Unipolar electrode system for an implantable defibrillator: new experimental approaches

Optimized first phase tilt in "parallel-series" biphasic waveform

INTRODUCTION

A biphasic defibrillation waveform can achieve a large second phase leading-edge voltage by a "parallel-series" switching system. Recently, such a system using two 30-microF capacitances demonstrated better defibrillation threshold than standard waveforms available in current implantable devices. However, the optimized tilt of such a "parallel-series" system had not been defined.

METHODS AND RESULTS

Defibrillation thresholds were evaluated for five different biphasic "parallel-series" waveforms (60/15 microF) and a biphasic "parallel-parallel" waveform (60/60 microF) in 12 anesthetized pigs. The five "parallel-series" waveforms had first phase tilts of 40%, 50%, 60%, 70%, and 80% with second phase pulse width of 3 msec. The "parallel-parallel" waveform had first phase tilt of 50% with second phase pulse width of 3 msec. The defibrillation lead system comprised a left pectoral "hot can" electrode (cathode) and a right ventricular lead (anode). The stored energy at defibrillation threshold of the "parallel-series" waveform with first phase tilts of 40%, 50%, 60%, 70%, and 80% was 7.0 +/- 2.1, 6.1 +/- 2.8, 6.8 +/- 2.8, 7.2 +/- 2.9, and 8.4 +/- 3.1 J, respectively. The stored energy of the "parallel-series" waveform with a 50% first phase tilt was 16% less than the nonswitching "parallel-parallel" waveform (7.3 +/- 2.8 J, P = 0.006).

CONCLUSIONS

A first phase tilt of 50% maximized defibrillation efficacy of biphasic waveforms implemented with a "parallel-series" switching system. This optimized "parallel-series" waveform was more efficient than the comparable "parallel-parallel" biphasic waveform having the same first phase capacitance and tilt.

Sawtooth first phase biphasic defibrillation waveform: a comparison with standard waveform in clinical devices

INTRODUCTION

A major limitation in a conventional truncated exponential waveform is the rapid drop in current that results in short duration of high current or longer duration with a lower average current. We hypothesized that increasing the first phase average current by boosting the decaying waveform prior to phase reversal may improve defibrillation efficacy.

METHODS AND RESULTS

To better simulate a "rectangular" waveform during the first phase, a "sawtooth" defibrillation waveform was constructed using "parallel-series" switching of capacitances (each 30 microF) during the first phase. This permitted a boost in the voltage late in the first phase. This sawtooth biphasic waveform (sawtooth) was compared to two clinical waveforms: a 135-microF capacitance (control-1) and a 90-microF capacitance (control-2) waveform. Defibrillation threshold (DFT) parameters were evaluated in 13 anesthetized pig models using a system consisting of a transvenous right ventricular apex lead (anode) and a left pectoral "hot can" electrode (cathode) system. DFT was determined by a "down-up down-up" protocol. The stored energy for sawtooth, control-1, and control-2 was 10.5 +/- 2.8 J, 12.3 +/- 3.7 J*, and 12.2 +/- 2.8 J*, respectively (*P < or = 0.01 vs sawtooth). The average current of the first phase for sawtooth, control-1, and control-2 was 7.6 +/- 1.3 A, 4.7 +/- 0.9 A*, and 6.2 +/- 0.9 A*, respectively (*P = 0.0001 vs sawtooth).

CONCLUSION

A sawtooth biphasic waveform utilizing a "parallel-series" switching system of smaller capacitors can improve defibrillation efficacy. A higher average current in the first phase generated by such a waveform may contribute to more efficient defibrillation by facilitating myocyte capture.

Comparative efficacy of subcutaneous mesh and plate electrodes for nonthoracotomy canine defibrillation

To determine the optimal configuration for the subcutaneous placement of electrodes for the performance of ventricular defibrillation without thoracotomy, internal defibrillation using four different subcutaneous electrodes was performed in 13 anesthetized dogs (7-12 Kg, mean +/- SD: 9.2 +/- 1.5 Kg). An electrode (7 cm2) was positioned transvenously in the superior vena cava with the following electrodes randomly implanted subcutaneously on the left chest: small mesh electrode (14 cm2), large mesh electrode (28 cm2), small titanium plate electrode (14 cm2), and large plate electrode (28 cm2). Ventricular fibrillation was induced by applying alternating current; a monophasic defibrillation wave was administered between the superior vena cava and the subcutaneous electrodes 10 seconds later. The energy level associated with a 50% successful defibrillation, as predicted by logistic regression analysis, was defined as the ED50. After the completion of the defibrillation protocol using the four subcutaneous electrodes, the small mesh electrode was sutured to the epicardium and the ED50 measurements were repeated. Energy ED50s were lower when the superior vena cava electrode was used as the cathode rather than as the anode. Of the subcutaneous electrodes, the large plate electrode showed the lowest energy ED50 (3.3 +/- 0.9 joules). The plate electrodes had lower energy ED50s than the mesh electrodes, and the large electrode had a lower energy ED50 than the small electrodes. Using the epicardium electrode, transient arrhythmias and ST elevation were observed following successful defibrillation; however, no arrhythmias or ST-T changes were observed following defibrillation using the subcutaneous electrodes.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical devices for treatment of arrhythmias

Electrical devices can be used for preventing and terminating tachycardia and for achieving hemodynamic improvement during a continuing tachycardia. Conventional approaches to tachycardia prevention include pacing at physiologic rates to prevent brady-cardia-related tachycardia or tachycardias associated with prolonged QT-interval syndromes. More exotic techniques, such as those involving stimulation during the refractory period, are undergoing investigation. Some tachycardias cannot be easily terminated or recur incessantly. Hemodynamics can be improved by pacing methods that result in a narrower QRS complex by coupled pacing and, in supraventricular tachycardias, by pacing rapidly enough to create atrioventricular block. Most clinical tachycardias are caused by reentry. Careful analysis of the timing of individual stimuli that successfully terminate tachycardias indicate that critical relations exist in the conduction velocity, refractoriness and physical properties and dimensions of the reentry circuit and the remaining myocardium. Elucidating these relations has permitted inferences into the mechanisms by which pacing terminates or accelerates tachycardias. A vast number of pacing patterns have evolved for use in tachycardia termination. None of these appear to be foolproof. There is widespread and justified concern about the risk of acceleration of tachycardia when antitachycardia pacing is used in the ventricle. Experience indicates that only a few patients are suitable for termination of ventricular tachycardia by pacing, but these carefully selected patients may do well. Both the results and the potential for widespread use may be better with pacing for termination of supraventricular tachycardia. Life-threatening tachycardias or fibrillation can be terminated by direct-current countershock. Although many technical problems remain, implantable cardioverter-defibrillators, possibly combined with antitachycardia pacemakers, will play an increasing role in the management or serious arrhythmias.

Improved defibrillation thresholds with large contoured epicardial electrodes and biphasic waveforms

A reduction in the shock strength required for defibrillation would allow use of a smaller automatic implantable cardioverter-defibrillator and would reduce the possibility of myocardial damage by the shock. Most internal defibrillation electrodes require 5 to 25 J for successful defibrillation in human beings and in dogs. In an attempt to lower the shock strength needed for defibrillation, we designed two large titanium defibrillation patch electrodes that were contoured to fit over the right and left ventricles of the dog heart, covering areas of approximately 33 and 39 cm2, respectively. In six anesthetized open-chest dogs, the electrodes were secured directly to the epicardium and ventricular fibrillation was induced by 60 Hz alternating current. Truncated exponential monophasic and biphasic shocks were given 10 sec later and defibrillation thresholds (DFTs) were determined. The DFT was 159 +/- 48 V, 3.2 +/- 1.9 J (mean +/- SD) for 10 msec monophasic shocks and 106 +/- 22 V, 1.3 +/- 0.4 J, for biphasic shocks with both phase durations equal to 5 msec (5-5 msec). The experiment was repeated in another six dogs in which the electrodes were secured to the pericardium. The mean DFT was not significantly higher than that for the electrodes on the epicardium: 165 +/- 27 V, 3.1 +/- 1.2 J for 10 msec monophasic shocks and 116 +/- 19 V, 1.6 +/- 0.5 J for 5-5 msec biphasic shocks. Low DFTs were also obtained with biphasic shocks in which the duration of the first phase was longer than that of the second.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of electrical pacing and automatic cardioversion defibrillation for normal cardiac function recovery

Antitachycardia pacing and low energy cardioversion terminated episodes of ventricular tachycardia and fibrillation and prevented extrasystoles in patients with acute myocardial infarction, coronary artery disease, and long QT syndrome. Low energy automatic defibrillation was used in 25 patients intraoperatively and during the early postoperative period. The Soviet prototype of an implantable cardioverter-defibrillator (ICAR-D) detects the arrhythmia automatically and delivers selectively an electrical impulse to the heart: 2.5-4 J for VT and 15-20 J for VF. In addition, the effectiveness of ICAR-D was studied in chronicle experiments in 10 dogs with follow-up of 4-6 months.