous contractions, low excitation threshold, the highest cell density, and the highest expression of Connexin-43 in the biphasic group. Subsequently, enriched CM were seeded on the networks of (1) cardiac fibroblasts (FB), (2) D4T endothelial cells (EC), or (3) a mixture of FB and EC that were precultured for 2 days prior to the addition of enriched CM. Biphasic field stimulation was also effective at improving electrical excitability of these cardiac organoids by improving the three-dimensional organization of the cells, increasing cellular elongation and enhancing Connexin-43 presence.

M2 muscarinic receptors induce airway smooth muscle activation via a dual, Gbetagamma-mediated inhibition of large conductance Ca2+-activated K+ channel activity

Airway smooth muscle is richly endowed with muscarinic receptors of the M(2) and M(3) subtype. Stimulation of these receptors inhibits large conductance calcium-activated K(+) (BK) channels, a negative feed back regulator, in a pertussis toxin-sensitive manner and thus facilitates contraction. The underlying mechanism, however, is unknown. We therefore studied the activity of bovine trachea BK channels in HEK293 cells expressing the M(2) or M(3) receptor (M(2)R or M(3)R). In M(2)R- but not M(3)R-expressing cells, maximal effective concentrations of carbamoylcholine (CCh) inhibited whole cell BK currents by 53%. This M(2)R-induced inhibition was abolished by pertussis toxin treatment or overexpression of the Gbetagamma scavenger transducin-alpha. In inside-out patches, direct application of 300 nm purified Gbetagamma decreased channel open probability by 55%. The physical interaction of Gbetagamma with BK channels was confirmed by co-immunoprecipitation. Interestingly, inhibition of phospholipase C as well as protein kinase C activities also reversed the CCh effect but to a smaller (approximately 20%) extent. Mouse tracheal cells responded similarly to CCh, purified Gbetagamma and phospholipase C/protein kinase C inhibition as M(2)R-expressing HEK293 cells. Our results demonstrate that airway M(2)Rs inhibit BK channels by a dual, Gbetagamma-mediated mechanism, a direct membrane-delimited interaction, and the activation of the phospholipase C/protein kinase C pathway.

Effect of electroporation on cardiac electrophysiology

Defibrillation shocks are commonly used to terminate life-threatening arrhythmias. According to the excitation theory of defibrillation, such shocks are aimed at depolarizing the membranes of most cardiac cells, resulting in resynchronization of electrical activity in the heart. If shock-induced transmembrane potentials are large enough, they can cause transient tissue damage due to electroporation. In this review, evidence is presented that electroporation of the heart tissue can occur during clinically relevant intensities of the external electrical field and that electroporation can affect the outcome of defibrillation therapy, being both pro- and antiarrhythmic.Here, we present experimental evidence for electroporation in cardiac tissue, which occurs above a threshold of 25 V/cm as evident from propidium iodide uptake, transient diastolic depolarization, and reductions of action potential amplitude and its derivative. These electrophysiological changes can induce tachyarrhythmia, due to conduction block and possibly triggered activity; however, our findings provide the foundation for future design of effective methods to deliver genes and drugs to cardiac tissues, while avoiding possible side effects such as arrhythmia and mechanical stunning.

Age differences in cholinergic airway responsiveness in relation with muscarinic receptor subtypes

Children seem more susceptible to increased airway reactivity than adults. Such an age-dependent discrepancy in airway reactivity may involve different airway smooth muscle functions. Therefore, we compared the in vivo and in vitro responsiveness of airway smooth muscles between two age groups of animals. Rats of 6 and 21 weeks old were challenged in vivo with acetylcholine (ACh) infused intravenously and airway resistance (R(aw)) was measured. Tracheal muscle was also isolated and the isometric force developed to ACh or KCl was measured. Furthermore, the level of genes encoding muscarinic receptor subtypes (M(1-3)) and acetylcholinesterase (AChE) expressed in the tracheal muscle was determined by RT-PCR. In results, the basal R(aw) was similar in the two age groups. The R(aw) at each ACh dose was significantly greater in young rats than older rats (p<0.05, n=22-27). Tracheal muscles from young rats were more sensitive to ACh than older rats (p<0.05, n=20-21), while receptor-independent muscle contraction to KCl was greater in older rats (p<0.05, n=10-19). Genes encoding AChE, M(2) and M(3) muscarinic receptors were more highly expressed in the tracheal muscles from young than older rats (p<0.05, n=4-6). In conclusion, airway smooth muscle in young rat is more sensitive to cholinergic stimulation in vivo and in vitro compared to older rats, which may be due to a higher expression of M(2) and M(3) muscarinic receptors in airway smooth muscle.

The airway cholinergic system: physiology and pharmacology

The present review summarizes the current knowledge of the cholinergic systems in the airways with special emphasis on the role of acetylcholine both as neurotransmitter in ganglia and postganglionic parasympathetic nerves and as non-neuronal paracrine mediator. The different cholinoceptors, various nicotinic and muscarinic receptors, as well as their signalling mechanisms are presented. The complex ganglionic and prejunctional mechanisms controlling the release of acetylcholine are explained, and it is discussed whether changes in transmitter release could be involved in airway dysfunctions. The effects of acetylcholine on different target cells, smooth muscles, nerves, surface epithelial and secretory cells as well as mast cells are described in detail, including the receptor subtypes involved in signal transmission.

Surgical Open-Chest Ventricular Defibrillation:. Triphasic Waveforms Are Superior to Biphasic Waveforms

Triphasic shocks have been evaluated for endocardial defibrillation but not for open-chest epicardial defibrillation. The purpose of this study was to compare the efficacy and safety of biphasic versus triphasic shocks for epicardial defibrillation in a porcine model. Twenty-two adult swine (18-28 kg) were deeply anesthetized and intubated. After 30 seconds electrically induced VF, each pig received truncated exponential biphasic (7.2-ms positive pulse duration and 7.2-ms negative pulse duration, total waveform duration 14.4 ms) and triphasic (4.8/4.8/4.8 ms, total waveform duration 14.4 ms) epicardial shocks. Pigs in group 1 (n = 11) received epicardial biphasic and triphasic shocks from large hand held paddle electrodes (44.2 cm2); pigs in group 2 (n = 11) received shocks from small paddle electrodes (15.9 cm2). Shocks were given at five selected energy levels (3-30 J) in random sequence. Four shocks were delivered at each energy level to construct an energy versus percentage of success curve. In group 1 (large paddle electrodes), percentage of shock success was significantly higher for triphasic shocks at the energy levels of 3, 5, 10, and 20 J compared to biphasic shocks. In group 2 (small paddle electrodes), triphasic shocks yielded a significantly higher percentage of shock success than biphasic shocks at the energy levels of 5, 10, and 20 J). Shock induced ventricular tachycardia was similar for both waveforms; asystole was rare. For open-chest defibrillation, triphasic waveform shocks were superior to biphasic waveform shocks for VF termination at energy levels of 3-20 J and were as safe as biphasic shocks.

Clinical efficacy of a wearable defibrillator in acutely terminating episodes of ventricular fibrillation using biphasic shocks

The Wearable Cardioverter Defibrillator (WCD) automatically detects and treats ventricular tachyarrhythmias without the need for assistance from a bystander, while at the same time allowing the patient to ambulate freely. It represents an alternative to emergency medical services for outpatient populations with a temporary risk of sudden cardiac death. While the original devices used a monophasic truncated exponential waveform for cardioversion/defibrillation shocks, a new, biphasic shock was developed for the next device generation. In 12 patients undergoing electrophysiological testing for ventricular tachyarrhythmias, termination of electrically induced ventricular fibrillation (VF) was attempted via the WCD. In 22 episodes, induced VF was promptly terminated by the first 70 J (n=12) or 100 J (n=10) biphasic shocks. Time between arrhythmia initiation and shock delivery was 22 +/- 6 seconds (70 J) and 21 +/- 6 seconds (100 J) (P=NS). The measured transthoracic impedance was 71 +/- 5 Ohms (64-79 Ohms) for the 70 J shock and 64 +/- 8 Ohms (47-72 Ohms) for the 100 J shock. The present study demonstrates that a single low energy biphasic shock delivered by the WCD, reliably terminates electrically induced VF (100% of episodes). The results of this study suggest that there is an acceptable safety margin to the maximum output of the device (150 J). Despite our promising data, we recommend that programming all shocks for maximum energy output should be done when using the WCD in ambulatory patients.

The use of ipratropium bromide for the management of acute asthma exacerbation in adults and children: a systematic review

Ipratropium bromide is a quaternary anticholinergic bronchodilator that is commonly used to treat obstructive lung disease. Although ipratropium is not usually employed as a first-line bronchodilator to treat chronic asthma, it has been used extensively in hospital emergency departments as adjunctive therapy for the emergency treatment of acute asthma exacerbation. This review will summarize the physiological actions of ipratropium and the rationale for its use as an anticholinergic bronchodilator. Evidence available from randomized trials and from two meta-analyses is summarized to determine whether the addition of inhaled ipratropium to inhaled beta2-agonist therapy is effective in the treatment of acute asthma exacerbation in children and adults. Published reports of randomized, controlled trials assessing the use of ipratropium and concurrent beta2-agonists in adult acute asthma exacerbation were identified by a search of electronic databases, as well as by hand searching. Data from 10 studies of adult asthmatics, reporting on a total of 1377 patients, were pooled in a meta-analysis using a weighted-average method. Use of nebulized ipratropium/beta2-agonist combination therapy was associated with a pooled 7.3% improvement in forced expiratory volume in 1 sec [95% confidence interval (CI), 3.8-10.9%] and a 22.1% improvement in peak expiratory flow (95% CI, 11.0-33.2%) compared with patients who received beta2-agonist without ipratropium. For the three trials in adults reporting hospital admission data (n = 1064), adult patients receiving ipratropium had a relative risk of hospitalization of 0.80 (95% CI, 0.61-1.06). Similarly, randomized controlled studies of pediatric asthma exacerbation and a meta-analysis of pediatric asthma patients suggest that ipratropium added to beta2-agonists improves lung function and also decreases hospitalization rates, especially among children with severe exacerbations of asthma. The adult and pediatric studies did not report any severe adverse effects attributable to ipratropium when it was used in conjunction with beta2-agonists. In conclusion, there is a modest statistical improvement in airflow obstruction when ipratropium is used as an adjunctive to beta2-agonists for the treatment of acute asthma exacerbation. In pediatric asthma exacerbation, use of ipratropium also appears to improve clinical outcomes; however, this has not been definitively established in adults. It would seem reasonable to recommend the use of combination ipratropium/beta2-agonist therapy in acute asthmatic exacerbation, since the addition of ipratropium seems to provide physiological evidence of benefit without risk of adverse effects.

Bench to bedside: resuscitation from prolonged ventricular fibrillation

Ventricular fibrillation (VF) remains the most common cardiac arrest heart rhythm. Defibrillation is the primary treatment and is very effective if delivered early within a few minutes of onset of VF. However, successful treatment of VF becomes increasingly more difficult when the duration of VF exceeds 4 minutes. Classically, successful cardiac arrest resuscitation has been thought of as simply achieving restoration of spontaneous circulation (ROSC). However, this traditional approach fails to consider the high early post-cardiac arrest mortality and morbidity and ignores the reperfusion injuries, which are manifest in the heart and brain. More recently, resuscitation from cardiac arrest has been divided into two phases; phase I, achieving ROSC, and phase II, treatment of reperfusion injury. The focus in both phases of resuscitation remains the heart and brain, as prolonged VF remains primarily a two-organ disease. These two organs are most sensitive to oxygen and substrate deprivation and account for the vast majority of early post-resuscitation mortality and morbidity. This review focuses first on the initial resuscitation (achieving ROSC) and then on the reperfusion issues affecting the heart and brain.

Electrophysiology of ventricular fibrillation and defibrillation

The survival rate from ventricular fibrillation is very high for short-duration fibrillation (<30 secs) but decreases to approximately 3% to 30% in out-of-hospital conditions. During short-duration fibrillation, action potentials occur rapidly with no intervening period of electrical diastole; a shock defibrillates by interacting with the fibrillation action potential to produce a uniformly long postshock extension of refractoriness. In contrast, during long-duration fibrillation, ischemia-induced degradation of cellular electrophysiology occurs, which causes intervening periods of electrical diastole between fibrillation action potentials and, thus, slowing of fibrillation frequency. A successful defibrillation shock must now not only prolong refractoriness when delivered during the action potential but must also excite cells during the periods of depolarized diastole. Biphasic waveforms enhance both effects by causing premature membrane repolarization with the first pulse, thereby allowing sodium channel recovery from inactivation so that the second pulse produces better-formed responses both during the cellular action potential and during the depolarized diastole. Therefore, biphasic waveforms remain superior to monophasic waveforms for treatment of long-duration fibrillation. Improved understanding of the ischemia-induced changes in cellular electrophysiology will suggest further improvements in both defibrillator waveforms and resuscitation techniques.

Success and failure of the defibrillation shock: insights from a simulation study

INTRODUCTION

This simulation study presents a further inquiry into the mechanisms by which a strong electric shock fails to halt life-threatening cardiac arrhythmias.

METHODS AND RESULTS

The research uses a model of the defibrillation process that represents a sheet of myocardium as a bidomain. The tissue consists of nonuniformly curved fibers in which spiral wave reentry is initiated. Monophasic defibrillation shocks are delivered via two line electrodes that occupy opposite tissue boundaries. In some simulation experiments, the polarity of the shock is reversed. Electrical activity in the sheet is compared for failed and successful shocks under controlled conditions. The maps of transmembrane potential and activation times calculated during and after the shock demonstrate that weak shocks fail to terminate the reentrant activity via two major mechanisms. As compared with strong shocks, weak shocks result in (1) smaller extension of refractoriness in the areas depolarized by the shock, and (2) slower or incomplete activation of the excitable gap created by deexcitation of the negatively polarized areas. In its turn, mechanism 2 is associated with one or more of the following events: (a) lack of some break excitations, (b) latency in the occurrence of the break excitations, and (c) slower propagation through deexcited areas. Reversal of shock polarity results in a change of the extent of the regions of deexcitation, and thus, in a change in defibrillation threshold.

CONCLUSION

The results of this study indicate the paramount importance of shock-induced deexcitation in both defibrillation and postshock arrhythmogenesis.

Electrophysiologic deterioration after one-minute fibrillation increases relative biphasic defibrillation efficacy

INTRODUCTION

The probability of survival decreases to 70% after 2 minutes of ventricular fibrillation. Biphasic shocks are more effective than monophasic shocks in terminating short-duration (<30 sec) ventricular fibrillation. We tested the hypotheses that developing ischemia changes the electrophysiologic characteristics of fibrillation and that the relative efficacy of biphasic shocks increases as electrophysiologic characteristics deteriorate.

METHODS AND RESULTS

Monophasic (12 msec) and biphasic (6/6 msec) shocks (1 to 4 A) were tested in random order in isolated rabbit hearts after 1-minute ischemic fibrillation. Monophasic action potentials showed only a sporadic occurrence of electrical diastole after 5 seconds of fibrillation (24% of action potentials in the right ventricle and 18% in the left ventricle). After 60 seconds of fibrillation, diastole (17.83+/-1.14 msec in the right ventricle and 21.52+/-1.16 msec in the left ventricle) appeared after almost every action potential (P < 0.0001 compared with 5 sec), despite a lack of change in fibrillation cycle length and dominant frequency. Monophasic I50 was 2.89 A, and biphasic I50 was 1.4 A (77% reduction in energy). Normalized curve width decreased 28%. Retrospective analysis showed that shocks delivered early in the fibrillation action potential had a greater probability of succeeding (89%) than shocks delivered late (30%; P < 0.001).

CONCLUSION

After 1-minute ischemic fibrillation, diastolic intervals occur during fibrillation. Therefore, defibrillation shocks have an approximately 29% probability of interacting with the fibrillation action potential during diastole. At this time, biphasic shocks produced a more deterministic defibrillation threshold and became even more efficacious (I50 B/M = 0.48) than at short fibrillation durations (I50 B/M = 0.7).

Regional differences in arrhythmogenic aftereffects of high intensity DC stimulation in the ventricles

Regional differences of the aftereffects of high intensity DC stimulation were investigated in isolated rabbit hearts stained with a voltage-sensitive dye (di-4-ANEPPS). Optical action potential signals were recorded from the epicardial surface of the right and left ventricular free wall (RVep, LVep) and from the right endocardial surface of the interventricular septum (IVS). Ten-millisecond monophasic DC stimulation (S2, 20-120 V) was applied to the signal recording spots during the early plateau phase of the action potential induced by basic stimuli (S1, 2.5 Hz). There was a linear relationship between S2 voltage and the S2 field intensity (FI). S2 caused postshock additional depolarization, giving rise to a prolongation of the shocked action potential. With S2 > or = 40 V (FI > or = approximately 20 V/cm), terminal repolarization of action potential was inhibited, and subsequent postshock S1 action potentials for 1-5 minutes were characterized by a decrease in the maximum diastolic potential and a decrease in the amplitude and a slowing of their upstroke phase. The higher the S2 voltage, the larger the aftereffects. The changes in postshock action potential configuration in RVep were significantly greater than those observed in LVep and IVS when compared at the same levels of S2 intensity. In RVep, 12 of 20 shocks of 120 V resulted in a prolonged refractoriness to S1 (> 1 s), and the arrest was often followed by oscillation of membrane potential. Ventricular tachycardia or fibrillation ensued from the oscillation in five cases. No such long arrest or serious arrhythmias were elicited in LVep and IVS. These results suggest that RVep is more susceptible than LVep and IVS for arrhythmogenic aftereffects of high intensity DC stimulation.

Effects of inhalation exposures to an M1-receptor agonist on ventilation in rhesus monkeys

Information was needed on effects of possible occupational inhalation exposure to an M1-receptor agonist (xanomeline) such as might occur during the manufacturing process. Both acute and repeated inhalation exposures to xanomeline were carried out in six male rhesus monkeys using a head-dome exposure system. Exposure concentrations ranged from 0.3 to 10 mg/m3. The exposure durations were up to 2 weeks. Decreases in tidal volume and increases in respiratory frequency were both time and concentration related during acute exposures. These effects were blocked with atropine pre-treatment. Correlation with pulmonary resistance measurements in two monkeys suggested that these were bronchoconstrictive changes that increased with severity with time at a given concentration and with concentration when measured after a constant exposure time. The dose-response was relatively steep with 10 mg/m3 becoming intolerable to the monkeys after approximately 15 minutes, but no measurable effects were observed at 0.3 mg/m3 after up to 4 hours of exposure. To investigate the effects of repeated exposures, monkeys were exposed for 4 hr/day, 5 days/wk for 2 weeks to 0.0 (air only), 0.3, and 1.2 mg xanomeline/m3 of air. When compared to the air-only exposure, 0.3 mg/m3 caused no significant changes in tidal volume. In contrast, 1.2 mg/m3 caused a rapid and significant decrease in tidal volume that was sustained throughout the 4-hr exposure. A slower rise in breathing frequency also occurred. Repeated exposures did not alter the effects seen after a single exposure. It is concluded that xanomeline, a M1-receptor agonist, can acutely alter normal ventilation in non-human primates at airborne concentrations > or = 0.6 mg/m3 and should be carefully controlled in a manufacturing environment. The no-observed-effect concentration was 0.3 mg/m3.

Separation between virtual sources modifies the response of cardiac tissue to field stimulation

INTRODUCTION

While it is now understood that the tissue geometry and the electric field distribution are important in generating virtual electrodes, the effects of interaction between a collection of electrodes have not been examined. To develop a basis for understanding such interactions, we have studied a single pair of oppositely polarized virtual sources. Although such oppositely polarized pairs of virtual electrodes can be generated by a variety of field distributions and tissue geometries, we examine one simple system that incorporates the salient features of source interaction.

METHODS AND RESULTS

Our model system is a homogeneous tissue strip stimulated by a uniform extracellular field. To clarify virtual source interaction, we show that field stimulated tissue can be equivalently polarized by a set of intracellular current sources with magnitude and distribution defined by the generalized activating function. In our model system, an intracellular current source is produced at one edge of the tissue and an intracellular current sink at the other. Therefore, the tissue length acts to modulate the overlap, or interaction, between the polarizations arising from each source. To quantify the effects of source interaction, the chronaxie and rheobase values of the strength-duration relation were determined for source separations varying between 1.0 cm and 100 microm (active membrane dynamics were modeled with the Luo-Rudy phase I formulation). At all separations >3.0 mm, the chronaxie was constant at 3.09 msec and the rheobase was 0.38 V/cm. Under 0.2 mm, the chronaxie decreased to 0.55 msec while the rheobase increased linearly with the inverse of source separation. The dependence of these parameters on separation primarily reflects passive electrotonic interactions between the two virtual electrodes. However, the exact values are strongly dependent upon active tissue properties-largely the inward rectifier potassium channel and activation of the sodium current.

CONCLUSION

Tissue excitation in response to field stimulation is strongly modulated by the proximity of, and therefore the interaction between, oppositely polarized virtual electrode sources.

Effects of electroporation on the transmembrane potential distribution in a two-dimensional bidomain model of cardiac tissue

INTRODUCTION

Defibrillation shocks, when delivered through internal electrodes, establish transmembrane potentials (Vm) large enough to electroporate the membrane of cardiac cells. The effects of such shocks on the transmembrane potential distribution are investigated in a two-dimensional rectangular sheet of cardiac muscle modeled as a bidomain with unequal anisotropy ratios.

METHODS AND RESULTS

The membrane is represented by a capacitance Cm, a leakage conductance g(l) and a variable electroporation conductance G, whose rate of growth depends exponentially on the square of Vm. The stimulating current Io, 0.05-20 A/m, is delivered through a pair of electrodes placed 2 cm apart for stimulation along fibers and 1 cm apart for stimulation across fibers. Computer simulations reveal three categories of response to Io: (1) Weak Io, below 0.2 A/m, cause essentially no electroporation, and Vm increases proportionally to Io. (2) Strong Io, between 0.2 and 2.5 A/m, electroporate tissue under the physical electrode. Vm is no longer proportional to Io; in the electroporated region, the growth of Vm is halted and in the region of reversed polarity (virtual electrode), the growth of Vm is accelerated. (3) Very strong Io, above 2.5 A/m, electroporate tissue under the physical and the virtual electrodes. The growth of Vm in all electroporated regions is halted, and a further increase of Io increases both the extent of the electroporated regions and the electroporation conductance G.

CONCLUSION

These results indicate that electroporation of the cardiac membrane plays an important role in the distribution of Vm induced by defibrillation strength shocks.

Altered airway and cardiac responses in mice lacking G protein-coupled receptor kinase 3

Contraction and relaxation of airway smooth muscles is mediated, in part, by G protein-coupled receptors (GPCRs) and dysfunction of these receptors has been implicated in asthma. Phosphorylation of GPCRs, by G protein-coupled receptor kinase (GRK), is an important mechanism involved in the dampening of GPCR signaling. To determine whether this mechanism might play a role in airway smooth muscle physiology, we examined the airway pressure time index and heart rate (HR) responses to intravenous administration of the cholinergic agonist methacholine (MCh) in genetically altered mice lacking one copy of GRK2 (GRK2 +/-), homozygous GRK3 knockout (GRK3 -/-), and wild-type littermates. (GRK2 -/- mice die in utero.) GRK3 -/- mice demonstrated a significant enhancement in the airway response to 100 and 250 microgram/kg doses of MCh compared with wild-type and GRK2 +/- mice. GRK3 -/- mice also displayed an enhanced sensitivity of the airway smooth muscle response to MCh. In addition, GRK3 -/- mice displayed an altered HR recovery from MCh-induced bradycardia. Although direct stimulation of cardiac muscarinic receptors measured as vagal stimulation-induced bradycardia was similar in GRK3 -/- and wild-type mice, the baroreflex increase in HR associated with sodium nitroprusside-induced hypotension was significantly greater in GRK3 -/- than wild-type mice. Therefore, these data demonstrate that in the mouse, GRK3 may be involved in modulating the cholinergic response of airway smooth muscle and in regulating the chronotropic component of the baroreceptor reflex.

Cardiac parasympathetic stimulation via QRS-synchronous low-energy shocks in humans

INTRODUCTION

In patients receiving test shocks to verify lead connections at implantation, we anecdotally have observed postshock delay. The purpose of this study was to determine whether QRS-synchronous low-energy shocks delivered by implantable defibrillators result in postshock cycle length prolongation, and to determine the mechanism of this phenomenon.

METHODS AND RESULTS

Twenty-five patients undergoing defibrillator testing were studied, three with epicardial patches and 22 with transvenous leads. Each patient received QRS-synchronous shocks of 0.2, 0.4, 0.6, and 2.0 J in random order. Patients were further randomized to receive either saline or 2.0 mg atropine intravenously, and then given a second sequence of shocks. At baseline, the postshock cycle length (1,035+/-245 msec) was significantly longer than the preshock cycle length (968+/-177 msec, P = 0.01). In patients with a coronary sinus (CS) or superior vena cava (SVC) lead, the mean prolongation was 91+/-160 msec, compared with 12+/-106 msec for patients without such a lead (P < 0.0001). All energy levels resulted in significant postshock prolongation compared with preshock cycle lengths (P < 0.05). Postshock prolongation before atropine was 76+/-162 msec, compared with -13+/-52 msec afterward (P < 0.00001). Biphasic shocks resulted in greater postshock prolongation than monophasic shocks of equal energy.

CONCLUSION

Low-energy shocks delivered during the QRS complex cause postshock cycle length prolongation in man. This effect required the presence of a CS or SVC lead. Atropine inhibited this effect, suggesting the phenomenon was mediated by direct cardiac parasympathetic nerve stimulation by the intracardiac shock.

Distribution of muscarinic receptor subtypes in rat small intestine

Despite its great promise, small intestinal transplantation in some patients is complicated by difficult postoperative management. The reasons for this are complex. In a rat model of small intestinal transplantation, frequencies of migrating myoelectric complexes during fasting are reduced in ileal isografts and muscarinic receptor density is decreased. We hypothesized that the distribution of muscarinic 1 receptors localized to enteric neurons is altered after small intestinal transplantation. Distal small intestine was orthotopically transplanted in Lewis-to-Lewis donor-recipient combinations. At 3 months, transplanted and normal ileum was obtained to prepare membrane fractions. [N-methyl-3H]Scopolamine served as ligand, while scopolamine methylbromide, pirenzepine, and methoctramine were used in competitive homologous and heterologous displacement experiments. Receptor subtype models were examined by nonlinear regression analysis. In normal and transplanted ileum, heterologous displacement was consistent with three site models (P < 0.05). In normals, the muscarinic 1 receptor subtype was most abundant, with a relative distribution of 69 to 78%. There was a relative distribution of 13 to 16% for muscarinic 3 receptor subtype. After transplantation, the muscarinic 1 subtype decreased to a mean of 45% but the muscarinic 3 subtype increased to a mean of 42%. Using pirenzepine, mean pKD values were not different between the two groups. It is concluded that the decrease in muscarinic 1 receptor subtype after transplantation could be related to neuronal cell loss or to downregulation of the expression of muscarinic 1 receptors. The results did not support defective posttranslational processing of receptor proteins.

Spatiotemporal effects of syncytial heterogeneities on cardiac far-field excitations during monophasic and biphasic shocks

INTRODUCTION

It has recently been postulated that syncytial (anatomic) heterogeneities inherent within cardiac tissue might represent a significant mechanism underlying field-induced polarization of the bulk myocardium. This simulation study examines and characterizes the spatiotemporal excitatory dynamics associated with this newly hypothesized mechanism.

METHODS AND RESULTS

Two-dimensional regions of syncytially heterogeneous cardiac tissue were simulated with active membrane kinetics. Heterogeneities were manifested via random spatial variations of intracellular volume fractions over multiple length scales. Excitation thresholds were determined for uniform rectangular monophasic (M) and symmetric biphasic (B) far-field stimuli, from which strength-duration and strength-interval relationships were constructed. For regions measuring 5.4 x 5.4 mm, baseline diastolic thresholds for longitudinal (L) and transverse (T) shocks of 5-msec total duration averaged (in V/cm, n = 10) M-L = 2.87+/-0.26, M-T = 6.71+/-0.83, B-L = 3.22+/-0.25, and B-T = 7.93+/-0.51. These thresholds decreased by 15% to 25% when the region sizes were increased to 10.8 x 10.8 mm. Strength-duration relationships correlated strongly with the Weiss-Lapicque hyperbolic relationship, with rheobases and chronaxies of 2.33 V/cm and 1.15 msec for M-L stimuli, and 2.28 V/cm and 2.04 msec for B-L stimuli. Strength-interval relationships for M-L and B-L stimuli decreased monotonically with increasing coupling intervals, with similar minimum coupling intervals at absolute refractoriness. However, the B-L thresholds were substantially less sensitive to changes in coupling intervals than their M-L counterparts.

CONCLUSION

This study provides strong additional support for and understanding of the syncytial heterogeneity hypothesis and its manifested properties. Furthermore, these results predict that syncytial heterogeneities of even modest proportions could represent a significant mechanism contributing to the far-field excitation process.

Muscarinic receptors and control of airway smooth muscle

Contraction of airway smooth muscle is mediated by M3 muscarinic receptors on the airway smooth muscle. However, there is no evidence suggesting that hyperresponsiveness results from any alterations in function of these M3 muscarinic receptors. In contrast, there is clearly increased release of the neurotransmitter acetylcholine in animal models of hyperactivity and in asthma. Release of acetylcholine is controlled by inhibitory M2 muscarinic receptors, and it appears that it is these M2 receptors that are dysfunctional in animal models of hyperresponsiveness. Allergen-induced M2 receptor dysfunction is absolutely dependent upon an influx of eosinophils into the airways. Activated eosinophils release major basic protein, which binds to M2 receptors and prevents binding of acetylcholine. Thus, the normal negative feedback control of acetylcholine release is lost, and acetylcholine release is increased. In conclusion, loss of function of inhibitory M2 muscarinic receptors on the airway parasympathetic nerves causes vagally mediated bronchoconstriction and hyperresponsiveness following antigen challenge.

Progressive depolarization: a unified hypothesis for defibrillation and fibrillation induction by shocks

Experimental studies of defibrillation have burgeoned since the introduction of the upper limit of vulnerability (ULV) hypothesis for defibrillation. Much of this progress is due to the valuable work carried out in pursuit of this hypothesis. The ULV hypothesis presented a unified electrophysiologic scheme for linking the processes of defibrillation and shock-induced fibrillation. In addition to its scientific ramifications, this work also raised the possibility of simpler and safer means for clinical defibrillation threshold testing. Recent results from an optical mapping study of defibrillation suggest, however, that the experimental data supporting the ULV hypothesis could instead be interpreted in a manner consistent with traditional views of defibrillation such as the critical mass hypothesis. This review will describe the evidence calling for such a reinterpretation. In one regard the ULV hypothesis superseded the critical mass hypothesis by linking the defibrillation and shock-induced fibrillation processes. Therefore, this review also will discuss the rationale for developing a new defibrillation hypothesis. This new hypothesis, progressive depolarization, uses traditional defibrillation concepts to cover the same ground as the ULV hypothesis in mechanistically unifying defibrillation and shock-induced fibrillation. It does so in a manner consistent with experimental data supporting the ULV hypothesis but which also takes advantage of what has been learned from optical studies of defibrillation. This review will briefly describe how this new hypothesis relates to other contemporary viewpoints and related experimental results.

Low-energy impedance-compensating biphasic waveforms terminate ventricular fibrillation at high rates in victims of out-of-hospital cardiac arrest. LIFE Investigators

INTRODUCTION

New automatic external defibrillators (AEDs), which are smaller, lighter, easier to use, and less costly make the goal of widespread AED deployment and early defibrillation for out-of-hospital cardiac arrest feasible. The objective of this study was to observe the performance of a low-energy impedance-compensating biphasic waveform in the out-of-hospital setting on 100 consecutive victims of sudden cardiac arrest.

METHODS AND RESULTS

AEDs incorporating a 150-J impedance-compensating biphasic waveform were used by 12 EMS systems. Data were obtained from the AED PC card reporting system. Defibrillation was defined as conversion to an organized rhythm or to asystole. Endpoints included: defibrillation efficacy for ventricular fibrillation (VF); restoration of an organized rhythm at the time of patient transfer to an advanced life support (ALS) team or to the emergency department (ED); and time from AED power-on to first defibrillation. The AED correctly identified 44 of 100 patients presenting in VF as requiring a shock (100% sensitivity) and 56 of 100 patients not in VF as not requiring a shock (100% specificity). The time from 911 call to first shock delivery averaged 8.1 +/- 3.0 minutes. A single 150-J biphasic shock defibrillated the initial VF episode in 39 of 44 (89%) patients. The average time from power-on to first defibrillation was 25 +/- 17 seconds. At patient transfer to ALS or ED care, an organized rhythm was present in 34 of 44 (77%) patients presenting with VF. Asystole was present in 7 (16%) and VF in 3 (7%).

CONCLUSIONS

Low-energy impedance-compensating biphasic waveforms terminate long-duration VF at high rates in out-of-hospital cardiac arrest. Use of this waveform allows AED device characteristics consistent with widespread AED deployment and early defibrillation.

The effects of ventricular fibrillation duration and a preceding unsuccessful shock on the probability of defibrillation success using biphasic waveforms in pigs

INTRODUCTION

While the defibrillation threshold has been reported to increase with ventricular fibrillation (VF) duration for monophasic waveforms, the effect of VF duration for biphasic waveforms is unknown.

METHODS AND RESULTS

The ED 50 requirements (the 50% probability of defibrillation success) for an endocardial lead system, which included a subcutaneous array, were determined by logistic regression using a recursive up-down algorithm for a biphasic waveform (6/6 msec). The study was performed in two parts, each with eight pigs. In part 1, ED 50 was compared for shocks delivered after 10 seconds of VF and for shocks delivered after 20 seconds of VF following a failed first shock at 10 seconds. Energy at ED 50 decreased from 6.5 +/- 0.9 J for shocks delivered after 10 seconds of VF to 4.9 +/- 0.8 J (P < 0.01) for shocks delivered after 20 seconds. To determine if improved second shock efficacy was a result of preconditioning by the failed first shock or a function of VF duration, part 2 of the study compared defibrillation efficacy between shocks delivered after 10 seconds of VF with shocks delivered after 20 seconds of VF with and without a failed first shock at 10 seconds. Mean energy at ED 50 decreased from 10.1 +/- 2.4 J for shocks delivered after 10 seconds of VF to 7.9 +/- 2.4 J (P < 0.01) and 7.5 +/- 3.2 J (P < 0.01) for shocks delivered after 20 seconds of VF with and without a failed first shock, respectively. The mean energy at ED 50 for shocks delivered after 20 seconds of VF with and without a failed first shock was not significantly different (P = 0.53). A strong linear correlation for energy at ED 50 was found between shocks delivered after 10 seconds of VF and shocks delivered after 20 seconds of VF following a failed first shock (r = 0.95, P < 0.01).

CONCLUSION

(1) As opposed to monophasic shocks, ED 50 is significantly lower for biphasic shocks delivered after 20 seconds of VF compared with shocks delivered after 10 seconds of VF in pigs. (2) An unsuccessful biphasic shock in pigs does not affect the defibrillation efficacy for a subsequent shock. (3) ED 50 for a biphasic shock delivered after 20 seconds of VF is linearly related to ED 50 for a shock delivered after 10 seconds of VF.

Membrane refractoriness and excitation induced in cardiac fibers by monophasic and biphasic shocks

INTRODUCTION

This modeling study examines the effect of low-intensity monophasic and biphasic waveforms on the response of a refractory cardiac fiber to the defibrillation shock.

METHODS AND RESULTS

Two cardiac fiber representations are considered in this study: a continuous fiber and a discrete fiber that incorporates gap junctions. Each fiber is undergoing a propagating action potential. Shocks of various strengths and coupling intervals are delivered extracellularly at fiber ends during the relative refractory period. In a continuous fiber, monophasic shock strengths of three times the diastolic threshold either elicit no response or, for coupling intervals above 380 msec, reinitiate propagation. In contrast, biphasic shocks of same strength are capable of terminating the existing wavefronts by either invoking a nonpropagating response (coupling intervals 370 to 382 msec) that prolongs the refractory period or inducing wavefront collision (coupling intervals above 400 msec). The fiber response is similar for other shock strengths and when cellular discontinuity is accounted for. Thus, for a refractory fiber, biphasic shocks have only a small "vulnerable" window of coupling intervals over which propagation is reinitiated.

CONCLUSION

At short coupling intervals, a significant extension of refractoriness is generated at regions where the biphasic shock induced hyperpolarization followed by depolarization. At large coupling intervals, the enhanced efficacy of biphasic shocks is associated with their ability to induce wavefront collision, thus decreasing the probability of reinitiating fibrillation. Overall, the defibrillation shock affects the tissue through the induced large-scale hyperpolarization and depolarization, and not through the small-scale transmembrane potential oscillations at cell ends.

[Influence of amiodarone on defibrillation threshold and perioperative complications in patients with implantable cardioverter-defibrillator with transvenous electrodes and biphasic shocks]

In order to evaluate the safety of implantation of a third generation automatic cardioverter-defibrillator (ICD) in patients with amiodarone therapy, we retrospectively analyzed implantation results and perioperative complications in 17 consecutive patients with continued amiodarone therapy and 38 patients without antiarrhythmics at first ICD implantation.ICD implantation could be performed successfully in all 55 patients using a transvenous electrode system. In one of 17 patients with amiodarone (6%) versus three of 38 patients without antiarrhythmics (8%), additional implantation of a subcutaneous patch electrode was necessary to achieve a defibrillation threshold </=24 Joules (p=n.s.). Intraoperatively tested defibrillation threshold was 13+/-5 Joules in the amiodarone group versus 12+/-5 Joules in patients without antiarrhythmics (p=n.s.). The incidence of perioperative complications was not different between patients with amiodarone and patients without antiarrhythmics.In summary, patients with amiodarone did not have higher defibrillation thresholds or a higher incidence of perioperative complications than patients without antiarrhythmics. The occasionally postulated discontinuation of amiodarone with subsequent waiting for the elimination of this antiarrhythmic drug is unnecessary. For final clarification of the influence of amiodarone-therapy on the defibrillation threshold in patients with third generation ICDs, a prospective randomized study in a large patiente cohort with standardized study conditions would be required.

[Mechanisms of electrical defibrillation]

Ventricular fibrillation has been described as a "chaotic, random, asynchronous electrical activity of the ventricles due to repetitive reentrant excitation and/or rapid focal discharge". Reentrant and non-reentrant mechanisms are responsible for the initiation of ventricular fibrillation. After fibrillation has been induced, it is thought that multiple, disorganized, wandering wavelets follow constantly changing reentrant pathways. Electrical defibrillation is the only valid therapeutic approach for ventricular fibrillation. A successful defibrillation shock must be of sufficient strength to stop fibrillation but must not be so strong that damage to the myocardium occurs. The clinical use of the implantable cardioverter/defibrillator device has significantly stimulated research in the field of cardiac defibrillation. In order to develop more efficient shock waveforms and electrode configurations for smaller, and also longer lasting devices, we need a better understanding of the basic mechanisms of defibrillation. The development of computerized electrical mapping systems, capable of recording before, during and after a defibrillation shock, optical recording systems and microelectrodes, for action potential recording before and after the shock application and mathematical models have contributed much to the understanding of defibrillation mechanisms.An electrical shock hits the cardiac cells in different phases of their action potential. This results in 1) direct activation, 2) a "graded response", or 3) no effect. "Graded response" produces prolongation of the action potential and prolongs refractoriness without giving rise to a propagated activation front. Refractory period prolongation in an area that is still refractory at the time of the shock is critical for successful defibrillation. Mapping studies have shown that for successful defibrillation with monophasic shocks a minimal potential gradient of 5-7 V/cm is necessary (the exact value depends on the waveform and the orientation of the cells with respect to the electric field).Several hypotheses have been developed in order to explain the mechanisms that underlie successful defibrillation shocks. This paper will discuss the various theories. The "upper limit of vulnerability" hypothesis for defibrillation states that a successful defibrillation shock must stop existing activation fronts by directly exciting or by prolonging refractoriness just in front of the upcoming activation fronts and must not give rise to new activation fronts at the border of the directly excited area. Shocks slightly weaker then necessary to defibrillate stop fibrillation activation fronts, but give rise to new activation fronts that reinitiate fibrillation. These new activation fronts arise at a "critical point," where a critical shock potential gradient interferes with a critical degree of tissue refractoriness. Mappping studies support the "upper limit of vulnerability" hypothesis of defibrillation but not all defibrillation failures, however, can be explained by this hypothesis.Clinical data and experimental results have shown that biphasic shocks may have lower defibrillation thresholds than monophasic shocks. The advantage of defibrillation with a biphasic waveform is not yet clearly understood. We discuss some possible reasons why some biphasic waveforms have lower defibrillation thresholds than monophasic waveforms.

Polarity reversal improves defibrillation efficacy in patients undergoing transvenous cardioverter defibrillator implantation with biphasic shocks

The purpose of this study was to determine the influence of polarity reversal on DFT in patients undergoing implantation of nonthoracotomy defibrillators with biphasic shocks. Previous studies have shown higher defibrillation efficacy with using the distal electrode as anode implantation of nonthoracotomy defibrillators and monophasic shocks. However, it is as yet unclear whether biphasic shock defibrillation will also be influenced by polarity reversal. Using a transvenous lead system with a proximal electrode in the superior caval vein and a distal electrode in the RV apex, 27 patients undergoing defibrillator implantation were randomized to DFT testing "initial" (distal electrode = cathode) or "reversed" polarity (distal electrode = anode). Defibrillation energy was reduced stepwise until defibrillation failure occurred. At this point, polarity was switched and testing continued until the lowest energy requirement was determined for both polarities. With reversed polarity, DFT was 11.1 +/- 5.7 J versus 13.3 +/- 5.8 J with polarity (P = 0.033). This means a 17% reduction of the DFT. In 10 patients, the threshold was lower with reversed, whereas in 3 patients it was lower with initial polarity. In conclusion, changing electrode polarity in transvenous implantable defibrillators with biphasic shocks may significantly influence defibrillation energy requirements. Therefore, polarity reversal should always be attempted before considering patch implantation.

Calcium dynamics in cultured heart cells exposed to defibrillator-type electric shocks

Spatial and temporal changes in intracellular calcium ion concentration and transmembrane voltage were recorded optically from single-isolated cultured chick-embryo heart cells exposed to high-voltage, defibrillator-type shocks. Fluorescence changes were measured during 5 msec electric shocks of field strengths up to 56 volts/cm in single myocytes stained with a Ca(++)-sensitive or voltage-sensitive dye. Shocks caused a reversible period of depolarization, elevated cytosolic Ca++, and refractoriness. Intracellular Ca++ elevation had two temporal phases: first, a Ca++ spike with morphology independent of shock intensity; and second, a prolonged Ca++ elevation with a shock-intensity-dependent magnitude and duration, and with greatest Ca++ elevation at the poles of the cell adjacent to the electrodes. The prolonged elevation (second phase) was initiated earlier at the anode-facing pole of the cell than at the cathode-facing pole. These results suggest that postshock Ca++ entry consists of two parts: early normal entry through excitation channels plus a prolonged elevation which may be related to cellular damage.

Improved sensing signals after endocardial defibrillation with a redesigned integrated sense pace defibrillation lead

Adequate sensing is a basic requirement for appropriate therapy with ICDs. Integrated sense pace defibrillation leads, which facilitate ICD implantation, show a close proximity of sensing and defibrillation electrodes that might affect the sensing signal amplitude by the high currents of internal defibrillation. In 99 patients, we retrospectively examined two integrated sense pace defibrillation leads, either both with a distance of 6 mm between the tip of the lead (sensing cathode) and the right ventricular defibrillation electrode (sensing anode) or one with a distance of 12 mm. Three seconds after a shock of 20 J, mean sensing signal amplitude during sinus rhythm (SR) decreased from 10.5 +/- 4.3 mV to 5.1 +/- 3.7 mV (P < 0.001) for the 6-mm lead, but showed no significant decrease for the 12-mm lead. The degree of signal reduction was inversely related to the time passed since defibrillation. Significant differences in reduction of sensing signal amplitude concerning monophasic and biphasic shocks could not be observed. Mean sensing signal amplitude of VF after shocks that failed to terminate it decreased in the same order as during SR (from 8.3 +/- 4.1 mV to 4.1 +/- 3.2 mV), but resulted in no failure of redetection during ongoing VF. DFTs did not differ for the 6-mm and the 12-mm lead. In conclusion, close proximity of the right ventricular defibrillation coil to the sensing tip of an integrated sense pace defibrillation lead causes energy and time related reduction in sensing signal amplitude after defibrillation, and might cause undersensing in the postshock period. A new lead design with a more proximal position of the right ventricular defibrillation coil avoids these problems without impairing DFTs.

Physiological perspectives of therapy in bronchial hyperreactivity

PURPOSE

This paper reviews the literature on the aetiology and therapy of bronchial hyperreactivity to describe the underlying pathophysiology, identify patients at risk and update knowledge on new and existing therapies.

SOURCE

Information was obtained from monograms on New Drugs for Asthma, Respiratory Medicine: recent advances, Agents and Actions Supplements, Pulmonary Pharmacology, Anesth Analg, the European Journal of Respiration and a Medline literature search.

PRINCIPAL FINDINGS

Reduced airway calibre, increased bronchial contractility, altered permeability of the bronchial mucosa, humoral and cellular mediators, and dysfunctional neural regulation are critical factors for bronchial hyperreactivity, a characteristic feature of hyperreactive airways which results in bronchoconstriction after exposure to varied stimuli. Preoperative anaesthetic considerations in these patients include FEV1 and PEFR testing to assess the severity and for optimal control of the condition. Bronchospasm causing hypoxaemia is the major intraoperative problem anticipated in these patients. Current therapeutic management of bronchoconstriction focuses on the beta 2 agonists, theophylline and steroids. Besides relaxing the airway smooth muscle these agents are all capable of altering bronchial inflammatory responses. Future developments of therapy are directed towards the inflammatory components of the disease.

CONCLUSION

This review has presented background information on physiological mechanisms of smooth muscle contractility, pathophysiological alterations of bronchial contractility and the pharmacological basis of therapy in bronchoconstrictive disease. Information is presented to enable the prompt arrest and reversal of airway constriction, and to maintain prophylactic treatment during the perioperative period. Intraoperative bronchospasm is managed by adequate oxygenation and reversal of bronchoconstriction.

Effects of high energy shocks on pacing impedance during transvenous ICD implantation

The purpose of the current study was to characterize the effects of transvenous ICD shocks on myocardial impedance. Rather than recording impedance during shocks, it was measured during continuous pacing in order to minimize confounding effects such as electrode polarization. Pacing impedance (reflecting the combined impedances of the electrode-tissue interface, myocardium, and blood pool) was measured every 5 seconds before and after 58 single shocks in 22 patients undergoing ICD implantation with a Transvene (n = 14) or Endotak (n = 8) lead. There was a progressive and long-lasting decrease in impedance after shocks. The magnitude of this change was similar for 0.6-J test shocks and shocks > or = 5 J (28 +/- 32 omega vs 23 +/- 16 omega; P = 0.8). However, the drop in impedance was more abrupt after high energy shocks. Because impedance continued to decline throughout the 5-minute interval between shocks, successive shocks had a cumulative effect, with a decrease of 46 +/- 42 omega after four discharges. In conclusion, a progressive decline in pacing impedance is a characteristic response to transvenous ICD discharges.

Involvement of the M2 muscarinic receptor in contractions of the guinea pig trachea, guinea pig esophagus, and rat fundus

The involvement of the M2 muscarinic receptor in contractile responses of the guinea pig trachea, guinea pig esophagus, and rat fundus was investigated. In the standard assay, oxotremorine-M elicited contractions of the trachea with an EC50 value of approximately 73 nanoM.--2- -(Diethylamino)methyl- -1-piperidinyl-acetyl--5,11- dihydro-6H-pyrido-2,3-b--1,4- benzodiazepine-6-one (AF-DX 116) at 1 and 10 microM antagonized these contractions by 2.1- and 9.0-fold increases in the EC50 value for oxotremorine-M. These effects are consistent with antagonism of an M3-mediated contractile response. In subsequent experiments, the M3 receptors were first inactivated selectively by incubation with N-(2-chloroethyl)-4- piperidinyl diphenylacetate (4-DAMP mustard) (40 nanoM) for 1 hr in the presence of AF-DX 116 (1 microM) followed by extensive washing. In 4-DAMP mustard treated trachea, oxotremorine-M elicited contractions with an EC50 value of 0.31 microM in the presence of histamine (10 microM) and forskolin (4 microM). Under these conditions, AF-DX 116 at 1 and 10 microM antagonized contractions to oxotremorine-M by 8- and 59-fold increases in the EC50, respectively, while para- fluorohexahydrosiladiphenidol(p-F-HHSiD) (0.1 microM) had no effect. These effects are consistent with a contraction being mediated by an M2 receptor. In the guinea pig esophagus and rat fundus, AF-DX 116 and p-F-HHSiD blocked contractions measured under similar conditions with magnitudes intermediate between what would be expected from an M2 and an M3 receptor, suggesting that perhaps both subtypes contribute to the overall contractile response under these conditions. In addition, contractions of the guinea pig trachea measured in the presence of histamine and forskolin were pertussis toxin sensitive. These results that, in the trachea, M2 receptors can dominate the contractile response after a majority of the M3 receptors have been inactivated, whereas in the guinea pig esophagus and rat fundus, M2 receptors may contribute to, but do not play a dominant role in the overall response.

Modulation of agonist-induced phosphoinositide metabolism, Ca2+ signalling and contraction of airway smooth muscle by cyclic AMP-dependent mechanisms

1. The effects of increased cellular cyclic AMP levels induced by isoprenaline, forskolin and 8-bromoadenosine 3':5'-cyclic monophosphate (8-Br-cyclic AMP) on phosphoinositide metabolism and changes in intracellular Ca2+ elicited by methacholine and histamine were examined in bovine isolated tracheal smooth muscle (BTSM) cells. 2. Isoprenaline (pD2 (-log10 EC50) = 6.32 +/- 0.24) and forskolin (pD2 = 5.6 +/- 0.05) enhanced cyclic AMP levels in a concentration-dependent fashion in these cells, while methacholine (pD2 = 5.64 +/- 0.12) and histamine (pD2 = 4.90 +/- 0.04) caused a concentration-related increase in [3H]-inositol phosphates (IP) accumulation in the presence of 10 mM LiCl. 3. Preincubation of the cells (5 min, 37 degrees C) with isoprenaline (1 microM), forskolin (10 microM) and 8-Br-cyclic AMP (1 mM) did not affect the IP accumulation induced by methacholine, but significantly reduced the maximal IP production by histamine (1 mM). However, the effect of isoprenaline was small (15.0 +/- 0.6% inhibition) and insignificant at histamine concentrations between 0.1 and 100 microM. 4. Both methacholine and histamine induced a fast (max. in 0.5-2 s) and transient increase of intracellular Ca2+ concentration ([Ca2+]i) followed by a sustained phase lasting several minutes. EGTA (5 mM) attenuated the sustained phase, indicating that this phase depends on extracellular Ca2+. 5. Preincubation of the cells (5 min, 37 degrees C) with isoprenaline (1 microM), forskolin (10 microM) and 8-Br-cyclic AMP (1 microM) significantly attenuated both the Ca(2+)-transient and the sustained phase generated at equipotent IP producing concentrations of 1 microM methacholine and 100 microM histamine (approx. 40% of maximal methacholine-induced IP response), but did not affect changes in [Ca2+]i induced by 100 microM methacholine (95.2 +/- 3.5% of maximal methacholine-induced IP response). 6. Significant correlations were found between the isoprenaline-induced inhibition of BTSM contraction and inhibition of Ca2+ mobilization or influx induced by methacholine and histamine, that were similar for each contractile agonist. 7. These data indicate that (a) cyclic AMP-dependent inhibition of Ca2+ mobilization in BTSM cells is not primarily caused by attenuation of IP production, suggesting that cyclic AMP induced protein kinase A (PKA) activation is effective at a different level in the [Ca2+]i homeostasis, (b) that attenuation of intracellular Ca2+ concentration plays a major role in beta-adrenoceptor-mediated relaxation of methacholine- and histamine-induced airway smooth muscle contraction, and (c) that the relative resistance of the muscarinic agonist-induced contraction to beta-adrenoceptor agonists, especially at (supra) maximal contractile concentrations is largely determined by its higher potency in inducing intracellular Ca2+ changes.

Biphasic defibrillation using a single capacitor with large capacitance: reduction of peak voltages and ICD device size

The volume of current implantable cardioverter defibrillators (ICD) is not convenient for pectoral implantation. One way to reduce the size of the pulse generator is to find a more effective defibrillation pulse waveform generated from smaller volume capacitors. In a prospective randomized crossover study we compared the step-down defibrillation threshold (DFT) of a standard biphasic waveform (STD), delivered by two 250-microF capacitors connected in series with an 80% tilt, to an experimental biphasic waveform delivered by a single 450-microF capacitor with a 60% tilt. The experimental waveform delivered the same energy with a lower peak voltage and a longer duration (LVLD). Intraoperatively, in 25 patients receiving endocardial (n = 12) or endocardial-subcutaneous array (n = 13) defibrillation leads, the DFT was determined for both waveforms. Energy requirements did not differ at DFT for the STD and LVLD waveforms with the low impedance (32 +/- 4 omega) endocardial-subcutaneous array defibrillation lead system (6.4 +/- 4.4 J and 5.9 +/- 4.2 J, respectively) or increased slightly (P = 0.06) with the higher impedance (42 +/- 4 omega) endocardial lead system (10.4 +/- 4.6 J and 12.7 +/- 5.7 J, respectively). However, the voltage needed at DFT was one-third lower with the LVLD waveform than with the STD waveform for both lead systems (256 +/- 85 V vs 154 +/- 51 V and 348 +/- 76 V vs 232 +/- 54 V, respectively). Thus, a single capacitor with a large capacitance can generate a defibrillation pulse with a substantial lower peak voltage requirement without significantly increasing the energy requirements. The volume reduction in using a single capacitor can decrease ICD device size.

Changes in the amplitude of endocardial electrograms following defibrillator discharge: comparison of two lead systems

Changes in the amplitude of endocardial electrograms after an unsuccessful shock attempt have been demonstrated to cause failure of redetection of ventricular fibrillation in patients using an integrated sense-pace defibrillating lead system. Thus, the objective of this study was to compare the effects of defibrillator shocks on the amplitude of endocardial electrograms in 26 patients using two different nonthoracotomy systems, a previous lead (model 0062) or a redesigned version (model 0072). At implant, bipolar endocardial electrograms were obtained before each shock application, during initial detection and redetection of ventricular fibrillation in case the applied shock was unsuccessful, and during intervals of 5, 10, 20, 30, 60, and 120 seconds after each shock delivery. No significant difference was noted in endocardial amplitudes between the lead models 0062 and 0072 during baseline sinus rhythm (12.2 +/- 4.6 mV vs 11.4 +/- 3.8 mV), and during initial ventricular fibrillation (7.0 +/- 2.4 mV vs 7.6 +/- 2.3 mV). During redetection of ventricular fibrillation, however, there was a significant difference (P = 0.0006) in endocardial amplitudes (3.4 +/- 1.9 mV vs 6.6 +/- 2.3 mV) between both leads tested. Comparing lead models 0062 and 0072, marked differences were found in endocardial amplitudes during sinus rhythm 5, 10, and 20 seconds after successful arrhythmia termination: 2.8 +/- 1.9 mV vs 8.6 +/- 2.9 mV (P < 0.0001), 4.6 +/- 2.9 mV vs 9.2 +/- 3.2 mV (P = 0.0007), and 6.4 +/- 4.0 mV vs 10.5 +/- 3.6 mV (P = 0.01). At predischarge testing, failure of redetection of ventricular fibrillation was documented in two patients with the lead model 0062 requiring external defibrillation to restore sinus rhythm. These findings demonstrate a significant less postshock attenuation of the endocardial electrogram amplitudes during persistent ventricular fibrillation after an unsuccessful shock attempt as well as during sinus rhythm immediately following an effective shock delivery using the redesigned lead system model 0072 compared to the electrogram amplitudes obtained in patients using the previous lead model 0062.

Impedance to defibrillation countershock: does an optimal impedance exist?

Defibrillation is thought to occur because of changes in the transmembrane potential that are caused by current flow through the heart tissue. Impedance to electric countershock is an important parameter because it is determined by the magnitude and distribution of the current that flows for a specific shock voltage. The impedance is comprised of resistive contributions from: (1) extra-tissue sources, which include the defibrillator, leads, and electrodes; (2) tissue sources, which include intracardiac and extra-cardiac tissue; and (3) the interface between electrode and tissue. Tissue sources dominate the impedance and probably contribute to the wide range of impedance values presented to the defibrillation pulse. Because impedance is not constant within or between subjects, defibrillators must be designed to accommodate these differences without compromising patient safety or therapeutic efficacy. Experimental investigations in animals and humans suggest that impedance changes at several different time scales ranging from milliseconds to years. These alterations are believed to be a result of both electrochemical and physiological mechanisms. It is commonly thought that impedance is optimized when it has been decreased to a minimum, since this allows the most current flow for a given voltage shock. However, if the impedance is lowered by changing the location or size of the electrodes in such a way that current flow is decreased in part of the heart even though current flow is increased elsewhere, then the total voltage, current, and energy needed for defibrillation may increase, not decrease, even though impedance is decreased. A simple boundary element computer model suggests that the most even distribution of current flow through the heart is achieved for those electrode locations in which the impedance across the heart is at or near the maximum cardiac impedance for any location of these particular electrodes. Thus, the optimum shock impedance is achieved when impedance is minimized for extra-tissue and extra-cardiac tissue sources and is at or near a maximum for intracardiac tissue sources.