Implantable cardioverters and defibrillators

Infrared neurostimulation inex-vivorat sciatic nerve using 1470 nm wavelength

Objective.To design and implement a setup forex-vivooptical stimulation for exploring the effect of several key parameters (optical power and pulse duration), activation features (threshold, spatial selectivity) and recovery characteristics (repeated stimuli) in peripheral nerves.Approach.A nerve chamber allowing ex-vivo electrical and optical stimulation was designed and built. A 1470 nm light source was chosen to stimulate the nerve. A photodiode module was implemented for synchronization of the electrical and optical channels.Main results. Compound neural action potentials (CNAPs) were successfully generated with infrared light pulses of 200-2000µs duration and power in the range of 3-10 W. These parameters determine a radiant exposure for stimulation in the range 1.59-4.78 J cm^-2. Recruitment curves were obtained by increasing durations at a constant power level. Neural activation threshold is reached at a mean radiant exposure of 3.16 ± 0.68 J cm^-2and mean pulse energy of 3.79 ± 0.72 mJ. Repetition rates of 2-10 Hz have been explored. In eight out of ten sciatic nerves (SNs), repeated light stimuli induced a sensitization effect in that the CNAP amplitude progressively grows, representing an increasing number of recruited fibres. In two out of ten SNs, CNAPs were composed of a succession of peaks corresponding to different conduction velocities.Significance.The reported sensitization effect could shed light on the mechanism underlying infrared neurostimulation. Our results suggest that, in sharp contrast with electrical stimuli, optical pulses could recruit slow fibres early on. This more physiological order of recruitment opens the perspective for specific neuromodulation of fibre population who remained poorly accessible until now. Short high-power light pulses at wavelengths below 1.5µm offer interesting perspectives for neurostimulation.

Implantable defibrillators impedance measurement using pacing pulses versus shock delivery with intact and modified high voltage lead system

At present the only method for measuring the high voltage system lead impedance in patients with an ICD is to deliver a low energy test shock. This is painful, requires sedation, and carries a risk of ventricular fibrillation induction. We sought to assess the shock lead and electrode function by calculating IMP using low voltage pacing pulses, and compared it to the measured impedance of a shock through the same lead. This was performed in both an intact and a modified lead system in order to mimic common clinical scenarios that alter lead system IMP (e.g., lead fracture). In an anesthesized canine model (n = 12) a standard (S) transvenous defibrillation lead (TDL), a modified (M) TDL (two-thirds of coil covered with heat-shrunk tubing), an active can (AC), and a M epicardial patch (EP) (two of four coils were disconnected) were used. Three configurations (C) were tested: C1:S/TDL-->AC, C2:M/TDL-->AC, and C3:M/TDL-->MEP. A measured IMP was obtained by an ICD using a 5-J shock as control. IMP was calculated using a 5-J shock, pacing pulses of 10-, 5-, 2-, and 1-V amplitude, as well as from a square wave drive train of low amplitude/high frequency signals (1 and 0.2 V, at 10 kHz) in all Cs. Ohm's law (V = IR) was utilized for measuring calculated IMP. As the surface area of the high voltage lead system decreased, the mean measured IMP (control) increased from C 1 to 3 (63 +/- 10, 95 +/- 4, and 127 +/- 20 omega, respectively). The correlation of calculated IMP from all Cs to measured impedance (control) remained high throughout the IMP range (range of correlation coefficient (r): 0.921-0.981). Calculated IMP using delivery of pacing pulses is highly correlated to IMP measured during shock delivery. This correlation remains high over a clinically significant range of high voltage lead system IMP changes. This study suggests that pacing pulses can be used to predict the IMP changes in the high voltage lead system which may occur clinically, reducing the need to deliver a shock for IMP measurement.

Effects of respiration phase on ventricular defibrillation threshold in a hot can electrode system

The impedance of defibrillation pathways is an important determinant of ventricular defibrillation efficacy. The hypothesis in this study was that the respiration phase (end-inspiration versus end-expiration) may alter impedance and/or defibrillation efficacy in a "hot can" electrode system. Defibrillation threshold (DFT) parameters were evaluated at end-expiration and at end-inspiration phases in random order by a biphasic waveform in ten anesthetized pigs (body weight: 19.1 +/- 2.4 kg; heart weight: 97 +/- 10 g). Pigs were intubated with a cuffed endotracheal tube and ventilated through a Drager SAV respirator with tidal volume of 400-500 mL. A transvenous defibrillation lead (6 cm long, 6.5 Fr) was inserted into the right ventricular apex. A titanium can electrode (92-cm2 surface area) was placed in the left pectoral area. The right ventricular lead was the anode for the first phase and the cathode for the second phase. The DFT was determined by a "down-up down-up" protocol. Statistical analysis was performed with a Wilcoxon matched pair test. The median impedance at DFT for expiration and inspiration phases were 37.8 +/- 3.1 omega, and 39.3 +/- 3.6 omega, respectively (P = 0.02). The stored energy at DFT for expiration and inspiration phases were 5.7 +/- 1.9 J and 6.0 +/- 1.0 J, respectively (P = 0.594). Shocks delivered at end-inspiration exhibited a statistically significant increase in electrode impedance in a " hot can" electrode system. The finding that DFT energy was not significantly different at both respiration phases indicates that respiration phase does not significantly affect defibrillation energy requirements.

Charge-burping theory correctly predicts optimal ratios of phase duration for biphasic defibrillation waveforms

BACKGROUND

For biphasic waveforms, it is accepted that the ratio of the duration of phase 2 to the duration of phase 1 (phase-duration ratio) should be < or = 1. The charge-burping theory postulates that the beneficial effects of phase 2 are maximal when it completely removes the charge delivered by phase 1. It predicts that the phase-duration ratio should be < 1 when the time constant of the defibrillation system (tau s) exceeds the time constant of the cell membrane (tau m) but > 1 when tau s < tau m. This study tested the hypothesis that the optimal phase-duration ratio depends on tau s (the product of the defibrillator capacitance and pathway resistance).

METHODS AND RESULTS

In a canine model of transvenous defibrillation (n = 8), we determined stored-energy defibrillation thresholds (DFTs) for biphasic waveforms from conventional capacitors (140 microF. tau s = 7.1 +/- 0.8 ms) and very small capacitors (40 microF. tau s = 2.0 +/- 0.2 ms). Each capacitance was tested with phase-duration ratios of 0.5, 1, 2, and 3. The duration of phase 1 approximated the optimal monophasic waveform, 6.3 +/- 0.7 ms for 140-microF waveforms and 2.8 +/- 0.2 ms for 40-microF waveforms. For 140-microF waveforms, the DFT was lower for phase-duration ratios < or = 1 than for phase-duration ratios > 1 (P = .0003). The reverse was true for 40-microF capacitors (P = .0008). There was a significant interaction between the effects of capacitance and phase-duration ratio on DFT (P = .0002). The lowest DFT for 40-microF waveforms was less than the lowest DFT for 140-microF waveforms (4.9 +/- 2.5 versus 6.4 +/- 2.4 J, P < .05).

CONCLUSIONS

The optimal phase-duration ratio is < or = 1 for conventional capacitors and > 1 for small capacitors. This supports the predictions of the charge-burping theory.

Adequacy of implantable cardioverter-defibrillator lead placement for tachyarrhythmia detection by sinus rhythm electrogram amplitude

This study examines whether the current clinical practice of using a 5 mV minimum amplitude during normal sinus rhythm (NSR) ensures adequate detection during subsequent episodes of ventricular fibrillation (VF) at the time of the implantable cardioverter-defibrillator (ICD) threshold testing. Risk of nondetection occurs with ICDs when a substantial portion of the individual cardiac events on an electrogram goes undetected. Detection risk was assessed by 2 methods: percentage of missed cardiac events (incidence of signal dropout), and mean electrogram amplitude. During ICD implantation and testing in 27 patients utilizing 41 lead positions, 135 episodes of VF were induced and analyzed. On 64 occasions, the countershock was not successful in achieving cardioversion, and the continuing electrical activity was analyzed as a separate group of postshock waveforms. Thresholds of 1 and 2 mV were applied to each individual cardiac depolarization in a VF episode. Significant risk of nondetection was assumed when > or = 10% of individual events displayed dropout. Underdetection by signal dropout occurred in 11 of 135 preshock arrhythmia signals (8.1%) from 3 patients at a 2 mV threshold, and in 6 of 135 signals (4.4%) at a 1 mV threshold. A mean NSR amplitude > or = 5 mV was associated with significantly lower risk of nondetection during subsequent VF episodes at both 1 and 2 mV thresholds (largest p < 0.001). Similar results were observed in analysis of postshock arrhythmia signals. Further examination of signal dropout and linear regression criteria suggest that in order to eliminate the possibility of nondetection at a 1 mV threshold, minimum NSR amplitudes of 8.5 and 10.0 mV, respectively, are required.

Incidence of ICD lead related complications during long-term follow-up: comparison of epicardial and endocardial electrode systems

The aim of this study was to evaluate the long-term stability of epicardial and endocardial lead systems for third-generation cardioverter defibrillators (ICDs) and to assess the usefulness of diagnostic tools. One hundred forty patients with 61 epicardial (43.6%) and 79 nonthoracotomy systems (56.4%) were followed for 25 +/- 19 months. A total of 18 (12.9%) lead related complications were documented. Complications of epicardial systems were detected in 10 patients (16.4%) during a follow-up time of 36 +/- 8 months: crinkling of patch electrodes in 6 patients (9.8%), insulation breakage of sensing electrodes in 2 patients (3.3%), and adapter defect in 2 patients (3.3%). Eight of the patients (10.1%) with transvenous-subcutaneous systems had lead related complications during a 13 +/- 6 months follow-up: fracture of the subcutaneous patch lead in 2 patients (2.5%), dislodgement of the right ventricular lead in 2 patients (2.5%), dislodgement of the superior vena cava lead in 2 patients (2.5%), insulation breakage of sensing electrodes in 1 patient (1.3%), and connector defect in 1 patient (1.3%). There was no significant difference in the incidence of lead related complications between epicardial and endocardial systems (P > 0.05). Fractures, dislodgements, and crinklings were documented within the first 8 +/- 5 months by regular chest X ray. Defects of insulation, adapter, or connector were detected 22 +/- 10 months after implantation and were associated with delivery of multiple inappropriate ICD therapies. An operative lead revision was indicated for 4 epicardial (6.6%) and 6 endocardial (7.6%) lead systems.

CONCLUSIONS

Endocardial lead systems offer a similar long-term stability as compared to epicardial lead systems. Chest X ray is the most useful tool to detect lead fracture, dislodgment, and patch crinkling. Marker recordings or real-time electrograms have not been helpful in this series to identify patients with suspected lead defects prior to the experience of inappropriate ICD discharges.

Impedance to defibrillation countershock: does an optimal impedance exist?

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

Optimal electrode configuration for pectoral transvenous implantable defibrillator without an active can

A new 83 cm3 implantable cardioverter-defibrillator (ICD) designed for pectoral implantation has been implanted most frequently using right ventricular and superior vena cava (RV-->SVC) electrodes; a patch electrode (RV-->patch + SVC) has been added when necessary to decrease the defibrillation threshold (DFT). The goal of this prospective study was to compare biphasic waveform DFTs for 3 electrode configurations: RV-->patch, RV-->SVC, and RV-->patch + SVC in 25 consecutive patients. The patch was positioned in a left retro-pectoral pocket, and the SVC electrode was positioned with the tip at the junction of the SVC and innominate vein. In the first 15 patients, all 3 electrode configurations were tested in random order; in the last 10 patients, only the RV-->patch and RV-->patch + SVC configurations were tested. In the first 15 patients, the stored-energy DFT for the RV-->SVC configuration (15.2 +/- 7.7 J) was higher (p < 0.001) than the DFT for the RV-->patch configuration (11.3 +/- 6.2 J) and the RV-->patch + SVC configuration (10.0 +/- 5.8 J). For all 25 patients, the DFT was lower for the RV-->patch + SVC configuration (9.7 +/- 5.1 J) than for the RV-->patch configuration (12.4 +/- 6.6 J, p = 0.005). The pathway resistance was highest for the RV-->patch configuration (72 +/- 9 omega), lower for the RV-->SVC configuration (63 +/- 6 omega, p < 0.01), and lowest for the RV-->patch + SVC configuration (46 +/- 3 omega, p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of normothermic and hypothermic cardiopulmonary bypass on defibrillation energy requirements and transmyocardial impedance. Implications for implantable cardioverter-defibrillator implantation

The influence of normothermic and hypothermic cardiopulmonary bypass on defibrillation energy requirements and transcardiac impedance is not well characterized. However, this relationship is of clinical importance during automatic defibrillator implantation done with concomitant cardiac surgery, and there is anecdotal information that criteria for successful implantation are harder to achieve after such operations. We studied the effect of controlled hypothermia on defibrillation energy requirements and transcardiac impedance in a canine model of cardiopulmonary bypass in which 26 animals underwent right atrial and femoral arterial cannulation, as well as continuous hemodynamic and intramyocardial temperature monitoring. The defibrillation energy requirements were evaluated at 60-minute intervals with an epicardial patch system, and transcardiac impedance was measured before and after the multiple inductions and terminations of ventricular fibrillation. In group 1 (n = 10) defibrillation energy requirements were evaluated immediately after initiation of cardiopulmonary bypass at 37 degrees C (T0), after gradual cooling to 28 degrees C (T1), and after rewarming to 37 degrees C (T2). Group 2 (n = 16) comprised time controls that were identically instrumented and studied, but maintained at 37 degrees C throughout. Percent successful defibrillation was plotted against delivered energy, and the raw data fit by logistic regression. The energy at which 50% of shocks were successful (E50) was 3.23 +/- 0.89 joules at T0, 5.12 +/- 1.85 joules at T1, and 4.42 +/- 1.22 joules at T2 in group 1; this was not significantly different from the corresponding group 2 E50 values, which were 3.11 +/- 1.39 joules, 4.95 +/- 2.47 joules, and 5.59 +/- 3.18 joules, respectively. Both groups demonstrated a significant increase in E50 during the first hour of cardiopulmonary bypass (mean increase from T0 to T1 was 1.89 joules in group 1 and 1.84 joules in group 2, p < 0.05). Transmyocardial impedance fell progressively during the group 2 experiments from 73.6 +/- 12.9 omega at the beginning of the T0 shock series to 61.4 +/- 8.9 omega at the end of the T2 shock series. A similar reduction in transmyocardial impedance was observed during the course of all the group 1 experiments; however, at the beginning of the T1 shock series impedance was significantly elevated to 77.4 +/- 12.3 omega (p < 0.05 compared with group 2 and with end T0 in group 1). There was no relationship between defibrillation energy requirements and transcardiac impedance; there was also no correlation between either of these parameters and intramyocardial extracellular pH or left ventricular end-diastolic pressure.(ABSTRACT TRUNCATED AT 400 WORDS)

Results of delivered therapy for VT or VF in patients with third-generation implantable cardioverter defibrillators

Third-generation implantable cardioverter defibrillators (ICDs) offer tiered therapy and can provide significant advantage in the management of patients with life-threatening arrhythmias. Three different types of ICDs were implanted in 21 patients with ventricular tachycardia (VT) or ventricular fibrillation (VF). Arrhythmia presentation was VT (76%), VF (10%), or both (14%). The mean left ventricular ejection fraction for the group was 32.4 +/- 7%. No surgical mortality occurred. Prior to discharge individual EPS determined the final programmed settings of the ICDs. During a mean follow-up of 13 +/- 1.4 months (range 2-20) the overall patient survival was 85.7%. No sudden arrhythmic or cardiac death occurred. Twenty of 21 patients (95%) received therapy by their device. In 14 patients (67%) antitachycardia pacing (ATP) was programmed "on," 13 of which was self-adaptative autodecremental mode. There were 247 VT episodes, 231 of which were subjected to ATP with 97% success and 3% acceleration or failure. Low energy shocks reverted all other VT episodes. VF episodes were successfully reverted by a single shock (93%), two shocks (6%), or three shocks (1%). We conclude that ATP therapy of VT is successful in the large majority of episodes with rare failures, and that VF episodes are generally terminated by a single ICD shock.

Position of epicardial patch electrodes for implantable defibrillation significantly affects shock strength requirements

OBJECTIVE

To assess the impact of epicardial patch electrode position on internal defibrillation efficacy.

METHODS

Two mesh patch electrodes (13 cm2) were positioned on the epicardium of acute, isoflurane-anesthetized pigs (n = 7, 40-47 kg). Defibrillation efficacy was determined for three different patch positions: P1 = anterior-basal right ventricle (RV) and lateral-apical left ventricle (LV); P2 = lateral RV and lateral LV; and P3 = anterior-basal septal region and posterior-apical septal region. To quantify defibrillation efficacy, single capacitor discharge, fixed-tilt (68%) biphasic waveforms were delivered to the heart 10 seconds after initiation of ventricular fibrillation. Initial shock intensities were selected using an up/down protocol. Conversion data were used to construct sigmoidal curves relating probability of defibrillation to energy delivered, peak voltage, and peak current in each animal.

RESULTS

Mean peak voltage and current at 50% defibrillation probability were 40% higher for P2 than they were for either P1 or P3 (p < 0.05). Similarly, mean energy delivered was 75% higher for P2. In this pig model, position of epicardial patch electrodes affects defibrillation efficacy.

CONCLUSION

Apical-to-basal shock vectors (P1 and P3) yielded significantly lower defibrillation shock strength requirements than did a lateral-wall-to-lateral-wall vector (P2), which was perpendicular to the intraventricular septum. These data may help explain the disparity in defibrillation thresholds observed in the human population of patients undergoing implantable cardioverter defibrillator testing with epicardial patch electrodes.

Experience with two different nonthoracotomy systems for implantable defibrillator in 170 patients

Implantation of a nonthoracotomy system (Medtronic PCD or CPI Endotak) was attempted in 170 patients with ventricular tachycardia (VT) or ventricular fibrillation (VF) not requiring concomitant cardiac surgery. A nonthoracotomy system could be successfully implanted in 95 of the 115 patients with the PCD system and 49 of 55 patients receiving the Endotak lead system. In 26 patients with failed nonthoracotomy system because of defibrillation threshold (DFT) > 25 joules (J), an epicardial system was implanted at the same setting. Patients receiving the two lead systems were comparable with regard to age, sex, and ejection fraction. However, since the PCD system offers tiered therapy multiprogrammable options, all attempts were made to implant this lead system in patients with VT that could be pace terminated. Mean DFT (15 +/- 4.7 vs 17 +/- 4.6 J; P = 0.03) and implant time (2.5 +/- 0.6 vs 3.3 +/- 0.7 hours; P = 0.02) were less with the Endotak lead system. There was no perioperative mortality. During a mean follow-up of 20 +/- 4 months, there were eight instances of lead dislodgment in patients receiving the PCD system. There were four nonsudden cardiac deaths and one sudden death in the Endotak group and three nonsudden deaths in the PCD group. Sudden cardiac death and total survival using the intention-to-treat analysis during this follow-up period were 99% and 95%, respectively. In conclusion, successful implantation, perioperative mortality, and survival rate are comparable with both lead systems; however, incorporating two defibrillating electrodes in one lead minimizes lead dislodgment and reduces implant time.

Advances in ICD lead systems

We have presented both technical and clinical data indicating that transvenous lead systems are comparable to standard epicardial ICD systems with respect to efficacy. In addition there may be a reduction in morbidity when compared with the standard epicardial implant. We have shown for the Endotak and Transvene systems that each has its own idiosyncrasies, advantages, and disadvantages. Other lead systems are too early in their clinical evaluations for such definitive statements to be made. Regarding long-term reliability, the issues of mechanical integrity of the insulation and conductor materials, as well as the stability (or otherwise) of sensing and defibrillating characteristics remains to be determined for all lead systems. Similarly, compatibility between lead systems and pulse generators made by different manufacturers cannot be assumed, but needs to be prospectively examined in rigorous clinical trials.

Effect of capacitor size and pathway resistance on defibrillation threshold for implantable defibrillators

BACKGROUND

The time constant of truncated exponential pulses used with implantable defibrillators is determined by the output capacitor size and defibrillation pathway resistance. The optimal capacitor size is unknown.

METHODS AND RESULTS

This study compared defibrillation threshold (DFT) for standard 120-microF capacitors (DFT120) and smaller 60-microF capacitors (DFT60) at implantation of cardioverter-defibrillators in 67 patients using epicardial electrodes (15 patients) or one of four transvenous electrode configurations (52 patients). Paired comparisons of DFT60 and DFT120 were made for 44 defibrillation pathways using monophasic pulses and for 53 pathways using biphasic pulses. Truncated exponential pulses with 65% tilt were used. Pooled data from all electrode configurations showed a significant inverse correlation between pathway resistance and the ratio of stored energy DFT60 to DFT120 (monophasic pulses: r = .75, P = .0001; biphasic pulses: r = .68, P = .0001). Data from all electrode configurations formed a continuum with 120-microF capacitors superior for low-resistance pathways and 60-microF capacitors superior for high-resistance pathways. For pathways with resistance < or = 40 omega, the modest advantage of 120-microF capacitors applied primarily to pathways with low DFTs: 8.2 +/- 6.1 versus 9.6 +/- 5.4 J (P = .001) for monophasic pulses and 4.1 +/- 2.8 versus 5.1 +/- 3.1 J (P < .02) for biphasic pulses. The greater advantage of 60-microF capacitors for pathways with resistance > or = 61 omega applied to pathways with higher DFTs: 12.4 +/- 4.3 versus 23.1 +/- 6.4 J (P = .0001) for monophasic pulses and 8.5 +/- 4.9 versus 12.5 +/- 6.4 J (P = .0001) for biphasic pulses. For pathways using monophasic 120-microF pulses versus 95% for 60-microF pulses. Similarly, the DFT was < or = 10 J for 48% of pathways using biphasic 120-microF capacitors versus 83% for 60-microF pulses.

CONCLUSIONS

In comparison with conventional 120-microF capacitors, 60-microF capacitors had clinically insignificant higher DFTs for low-resistance pathways and clinically important lower DFTs for high-resistance pathways. Optimal capacitance is inversely related to pathway resistance for clinical defibrillation pathways and waveforms.

Tachycardia detection by antitachycardia devices: present limitations and future strategies

Early experience with three generations of implantable devices has demonstrated the need to further refine the accuracy of automated rhythm analysis. Although initial experience with commercially utilized morphological waveform analysis has been disappointing, other time and frequency domain electrogram features have been developed and show potential promise for future devices. While single chamber algorithms for rate and rate variations have theoretical appeal because of their limited power demands, practical experience has demonstrated that inaccurate arrhythmia diagnosis continues to occur by antitachycardia devices that utilize them. Technological advancement in hardware manufacturing and the design of increasingly more efficient software algorithms for tachycardia detection will continue to yield lower power digital circuitry, to increase device battery power and life, and to permit more and more accurate automated arrhythmia diagnosis and treatment by antitachycardia devices. Two chamber sensing has been available for decades in dual chamber antibradycardia pacemakers. The introduction of this technology into antitachycardia devices is not only inevitable but should dramatically improve the precision of diagnosis in future generation devices.

Perioperative complications of cardioverter-defibrillator implantation: the Emory experience

Over a 5-year period, 110 cardioverter-defibrillators (109 epicardial, 1 transvenous) were implanted consecutively in selected patients with ventricular tachyarrhythmias. The perioperative course of this patient population was examined to determine the associated morbidity and mortality of the procedure. Patients were predominantly male, with coronary artery disease and a decreased left ventricular ejection fraction. Most underwent median sternotomy for implantable cardioverter defibrillator implantation. The incidence of perioperative mortality was found to be 2.7%. New-onset atrial fibrillation or flutter occurred in 17.3% of the patients during the postoperative period and aggravation of ventricular tachyarrhythmias in 19.1%. The ICD system became infected in 2.7% of the patients and the mediastinal incision site infected in 2.4%. Pneumonia developed in 4.5%. Other complications included significant blood loss, ICD pocket hematomas, and lead dislodgement. There is an appreciable incidence of morbidity and mortality associated with ICD implantation.

Why do some patients have high defibrillation thresholds at defibrillator implantation? Answers from basic research

Implantable cardioverter defibrillators reduce the risk of sudden cardiac death in patients with ventricular tachyarrhythmias. However, for the few patients with unacceptably high defibrillation thresholds at implantation the risk of sudden death may remain high. If a small number of defibrillation attempts are used to determine a defibrillation threshold, then a high defibrillation threshold may occur in some patients due to the probabilistic nature of defibrillation: a small percentage of shocks will fail even at optimal shock strengths. Basic investigations have suggested mechanisms for high defibrillation thresholds in other patients. The extracellular potential gradients produced by a shock correlate with ability to defibrillate and may be used to classify mechanisms for high defibrillation thresholds. Computerized mapping studies have demonstrated that extracellular potential gradient fields produced by defibrillation shocks are uneven with high gradient areas close to the electrodes and low gradient areas distant from the electrodes. A high defibrillation threshold may occur because: (1) a shock creates a subthreshold potential gradient in the low gradient areas; (2) a patient has a higher minimum potential gradient threshold than other patients; or (3) a shock leads to refibrillation in the high gradient areas. This article reviews experimental evidence to support each of these three possibilities then suggests experimental and clinical investigations that may clarify the causes of high defibrillation thresholds in patients.

Effect of sotalol on ventricular fibrillation and defibrillation in humans

Antiarrhythmic drugs are frequently administered to patients receiving implanted cardioverter defibrillators. Some of these drugs may decrease the efficacy of defibrillation shocks from the defibrillator. Sotalol, a drug with beta-blocking and class III antiarrhythmic properties, lowers defibrillation energy requirements in experimental animals and may do so in humans. Oral sotalol 171 +/- 58 mg was administered before and after device implantation in 25 patients receiving implanted defibrillators. During sotalol therapy, the lowest energy required for successful defibrillation was 5.9 +/- 3.4 J (range 2-15J). In a concurrent nonrandomized comparison group of 23 patients, including 18 treated with amiodarone, the lowest successful energy was 16 +/- 10 J (p < 0.01). In 5 sotalol patients, ventricular fibrillation (VF) could not be induced at all (1 patient) or more than 2 or 3 times (4 patients) despite repeated 60 Hz stimulation. The induced VF had a pronounced tendency to terminate spontaneously, with the termination occurring at up to 23 seconds after the offset of 60 Hz stimulation. The cycle length of the VF was 236 +/- 34 msec, significantly greater than in patients not given drug therapy (191 +/- 21 msec, p < 0.01). In 10 patients, but none of the controls, intracardiac electrograms during surface electrocardiographic VF were regular, monoform, and without low-amplitude diastolic activity. In addition, monophasic action potentials during apparent VF showed maintenance of distinct and normal morphology. The ventricular effective refractory period increased after sotalol (249.4 +/- 19 to 278.4 +/- 24 msec; p < 0.03) and the maximum heart rate response to exercise was limited to 120 +/- 28 beats/min.(ABSTRACT TRUNCATED AT 250 WORDS)

Effective defibrillation in pigs using interleaved and common phase sequential biphasic shocks

Previous studies have shown that low internal defibrillation thresholds (DFTs) can be attained by using two pairs of electrodes and combining biphasic shocks with sequential timing. The purpose of this two-part study was to test the defibrillation efficacy of two new shock sequences, an interleaved biphasic, and a common phase sequential biphasic, that utilized two pairs of electrodes and were developed from the concept of sequential biphasic shocks. In the first part, defibrillation catheters were placed in the right ventricle and the superior vena cava of six anesthetized pigs. A small patch electrode was placed on the LV apex through a subxiphoid incision and a cutaneous patch was placed on the left thorax. The mean DFT energies for the interleaved biphasic (5.2 +/- 0.4 J) and the common phase sequential biphasic waveforms (5.4 +/- 0.4 J) were substantially less (P < 0.0001) than those for either the sequential monophasic (10.6 +/- 1.0 J) or single biphasic waveforms (9.0 +/- 1.0 J). In the second study, which used nine anesthetized pigs, the importance of phase reversal was demonstrated by the finding that the DFT energy of a common phase sequential biphasic shock (6.2 +/- 0.4 J) was much less than a common phase sequential monophasic shock (17.9 +/- 1.3 J, P < 0.0001); furthermore, the average DFT for four common phase sequential biphasic configurations (5.7 +/- 0.2 J) was much less than for a configuration that was similar except that current flow was not reversed in one phase so that no biphasic effect was present (19.7 +/- 1.2 J). The efficacy of common phase sequential biphasics was comparable to that of sequential biphasics. The effectiveness of sequential biphasics, interleaved biphasics, and common phase sequential biphasics is possibly due to two mechanisms: (A) an increase in the potential gradient during a later phase in regions that were low during the first phase, and (B) the exposure of most of the myocardium to a biphasic shock that reduces the minimum extracellular potential gradient needed to defibrillate.

Experience with three different third-generation cardioverter-defibrillators in patients with coronary artery disease or cardiomyopathy

Clinical investigations are being performed in multiprogrammable devices whose therapeutic options include antitachycardia pacing, cardioversion, defibrillation and bradycardia pacing. Three different third-generation devices were implanted in 46 research patients at 1 clinical center to document their safety and efficacy for the treatment of malignant ventricular arrhythmias. Additionally, the purpose of the study was to determine if antitachycardia pacing is a desirable and frequently used feature of tiered devices. The Medtronic PCD was implanted in 15 patients (12 men, mean age 60 +/- 13 years, mean ejection fraction 40 +/- 15%), the Ventritex Cadence in 21 patients (17 men, mean age 65 +/- 10 years, mean ejection fraction 38 +/- 12%), and the CPI VENTAK PRx in 10 patients (8 men, mean age 63 +/- 14 years, mean ejection fraction 31 +/- 8%). All patients presented with cardiac arrest or ventricular tachycardia. During follow-up of 10 +/- 6 months (range 1 to 19), 70% of the 20 patients with antitachycardia pacing activated used the feature for spontaneous ventricular tachycardia. The antitachycardia pacing parameters were reprogrammed 20 times in 15 patients. Two thousand six hundred thirty-eight of 2,675 (98%) antitachycardia pacing attempts successfully terminated spontaneous tachyarrhythmias. Low-energy cardioversion or defibrillation terminated tachyarrhythmias in patients where pacing was unsuccessful. One hundred forty-eight episodes of tachycardia were successfully treated directly by shocks in 16 of 46 patients (35%). There were no deaths due to device failure. This initial single-center clinical experience suggests that the PCD, Cadence and VENTAK PRx are all safe and effective tiered therapy devices for the treatment of malignant ventricular arrhythmias. Antitachycardia pacing successfully terminated most episodes of ventricular tachycardia; in patients with this feature activated, it was used frequently but required reprogramming to achieve high levels of success.

Heart transplantation after cardioverter-defibrillator implantation. A case control study

A case control study was performed to determine whether previous implantable cardioverter-defibrillator (ICD) insertion adversely affects outcome after heart transplantation. Six male heart transplant recipients who had undergone ICD insertion 12 +/- 5 months before heart transplantation were compared to a cohort of six heart transplant recipients who were matched according to age, preoperative status and hemodynamics, date of transplantation, graft ischemic time, history of a previous cardiac operation, and duration of follow-up. There were no significant differences in operating room time, chest tube drainage, time to extubation, and the duration of intensive care unit or hospital stay between the two groups. Furthermore, there were no significant differences in the number of units of packed cells, fresh frozen plasma, platelets and cryoprecipitate transfused. The number of treated rejection episodes and the number of patients requiring intravenous antibiotics for infection in the first 90 days was identical between groups. It was concluded that heart transplantation after ICD implantation did not appear to carry more risk than heart transplantation after a previous cardiac operation. Our limited experience supports the potential use of the ICD in patients with life-threatening ventricular dysrhythmias who are awaiting transplantation.

Predictors of defibrillation efficacy in patients undergoing epicardial defibrillator implantation. The Multicenter Pacemaker-Cardioverter-Defibrillator (PCD) Investigators Group

OBJECTIVES

The objective of this study was to identify predictors of defibrillation threshold in patients undergoing epicardial defibrillator implantation.

BACKGROUND

Factors that predict epicardial defibrillation efficacy are poorly defined.

METHODS

The data from 375 consecutive adult patients were reviewed. After exclusion of 137 patients in whom defibrillation threshold was not obtained, 238 patients (32 women and 206 men) with a mean age of 58.9 +/- 13.3 years formed the study group. Coronary heart disease was present in 175 patients and the mean left ventricular ejection fraction was 35.8 +/- 15.4%. At device implantation, three epicardial patch sizes were available and shocks could be delivered over one current pathway (two patches) or over two current pathways (three patches with simultaneous or sequential shocks). Defibrillation threshold was defined as the lowest programmed energy that successfully defibrillated the heart, provided there had been an unsuccessful shock at a lower energy level or successful defibrillation at < or = 5 J.

RESULTS

The mean defibrillation threshold was 8.6 +/- 5.3 J. With univariate analysis, female gender, sequential shocks with three patches, higher left ventricular ejection fraction and lower New York Heart Association functional class predicted a lower defibrillation threshold. In the multivariate analysis, female gender (coefficient -3.9; 95% confidence interval [CI] -1.9 to -5.0 J), ejection fraction (coefficient -0.6; CI -0.1 to -1.0 J/decile) and sequential shocks (coefficient -2.5; CI -1.0 to -4.0 J) were independently associated with a lower defibrillation threshold. Total epicardial patch conductive surface area normalized to body surface area reached borderline significance (coefficient 0.004; CI 0 to 0.01; p = 0.10). Antiarrhythmic drug use, including amiodarone, did not predict defibrillation threshold.

CONCLUSIONS

Female gender, high left ventricular ejection fraction and the use of sequential pulse shocks were important determinants of improved defibrillation efficacy.

The effect of phase separation on biphasic waveform defibrillation

It has been hypothesized that the defibrillation efficacy of a biphasic shock is caused by the large change in voltage between the two phases. This study examined the effects of separating the two phases in time thus splitting in half the rapid voltage change at phase reversal. The study was performed in three parts each using six dogs. Part I determined defibrillation thresholds (DFTs) for two exponentially truncated biphasic waveforms (3.5/2 msec and 6/6 msec) with interphase time delays of 0, 1, 2, 3, 4, 6, 8, and 10 msec. In Part II, probability of success curves were generated using an up down method with 15 shocks for each delay for the 3.5/2 msec biphasic waveform with interphase delays of 0, 2, 3, 4, and 5 msec. In Part III, DFTs were determined using a 3.5/2 msec and 6/6 msec biphasic as well as a third waveform that consisted of two sequential 6-msec pulses of the same polarity with interphase delays of 0, 5, 10, 15, 20, 25, 50, and 100 msec. In all three parts the defibrillating cathode was a 6.17 cm2 transvenous spring electrode positioned in the RV apex and the anode was a 113 cm2 cutaneous left chest wall electrode patch. With all waveforms, the trailing edge voltage of the first phase was equal to the negative of the leading edge voltage of the second phase. There was no statistical difference in DFTs or in 50% successful defibrillation points for phase separations from 0 to 6 msec and 0 to 5 msec for Parts I and II, respectively. In Part I there was a significant increase in DFTs for phase separations of 8 and 10 msec compared to a phase separation of 0 msec. In Part III there was no significant difference for separations of 0 and 5 msec; however, there was a significant increase in DFT requirements for separations from 5 to 50 msec, which then decreased with a separation of 100 msec for all three waveforms tested. In conclusion, defibrillation efficacy was unchanged with phase separations up to 6 msec. With phase separation, the rapid voltage change during phase reversal is split in half and, thus, cannot explain the improved efficacy of biphasic waveforms.

Changing trends in therapy delivery with a third generation noncommitted implantable defibrillator: results of a large single center clinical trial

Thirty-four patients underwent implantation of a third generation ICD, the 4210 ATP, for sudden cardiac death or ventricular tachycardia. This device incorporates significant telemetry logs as well as a detailed analysis of each arrhythmia episode detected. During the period of clinical follow-up, a mean of 12.2 months, a total of 26,569 VT or VF detections were made. The vast majority of these were either due to atrial fibrillation, nonsustained VT, or "noise" detection, and only 6% led to device therapy. ATP was successful in 86.3% of episodes, with 3.5% accelerations and 2.4% failure of ATP trains. The majority of inappropriate therapy episodes were clustered in seven patients, and all were easily diagnosed with the aid of the extensive telemetry logs and sense histories. Of five late deaths, three were from congestive heart failure, one from cerebrovascular accident, and one unknown. These data reveal that this "tiered" therapy noncommitted ICD performs to expectations; the stored data is of significant value in diagnosing the cause of ICD therapy. In addition, ATP is an effective modality for termination of VT.

The role of an engineering oriented medical research group in developing improved methods and devices for achieving ventricular defibrillation: the University of Missouri experience

Physical scientists and engineers have played important roles in helping to expand our understanding of the factors that influence the defibrillation process and in developing improved methods and devices for achieving cardiac ventricular defibrillation. The long-term experience of one engineering oriented group, based in a clinical department of a medical school, is summarized. Emphasized are the features of a series of research defibrillators that facilitated the generation of an extensive experimental database from studies in dogs and calves, the development of the first automatic implantable defibrillator to be successfully used in dogs, and studies that furnished the rationale for the widespread use of the uniphasic truncated exponential waveform and for the increasing interest in a variety of biphasic and multiphasic waveforms. Also considered are studies concerning the scaling of the defibrillatory shock with subject size and the role of compound units, defibrillation threshold, and contour graphs in the presentation and interpretation of data.

Failure of defibrillator paddle as mimic for subcutaneous patch lead during nonthoracotomy implantable cardioverter defibrillator lead configuration assessment

It is a common, although virtually unsubstantiated, practice to assess the efficacy of nonthoracotomy lead systems for implantable cardioverter defibrillators using a defibrillator paddle as mimic for the subcutaneous patch lead. We report a case in which an adequate defibrillation threshold was documented with the nonthoracotomy lead system using a defibrillator paddle but not following implantation of the true subcutaneous patch lead. This case suggests that the substitution of a defibrillator paddle for the subcutaneous patch lead during nonthoracotomy lead system evaluation may have significant limitations in assessing lead configuration efficacy.

Use of event markers during exercise testing to optimize morphology criterion programming of implantable defibrillator

The present generation implantable defibrillator introduced on-line event markers that can be used to evaluate tachycardia detection in the electrophysiological testing mode. These markers were used to assess the appropriateness of programming the morphology criterion for detection of ventricular tachycardia. Twenty-one consecutive patients (19 men, 2 women) performed 29 bicycle exercise tests with real-time recording of the electrocardiogram and the event markers on a multichannel recorder. Mean ejection fraction was 29% (range 15%-69%). Seven patients were taking antiarrhythmic agents. Twelve patients satisfied the morphology criterion at rest (n = 1) or during exercise (group I), and nine patients did not (group II). One patient was excluded from analysis because of continuous ventricular pacing. Mean peak heart rate was 130 beats/min in group I and 125 beats/min in group II. No statistical differences existed between the groups in relation to cycle length of ventricular tachycardia and mode of induction of arrhythmia, QRS duration on the electrocardiogram, during native rhythm, amplitude and duration of defibrillation patch R wave, calculated duty cycle at peak heart rate, and number of discharges of the automatic implantable cardioverter defibrillator at 2 to 14 months of follow-up time. It is concluded that clinical, electrocardiographic, and implantation data are unreliable in predicting satisfaction of the morphology criterion during high heart rates in native rhythm. Formal exercise testing in the electrophysiological mode enables a rational decision to be made about the appropriateness of the use of probability density function in each patient.

Failure of a second and third generation implantable cardioverter defibrillator to sense ventricular tachycardia: implications for fixed-gain sensing devices

Failure to sense ventricular tachycardia and/or ventricular fibrillation by implantable cardioverter defibrillators (ICDs) is rare. We report a case in which persistent undersensing of monomorphic and polymorphic ventricular tachycardia occurred with a second and third generation ICD using fixed-gain sensing. This occurred despite adequate R wave sensing during sinus rhythm. The use of an endocardial sensing lead did not correct the problem. Failure to sense ventricular tachycardia in the third generation device with fixed-gain sensing occurred late after implantation and was discovered only at follow-up electrophysiology testing of the ICD. This problem could not be corrected by reprogramming of the device, and was not related to lead dislodgement. Placement of a new device with an automatic-gain sensing algorithm and use of previously implanted epicardial leads with better sensing characteristics provided appropriate sensing of ventricular tachyarrhythmias. The case illustrates the importance of testing the sensing of all ventricular arrhythmias in patients with fixed-gain ICD's. Follow-up electrophysiology testing and evaluation of epicardial and endocardial leads may be necessary in certain cases to ensure adequate sensing of ventricular tachyarrhythmias late after implantation.

Three-dimensional potential gradient fields generated by intracardiac catheter and cutaneous patch electrodes

BACKGROUND

Defibrillation may be improved if electrode configurations can be found that create a larger and more even voltage gradient field across the heart. This study determined the magnitude of the shock gradient fields generated by four nonthoracotomy electrode configurations for defibrillation.

METHODS AND RESULTS

In six dogs, a catheter was inserted containing a right ventricular apical electrode (V) and a right atrial electrode (A). A cutaneous patch electrode (P) was placed on the left lateral thorax. Shock potentials were recorded simultaneously from 128 electrodes in the left ventricular and right ventricular subepicardium and subendocardium, ventricular septum, and atria. With the chest closed, 50-mA shocks were given during diastole via the following lead configurations: V----A (V, cathode; A, anode); V----P; V----A+P; and V+A----P. Potential gradients were calculated at the subepicardium and subendocardium in millivolts per centimeter per volt of shock. In most dogs, the V----A+P configuration produced higher gradients throughout the ventricles than did V----A, V----P, or V+A----P. The maximum potential gradient was smaller for the V+A----P configuration than for V----A, V----P, or V----A+P. The gradient fields for the configurations with the catheter alone or combined with P were uneven.

CONCLUSIONS

It is possible to estimate shock gradient fields in three dimensions. Of the four configurations tested, V----A+P produced the highest gradients and V+A----P produced the lowest high gradient. The gradient fields were uneven throughout the ventricles.

Infections involving implanted cardioverter defibrillator devices

Infection is one of the most serious complications in patients with an implanted cardioverter defibrillator. The diagnosis is facilitated by computed tomographic and radionuclide imaging. Infection may be caused by intraoperative contamination or hematogenous seeding. In view of the serious consequences, the emphasis should be on prevention of these events. Perioperative antibiotic prophylaxis is common practice but the utility of prophylactic antibiotic remote from surgery is questionable. Strict adherence to asepsis and a meticulous surgical technique are essential. Identification of risk factors in the individual patient allows a patient-tailored treatment policy that may add to infection prevention. If implant infection does occur, complete removal of the system is most successful.

Mixed venous oxygen saturation for differentiating stable from unstable tachycardias

Current antitachycardia systems are incapable of adequately distinguishing stable from unstable tachycardias. Previously, integration of a pressure sensor or an impedance sensor, together with electrogram analysis, has been investigated as an improved method of identifying unstable arrhythmias. A mixed venous oxygen saturation sensor was investigated for differentiating stable from unstable paced and induced tachycardias in 10 patients. During rapid pacing at 600, 500, 400, 350, 300, and 250 msec cycle lengths, mixed venous oxygen saturation decreased as cycle length decreased. For any given cycle length, rapid ventricular pacing tended to result in greater mixed venous oxygen desaturation compared with atrial pacing. Mixed venous oxygen saturation decreased similarly during induced ventricular tachycardias at cycle lengths greater than 230 msec. However, ventricular tachycardias at cycle lengths less than or equal to 230 msec and ventricular fibrillation had no effect on mixed venous oxygen saturation until after termination. Subsequently, a mixed venous oxygen saturation-tiered therapy algorithm (cycle length less than or equal to 230 msec = unstable; cycle length greater than 230 msec and MVO2 greater than or equal to 6% over 30 seconds = unstable) was developed and was tested retrospectively in 113 paced and induced tachyarrhythmias in these 10 patients for detecting unstable tachycardias (defined as a decrease from baseline systolic arterial pressure of greater than or equal to 50 mm Hg at 15 seconds). The mixed venous oxygen algorithm had 93% sensitivity and 96% specificity compared with rate-only (rate greater than or equal to 170 beats/min) detection with 93% sensitivity and 71% specificity.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of electrical axis of stimulation on excitation of cardiac muscle cells

Orthogonal sequential shock can defibrillate the heart with greater efficacy compared with single shock defibrillation. In this study we tested the hypothesis that cardiac cells have a preferred orientation in their response to excitatory extracellular electric fields, so that orthogonal shocks may stimulate distinct populations of cells. A micropaddle electrode system was used to deliver rectangular pulses for extracellular field stimulation of individual heart cells. We found that single frog and guinea pig ventricular myocytes are excitable with rectangular pulse field stimulation over a wide range of pulse durations, ranging from 10 msec to as little as 20 microseconds. The excitation field strength varies inversely with pulse duration as described by the Weiss-Lapicque equation, although the frog myocytes show a significant "notch" at pulse durations of approximately 1-2 msec, and the guinea pig myocytes are more excitable than predicted for pulse durations of less than 0.2 msec. Every myocyte tested was more excitable when the long axis of the cell was oriented parallel to the stimulating field than when perpendicular to the field. For 2-msec pulses, the difference in field strength was a factor of 5.8 +/- 2.0 (n = 30) for frog and 2.6 +/- 0.5 (n = 23) for guinea pig myocytes. Complete excitation strength-duration curves were obtained in seven frog and 14 guinea pig cells for both parallel and perpendicular cell orientations.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction in defibrillator shocks with an implantable device combining antitachycardia pacing and shock therapy

Implantable defibrillators reduce the risk of sudden death in patients with malignant ventricular arrhythmias, but significant restriction in quality of life can occur as a result of frequent device activation. To determine if a device that provides both antitachycardia pacing and shock therapy can safely reduce the frequency of shocks after implantation, 46 consecutive patients undergoing initial implantation of a defibrillator were studied. In all patients, the implanted device provided antitachycardia pacing and shock therapy. Detected tachycardia characteristics and the results of therapy were stored in the device's memory. There were 42 men and 4 women, aged 26 to 71 years (mean 58.7 +/- 13.5). Left ventricular ejection fraction ranged from 13% to 67% (mean 32.2 +/- 13.4%) and 31 patients had experienced one or more episodes of cardiac arrest. Induced arrhythmias included sustained monomorphic ventricular tachycardia in 38 patients, nonsustained polymorphic ventricular tachycardia in 2 and ventricular fibrillation in 4. Over a total follow-up period of 255 patient-months (range 1 to 13, mean 6.1), 25 patients experienced spontaneous arrhythmic events. In 22 patients, 909 episodes of tachycardia were treated by antitachycardia pacing, which was successful on 840 occasions (92.4%). Acceleration of ventricular tachycardia by pacing therapy was estimated to have occurred 39 times. Syncope occurred once during pacing-induced acceleration of ventricular tachycardia. Forty-four episodes of tachycardia in seven patients were treated directly by shocks because of short tachycardia cycle length; 88% of all detected tachycardias were treated without the need for shocks. Four patients died from cardiorespiratory failure and one patient died suddenly without any detected tachyarrhythmia.(ABSTRACT TRUNCATED AT 250 WORDS)

Patients with a high defibrillation threshold: clinical characteristics, management, and outcome

Of 125 patients prepared to receive implantable cardioverter-defibrillators (ICDs) with the patch-patch configuration of the difibrillating electrodes, 23 (18%) had high (greater than or equal to 25 joules) defibrillation thresholds (DFTs). These patients had lower left ventricular ejection fractions (27 +/- 12 vs 34 +/- 13; p less than 0.03) and a higher incidence of previous heart surgery (47% vs 19%; p less than 0.01) than patients with normal DFTs but did not differ in age, type of heart disease, incidence of concomitant heart surgery, or use of antiarrhythmic medication. Defibrillators were implanted in 18 of these 23 patients, 12 during the initial surgery and six after repeat DFT testing 2 weeks later. After 22 +/- 11 months of follow-up, four patients with ICDs died (two suddenly, and two of nonsudden cardiac causes). Two patients without ICDs died of nonsudden cardiac causes. Appropriate shocks were received by five patients (29%) including both who died suddenly later. A high DFT may be more common than previously stated. It is associated with poor ventricular function and previous heart surgery. Repeated DFT testing may be useful in some patients. A high DFT does not preclude successful ICD shocks, but other therapies may provide better results.

Safety of external cardioversion/defibrillation in patients with internal defibrillation patches and no device

Placement of prophylactic epicardial defibrillation patches at time of open-heart surgery in patients at risk for postoperative arrhythmias has been strongly questioned. Concern has centered on the ability to safely perform subsequent external defibrillation if needed. From 61 patients who were treated with a two-stage strategy we identified 17 who, while wearing epicardial patches and no generator, received external cardioversion/defibrillation for 20 episodes of hemodynamically unstable ventricular arrhythmias. All the patients had one small and one large patch. Eighteen of the episodes were induced during electrophysiological testing (with transthoracic shocks delivered via pad electrodes oriented in an apex-posterior configuration) and two were spontaneous. The episodes occurred at 21 +/- 27 days from patch implant. Thirteen episodes (65%) were converted with one shock at an energy level of 185 +/- 65 J. Seven (35%) required a second shock at 351 +/- 22 J. The accumulated energy requirement was 286 +/- 205 J. No adverse outcomes were noted. The number of episodes requiring more than one shock and the energy requirements were not different from those in a control group of 20 similar arrhythmias treated with the same equipment. Under these conditions, external cardioversion/defibrillation in patients with one large and one small epicardial defibrillation patch was uniformly successful. Further data is needed in the out-of-hospital setting and on the results of external defibrillation in patients with two large patches.