Importance of vagally mediated bradycardia for the induction of torsade de pointes in an in vivo model

Bradycardia-Induced Torsades de Pointes in Atrioventricular Block

Bradycardia is known to prolong QT interval. Persistent bradycardia and high-grade atrioventricular (AV) block may lead to persistently prolonged QTc interval with a risk for life-threatening ventricular arrhythmias, which needs addressing the underlying cause. We present the case of a patient with persistent sinus bradycardia with a high-grade AV block leading to persistently prolonged QTc without any reversible etiology that resulted in torsades de pointes. The underlying treatment involved shortening the QTc by increasing the heart rate to prevent any further episodes of polymorphic ventricular tachycardia.

Atypical trigeminocardiac reflex during intranasal dexmedetomidine sedation of a 6-month-old premature baby

The trigeminocardiac reflex (TCR) is an established brainstem reflex leading to parasympathetic dysrhythmias-including haemodynamic irregularities, apnoea and gastric hypermotility-during stimulation of any sensory branches of the trigeminal nerve. Most of the clinical knowledge about TCR was gathered from general anaesthesia observations, not from procedural sedation.We present a case of a 6-month-old premature baby experiencing the reflex twice under dexmedetomidine-propofol-sedation while undergoing ophthalmic and ear examination. This was interpreted as cross-over sensitisation between the facial and trigeminal cranial nerve (N V and N VII).The present case demonstrates that different TCR subtypes can occur during the same anaesthetic procedure. Triggering TCR seems to be based on several factors and not just on a single stimulus as often presumed. Therefore, for premature babies, there is a risk for TCR under procedural sedation, and we recommend using glycopyrrolate as preventive treatment.

Long-Term Endurance Exercise Training Alters Repolarization in a New Rabbit Athlete’s Heart Model

In the present study, the effect of long-term exercise training was investigated on myocardial morphological and functional remodeling and on proarrhythmic sensitivity in a rabbit athlete’s heart model. New-Zealand white rabbits were trained during a 12-week long treadmill running protocol and compared with their sedentary controls. At the end of the training protocol, echocardiography, in vivo and in vitro ECG recordings, proarrhythmic sensitivity with dofetilide (nM) were performed in isolated hearts, and action potential duration (APD) measurements at different potassium concentrations (4.5 and 2 mM) were made in the isolated papillary muscles. Expression levels of the slow component of delayed rectifier potassium current and fibrosis synthesis and degradation biomarkers were quantified. Echocardiography showed a significantly dilated left ventricle in the running rabbits. ECG PQ and RR intervals were significantly longer in the exercised group (79 ± 2 vs. 69 ± 2 ms and 325 ± 11 vs. 265 ± 6 ms, p < 0.05, respectively). The in vivo heart rate variability (HRV) (SD of root mean square: 5.2 ± 1.4 ms vs. 1.4 ± 0.2 ms, p < 0.05) and Tpeak-Tend variability were higher in the running rabbits. Bradycardia disappeared in the exercised group in vitro. Dofetilide tended to increase the QTc interval in a greater extent, and significantly increased the number of arrhythmic beats in the trained animals in vitro. APD was longer in the exercised group at a low potassium level. Real-time quantitative PCR (RT-qPCR) showed significantly greater messenger RNA expression of fibrotic biomarkers in the exercised group. Increased repolarization variability and higher arrhythmia incidences, lengthened APD at a low potassium level, increased fibrotic biomarker gene expressions may indicate higher sensitivity of the rabbit “athlete’s heart” to life-threatening arrhythmias.

Symptomatic Premature Ventricular Contractions in Vasovagal Syncope Patients: Autonomic Modulation and Catheter Ablation

Introduction

There has been limited reports about the comorbid premature ventricular contractions (PVCs) and vasovagal syncope (VVS). Deceleration capacity (DC) was demonstrated to be a quantitative evaluation to assess the cardiac vagal activity. This study sought to report the impact of autonomic modulation on symptomatic PVCs in VVS patients.

Methods and Results

Twenty-six VVS patients with symptomatic idiopathic PVCs were consecutively enrolled. Identification and catheter ablation of left atrial ganglionated plexi (GP) and PVCs were performed in 26 and 20 patients, respectively. Holter 24 h-electrocardiograms were performed before and after the procedure to evaluate DC and PVCs occurrence. Eighteen patients were subtyped as DC-dependent PVCs (D-PVCs) and eight as DC-independent PVCs groups (I-PVCs). In D-PVCs group, circadian rhythm of hourly PVCs was positively correlated with hourly DC (P < 0.05) while there was no correlation in I-PVCs group (P > 0.05). Fifty-three GPs with positive vagal response were successfully elicited (2.0 ± 0.8 per patient). PVCs failed to occur spontaneously nor to be induced in six patients. In the remaining 20 patients, PVCs foci identified were all located in the ventricular outflow tract region. Post-ablation DC decreased significantly from baseline (P < 0.05). During mean follow-up of 10.64 ± 6.84 months, syncope recurred in one patient and PVCs recurred in another. PVCs burden of the six patients in whom neither catheter ablation nor antiarrhythmic drugs were applied demonstrated a significant decrease during follow-up (P = 0.037).

Conclusion

Autonomic activities were involved in the occurrence of symptomatic idiopathic PVCs in some VVS patients. D-PVCs might be facilitated by increased vagal activities. Catheter ablation of GP and PVCs foci may be an effective, safe treatment in patients with concomitant VVS and idiopathic PVCs.

Inter-individual variability and modeling of electrical activity: a possible new approach to explore cardiac safety?

Safety pharmacology aims to predict rare side effects of new drugs. We explored whether rare pro-arrhythmic effects could be linked to the variability of the effects of these drugs on ion currents and whether taking into consideration this variability in computational models could help to better detect and predict cardiac side effects. For this purpose, we evaluated how intra- and inter-individual variability influences the effect of hERG inhibition on both the action potential duration and the occurrence of arrhythmias. Using two computer simulation models of human action potentials (endocardial and Purkinje cells), we analyzed the contribution of two biological parameters on the pro-arrhythmic effects of several hERG channel blockers: (i) spermine concentration, which varies with metabolic status, and (ii) L-type calcium conductance, which varies due to single nucleotide polymorphisms or mutations. By varying these parameters, we were able to induce arrhythmias in 1 out of 16 simulations although conventional modeling methods to detect pro-arrhythmic molecules failed. On the basis of our results, taking into consideration only 2 parameters subjected to intra- and inter-individual variability, we propose that in silico computer modeling may help to better define the risks of new drug candidates at early stages of pre-clinical development.

The high frequency relationship: implications for torsadogenic hERG blockers

BACKGROUND AND PURPOSE

Ventricular arrhythmias induced by human ether-a-go-go related gene (hERG; Kv 11.1 channel) blockers are a consequence of alterations in ventricular repolarisation in association with high-frequency (HF) oscillations, which act as a primary trigger; the autonomic nervous system plays a modulatory role. In the present study, we investigated the role of β1 -adrenoceptors in the HF relationship between magnitude of heart rate and QT interval changes within discrete 10 s intervals (sorted into 5 bpm heart rate increments) and its implications for torsadogenic hERG blockers.

EXPERIMENTAL APPROACH

The HF relationship was studied under conditions of autonomic blockade with atenolol (β1 -adrenoceptor blocker) in the absence or presence of five hERG blockers in beagle dogs. In total, the effects of 14 hERG blockers on the HF relationship were investigated.

KEY RESULTS

All the torsadogenic hERG blockers tested caused a vertical shift in the HF relationship, while hERG blockers associated with a low risk of Torsades de Pointes did not cause any vertical shift. Atenolol completely prevented the effects four torsadogenic agents (quinidine, thioridazine, risperidone and terfenadine) on the HF relationship, but only partially reduced those of dofetilide, leading to the characterization of two types of torsadogenic agent.

CONCLUSIONS AND IMPLICATIONS

Analysis of the vertical shift in the HF relationship demonstrated that signs of transient sympathetic activation during HF oscillations in the presence of torsadogenic hERG blockers are mediated by β1 -adrenoceptors. We suggest the HF relationship as a new biomarker for assessing Torsades de pointes liability, with potential implications in both preclinical studies and the clinic.

Role of autonomic reflex arcs in cardiovascular responses to air pollution exposure

The body responds to environmental stressors by triggering autonomic reflexes in the pulmonary receptors, baroreceptors, and chemoreceptors to maintain homeostasis. Numerous studies have shown that exposure to various gases and airborne particles can alter the functional outcome of these reflexes, particularly with respect to the cardiovascular system. Modulation of autonomic neural input to the heart and vasculature following direct activation of sensory nerves in the respiratory system, elicitation of oxidative stress and inflammation, or through other mechanisms is one of the primary ways that exposure to air pollution affects normal cardiovascular function. Any homeostatic process that utilizes the autonomic nervous system to regulate organ function might be affected. Thus, air pollution and other inhaled environmental irritants have the potential to alter both local airway function and baro- and chemoreflex responses, which modulate autonomic control of blood pressure and detect concentrations of key gases in the body. While each of these reflex pathways causes distinct responses, the systems are heavily integrated and communicate through overlapping regions of the brainstem to cause global effects. This short review summarizes the function of major pulmonary sensory receptors, baroreceptors, and carotid body chemoreceptors and discusses the impacts of air pollution exposure on these systems.

Assessment of efficacy of proarrhythmia biomarkers in isolated rabbit hearts with attenuated repolarization reserve

Isolated hearts with reduced repolarization reserve would be suitable for assessing the proarrhythmic liability of drugs. However, it is not known which proarrhythmia biomarkers indicate the increased susceptibility to torsades de pointes arrhythmia (TdP) in such experimental setting. Thus, we estimated the efficacy of proarrhythmia biomarkers in isolated hearts with attenuated repolarization reserve. Langendorff-perfused rabbit hearts were used. Repolarization reserve was reduced by concomitant inhibition of the rapid (IKr) and slow (IKs) delayed rectifier potassium currents by dofetilide and HMR-1556, respectively. Rate corrected QT (QTc) interval and beat-to-beat variability of the QT interval measured in sinus rhythm or irrespective of rhythm even during arrhythmias (sinus and absolute QT variability, respectively) were tested. QTc failed to predict increased proarrhythmic risk. Sinus QT variability indicated proarrhythmic liability when low concentration of dofetilide was used. However, when arrhythmias compromised sinus variability measurement during coperfusion of catecholamines and elevated concentration of dofetilide, only absolute QT variability indicated increased proarrhythmic risk. Absolute QT variability parameters seem to be the most practical and sensitive biomarkers of proarrhythmic liability in rabbit hearts with reduced repolarization reserve. Absolute QT variability parameters could serve as surrogates for torsades de pointes in drug-safety investigations in isolated rabbit hearts with attenuated repolarization reserve.

Comparative electrocardiographic, autonomic and systemic inflammatory responses to soy biodiesel and petroleum diesel emissions in rats

CONTEXT

Biodiesel fuel represents an alternative to high particulate matter (PM)-emitting petroleum-based diesel fuels, yet uncertainty remains regarding potential biodiesel combustion emission health impacts.

OBJECTIVE

The purpose of this study was to compare cardiovascular responses to pure and blended biodiesel fuel emissions relative to petroleum diesel exhaust (DE).

MATERIALS AND METHODS

Spontaneously hypertensive rats were exposed for 4 h per day for four days via whole body inhalation to combustion emissions (based on PM concentrations 50, 150 or 500 μg/m(3) or filtered air) from pure (B100) or blended (B20) soy biodiesel, or to pure petroleum DE (B0). Electrocardiogram (ECG) and heart rate variability (HRV, an index of autonomic balance) were monitored before, during and after exposure while pulmonary and systemic inflammation were assessed one day after the final exposure. ECG and HRV data and inflammatory data were statistically analyzed using a linear mixed model for repeated measures and an analysis of variance, respectively.

RESULTS

B100 and B0, but not B20, increased HRV during all exposure days at the highest concentration indicating increased parasympathetic tone. Electrocardiographic data were mixed. B100 and B0, but not B20, caused significant changes in one or more of the following: serum C-reactive protein, total protein, low density lipoprotein (LDL) and high density lipoprotein (HDL) cholesterol, and blood urea nitrogen (BUN) and plasma angiotensin converting enzyme (ACE) and fibrinogen.

DISCUSSION AND CONCLUSIONS

Although responses to emissions from all fuels were mixed and relatively mild, some findings point to a reduced cardiovascular impact of blended biodiesel fuel emissions.

Risk of cardiac arrhythmias during hypoglycemia in patients with type 2 diabetes and cardiovascular risk

Recent trials of intensive glycemic control suggest a possible link between hypoglycemia and excess cardiovascular mortality in patients with type 2 diabetes. Hypoglycemia might cause arrhythmias through effects on cardiac repolarization and changes in cardiac autonomic activity. Our aim was to study the risk of arrhythmias during spontaneous hypoglycemia in type 2 diabetic patients with cardiovascular risk. Twenty-five insulin-treated patients with type 2 diabetes and a history of cardiovascular disease or two or more risk factors underwent simultaneous continuous interstitial glucose and ambulatory electrocardiogram monitoring. Frequency of arrhythmias, heart rate variability, and markers of cardiac repolarization were compared between hypoglycemia and euglycemia and between hyperglycemia and euglycemia matched for time of day. There were 134 h of recording at hypoglycemia, 65 h at hyperglycemia, and 1,258 h at euglycemia. Bradycardia and atrial and ventricular ectopic counts were significantly higher during nocturnal hypoglycemia compared with euglycemia. Arrhythmias were more frequent during nocturnal versus daytime hypoglycemia. Excessive compensatory vagal activation after the counterregulatory phase may account for bradycardia and associated arrhythmias. QT intervals, corrected for heart rate, >500 ms and abnormal T-wave morphology were observed during hypoglycemia in some participants. Hypoglycemia, frequently asymptomatic and prolonged, may increase the risk of arrhythmias in patients with type 2 diabetes and high cardiovascular risk. This is a plausible mechanism that could contribute to increased cardiovascular mortality during intensive glycemic therapy.

Autonomic involvement in idiopathic premature ventricular contractions

OBJECTIVE

To investigate autonomic involvement in different types of idiopathic premature ventricular contractions (PVCs) grouped by heart rate (HR) dependency.

METHODS

One hundred and sixty PVC patients and 31 controls were enrolled. Holter ECG was used to evaluate PVC occurrence, and spectral analysis of heart rate variability (HRV) was calculated to represent cardiac autonomic control. PVCs were divided into fast rate-dependent (F-PVC), slow rate-dependent (S-PVC), and HR-independent PVC (I-PVC) based on the relationship between hourly PVC density and hourly HR. HRV among different types of PVCs were compared, and the association between PVC density with HR and HRV were analyzed. Furthermore, autonomic changes assessed by consecutive 5-min HRV in 30 min before PVC episodes were investigated.

RESULTS

In 160 subjects, there were 73 F-PVC, 56 S-PVC, and 31 I-PVC. Hourly PVC density was positively associated with sympathetic indice (LF/HF) and negatively related to vagal indices (HF and HFnu) in F-PVC, and this trend was reversed in S-PVC. During 30 min before PVC onset, RR interval and HFnu decreased significantly with LF/HF showing an increasing trend in F-PVC, while in S-PVC both RR interval and HF increased significantly. It was noted that those changes were most evident during the last 5 min before PVC onset. In addition, PVC density in F-PVC was significantly decreased by β-blocker treatment.

CONCLUSIONS

HR dependency reflected autonomic modulation of idiopathic PVCs. F-PVC and S-PVC might be facilitated by sympathetic activation and vagal activation, respectively. HR dependency and the related autonomic mechanisms should be considered when treating idiopathic PVCs.

The clinical significance of QT interval prolongation in anesthesia and pain management: what you should and should not worry about

The most feared drug-induced complication is fatal cardiac arrest. Torsades de pointes (TdP) is a polymorphic ventricular tachycardia occurring in the setting of a QT interval prolongation and is the most frequent type of drug-induced pro-arrhythmia. The most common mechanism of QT prolongation and TdP is blockade of the rapid component of the delayed rectifier repolarizing potassium conductance IKr. Anesthesiologists have extensive experience with QT prolonging drugs, but there are relatively few reports of TdP occurring in the perioperative setting. Nevertheless, regulatory concern regarding the drug droperidol resulted in a significant reduction in its use. Concern regarding two other agents that potently block IKr, i.e., sevoflurane and methadone, has grown, and practitioners are worried that these valuable agents may meet the same fate. In this review, the data regarding the TdP risk of droperidol, sevoflurane, and methadone are compared with particular emphasis on the different settings in which they are employed. While the three drugs are potent IKr inhibitors, little evidence exists to suggest that droperidol or sevoflurane are associated with significant proarrhythmia in the perioperative setting. Due to factors such as inhibition of the parasympathetic nervous system, prevention of hypoxia and hypercarbia, and attention to serum electrolytes, TdP is a very rare occurrence in the perioperative environment. Methadone, however, is typically given to outpatients, over long periods, and in combination with agents that inhibit its metabolism or are QT prolonging in their own right. Thus, pre- and post-drug electrocardiograms may be appropriate when prescribing methadone for outpatients, while the much lower risk for TdP (and the difficulties inherent in QT measurement in the perioperative period) render this approach unfruitful and worthy of reevaluation.

Overt and latent cardiac effects of ozone inhalation in rats: evidence for autonomic modulation and increased myocardial vulnerability

BACKGROUND

Ozone (O₃) is a well-documented respiratory oxidant, but increasing epidemiological evidence points to extrapulmonary effects, including positive associations between ambient O₃ concentrations and cardiovascular morbidity and mortality.

OBJECTIVE

With preliminary reports linking O₃ exposure with changes in heart rate (HR), we investigated the hypothesis that a single inhalation exposure to O₃ will cause concentration-dependent autonomic modulation of cardiac function in rats.

METHODS

Rats implanted with telemeters to monitor HR and cardiac electrophysiology [electrocardiography (ECG)] were exposed once by whole-body inhalation for 4 hr to 0.2 or 0.8 ppm O₃ or filtered air. A separate cohort was tested for vulnerability to aconitine-induced arrhythmia 24 hr after exposure.

RESULTS

Exposure to 0.8 ppm O₃ caused bradycardia, PR prolongation, ST depression, and substantial increases in atrial premature beats, sinoatrial block, and atrioventricular block, accompanied by concurrent increases in several HR variability parameters that were suggestive of increased parasympathetic tone. Low-O₃ exposure failed to elicit any overt changes in autonomic tone, heart rhythm, or ECG. However, both 0.2 and 0.8 ppm O₃ increased sensitivity to aconitine-induced arrhythmia formation, suggesting a latent O₃-induced alteration in myocardial excitability.

CONCLUSIONS

O₃ exposure causes several alterations in cardiac electrophysiology that are likely mediated by modulation of autonomic input to the heart. Moreover, exposure to low O₃ concentrations may cause subclinical effects that manifest only when triggered by a stressor, suggesting that the adverse health effects of ambient levels of air pollutants may be insidious and potentially underestimated.

Cardiac ventricular repolarization reserve: a principle for understanding drug-related proarrhythmic risk

Cardiac repolarization abnormalities can be caused by a wide range of cardiac and non-cardiac compounds and may lead to the development of life-threatening Torsades de Pointes (TdP) ventricular arrhythmias. Drug-induced torsades de pointes is associated with unexpected and unexplained sudden cardiac deaths resulting in the withdrawal of several compounds in the past. To better understand the mechanism of such unexpected sudden cardiac deaths, the concept of repolarization reserve has recently emerged. According to this concept, pharmacological, congenital or acquired impairment of one type of transmembrane ion channel does not necessarily result in excessive repolarization changes because other repolarizing currents can take over and compensate. In this review, the major factors contributing to repolarization reserve are discussed in the context of their clinical significance in physiological and pathophysiological conditions including drug administration, genetic defects, heart failure, diabetes mellitus, gender, renal failure, hypokalaemia, hypothyroidism and athletes' sudden deaths. In addition, pharmacological support of repolarization reserve as a possible therapeutic option is discussed. Some methods for the quantitative estimation of repolarization reserve are also recommended. It is concluded that repolarization reserve should be considered by safety pharmacologists to better understand, predict and prevent previously unexplained drug-induced sudden cardiac deaths.

The two-pore domain potassium channel KCNK5: induction by estrogen receptor alpha and role in proliferation of breast cancer cells

The growth of many human breast tumors requires the proliferative effect of estrogen acting via the estrogen receptor α (ERα). ERα signaling is therefore a clinically important target for breast cancer prevention and therapeutics. Although extensively studied, the mechanism by which ERα promotes proliferation remains to be fully established. We observed an up-regulation of transcript encoding the pH-sensitive two-pore domain potassium channel KCNK5 in a screen for genes stimulated by 17β-estradiol (E2) in the ERα(+) breast cancer cell lines MCF-7 and T47D. KCNK5 mRNA increased starting 1 h after the onset of E2 treatment, and protein levels followed after 12 h. Estrogen-responsive elements are found in the enhancer region of KCNK5, and chromatin immunoprecipitation assays revealed binding of ERα to the KCNK5 enhancer in E2-treated MCF-7 cells. Cells treated with E2 also showed increases in the amplitude of pH-sensitive potassium currents, as assessed by whole-cell recordings. These currents are blocked by clofilium. Although confocal microscopy suggested that most of the channels are located in intracellular compartments, the increase in macroscopic currents suggests that E2 treatment increases the number of active channels at the cell surface. Application of small interfering RNA specific for KCNK5 decreased pH-sensitive potassium currents and also reduced the estrogen-induced proliferation of T47D cells. We conclude that E2 induces the expression of KCNK5 via ERα(+) in breast cancer cells, and this channel plays a role in regulating proliferation in these cell lines. KCNK5 may therefore represent a useful target for treatment, for example, of tamoxifen-resistant breast cancer.

Renal defects in KCNE1 knockout mice are mimicked by chromanol 293B in vivo: identification of a KCNE1-regulated K+ conductance in the proximal tubule

Non-technical summary

The kidney plays a critical role in regulating body fluid volume and blood pressure by conserving ions, solutes and water. Knowing the processes that underpin the handling of ions, solutes and water by the kidney is essential to our understanding of fluid and blood pressure regulation. Movement of ions is mediated by specific transport proteins found in the membranes of kidney cells. These proteins are regulated by additional proteins, called accessory proteins. In the current study, we have examined the role of the accessory protein KCNE1 in regulating a channel, KCNQ1, which is important in kidney function. We have observed that in the absence of KCNE1 the kidney has difficulty conserving sodium, chloride and water. However, by using specific inhibitors of these proteins we have also determined that although KCNE1 has a role in kidney function, the mechanism of its action is unlikely to be by regulating the protein KCNQ1.

Abstract

KCNE1 is a protein of low molecular mass that is known to regulate the chromanol 293B and clofilium-sensitive K⁺ channel, KCNQ1, in a number of tissues. Previous work on the kidney of KCNE1 and KCNQ1 knockout mice has revealed that these animals have different renal phenotypes, suggesting that KCNE1 may not regulate KCNQ1 in the renal system. In the current study, in vivo clearance approaches and whole cell voltage-clamp recordings from isolated renal proximal tubules were used to examine the physiological role of KCNE1. Data from wild-type mice were compared to those from KCNE1 knockout mice. In clearance studies the KCNE1 knockout mice had an increased fractional excretion of Na⁺, Cl⁻, HCO₃⁻ and water. This profile was mimicked in wild-type mice by infusion of chromanol 293B, while chromanol was without effect in KCNE1 knockout animals. Clofilium also increased the fractional excretion of Na⁺, Cl⁻ and water, but this was observed in both wild-type and knockout mice, suggesting that KCNE1 was regulating a chromanol-sensitive but clofilium-insensitive pathway. In whole cell voltage clamp recordings from proximal tubules, a chromanol-sensitive, K⁺-selective conductance was identified that was absent in tubules from knockout animals. The properties of this conductance were not consistent with its being mediated by KCNQ1, suggesting that KCNE1 regulates another K⁺ channel in the renal proximal tubule. Taken together these data suggest that KCNE1 regulates a K⁺-selective conductance in the renal proximal tubule that plays a relatively minor role in driving the transport of Na⁺, Cl⁻ and HCO₃⁻.

Biomarkers and endogenous determinants of dofetilide-induced torsades de pointes in α(1) -adrenoceptor-stimulated, anaesthetized rabbits

BACKGROUND AND PURPOSE

Torsades de pointes (TdP) liability is a stochastic event, which indicates that unidentified factors have an important role in facilitating the initiation of TdP by increasing the probability of TdP occurrence. We sought to identify factors that facilitate drug-induced TdP.

EXPERIMENTAL APPROACH

We studied dofetilide-induced TdP in pentobarbital-anaesthetized, phenylephrine-sensitized rabbits, seeking biomarkers that discriminated between the animals that experienced TdP ('TdP+' animals) and those that did not ('TdP-' animals). As novel variables, the beat-to-beat variability and instability of ECG intervals were measured at preset times, irrespective of whether or not hearts were in stable sinus rhythm ('absolute' variability and instability). Autonomic activity was also determined.

KEY RESULTS

Dofetilide delayed repolarization and induced arrhythmias prior to TdP. The variability of the coupling interval and shape of arrhythmic beats before TdP were significantly greater in the 'TdP+' group than in the 'TdP-' group. Accordingly, the 'absolute' variability and instability of the ECG intervals were significantly elevated in the 'TdP+' group. Phenylephrine increased significantly the up-baroreflex sensitivity in the 'TdP+' group before dofetilide administration.

CONCLUSIONS AND IMPLICATIONS

'Preceding' arrhythmias have characteristics that permit prediction of TdP occurrence: the more chaotic the ventricular rhythm, the greater the probability of TdP initiation. This suggests that complexity of the arrhythmic beats may play an important mechanistic role in TdP genesis. The electrical instability quantified by the novel 'absolute' variability and instability parameters correlates with the probability of TdP occurrence. Baroreflex may contribute to TdP genesis in vivo.

Ventricular arrhythmias during race recovery in Standardbred Racehorses and associations with autonomic activity

BACKGROUND

A lack of information on normal heart rhythm at maximal effort hampers investigation of poor performance and sudden death in Standardbred racing.

HYPOTHESIS/OBJECTIVE

To characterize rhythm variations during scheduled racing in clinically normal Standardbred horses.

ANIMALS

Two hundred and eighty-eight Standardbred horses competing in 40 scheduled races at a Southern Ontario racetrack.

METHODS

Observational study, convenience sampling: Heart rhythm was monitored by ECG from harnessing to postrace recovery and assessed visually and by examining heart rate intervals. Rhythm variations were used as response variables in multivariate analysis of race records detailing signalment, race, and race outcome.

RESULTS

Monitoring involved 345 individual horse or race events. Occasional, isolated premature cycles, only, occurred during the race. Postrace, sudden cardiac slowing (punctuated deceleration [PD]) appeared in 42 events (12.2%). Only premature ventricular complexes were exhibited in 40 events postrace (11.6%), whereas 55 (15.9%) exhibited complex ventricular arrhythmias (CVA) including torsades-like polymorphic ventricular tachycardia, 34.5% of these being closely associated with PD (odds ratio = 8). Predispositions to CVA were found for horses parked at the 1/2 mile (odds ratio = 3), and trotters breaking in the stretch (odds ratio = 38). Horses spontaneously reverted to sinus rhythm and no sudden death events were encountered.

CONCLUSIONS AND CLINICAL IMPORTANCE

Arrhythmias occur frequently in racing Standardbreds during cardiac deceleration, often associated with sudden, rapid increases in vagal tone. Circumstances imposing unusual demand and racing at the trot appear to predispose. Findings provide insight into possible mechanisms of sudden death.

Rabbit models of heart disease

Human heart disease is a major cause of death and disability. A variety of animal models of cardiac disease have been developed to better understand the etiology, cellular and molecular mechanisms of cardiac dysfunction and novel therapeutic strategies. The animal models have included large animals (e.g. pig and dog) and small rodents (e.g. mouse and rat) and the advantages of genetic manipulation in mice have appropriately encouraged the development of novel mouse models of cardiac disease. However, there are major differences between rodent and human hearts that raise cautions about the extrapolation of results from mouse to human. The rabbit is a medium-sized animal that has many cellular and molecular characteristics very much like human, and is a practical alternative to larger mammals. Numerous rabbit models of cardiac disease are discussed, including pressure or volume overload, ischemia, rapid-pacing, doxorubicin, drug-induced arrhythmias, transgenesis and infection. These models also lead to the assessment of therapeutic strategies which may become beneficial in human cardiac disease.

Ju Chen – University of California, San Diego, Department of Medicine, La Jolla, CA, USA

Robert Ross – University of California, San Diego, Cardiology Section, San Diego, CA, USA

Model systems for the discovery and development of antiarrhythmic drugs

Cardiovascular diseases are the leading cause of mortality worldwide and about 25% of cardiovascular deaths are due to disturbances in cardiac rhythm or "arrhythmias". Arrhythmias were traditionally treated with antiarrhythmic drugs, but increasing awareness of the risks of presently available antiarrhythmic agents has greatly limited their usefulness. Most common treatment algorithms still involve small molecule drugs, and antiarrhythmic agents with improved efficacy and safety are sorely needed. This paper reviews the model systems that are available for discovery and development of new antiarrhythmic drugs. We begin with a presentation of screening methods used to identify specific channel-interacting agents, with a particular emphasis on high-throughput screens. Traditional manual electrophysiological methods, automated electrophysiology, fluorescent dye methods, flux assays and radioligand binding assays are reviewed. We then discuss a variety of relevant arrhythmia models. Two models are widely used in testing for arrhythmogenic actions related to excess action potential prolongation, an important potential adverse effect of chemical entities affecting cardiac rhythm: the methoxamine-sensitized rabbit and the dog with chronic atrioventricular block. We then go on to review models used to assess potential antiarrhythmic actions. For ventricular arrhythmias, chemical induction methods, cardiac or neural electrical stimulation, ischaemic heart models and models of cardiac channelopathies can be used to identify effective antiarrhythmic agents. For atrial arrhythmias, potentially useful models include vagally-maintained atrial fibrillation, acute asphyxia with atrial burst-pacing, sterile pericarditis, Y-shaped atria surgical incisions, chronic atrial dilation models, atrial electrical remodelling due to sustained atrial tachycardia, heart failure-related atrial remodelling, and acute atrial ischaemia. It is hoped that the new technologies now available and the recently-developed models for arrhythmia-response assessment will permit the introduction of newer and more effective antiarrhythmic therapies in the near future.