Short biphasic pulses from 90 microfarad capacitors lower defibrillation threshold

ICD therapy in the new millennium

Remarkable progress has been made in the 15 years since ICD therapy was approved for human use. The early "shock boxes" had almost no diagnostic capabilities and required thoracotomy for epicardial patch implantation with typical duration of hospitalization of about a week. Pulse-generator longevity was less than 2 years. Modern devices provide detailed information about the morphology and rate of electrocardiographic signals before, during, and after arrhythmia therapy. The down-sizing of pulse generators and improvements in lead design and shock waveforms allow the simplicity of defibrillator implantation to approach that of pacemakers, with defibrillation thresholds comparable with those initially observed with epicardial patches. Despite the marked reduction in size and increase in diagnostic capabilities, device longevity is now longer than 6 years. Routine outpatient ICD implantation is presently feasible and will increase in frequency if ongoing primary prevention trials prove beneficial. Further advances in lead technology and arrhythmia discrimination should increase the efficacy and reliability of therapy. Finally, devices have the capabilities to treat multiple problems in addition to life-threatening ventricular arrhythmias including atrial arrhythmias and congestive heart failure.

Ventricular defibrillation with triphasic waveforms

BACKGROUND

It has been reported that triphasic defibrillation waveforms cause less myocardial injury than biphasic waveforms. This study compared the defibrillation thresholds (DFTs) of triphasic and biphasic waveforms.

METHODS AND RESULTS

++DFTs were determined for a transvenous lead system and a 300-microF-capacitor defibrillator. In 8 pigs (group 1), DFTs were determined for 5 triphasic waveforms with tilts of 80%, 83%, and 86% and for 1 biphasic waveform. DFTs were determined in another 8 pigs (group 2) for 2 triphasic and 4 biphasic waveforms with tilts of 43%, 49%, and 56%. In both groups, a biphasic waveform from a 140-microF-capacitor defibrillator was also evaluated, and both shock polarities were tested for each waveform. In group 1, with the 300-microF-capacitor defibrillator, the leading-edge voltage and energy stored at DFT were significantly lower for triphasic waveforms with phase-duration ratios of 50/33/17 and an anode at the right ventricular electrode for phase 1 than for biphasic waveforms (P<0.001). In group 2, the stored energy of triphasic waveforms with 56% and 49% tilt was significantly lower than that of biphasic waveforms with the same tilts for anodal but not cathodal phase 1 at the right ventricular electrode. Electrode polarity significantly affected the DFT of triphasic waveforms for both studies.

CONCLUSIONS

Some 80% tilt triphasic waveforms defibrillate more efficiently than biphasic waveforms with a 300-microF-capacitor defibrillator. The triphasic waveforms for both groups were not superior to 140-microF-capacitor biphasic waveforms. The efficacy of triphasic waveforms depends on phase durations and electrode polarity.

Theoretical predictions of the optimal monophasic and biphasic defibrillation waveshapes

The truncated decaying exponential waveshape has become the de facto standard for implantable cardiac defibrillators. However, the optimal defibrillation waveshape with respect to delivered energy remains unknown. To this end, this study has derived the theoretically optimal waveshapes for monophasic and biphasic defibrillation shocks as predicted from a lumped-component model of cardiac tissue in conjunction with the "charge-banking" and "charge-burping" hypotheses of defibrillation. These derivations predict that a truncated ascending exponential waveshape--with a shock time constant, tau s, always equal to the underlying tissue time constant, tau m--minimizes the delivered energy required for defibrillation. These predictions are qualitatively consistent with available experimental data. Thus, to the extent that "charge-banking" and "charge-burping" are assumed to be valid and accurate models of defibrillation, these derivations identify the theoretical "gold standards" of defibrillation waveshapes requiring minimum delivered energy.

Preliminary clinical results of a biphasic waveform and an RV lead system

Biphasic defibrillation waveforms have provided a reduction in defibrillation thresholds in transvenous ICD systems. Although a variety of biphasic waveforms have been tested, the optimal pulse durations and tilts have yet to be identified. A multicenter clinical study was conducted to evaluate the performance of a new ICD biphasic waveform and new RV active fixation steroid eluting lead system. Fifty-three patients were entered into the study. Mean age was 63 years with a mean ejection fraction of 36.8%. Primary indication for implantation was monomorphic ventricular tachycardia alone (54.7%). Forty-eight patients (90.6%) were implanted with an RV shocking lead and active can alone as the anodal contact. The ICD can was the cathode. In four cases (7.5%), an additional SVC or CS lead was used due to a high DFT with the RV lead alone. In an additional case, a chronic SVC lead was used although the RV-Can DFT was acceptable. DFT for all cases at implant was 9.8 +/- 3.7 J. Repeat testing at 3 months for a subset of patients showed a reduction in DFT (7.4 +/- 3.0 J), P value = 0.03. Sensing and pacing characteristics of the RV lead system remained excellent during the study period (acute 0.047 +/- 0.005 ms at 5.4 V and 9.9 +/- 6.2 mV R wave; chronic 0.067 +/- 0.11 ms at 5.4 V and 9.3 +/- 5.4 mV R wave). It is concluded that this lead system provides good acute and chronic sensing and pacing characteristics with good DFT values in combination with this waveform.

Effect of shock waveform on relationship between upper limit of vulnerability and defibrillation threshold

INTRODUCTION

The upper limit of vulnerability (ULV) correlates with the defibrillation threshold (DFT). The ULV can be determined with a single episode of ventricular fibrillation and is more reproducible than the single-point DFT. The critical-point hypothesis of defibrillation predicts that the relation between the ULV and the DFT is independent of shock waveform. The principal goal of this study was to test this prediction.

METHODS AND RESULTS

We studied 45 patients at implants of pectoral cardioverter defibrillators. In the monophasic-biphasic group (n = 15), DFT and ULV were determined for monophasic and biphasic pulses from a 120-microF capacitor. In the 60- to 110-microF group (n = 30), DFT and ULV were compared for a clinically used 110-microF waveform and a novel 60-microF waveform with 70% phase 1 tilt and 7-msec phase 2 duration. In the monophasic-biphasic group, all measures of ULV and DFT were greater for monophasic than biphasic waveforms (P < 0.0001). In the 60- to 110-microF group, the current and voltage at the ULV and DFT were higher for the 60-microF waveform (P < 0.0001), but stored energy was lower (ULV 17%, P < 0.0001; DFT 19%, P = 0.03). There was a close correlation between ULV and DFT for both the monophasic-biphasic group (monophasic r2 = 0.75, P < 0.001; biphasic r2 = 0.82, P < 0.001) and the 60- to 110-microF group (60 microF r2 = 0.81 P < 0.001; 110 microF r2 = 0.75, P < 0.001). The ratio of ULV to DFT was not significantly different for monophasic versus biphasic pulses (1.17 +/- 0.12 vs 1.14 +/- 0.19, P = 0.19) or 60-microF versus 110-microF pulses (1.15 +/- 0.16 vs 1.11 +/- 0.14, P = 0.82). The slopes of the ULV versus DFT regression lines also were not significantly different (monophasic vs biphasic pulses, P = 0.46; 60-microF vs 110-microF pulses, P = 0.99). The sample sizes required to detect the observed differences between experimental conditions (P < 0.05) were 4 for ULV versus 6 for DFT in the monophasic-biphasic group (95% power) and 11 for ULV versus 31 for DFT in the 60- to 110-microF group (75% power).

CONCLUSION

The relation between ULV and DFT is independent of shock waveform. Fewer patients are required to detect a moderate difference in efficacy of defibrillation waveforms by ULV than by DFT. A small-capacitor biphasic waveform with a long second phase defibrillates with lower stored energy than a clinically used waveform.

Strength-duration relationship for human transvenous defibrillation

BACKGROUND

One of the basic characteristics of electrical defibrillation is the strength-duration relationship, or the effect of pulse width on defibrillation efficacy. This relationship is important for understanding the mechanism of defibrillation and for the design of optimal waveforms. However, a detailed evaluation of the strength-duration relationship for human transvenous defibrillation has not been performed previously.

METHODS AND RESULTS

This was a prospective study of 29 patients undergoing initial defibrillator implantation with a uniform dual coil, transvenous lead. In each patient defibrillation thresholds were measured for either short (2, 3, 4, 6 ms) or long (6, 12, 18 ms) pulse durations, with the order of testing randomized. The shock waveform was a truncated monophasic pulse from a capacitor of 150 microF. The leading edge voltage at defibrillation threshold was 566+/-100 V for 2-ms pulses. Voltages declined exponentially with increasing pulse width reaching an asymptote by 6 ms (451+/-68 V, P<.05). Defibrillation threshold voltage was insensitive to longer pulse widths. Stored energy at defibrillation threshold showed a similar relationship with pulse width. In contrast, mean current decreased monotonically over the full range of pulse durations evaluated, and there was no evidence of a rheobase.

CONCLUSIONS

The shape of the strength-duration curve and the lack of rheobase current indicate a fundamental difference between cardiac stimulation and defibrillation. The relationship between pulse duration and defibrillation threshold voltage or stored energy is well modeled by a parallel capacitor resistor circuit with a time constant of 5.3 ms.

Application of models of defibrillation to human defibrillation data: implications for optimizing implantable defibrillator capacitance

BACKGROUND

Theoretical models predict that optimal capacitance for implantable cardioverter-defibrillators (ICDs) is proportional to the time-dependent parameter of the strength-duration relationship. The hyperbolic model gives this relationship for average current in terms of the chronaxie (t(c)). The exponential model gives the relationship for leading-edge current in terms of the membrane time constant (tau(m)). We hypothesized that these models predict results of clinical studies of ICD capacitance if human time constants are used.

METHODS AND RESULTS

We studied 12 patients with epicardial ICDs and 15 patients with transvenous ICDs. Defibrillation threshold (DFT) was determined for 120-microF monophasic capacitive-discharge pulses at pulse widths of 1.5, 3.0, 7.5, and 15 ms. To compare the predictions of the average-current versus leading-edge-current methods, we derived a new exponential average-current model. We then calculated individual patient time parameters for each model. Model predictions were validated by retrospective comparison with clinical crossover studies of small-capacitor and standard-capacitor waveforms. All three models provided a good fit to the data (r2=.88 to .97, P<.001). Time constants were lower for transvenous pathways (53+/-7 omega) than epicardial pathways (36+/-6 omega) (t(c), P<.001; average-current tau(m), P=.002; leading-edge-current tau(m), P<.06). For epicardial pathways, optimal capacitance was greater for either average-current model than for the leading-edge-current model (P<.001). For transvenous pathways, optimal capacitance differed for all three models (P<.001). All models provided a good correlation with the effect of capacitance on DFT in previous clinical studies: r2=.75 to .84, P<.003. For 90-microF, 120-microF, and 150-microF capacitors, predicted stored-energy DFTs were 3% to 8%, 8% to 16%, and 14% to 26% above that for the optimal capacitance.

CONCLUSIONS

Model predictions based on measured human cardiac-muscle time parameter have a good correlation with clinical studies of ICD capacitance. Most of the predicted reduction in DFT can be achieved with approximately 90-microF capacitors.

Lead system optimization for transvenous defibrillation

Lead systems that include an active pectoral shell reduce defibrillation thresholds and permit transvenous defibrillation in nearly all patients. A further improvement in defibrillation efficacy is desirable to allow for smaller pulse generators with a reduced maximum output. Accordingly, the purpose of this study was to compare defibrillation thresholds with multiple transvenous lead systems including those with an active pectoral shell to determine which system would optimize defibrillation energy requirements. This prospective study was performed on 21 consecutive patients. Each subject was evaluated with 3 lead configurations with the order of testing randomized. The configurations were a dual coil transvenous lead (lead), the distal right ventricular coil and pectoral pulse generator shell (unipolar), and all 3 components (triad). The right ventricular coil was the cathode for the first phase of the biphasic defibrillation waveform. Delivered energy at defibrillation threshold was 11.2 +/- 3.4 J for the lead configuration, 10.1 +/- 5.2 J for the unipolar configuration, and 7.8 +/- 3.6 J for the triad configuration (p <0.01). Leading edge voltage (p <0.01) and shock impedance (p <0.001) were also decreased for the triad configuration compared with the lead or unipolar configurations, whereas peak current was minimized with the unipolar configuration (p <0.01). We conclude that the combination of a dual coil, transvenous lead and an active pectoral shell reduces defibrillation energy requirements compared with either the lead alone or unipolar configuration. Moreover, the defibrillation thresholds were < or =15 J in all patients using the triad lead system.

Clinical predictors of transvenous biphasic defibrillation thresholds

Transvenous lead systems have become routine for defibrillator placement. However, previous studies of clinical predictors of an adequate nonthoracotomy defibrillation threshold (DFT) evaluated monophasic waveforms or more complex lead systems, including subcutaneous patches. Accordingly, this study is a prospective evaluation of the predictors of an adequate biphasic DFT in 114 consecutive patients undergoing cardioverter-defibrillator implantation with a single transvenous lead. For each subject, 38 parameters were assessed, including standard demographic, electrocardiographic, echocardiographic, and radiographic measurements. An adequate DFT (< or =20 J) was achieved in 92% of patients. Multivariable analysis revealed 2 independent factors predictive of a high threshold: echocardiographic measurements of left ventricular dilation (odds ratio = 0.16, 95% confidence interval 0.05 to 0.53, p = 0.003) and body size (odds ratio = 0.36, 95% confidence interval 0.17 to 0.73; p = 0.005). No patient with a normal left ventricular end-diastolic dimension had a high DFT, whereas 14% (9 of 66) of those with left ventricular dilation had elevated thresholds. When the DFT cutoff was lowered to 15 J, as is necessary with some downsized pulse generators, an adequate threshold was observed in 84% of patients and the same 2 independent predictors of high thresholds were found. These results indicate that an adequate transvenous DFT can be predicted from simple clinical parameters.

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.