Activation sequences following failed atrial defibrillation

OBJECTIVES

The purposes of this study were to examine the first activations following atrial defibrillation shocks to help understand how and where atrial fibrillation (AF) relapsed following failed shocks and to assess the difference in postshock activation between failed and successful shocks.

BACKGROUND

While many studies have investigated the mechanism of ventricular defibrillation, much less is known about the mechanisms of AF.

METHODS

Sustained AF was induced electrically after pericardial infusion of methylcholine in 10 sheep. Biphasic subthreshold shocks were delivered to three configurations: right atrium to distal coronary sinus (RA-CS), sequential shocks with RA-CS as the first pathway followed by proximal CS to superior vena cava as the second pathway (Sequential), and right ventricle to superior vena cava plus can (V-triad). In eight sheep, global atrial mapping was performed with 504 electrodes spaced 3 to 4 mm apart.

RESULTS

Earliest postshock activations mostly arose from the left atrium for V-triad but arose from either atrium for RA-CS and Sequential. Preshock AF cycle lengths were significantly shorter at the earliest activation sites than at seven of eight other sites globally distributed over both atria. In all type B successful episodes in which one or more rapid activations occurred after the shock and in 50 of the 72 failed episodes analyzed, activation fronts spread away from the earliest site in a focal pattern, and discrete nonfragmented activation complexes were present in the first derivatives of the electrograms. In the other 22 failed episodes, earliest activation fronts spread in a nonfocal pattern, and earliest postshock electrogram derivatives were fractionated. To better interpret the activation pattern in the fragmented regions, a 504 electrode plaque with 1.5-mm electrode spacing was placed on the right atrial appendage in two additional sheep. In 11 of 108 failed episodes, earliest postshock activation appeared inside the plaque and spread in a focal pattern with nonfragmented electrogram derivatives in 10 episodes and in a reentrant pattern with fragmented electrogram derivatives in the other.

CONCLUSIONS

(1) The electrode configuration influenced the location of earliest postshock activation. (2) Earliest postshock activation occurred where the preshock AF cycle length was short. (3) Earliest activations following all type B successful and most failed episodes were not fragmented and spread in a focal pattern. (4) The region of earliest postshock activation in the failed episodes without a focal postshock activation pattern exhibited regions of fragmented electrogram derivatives that may represent conduction block and possibly reentry.

Short-acting beta-adrenergic antagonist esmolol given at reperfusion improves survival after prolonged ventricular fibrillation

BACKGROUND

High catecholamine concentrations are cytotoxic to cardiac myocytes. We hypothesized that myocardial interstitial catecholamine levels are greatly elevated immediately after long-duration ventricular fibrillation (VF), defibrillation, and reperfusion and that the short-acting beta-antagonist esmolol administered at reperfusion would protect against this catecholamine surge and improve survival.

METHODS AND RESULTS

In part 1 of this study, catecholamines from myocardial interstitial fluid (ISF) and aortic and coronary sinus plasma were quantified by use of 3H-labeled radioenzymatic assay in 8 open-chest, anesthetized pigs. Eight minutes of electrically induced VF was followed by internal defibrillation and reperfusion. By 4 minutes of VF, ISF norepinephrine increased significantly, from 1.3+/-0.3 to 7.4+/-2.4 ng/mL. Epinephrine increased significantly, from 0.4+/-0.2 to 1.5+/-0.7 ng/mL. ISF norepinephrine and epinephrine peaked at 219.2+/-92.1 and 63.7+/-25.1 ng/mL after defibrillation and reperfusion and decreased significantly to 12.2+/-3.5 and 6.7+/-3.1 ng/mL 23 minutes after defibrillation. Transcardiac catecholamine changes were similar. In part 2, 8 minutes of VF was followed by external defibrillation in anesthetized, closed-chest pigs. Animals received 1.0 mg/kg esmolol (n=8) or saline (n=8) intravenously at the start of cardiopulmonary resuscitation (CPR). Advanced cardiac life support, including CPR and epinephrine, was delivered to both groups. Esmolol before reperfusion improved return of spontaneous circulation and 4-hour survival (7/8 versus 3/8 survivors, chi2 P<0.05).

CONCLUSIONS

Transcardiac and ISF norepinephrine and epinephrine levels are briefly massively elevated after 8 minutes of VF, defibrillation, and reperfusion. A short-acting beta-antagonist administered immediately after defibrillation improves return of spontaneous circulation and 4-hour survival after this prolonged VF.

Do clinically relevant transthoracic defibrillation energies cause myocardial damage and dysfunction?

Sufficiently strong defibrillation shocks will cause temporary or permanent damage to the heart. Weak defibrillation shocks do not cause any damage to the heart but also do not defibrillate. A relevant and practical question is what range of shock energies is most likely to defibrillate while not causing damage to the heart. This question is most difficult to answer in the pre-hospital defibrillation setting where the patients' size and shape vary, placement of the defibrillation patches vary, and the etiology of their arrhythmia varies. Unlike internal defibrillators, which are tested at implantation, efficacy of an external defibrillator is determined only once, when it is most needed. This review discusses shock damage and dysfunction caused by monophasic waveforms as well as biphasic waveforms. Evidence is presented suggesting that for perfused hearts, the threshold for damage is well above any shock size delivered clinically. For non-perfused hearts, both in humans and animals, evidence is presented that monophasic shocks of up to 5 J/kg do not cause any more cardiac damage/dysfunction than that associated with smaller shocks and that much of this damage is caused by the ischemic period itself rather than the shock. Although many patients can be defibrillated with 150 J (2.2 J/kg) biphasic shocks, some patients may require biphasic shocks up to 360 J (5 J/kg) to be defibrillated. Studies still need to be performed comparing the efficacy and damaging effects of 360 J biphasic shocks to 150 J biphasic shocks. Until those studies are completed, it seems reasonable to use the same 360 J (5 J/kg) energy limit for biphasic shocks as for monophasic shocks.

Effects of burst stimulation during ventricular fibrillation on cardiac function after defibrillation

The purpose of defibrillation is to rapidly restore blood flow and tissue perfusion following ventricular fibrillation (VF) and shock delivery. We tested the hypotheses that 1) a series of 1-ms pulses of various amplitudes delivered before the defibrillation shock can improve hemodynamics following the shock, and 2) this hemodynamic improvement is due to stimulation of cardiac or thoracic sympathetic nerves. Ten anesthetized pigs received a burst of either 15 or 30 1-ms pulses (0.1-10 A in strength) during VF, after which defibrillation was performed. ECG, arterial blood pressure, and left ventricular (LV) pressure were recorded. Defibrillation shocks and burst pulses were delivered from a right ventricular coil electrode to superior vena cava coil and left chest wall electrodes. Sympathetic blockade was induced with 1 mg/kg timolol and trials were repeated. The first half of this protocol was repeated in two animals that were pretreated with reserpine. Heart rate (HR) after 1-, 2-, 5-, and 10-A pulses was significantly higher than after control shocks without preceding pulse therapy. Mean and peak LV pressure measurements increased 38 and 72%, respectively, following shocks preceded by 5- and 10-A pulses compared with shocks preceded by no burst pulses. Mean and peak arterial pressures increased 36 and 43%, respectively, following shocks preceded by 5- and 10-A pulses compared with shocks preceded by no burst pulses. After beta-blockade, HR, mean and peak arterial pressures, and mean LV pressure were not significantly different after pulses of any strength compared with control shocks. LV peak pressure following the 10-A pulses was significantly higher than with no burst pulses but was significantly lower than the response to the 10-A pulses delivered without beta-blockade. HR, mean and peak arterial pressures, and mean and peak LV pressure responses after 15 or 30 5- or 10-A pulses were similar to the responses to the same pulses after beta-blockade. We conclude that a burst of 15-30 1-ms pulses delivered during VF can increase HR, arterial pressure, and LV pressure following defibrillation. beta-Blockade or reserpine pretreatment prevents most of this postshock increase in HR, arterial pressure, and LV pressure.

Comparison of six clinically used external defibrillators in swine

BACKGROUND

External defibrillation has long been practiced with two types of monophasic waveforms, and now four biphasic waveforms are also widely available. Although waveforms and clinical dosing protocols differ among defibrillators, no studies have adequately compared performance of the monophasic or the biphasic waveforms. This is the first study to compare defibrillation efficacy among biphasic external defibrillators, and does so as part of a study comparing all commonly available waveforms using their respective manufacturer-provided and clinically used doses.

METHODS AND RESULTS

Efficacy of six waveforms was tested in 852 short-duration ventricular fibrillation episodes in 14 swine. Protocol 1: 200-J monophasic damped sine (MDS) and monophasic truncated exponential (MTE) shocks were compared to 150-J biphasic shocks in six swine at the low-impedance of these animals. Protocol 2: Four commercially available biphasic defibrillators were compared using their respective manufacturer-recommended dose protocols in eight swine at low and simulated high-impedance. At low-impedance, all biphasic shocks achieved near-perfect success, while efficacy was significantly lower for MDS (67%) and MTE (30%) shocks. In protocol 2, first-shock success rates of the four biphasic defibrillators were uniformly high (97, 100, 100, and 94%) for low-impedance shocks, and decreased for high-impedance shocks (62, 92, 82, and 64%). There were statistically significant differences in efficacy among devices.

CONCLUSIONS

Commonly used MDS and MTE waveforms provide markedly dissimilar efficacies. Despite impedance-compensation schemes in biphasic defibrillators, impedance has an impact on their efficacy. At high-impedance, modest efficacy differences exist among clinically available biphasic defibrillators, reflecting differences in both waveforms and manufacturer-provided doses.

Identification of transmural necrosis along a linear catheter ablation lesion during atrial fibrillation and sinus rhythm

BACKGROUND

Determining whether a linear catheter radio frequency (RF) ablation lesion is transmural may be difficult, especially during atrial fibrillation. We hypothesized that changes in pacing thresholds and electrogram amplitude during atrial fibrillation and sinus rhythm could be used to assess whether a radiofrequency ablation resulted in transmural necrosis.

METHODS

A hexapolar, linear, RF ablation catheter was positioned between the caval veins in the right atrium of seven sheep. Pacing thresholds and electrogram amplitudes during atrial fibrillation and sinus rhythm were measured before and after the application of RF energy. Sites along the linear lesion were assessed histologically.

RESULTS

The electrogram amplitude in atrial fibrillation decreased significantly more at transmural sites (unipolar recording: 33 +/- 11% transmural vs. 22 +/- 13% non-transmural, p < or = 0.01; bipolar recording: 62 +/- 9% transmural vs. 43 +/- 15% non-transmural, p < or = 0.01). The electrogram amplitude in sinus rhythm decreased significantly more at transmural sites (unipolar recording: 49 +/- 18% transmural vs. 15 +/- 20% non-transmural, p < 0.001; bipolar recording: 63 +/- 17% transmural vs. 42 +/- 19% non-transmural, p = 0.002). The pacing threshold increased significantly more at sites with transmural necrosis (unipolar: increased by 378 +/- 103% transmural vs. 207 +/- 93% non-transmural, p < 0.001; bipolar: 370 +/- 80% transmural vs. 259 +/- 60% non-transmural, p < 0.001).

CONCLUSIONS

The amplitude of the atrial electrogram from an ablation catheter can be used to discriminate areas with transmural necrosis from those without transmural necrosis during either atrial fibrillation or sinus rhythm. Termination of atrial fibrillation may not be necessary to estimate the histologic characteristics of an ablation lesion.

Emission ratiometry for simultaneous calcium and action potential measurements with coloaded dyes in rabbit hearts: reduction of motion and drift

INTRODUCTION

Optical measurements of the cardiac calcium transient (Ca) and transmembrane action potential (AP) may be performed simultaneously with emission ratiometry to lessen motion artifacts and photobleaching effects. We examined changes in emission spectrum in perfused rabbit hearts coloaded with Rh237 and a green-emitting Ca dye (Fluo-4 or Oregon Green BAPTA 1) to determine wavelength bands for emission ratiometry and to test whether ratiometry reduces motion artifacts and drift.

METHODS AND RESULTS

A 488-nm laser illuminated hearts while a spectrofluorometer collected fluorescence from 489 to 838 nm at 1 kHz. Ratiometry with the Ca- and AP-insensitive emission band 663 to 685 nm (IS) as denominator and the Ca-sensitive band 510 to 532 nm as numerator lessened motion artifacts, which was quantified as a 1.4-fold increase in relative amplitude of the Ca (P < 0.05). Ratiometry with the AP-sensitive band 772 to 794 nm as denominator and the IS as numerator produced a 1.7-fold increase in relative amplitude of the AP (P < 0.05). The ratiometry decreased photobleaching-dependent drift by a factor of 0.6 (P < 0.05) for Ca and 0.45 (P < 0.05) for AP.

CONCLUSION

Simultaneous Ca and AP emission ratiometry reduces motion artifacts and drift in hearts with coloaded dyes.

Defibrillation with a minimally invasive direct cardiac massage device

OBJECTIVE

This study examined (1) the defibrillation efficacy of using a minimally invasive direct cardiac massage (MID-CM) device as one electrode of the defibrillation electrical circuit and (2) the effect on external defibrillation of defibrillating when the MID-CM device is in place and a pneumothorax is present.

METHODS

Part 1: in seven pigs, defibrillation thresholds (DFTs) were determined with a truncated exponential biphasic waveform. DFTs were determined for five electrode configurations: standard transthoracic defibrillation with electrodes on the left and right chest walls (1), with the MID-CM as one of the defibrillation electrodes pressed gently (2) or firmly (3) against the heart with the right chest wall patch as the second electrode, the same as (1) with the MID-CM device in place and the lungs at end-inspiration (4) or at end-expiration (5). Part 2: in six pigs, DFTs were determined with a monophasic damped sinusoidal waveform with external defibrillation electrodes (1) and with the device as one defibrillation electrode and the other electrode on either the anterior (2), lateral (3), or posterior right chest wall (4).

RESULTS

Part 1: the DFTs for (2) or (3) were not different (18.7+/-12.4 vs. 17.0+/-8.3 J), but both DFTs were lower than that for (1) (155+/-45 J). The DFT was elevated for (4) (205+/-69 J) compared with (1). For (5) only one animal could be defibrillated with shocks up to 360 J. Part 2: the DFTs for (2), (3) or (4) were not different (19.5+/-11.0, 25.4+/-9.4, 27.4+/-9.0 J), but all three were lower than the DFT for (1) (198+/-70 J).

CONCLUSIONS

Using the MID-CM device as one electrode of the defibrillation circuit markedly lowers the DFT compared with that for standard transthoracic defibrillation for both a monophasic and biphasic waveform. Defibrillation with the device in place and the chest opened elevates the DFT for external defibrillation much more during end-expiration than during end-inspiration.

Defibrillation threshold and cardiac responses using an external biphasic defibrillator with pediatric and adult adhesive patches in pediatric-sized piglets

Before recommendations for using an automatic external defibrillator on pediatric patients can be made, a protocol for the energy of a biphasic waveform energy dosing needs to be determined that will allow ventricular defibrillation of 8 year olds while causing only a minimal amount of cardiac damage to infants. Pediatric- and adult-sized electrode patches were alternately applied to 10 isoflurane-anesthetized piglets weighing 3.8-20.1 kg to approximate the body weights of newborns to children < 8 years old. The defibrillation threshold (DFT) was determined for biphasic truncated exponential waveform shocks. Additional shocks, varying from the DFT to 360 Joules (J), were delivered during sinus rhythm or following 30 s of ventricular fibrillation (VF). The DFT was 2.4+/-0.81 and 2.1+/-0.65 J/kg for pediatric and adult patches, respectively (P = N.S.). The change in left ventricular (LV) dP/dt from baseline as a function of shock strength was significantly different at 1 and 10 s after shocks of increasing energy that were delivered in sinus rhythm, and 1, 10, 20, and 30 s after defibrillation shocks. There was no significant difference in LV dP/dt with increasing shock energy at 60 s with either patch size. The time to return of sinus rhythm, ST-segment deviation, and cardiac output were also not significantly different from baseline 60 s following shocks of up to 360 J delivered during sinus rhythm or VF with either patch. The same amount of energy delivered with a biphasic external defibrillator successfully defibrillated VF whether adult or pediatric patches were used. Cardiac rhythm and hemodynamic variables were unaltered at 60 s after shocks delivered at energies of up to 360 J. These data suggest that there is a substantial safety margin above a DFT strength shock for this biphasic waveform in piglets.

Reduction of the internal atrial defibrillation threshold with balanced orthogonal sequential shocks

INTRODUCTION

The aim of this study was to determine the atrial defibrillation threshold (ADFT) of a first shock across the standard right atrium (RA) to distal coronary sinus (dCS) configuration followed by a second shock along the atrial septum with a standard sequential waveform (the second shock leading edge equaled the first shock trailing edge) and a balanced sequential waveform (the leading edges of both shocks were equal).

METHODS AND RESULTS

In nine sheep atrial fibrillation was induced with acetyl-beta-methylcholine and burst pacing. A catheter was placed with electrodes in the dCS, proximal coronary sinus (pCS), and RA. A J-shaped catheter was positioned with an electrode at Bachmann's bundle (BB) while another catheter was positioned with an electrode in the superior vena cava (SVC). The ADFTs of six single- and dual-pathway configurations were determined with single, standard sequential, or balanced sequential shocks. The ADFT of the RA-->dCS configuration (0.86 +/- 0.27 J, 159 +/- 29 V, 2.42 +/- 0.36 A) was significantly reduced when followed by an SVC-->pCS (0.58 +/- 0.17 J, 112 +/- 20 V, 1.64 +/- 0.39 A) or a BB-->pCS shock (0.64 +/- 0.16 J, 119 +/- 18 V, 1.81 +/- 0.38 A) with standard sequential shocks. With balanced sequential shocks, the peak voltage and current ADFTs were further significantly reduced (85 +/- 11 V and 1.24 +/- 0.21 A for second shock SVC-->pCS, and 93 +/- 13 V and 1.38 +/- 0.27 A for second shock BB-->pCS).

CONCLUSION

The ADFT of the standard RA-->dCS shock is significantly reduced when followed by a second shock along the atrial septum delivered between electrodes in the pCS and either SVC or BB and ADFT is further reduced with balanced sequential shocks.

Impact of myocardial ischemia and reperfusion on ventricular defibrillation patterns, energy requirements, and detection of recovery

BACKGROUND

Shocks that have defibrillated spontaneous ventricular fibrillation (VF) during acute ischemia or reperfusion may seem to have failed if VF recurs before the ECG amplifier recovers after shock. This could explain why the defibrillation threshold (DFT) for spontaneous VF appears markedly higher than for electrically induced VF.

METHODS AND RESULTS

The DFT for electrically induced VF (E-DFT) was determined in 15 pigs before ischemia, followed by left anterior ascending or left circumflex artery occlusion. VF was electrically induced 20 minutes after occlusion, followed 5 minutes later by reperfusion. Whether spontaneous or electrically induced, VF during occlusion or reperfusion was treated with up to 3 shocks at 1.5xE-DFT. If all 3 shocks failed, shock strength was increased. Thirty minutes after reperfusion, the other artery was occluded and the protocol was repeated. Defibrillation was considered successful if postshock sinus/idioventricular rhythm was present for > or = 30 seconds. VF recurring within 30 seconds after the shock was considered immediate or delayed if the first postshock activation complex in a rapidly restored ECG recording was VF or sinus/idioventricular rhythm, respectively. Defibrillation efficacy at 1.5xE-DFT was significantly higher for electrically induced ischemic VF (76%) than for spontaneous VF (31%). The incidence of delayed recurrence after electrically induced nonischemic (3%) or ischemic (20%) VF was significantly lower than after spontaneous VF (75%). Mean VF recurrence time after spontaneous VF was 4.6+/-5.3 seconds.

CONCLUSIONS

Spontaneous VF can be halted by a shock but then quickly restart before a standard ECG amplifier has recovered from postshock saturation, making it appear that the shock failed.

Biphasic waveform external defibrillation thresholds for spontaneous ventricular fibrillation secondary to acute ischemia

OBJECTIVES

The goal of this study was to determine if the defibrillation threshold (DFT) after spontaneous ventricular fibrillation (VF) secondary to acute ischemia differs from the DFT for electrically induced VF in the absence of ischemia in anesthetized, closed-chest dogs and pigs.

BACKGROUND

The efficacy of external defibrillators has been tested mainly in animals and humans using E-VF, yet external defibrillators are often used in patients to halt S-VF.

METHODS

Protocol 1: biphasic truncated exponential (BTE) waveform shocks were delivered through electrodes placed in an anterior-anterior (A-A) position (left and right lateral thorax) in nine dogs. After measuring the E-VF DFT, acute ischemia was induced with an angioplasty balloon in either the left anterior descending or left circumflex coronary artery, and the S-VF DFT was determined. Protocol 2: in a group of 12 pigs, the E-VF DFT and S-VF DFT were determined for electrodes in the A-A position and in the anterior-posterior position (A-P). Protocol 3: the E-VF DFT was determined in seven pigs. Then up to three shocks 1.5x the E-VF DFT were delivered to S-VF. If defibrillation did not occur, a step-up protocol was used until defibrillation occurred.

RESULTS

Protocol 1: the DFT for E-VF was 65 +/- 28 J (mean +/- SD) compared with 226 +/- 97 J for S-VF, p < 0.05. Protocol 2: the DFT was 152 +/- 58 J for E-VF and 315 +/- 123 J for S-VF for A-A electrodes. The DFT was 100 +/- 43 J for E-VF and 206 +/- 114 J for S-VF for A-P electrodes. Protocol 3: 11/37 shocks of strength 1.5x E-VF DFT (182 +/- 40 J) stopped the arrhythmia. The episodes of S-VF not halted by these shocks required energy levels of up to 400 J for defibrillation.

CONCLUSIONS

External defibrillation of S-VF induced by acute ischemia requires significantly more energy than VF induced by 60-Hz current in the absence of ischemia. A safety margin >1.5x the DFT for electrically induced VF may be necessary in BTE external defibrillators to defibrillate S-VF.

Endocardial wave front organization during ventricular fibrillation in humans

OBJECTIVES

This study was designed to characterize the organization of ventricular fibrillation (VF) on the endocardium of humans.

BACKGROUND

Most proposed mechanisms for the maintenance of VF postulate the propagation of a number of activation wave fronts that reenter to maintain the arrhythmia. We tested the hypothesis that, in patients undergoing internal cardioverter-defibrillator implantation, VF consists primarily of a few large wave fronts on the endocardium.

METHODS

Electrograms were recorded from a 36-electrode catheter in the left ventricle of 16 patients during VF. Activation times were chosen for a 2-s epoch for each fibrillation episode, and a two-dimensional Kolmogorov-Smirnov test was performed to determine if activation occurred randomly along the catheter over that time interval. The maximum cross-correlation was found for all possible pairs of electrodes on the catheter, and these values were plotted relative to the distance between the two electrodes. An exponential curve was then fit to the data, and a length constant was determined. Activation times were grouped into wave fronts along the catheter, and the lengths of the wave fronts were estimated.

RESULTS

The Kolmogorov-Smirnov test showed that activation was not random along the catheter in any of the patients studied. The correlation length determined was 9 +/- 2 cm. The number of wave fronts recorded by the catheter was 9.2 +/- 2.9 wave fronts/s. The length of the pathway of each wave front along the catheter was 6.5 +/- 4.5 cm.

CONCLUSIONS

Ventricular fibrillation is well organized on the endocardial surface of humans, consisting primarily of a few large wave fronts on the order of 6 to 9 cm.

Effect of electrode location in great cardiac vein on the ventricular defibrillation threshold

This study tested the hypothesis that the DFT could be lowered by delivering a weak auxiliary shock in conjunction with a stronger primary shock with the auxiliary shock electrode near the cardiac region where the primary shock electric field is weakest. This hypothesis was tested by determining the DFTs with the auxiliary shock delivered from different locations within the great cardiac vein (GCV). In 15 dogs, catheters with defibrillation electrodes were placed transvenously in the RV apex, the SVC, and the GCV. An active can electrode and the SVC electrodes were electrically coupled to serve as a return electrode for the RV and GCV electrodes. DFTs were determined for a primary shock through the RV electrode with and without a subsequent auxiliary shock of lower amplitude through the GCV electrode. The leading edge voltage and current at DFT were significantly lowered by addition of the auxiliary shock (17% and 19% decreased, respectively), but energy was not changed. The animals were divided into three groups according to the location of the GCV electrode. The leading edge voltage, current, and total delivered energy at the DFT were significantly lower in animals with the GCV electrode near the apex (22%, 24%, and 13% reduction, respectively) compared with those where the GCV electrode was positioned away from apex (8%, 10% reduction and 18% increase, respectively, P < 0.001). Application of an auxiliary shock to the apical region, near the region where previous studies have indicated that the RV primary shock has its weakest effects, caused the greatest decrease in DFT.

Comparison of the temperature profile and pathological effect at unipolar, bipolar and phased radiofrequency current configurations

With a multi-electrode catheter, phased radiofrequency (RF) delivers current between each electrode and a backplate as well as between adjacent electrodes. This study compared the tissue heating and lesion dimensions created by phased and standard RF. Ablation was performed on the in vivo thigh muscles in 5 pigs. Six lesions were created on each thigh muscle using phase angle 0 degrees RF, 127 degrees RF, 180 degrees RF with and without a backplate, and standard RF in bipolar and sequential unipolar configurations. Two plunge needles, each with 6 thermocouples 1 mm apart, were inserted into the tissue with one needle beside an electrode and the other midway between electrodes for tissue temperature measurement. The 0 degrees RF created lower tissue temperatures and smaller lesions between electrodes than those beside electrode. With 127 degrees and 180 degrees RF, tissue temperature and lesion dimensions between electrodes were similar to beside electrode, while the 127 degrees RF created higher tissue temperature and deeper lesions than 180 degrees RF (both with and without a backplate) at both sites. Standard RF bipolar ablation created similar tissue temperatures and lesion depths at both sites, but required greater power than the 127 degrees RF. Standard RF sequential unipolar ablation created only a slight temperature increase and no lesions between electrodes 3 and 4. As judged by tissue temperature, lesion depth and uniformity, and RF power requirement, 127 degrees RF may be a better energy configuration for linear ablation than the other RF modalities tested.

Right atrial septal electrode for reducing the atrial defibrillation threshold

BACKGROUND

The atrial defibrillation threshold (ADFT) energy of the standard lead configuration, right atrial appendage (RAA) to coronary sinus (CS), was reduced by >50% with the addition of a third electrode traversing the atrial septum in a previous study. This study determined whether the ADFT would be lowered by a more clinically practical third electrode placed in the right atrium along the atrial septum (RSP).

METHODS AND RESULTS

Sustained atrial fibrillation was induced in 8 closed-chest sheep with burst pacing and maintained with pericardial infusion of acetyl-beta-methylcholine chloride. A custom-made, dual-defibrillation catheter was placed with electrodes in the lateral RA, CS, and RSP. A separate defibrillation catheter was also placed in the RAA. ADFT characteristics of RAA-->CS and 6 other single- or sequential-shock configurations were determined in random order by using biphasic, truncated-exponential waveforms in a multiple-reversal protocol. The delivered-energy, peak-voltage, and peak-current ADFTs for the sequential-shock configuration CS-->RSP/RA-->RSP (0.53+/-0.31 J, 86+/-22 V, and 1.6+/-0.6 A, respectively) were significantly lower than those of RAA-->CS (1.14+/-0.64 J, 157+/-34 V, and 2.5+/-1.1 A, respectively). The ADFT characteristics of RAA-->CS and RA-->CS were not significantly different, nor were those of CS-->RSP/RA-->RSP and CS-->RSP/RAA--> RSP.

CONCLUSIONS

The ADFT of the standard RAA-->CS configuration may be markedly reduced with an additional electrode situated at the RSP.

Atrial defibrillation thresholds of electrode configurations available to an atrioventricular defibrillator

INTRODUCTION

Little investigation has been conducted to assess the atrial defibrillation thresholds of electrode configurations using electrodes designed for internal ventricular defibrillation (right ventricle [RV], superior vena cava [SVC], and pulse generator housing [Can]) combined with coronary sinus (CS) electrodes. We hypothesized that a CS-->SVC+Can electrode configuration would have a lower atrial defibrillation threshold than a standard configuration for defibrillation, RV-->SVC+Can. We also tested the atrial defibrillation thresholds of five other configurations.

METHODS AND RESULTS

In 12 closed chest sheep, we situated a two-coil (RV, SVC) defibrillation catheter, a left-pectoral subcutaneous Can, and a CS lead. Atrial fibrillation was burst induced and maintained with continuous infusion of intrapericardial acetyl-beta-methylcholine chloride. Using fixed-tilt biphasic shocks, we determined the atrial defibrillation thresholds of seven test configurations in random order according to a multiple-reversal protocol. The peak voltage and delivered energy atrial defibrillation thresholds of CS-->SVC+Can (168+/-67 V, 2.68+/-2.40 J) were significantly lower than those of RV-->SVC+Can (215+/-88 V, 4.46+/-3.40 J). The atrial defibrillation thresholds of the other test configurations were RV+CS-->SVC+Can: 146+/-59 V, 1.92+/-1.45 J; RV-->CS+SVC+Can: 191+/-89 V, 3.53+/-3.19 J; CS-->SVC: 188+/-98 V, 3.77+/-4.14 J; SVC-->CS+ Can: 265+/-145 V, 7.37+/-9.12 J; and SVC-->Can: 516+/-209 V, 24.5+/-15.0 J.

CONCLUSIONS

The atrial defibrillation threshold of CS-->SVC+Can is significantly lower than that of RV-->SVC+Can. In addition, the low atrial defibrillation threshold of RV+CS-->SVC+Can merits further investigation. Based on corroboration of low atrial defibrillation thresholds of CS-based configurations in humans, physicians might consider using CS leads with atrioventricular defibrillators.