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Circulation. 1997;95:1497-1504

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(Circulation. 1997;95:1497-1504.)
© 1997 American Heart Association, Inc.

Articles

Programming of Implantable Cardioverter-Defibrillators on the Basis of the Upper Limit of Vulnerability

Charles D. Swerdlow, MD; C. Thomas Peter, MD; Robert M. Kass, MD; Eli S. Gang, MD; William J. Mandel, MD; Chun Hwang, MD; David J. Martin, MD; Peng-Sheng Chen, MD

From the Division of Cardiology (C.D.S., C.T.P., E.S.G., W.J.M., C.H., D.J.M., P.-S.C.) and the Department of Cardiovascular Surgery (R.M.K.), Cedars-Sinai Medical Center, Los Angeles, Calif.

Correspondence to Charles D. Swerdlow, MD, 8635 W Third St, Suite 975 W, Los Angeles, CA 90048. E-mail swerdlow{at}ucla.edu.


*    Abstract
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 *Abstract
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Background A patient-specific measure of defibrillation efficacy that requires a minimum number of ventricular fibrillation (VF) episodes would be valuable for programming implantable cardioverter-defibrillators (ICDs). The upper limit of vulnerability (ULV) is the weakest shock strength at or above which VF is not induced when a stimulus is delivered during the vulnerable phase of the cardiac cycle. It correlates with the defibrillation threshold (DFT) and can be determined with a single episode of VF. The objective of this study was to test the hypothesis that ICDs programmed on the basis of the ULV convert spontaneous ICD-detected VF reliably.

Methods and Results We studied 100 consecutive patients at ICD implantation and during follow-up of 20±7 months. At implantation, the ULV and DFT were determined, and the ICD system was tested at a shock strength equal to the ULV+3 J. During follow-up, the strength of the first shock was programmed to the ULV+5 J for arrhythmias detected in the VF zone (cycle length <292±17 ms). We reviewed stored detection intervals and electrograms from spontaneous episodes of ICD-detected VF to determine the success rate for appropriate first shocks. The programmed first-shock strength was 17.5±5.2 J. During follow-up, there were 120 appropriate first shocks in 37 patients. The arrhythmia was rapid monomorphic ventricular tachycardia (VT) in 70% of episodes (31 patients), VF in 11% (13 patients), polymorphic VT in 1%, and unclassified in 17% (15 patients). The first shock was successful in 119 of 120 episodes (99%; 95% CI, 93% to 100%). One unclassified episode required two shocks. No patient had syncope associated with an ICD shock or arrhythmic death.

Conclusions ICD shocks can be programmed on the basis of the ULV, a measurement made in regular rhythm, without a direct measure of defibrillation efficacy.


Key Words: defibrillation • heart-assist device • upper limit of vulnerability


*    Introduction
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 *Introduction
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Implantable cardioverter-defibrillators have become accepted as first-line therapy for patients who have been resuscitated from VF or VT. Ideally, shocks from ICDs should be sufficiently strong to defibrillate but not strong enough to cause unnecessary battery depletion or adverse effects such as myocardial depression1 2 and conduction block.3 4 5 6 To achieve this goal, shock strength may be programmed in relation to the patient-specific DFT.7 8 9 However, multiple fibrillation-defibrillation test episodes are required to determine the DFT. To minimize risks8 10 11 12 13 14 15 16 17 of this testing at implantation, the number of fibrillation-defibrillation episodes may be limited to the minimum necessary to ensure defibrillation at the maximum output of the ICD.7 8 18 Shock strength is then programmed to this maximum value. However, this safety-margin strategy places all patients at risk for possible adverse effects of excessive shock strength during long-term follow-up. These factors have motivated investigators to seek an accurate, patient-specific measure of defibrillation efficacy that requires a minimum number of fibrillation-defibrillation episodes.

The ULV is the weakest shock strength at or above which VF is not induced when a stimulus is delivered during the vulnerable phase of the cardiac cycle. The ULV hypothesis of defibrillation links the ULV, a measurement made in regular rhythm, to the shock strength that defibrillates reliably.19 20 21 22 23 The ULV has been proposed as a patient-specific measure of defibrillation efficacy because it can be determined with a single episode of VF. Recently, we described a clinical method for determining the ULV24 that provides a good estimate of the shock strength that defibrillates with a 90% probability of success.25 A shock strength equal to this ULV+3 J provides an adequate defibrillation safety margin for VF induced at ICD implantation25 and 3 months later.26 The purpose of this prospective study was to test the hypothesis that patient-specific shock strengths programmed on the basis of the ULV, without regard to the DFT, convert spontaneous ICD-detected VF reliably.


*    Methods
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Patients
We studied 100 consecutive patients at implantation of a biphasic-waveform, transvenous ICD and during long-term follow-up. All patients gave written, informed consent according to a protocol approved by the Human Subjects Committee of Cedars-Sinai Medical Center. There were 73 men and 27 women with a mean age of 64±11 years. The mean left ventricular ejection fraction was 0.34±0.12. Sixty-eight patients had coronary artery disease, 27 patients had myocardial or valvular disease, and 5 patients had no structural heart disease. The clinical indication for ICD therapy was sustained monomorphic VT in 58 patients, VF in 35 patients, sustained polymorphic VT in 1 patient, and syncope with inducible sustained VT in 6 patients. At the time of implantation, antiarrhythmic drugs had been discontinued for 5 half-lives except for amiodarone (6 patients), which does not affect the relationship between ULV and DFT.24 Therapy with digoxin (14 patients), ß-blockers (21 patients), and calcium channel blockers (10 patients) was continued.

Implantation Procedure
Surgical technique. Implantations were performed under general anesthesia and after endotracheal intubation as described previously.25 27 A transvenous ICD (Medtronic Jewel model 7202D, 7218C, 7219C, or 7219D) and electrodes were inserted through a single left anterior axillary incision28 in all 100 patients.

Electrode configuration. A tripolar lead with a 5-cm defibrillation coil and bipolar pace-sense electrodes was positioned in the right ventricular apex via the cephalic or subclavian vein. It served as the cathode for the first phase of biphasic shocks. The pulse generator was positioned in a retropectoral pocket. In the first 70 patients who received model 7202D or 7219D pulse generators, the first-phase anode was a patch electrode positioned deep to the pulse generator (34 patients) or the patch combined with a coil electrode positioned at the junction of the innominate vein and superior vena cava (34 patients)27 or in the coronary sinus (2 patients). In the last 30 patients who received a pulse generator with an "active can" (Medtronic model 7218C or 7219C), the titanium shell of the ICD served as the first-phase anode.

Determination of the ULV. The main features of this method have been reported.24 25 29 Briefly, pacing was performed via the implanted bipolar electrodes at the right ventricular apex. The baseline drive (S1) was delivered at a cycle length of 500 ms. All six surface ECG limb leads were recorded simultaneously on a physiological recorder or computer screen. The T waves from all recorded leads were inspected to select the lead with the latest-peaking monophasic T wave. The interval from S1 to the peak of this T wave was measured initially and after every other change in shock strength. The first T-wave shock was delivered after eight S1s to coincide with the peak of this T wave. If VF was not induced, subsequent shocks were delivered 20 and 40 ms before the peak. If any of the three shocks induced VF, the strength of the next shock was increased; if none of the three shocks induced VF, the strength of the next shock was decreased. After VF was induced in patients 53 through 100, an additional shock with a coupling interval of 20 ms after the peak was given at the weakest shock strength that did not induce VF at 0, 20, and 40 ms before the peak. The ULV was defined as the weakest measured shock strength that did not induce VF.3 There was a 60-second recovery period between shocks that failed to induce VF for strengths of >=15 J and a 30-second interval for weaker shocks.

The strength of the first test shock was 15 J in patients 1 through 22 and 73 through 100. In patients 23 through 72, it was 20 J. The strength of the shocks was changed by 5-J steps for energies >15 J. For energies between 15 and 5 J, the step-down increment was 5 J, followed by a step-up increment of 2.5 J. At 5 J, the step-down increment was 2.5 J. If VF was not induced at a programmed shock strength of 2.5 J, the ULV was the corresponding measured shock strength. This procedure resulted in a resolution of 5 J above the 15-J level and 2.5 J below it. All shocks were biphasic pulses from 120-µF capacitors with 65% tilt in each phase delivered by an external defibrillator (Medtronic model 2394).

Determination of the DFT. To permit direct comparison of the DFT and the ULV, the sequence of shock strengths for DFT testing was identical to the sequence for ULV testing.25 29 VF was induced by T-wave shocks during ULV testing. If the ULV was determined before the DFT was determined, subsequent episodes of VF were induced by T-wave shocks of <=2.0 J. The DFT was defined as the lowest measured shock strength that terminated VF after 10 seconds. There was a 3-minute recovery period after each defibrillation shock. If the defibrillation test shock failed, a 200-J rescue shock from an external defibrillator was delivered 3 to 4 seconds later. Eighty-five patients also participated in an additional investigational protocol.25 27 29 30

ICD testing. In all patients, defibrillation was tested successfully at the programmable shock strength closest to the ULV+3 J25 by use of the implanted ICD system.

Data Recording
Voltage and current waveforms were recorded as described previously.31 For each phase of the biphasic pulse, we recorded the leading-edge voltage, trailing-edge voltage, leading-edge current, and trailing-edge current. Resistance and stored energy were calculated.25 31

Perioperative Complications
One patient developed hypotension requiring vasopressors after induction of anesthesia and before ICD implantation; a second patient developed pulmonary edema postoperatively. Both had severe left ventricular dysfunction. One patient required reintubation for 18 hours because of laryngeal edema. Two patients who were treated postoperatively with heparin because of mechanical heart valves developed wound hematomas requiring surgical drainage. The remaining patients had uncomplicated clinical courses.

ICD Programming
The Medtronic Jewel is a tiered-therapy, biphasic-waveform ICD.32 33 Its three tachyarrhythmia detection zones (VT, fast VT, and VF) were programmed on the basis of the cycle length of spontaneous and induced arrhythmias. We programmed a single detection zone (VF) in 48 patients, two zones (VF and VT) in 38 patients, and three zones (VF, VT, and fast VT) in 14 patients. For simplicity, in this report fast VT is not distinguished from VT.

VF detection. The mean programmed cycle length below which arrhythmias were classified in the VF zone was 292±17 ms. It was 340 ms in 4 patients, 320 ms in 36 patients, 300 ms in 28 patients, 280 ms in 17 patients, and <280 ms in 15 patients. In 89 patients, detection of VF required that 18 of 24 intervals in a rolling detection window be in the VF-detection zone. The number of required intervals was 12 of 16 in 8 patients and 24 of 30 in 3 patients.

VF therapy. The first shock was set to the programmable shock strength34 closest to the ULV+5 J. This shock strength was chosen because analysis of retrospective data indicated that it defibrillated reliably,24 and preliminary analysis of prospective data indicated that a shock strength equal to the ULV+3 J defibrillated reliably.25 The strengths of the second and third shocks were programmed 6 to 10 J greater than the immediately preceding shock. Subsequent shocks were programmed to the maximum value of 34 J.

ICD memory. The Medtronic Jewel models 7202 and 7219 store RR intervals preceding the last five VT or VF episodes and after the last therapy delivered in each episode. An arrhythmia episode begins with detection of VT or VF and ends with redetection of "sinus rhythm," which is defined as eight consecutive intervals with cycle lengths longer than the longest cycle length detected as VT or VF. These ICDs store a total of 10 seconds of bipolar electrograms recorded at the right ventricular apex. They were programmed to store all 10 seconds for the last detected arrhythmia (VT or VF). Thus, electrograms were not available for episodes of ICD-detected VF if a subsequent episode of VT or VF was detected before device interrogation. Neither electrograms nor intervals were available if antitachycardia pacing or cardioversion given for an arrhythmia detected in the VT zone accelerated the rhythm to the VF zone; the only recorded data corresponded to the initial detection of VT. The Medtronic Jewel model 7218 had sufficient memory to record intervals or global, coil-to-can electrograms for all episodes of ICD-detected VF.35 All ICDs stored the capacitor-charge time for the last delivered VT or VF shock.

Follow-up
Patients were followed up for a total of 1694 patient-months from the date of postoperative electrophysiological study until the date of study closure or the patient's death (2 patients). The mean duration of follow-up was 20±7 months (median, 20 months; range, 5 to 35 months). One patient died of heart failure and 1 of stroke. ICDs were interrogated at 1 month and 3 months postoperatively, every 3 months thereafter, and whenever patients complained of lightheadedness, palpitations, or shocks. Patients were specifically questioned about syncope and presyncope whenever a shock occurred.

Antiarrhythmic drugs. During follow-up, amiodarone was discontinued in 3 of the 6 patients who were receiving it at the time of ICD implantation. Overall, 12 patients received antiarrhythmic drugs during follow-up: 8 to diminish the frequency of VT, 1 to prevent atrial flutter, and 3 to prevent symptomatic, paroxysmal atrial fibrillation. Eight of these patients received amiodarone and 4 received sotalol.

ICD reprogramming. Electrophysiological testing of the chronically implanted ICD was recommended if any of the following events occurred: cardiac surgery, acute myocardial infarction, change in heart failure status of more than one New York Heart Association functional class, or a 25% increase in the dose of antiarrhythmic drugs other than ß-blockers, digoxin, or calcium antagonists.26 The chronic ULV and DFT were determined as reported previously.26 The strength of the first shock was then reprogrammed to 5 J above the chronic ULV.

Classification of Rhythms Detected as VF
The cardiac rhythms associated with ICD-detected VF were classified on the basis of data stored in the ICD in conjunction with clinical data. Previously reported, prospective criteria for stored detection intervals36 37 and stored electrograms38 39 40 were used to diagnose monomorphic VT, polymorphic VT, VF, atrial fibrillation, sinus tachycardia, and T-wave oversensing during sinus tachycardia. ECG recordings were used for classification whenever they were available. First shocks were considered appropriate if the rhythm was classified as monomorphic VT, polymorphic VT, or VF. They were also considered appropriate if the detected rhythm occurred as a result of VT therapy unless ECG recordings resulted in a different classification. First shocks were considered inappropriate if they did not conform to these classifications. Secondary arrhythmias were defined as arrhythmias that occurred when therapy for an arrhythmia detected in the VT zone accelerated the rhythm to the VF zone. These arrhythmias could be classified for patients with the Medtronic Jewel model 7218 but not for patients with models 7202 and 7219 because of the limited memory in these devices.

First-Shock Efficacy
The Medtronic Jewel records the outcome of each delivered VF therapy as effective, ineffective, undetermined, or conversion to VT. The outcome of each appropriate first shock was analyzed by use of stored postshock intervals, postshock electrograms (whenever available), and clinical data. The efficacy of first VF shocks was classified according to the recorded outcome unless this additional analysis produced specific, unequivocal evidence resulting in a different classification. The efficacy of VT shocks was not analyzed.

Statistical Analysis
Data are presented as mean±SD. Basic comparative statistics were calculated by the two-tailed, paired, or unpaired t test. The Pearson correlation coefficient was used to assess the correlation between DFT and ULV. We estimated 95% CIs for the distribution of unclassified secondary arrhythmias from the distribution for classified secondary arrhythmias by either the {chi}2 test or Fisher's exact test. The first-shock strength of the ULV+5 J was compared with the commonly used strengths of the DFT+10 J and twice the DFT. The number of patients whose first-shock strengths would be <25 J was determined for each of these programming strategies and compared by the {chi}2 test. This value of 25 J was selected because it approximates the maximum output of the next generation of smaller ICDs. Patients who received appropriate VF shocks were compared with the remaining patients with respect to age, sex, type of structural heart disease, left ventricular ejection fraction, and presenting arrhythmia. A value of P<.05 was used to reject the null hypothesis for single comparisons.


*    Results
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ULV
The ULV was determined at or before the peak in 98 patients and 20 ms after the peak in 2 (of the last 48) patients. The number of T-wave shocks required to determine the ULV was 8.3±2.4 in the 50 patients in whom the strength of the initial shock was 15 J and 11.4±2.5 J in the 50 patients in whom it was 20 J. The number of required VF episodes was zero in 2 patients, one in 49 patients, and two in 49 patients. If the ULV had been determined by a pure step-down protocol, the number of required VF episodes would have been zero or one in 95 patients. The time required to determine the ULV was recorded in the last 89 patients. It was 6±2 minutes when the initial shock was 15 J (39 patients) and 9±3 minutes when it was 20 J (50 patients).

ULV Versus DFT
Table 1Down summarizes values for DFT and ULV in all 100 patients. The ULV was 3.0±3.0 J greater than the DFT, with a range of -3.0 to 15.3 J. The correlation coefficient between DFT and ULV was .83 for stored energy (P<.001) and .82 for voltage (P<.001). Determination of the ULV required more total shocks than determination of the DFT (9.8±3.0 versus 4.1±0.8, P<.001) but fewer fibrillation-defibrillation episodes (1.5±0.6 versus 4.1±0.8, P<.001).


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  		Table 1. DFT and ULV (n=100)

First Shocks
The mean strength of first shocks was 17.5±5.2 J. During follow-up, there were 120 appropriate first shocks in 37 patients. These 37 patients did not differ from the remaining 63 patients with respect to any of the clinical variables analyzed, ULV (12.4±5.8 versus 12.7±5.1 J, P=.82), or DFT (9.2±5.2 versus 9.8±4.9 J, P=.52). The number of appropriate first shocks was 1 in 14 patients, 2 in 8 patients, 3 in 4 patients, 4 in 4 patients, 5 in 2 patients, 6 to 10 in 2 patients, and >=11 in 2 patients.

Arrhythmias Treated by First Shocks
Table 2Down summarizes the appropriately treated arrhythmias in the VF zone. Fig 1Down shows representative stored electrograms. Overall, 76% were primary arrhythmias and 24% were secondary arrhythmias. Seventy percent were documented monomorphic VT with cycle length shorter than the programmed cycle length for detection of VF (82 primary arrhythmias in 31 patients and 2 secondary arrhythmias in 1 patient). Eleven percent were documented VF (8 primary arrhythmias in 6 patients and 7 secondary arrhythmias in 7 patients). Nine of 29 secondary arrhythmias were classified as VF (7 arrhythmias in 7 patients) or monomorphic VT (2 arrhythmias in 1 patient). Twenty secondary arrhythmias (17% of all arrhythmias) in 15 patients could not be classified because only data regarding the initial detection of VT were available. The likely distribution of unclassified secondary arrhythmias was estimated from the distribution for classified secondary arrhythmias. Based on 95% CIs, 7 to 20 of the unclassified arrhythmias were VF and 0 to 6 of them were VT; on a per-patient basis, VF occurred in 5 to 15 of the patients who had unclassified arrhythmias and VT occurred in 0 to 4 of them. Table 2Down shows that there were no significant differences among the strengths of first shocks delivered for different arrhythmias. Overall, stored detection intervals were available for 98 episodes (82%), electrograms for 78 episodes (65%), and ECG recordings for 9 episodes (8%).


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  		Table 2. Arrhythmias Treated by First Shocks



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  		Figure 1. Representative stored electrograms and detection intervals from spontaneous arrhythmias treated by first shocks in the VF zone. Each panel shows electrogram and marker channel with corresponding detection intervals. Electrogram is recorded immediately before first detected arrhythmia and after charging of output capacitor is completed. Intervals between detected electrograms are classified on the basis of cycle length into VF zone (short double markers, FS), VT zone (medium-length double markers, TS), or sensed but not classified as an arrhythmia (medium-length single markers, VS). Recording speed is 25 mm/s. CD (charge delivery) denotes shock. Electrogram is recorded between right ventricular tip and ring electrodes in A and between right ventricular coil and active can in B and C. A, Sustained monomorphic VT with cycle length 260 ms is detected (FD) as VF. Shock converts rhythm to sinus tachycardia. Note change in electrogram morphology after shock. Vertical dotted lines enclosing "(Therapy Sequence)" indicate charging of output capacitor. In Jewel model 7219, markers are not recorded during charging. B, Primary VF. Repetitive, polymorphic VT degenerates to VF. Fibrillation is detected when 18 of last 24 intervals are in programmed VF zone (<320 ms). Detection intervals during charging and snapshot of electrogram between end of charging (long, broad marker, CE) and shock confirm that VF persists until shock is delivered. First postshock complex is paced (TP). C, Acceleration of VT to VF by autodecremental pacing. Eight autodecremental pacing pulses (TP) are delivered to treat monomorphic VT with average cycle length 270 ms. This is denoted as therapy (a). Postpacing rhythm is VF. Detection of VF after failed therapy requires that 12 of 16 intervals be in programmed VF zone (<260 ms). Intervals are marked VS after detection of VF occurs because detection is suspended during charging. VF shock is denoted as therapy (b). Note difference in electrogram morphology between initial detection of VT and snapshot of VF that precedes shock delivery.

First-Shock Success
The ICD recorded 116 of the 120 appropriate first shocks as successful, 1 as ineffective, and 3 as undetermined because the ICD was interrogated before it classified 8 consecutive intervals as sinus. These 3 shocks occurred in 1 patient during a clinical "storm" of monomorphic VT. ECG recordings and postshock RR intervals documented that each shock terminated the arrhythmia. The only ineffective shock was delivered to treat an unclassified secondary arrhythmia. Before this shock, the patient received four trials of ineffective antitachycardia pacing followed by a 2-J cardioversion that accelerated the rhythm. The strength of this first shock was 20 J, corresponding to the DFT+12 J or 2.5 times the DFT. The second shock at 26 J was effective. The cycle length for detection of VF was 320 ms.

Overall, the first shock was successful in 119 of 120 episodes (99%; 95% CIs, 93% to 100%). The only unsuccessful shock was delivered for an unclassified arrhythmia that could have been VT, VF, or atrial fibrillation induced by low-energy cardioversion. If we use 95% CIs to estimate the distribution of unclassified shocks, first shocks were successful in at least 84 of 85 VT episodes (99%) and at least 21 of 22 VF episodes (95%). On a per-patient basis, 95% CIs indicate that first shocks were successful in at least 30 of 31 patients with VT (97%) and at least 17 of 18 patients with VF (94%).

Syncope
No patient had syncope associated with an ICD shock. The patient who had the ineffective shock was the only patient who experienced presyncope. The charge time was recorded for 69 first-VF shocks with a shock strength of 18.1±5.7 J. It was 3.3±0.7 seconds.

Relationship of First-Shock Strength to DFT
Fig 2Down shows the difference between the first-shock strength and the DFT in all 100 patients. The shock was 8.0±3.0 J greater than the DFT (range, 2 to 20 J). It was within 5 J of the DFT in 28 patients and >10 J above the DFT in 17 patients. The ratio of the first shock to the DFT was 2.1±0.8 (range, 1.2 to 4.9). The mean first-shock strength was less than the strength that would have been programmed if a guideline of the DFT+10 J (20.6±5.0 J, P<.001) or twice the DFT (19.1±9.7 J, P=.01) had been used. Furthermore, a shock strength of twice the DFT exceeded the 34-J maximum output of the ICD in 7 patients. Fig 3Down shows a percentile plot of the programmed stored energy for the first shock in all 100 patients in comparison with values that would have been programmed if the DFT+10 J or twice the DFT had been used. The number of patients whose programmed shock strength was <=25 J was 93 for the ULV+5-J strategy. It was lower (78 patients, P=.008) for the twice-DFT strategy and not significantly different (90 patients, P=.62) for the DFT+10-J strategy.



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  		Figure 2. Difference between programmed strength of first shock and DFT for all 100 patients is shown on abscissa. Values represent center of a bin with a width of 2 J. Strength of first shock was 17.5±5.2 J or 8.0±3.0 J greater than DFT.



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  		Figure 3. Percentile plot of programmed, first-shock strength (J) equal to ULV+5 J compared with shock strengths that would have been programmed if guidelines of DFT+10 J or twice DFT were used. Cumulative percent of patients is shown on abscissa and DFT is shown on ordinate for all 100 patients. See text for details.

On a per-shock basis, the 120 delivered appropriate first shocks exceeded the corresponding DFTs by 7.4±2.4 J and were 2.0±0.6 times as strong as the DFT. The 37 patients who received appropriate VF shocks did not differ from the remaining 63 patients with respect to the difference between the strength of the first shock and the DFT (8.3±2.7 versus 7.9±3.2 J, P=.52) or the ratio of the first shock to the DFT (2.2±0.7 versus 2.1±0.8, P=.67).

ICD Reprogramming
Nine patients underwent chronic electrophysiological testing and subsequent reprogramming of the first shock. The chronic ULV increased by >=10 J in 4 patients and by 5 to 10 J in 1 patient. It differed from the acute ULV by <5 J in 4 patients. Only 1 patient received VF therapy after the shock strength was increased at reprogramming.


*    Discussion
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 *Discussion
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Previous studies have correlated the ULV with the shock strength required to defibrillate induced VF in animals19 41 42 and in humans at ICD implantation24 25 43 44 45 46 and 3 months later.26 This prospective study assessed the long-term results of programming ICDs on the basis of the ULV measured at implantation. We found a first-shock success rate of 99% for spontaneous arrhythmias detected in the VF zone despite a group-mean shock strength lower than that which would have been programmed if conventional strategies based on defibrillation testing were used. No patient had syncope associated with an ICD shock.

Programming Strategies for ICDs
Programming based on the ULV may be compared with conventional programming strategies.

Programming by DFT. The principal advantage is that patient-specific programming may limit excessive shock strength during follow-up. The principal disadvantage is that multiple fibrillation-defibrillation episodes are required at implantation. Although this testing generally is considered safe, complications include cerebral hypoperfusion,8 10 electroencephalographic changes10 associated with postoperative cognitive deficits,47 myocardial ischemia,11 systemic hypoperfusion,13 diminished left ventricular function,12 14 15 prolonged circulatory arrest due to intractable VF,15 16 17 and rarely, death.11 12

Programming by defibrillation safety margin. This strategy limits fibrillation-defibrillation testing to the minimum number of episodes necessary to determine whether there is a sufficient safety margin between the maximum output of the ICD and the shock strength required for consistent defibrillation.7 8 9 48 49 The first shock is then programmed to maximum output. The disadvantages apply to patients who do not require maximum-strength shocks during long-term follow-up. Maximum-output shocks may increase the risk of syncope during capacitor charging (see below). They may result in rapid battery depletion if the ICD capacitors charge frequently because of appropriate shocks, inappropriate shocks, or aborted shocks.50 In in vitro and animal experiments, excessively strong shocks cause myocardial depression1 2 and conduction block.4 5 6 In humans, multiple high-energy shocks have been associated with a poor prognosis, but it is not known whether they cause adverse clinical consequences or merely identify patients at high risk.51 52

Programming by ULV. This strategy both provides patient-specific programming and minimizes the number of fibrillation-defibrillation episodes. It thus combines the advantages of programming by DFT and programming by ICD safety margin. The principal disadvantage is the need for more total shocks during paced rhythm. Although clinical data from the present study and clinical24 and creatine kinase isoenzyme data25 from previous studies have identified no complications from ULV testing, subclinical effects have not been studied. The ULV method could be adapted to a "vulnerability" safety-margin approach. For example, if VF is not induced by a T-wave shock at 15 J, the ICD could be programmed to 20 J. This method would minimize both the number of shocks and the number of fibrillation-defibrillation episodes.

First-Shock Success: Conventional Programming
For biphasic-waveform, transvenous ICDs, several studies report success rates for appropriate first shocks as assessed by stored electrograms. Trappe et al53 reported that a shock strength equal to the DFT+10 J resulted in a success rate of 75%. Vijgen et al54 reported a success rate of 85% for a shock equal to or greater than the DFT+9 J. For patients whose DFTs were <=15 J, Heisel et al55 reported that the success rate for a shock strength of twice the DFT was 98%. With the maximum-output, safety-margin strategy for the same ICDs as in the present study, the success rate has been reported to be 90% for the active-can configuration and 95% for systems without the active can.32

First-Shock Success: Programming by ULV
In the present study, a shock strength equal to the ULV+5 J resulted in a 99% success rate despite a lower mean shock strength than programming based on the DFT. There are at least two plausible explanations for this apparent paradox. (1) The ULV permits an accurate estimate of the minimum shock strength that results in near 100% defibrillation because it is highly reproducible56 and corresponds to a shock strength with a high (90%) and highly predictable probability of defibrillation.25 This permits selection of a patient-specific shock strength that has a high success rate but not an excessive safety margin. In contrast, the DFT does not permit an accurate estimate of the minimum shock strength that results in uniformly successful defibrillation because it is less reproducible,56 and the probability of successful defibrillation at the DFT is lower and less predictable than at the ULV.8 57 To select a patient-specific shock strength that has a high success rate, an excessive safety margin must be programmed in some patients. However, in any individual patient, the shock strength programmed on the basis of the ULV may be higher than that programmed on the basis of the DFT, as shown in Fig 2Up. (2) Programming based on the DFT might have performed as well as programming based on the ULV in our patients. However, because the only unsuccessful shock in this study was programmed to the DFT+12 J or 2.5 times the DFT, it is unlikely that programming to the DFT+10 J or twice the DFT would have performed better.

Syncope
Because the time required to charge an ICD capacitor is longer for stronger shocks, the duration of the arrhythmia before the shock is also longer. Thus, the risk of syncope with possible resultant trauma may be higher when shocks are programmed to maximum output. For a 2-year-old Medtronic Jewel, the minimum charge time is 4.5 seconds for the 18-J mean first-shock strength in the present study versus 11 seconds for the maximum shock strength of 34 J. Syncope or presyncope has been reported to occur in 17% to 42% of appropriate shock episodes when the strength of the first shock is maximum.58 59 In the present study, no patient had syncope and only 1 had presyncope. This may be due in part to short charge times.

Strengths and Limitations of Conventional Programming Strategies
These strategies result in operative mortality of <=1% for biphasic-waveform, transvenous ICDs.32 60 However, as noted previously, DFT testing still results in rare mortality and finite morbidity. Programming to the DFT+10 J is associated with a significant first-shock failure rate,53 54 61 and programming to twice the DFT would not have been applicable to the 10% of patients in the present study whose DFTs exceeded 15 J. Furthermore, this latter strategy may limit the fraction of patients who are candidates for smaller, lower-output ICDs. Programming first shocks to maximum output results in occasional but important problems, such as rapid battery depletion due to frequent capacitor charging or syncope associated with long capacitor charge times. Patients who have frequent self-terminating arrhythmias in the VF zone (eg, torsade de pointes) present a particularly difficult problem for this strategy: A long detection time reduces the number of aborted shocks but places patients at risk for syncope because of both long detection times and charge times; a short detection time results in frequent aborted shocks and rapid battery depletion. ICDs have reduced the annual sudden death rate to <=2%.32 60 However, among patients who have arrhythmias in the VF zone, it may be as high as 8.7%,60 and ICDs have been less successful at decreasing total mortality.51 52 It is not known whether excessive shock strength contributes to morbidity and mortality from heart failure in some ICD recipients.

Limitations of the Present Study
1. The number of arrhythmias in the VF zone is limited, and the corresponding number of patients is even smaller. This is a consequence both of the relative infrequency of rapid VT and primary VF36 60 and of programming of an intermediate zone for fast VT in some patients.

2. In this study, most arrhythmias in the VF zone were rapid VT rather than VF. This finding is consistent with the observations of Raitt et al36 and previous studies of first-shock success rate in which stored electrograms or intervals were recorded.32 53 54 55 61 Thus, the first-shock success rate in this study applies to arrhythmias in the VF zone of ICDs as they are programmed in clinical practice. This differs from previous studies that correlated the ULV with the shock strength required to terminate induced VF. We have limited first-shock success data for true defibrillation; this limitation also applies to previous studies of first-shock success rates.

3. We did not compare programming based on the ULV with a conventional programming strategy. A prospective study would be needed for direct comparison of perioperative morbidity, first-shock success rates, syncope during capacitor charging, shock-induced morbidity, and pulse-generator longevity.

4. The vulnerability-safety-margin method has not been validated or compared with the defibrillation-safety-margin method.

5. Limitations of the ULV method have been described.25 29 We do not know how well it applies to patients taking antiarrhythmic drugs other than amiodarone.24

Clinical Implications
Because current programming methods prevent sudden death with acceptable morbidity, independent confirmation of our method is desirable before widespread clinical application. However, programming based on vulnerability testing may be recommended in patients at higher than usual risk for fibrillation-defibrillation testing. Examples may include patients in whom external-rescue defibrillation is unreliable or those with high-grade coronary stenoses or recent coronary interventions. Depending on the benefits of minimizing shock strength in a given patient, vulnerability testing may be performed with either a threshold or safety-margin strategy. A threshold strategy is preferred for patients who have frequent self-terminating arrhythmias in the VF zone (to prevent rapid battery depletion) and for those in whom it is important to minimize the risk of syncope during capacitor charging. Our present recommendations apply only to the strength of the first shock and assume that subsequent shocks will be programmed to a higher energy.


*    Selected Abbreviations and Acronyms
 
DFT = defibrillation threshold
ICD = implantable cardioverter-defibrillator
ULV = upper limit of vulnerability
VF = ventricular fibrillation
VT = ventricular tachycardia


*    Acknowledgments
 
This study was supported in part by a grant from the PM Foundation to Dr Swerdlow and by a FIRST award (HL-50259) and Specialized Center of Research (SCOR) grant CHL-52319 from the NIH. It was done during the tenure of an American Heart Association and Wyeth-Ayerst Established Investigator Award to Dr Chen.

Received August 21, 1996; revision received October 31, 1996; accepted November 18, 1996.


*    References
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