Chun Hwang

Underdetection of ventricular tachycardia by algorithms to enhance specificity in a tiered-therapy cardioverter-defibrillator

OBJECTIVESnThe goal of this study was to determine the incidence and clinical significance of underdetection in 125 patients treated with a tiered-therapy cardioverter-defibrillator, the Medtronic PCD.nnnBACKGROUNDnUnderdetection, distinct from undersensing, is a unique, potential complication of new algorithms that enhance specificity in tiered-therapy cardioverter-defibrillators. These algorithms may delay or prevent recognition of ventricular tachycardia even though electrograms are sensed accurately and RR intervals meet the programmed interval criterion.nnnMETHODSnUnderdetection was defined as delay in detection > 5 s at electrophysiologic study or symptomatic delay or detection failure at follow-up of 15 +/- 8 months.nnnRESULTSnWe identified six specific mechanisms of underdetection caused by algorithms to discriminate sustained ventricular tachycardia from sinus tachycardia, atrial fibrillation, ventricular fibrillation and nonsustained ventricular tachycardia. Underdetection caused detection delays in 13 (1.9%) of 677 induced ventricular tachyarrhythmia episodes in 12 patients (9.6%). During follow-up, underdetection occurred in 7 (9.9%) of 71 patients in whom ventricular tachycardia therapies were programmed. Failure to detect ventricular tachycardia occurred in 6 (0.6%) of 988 spontaneous ventricular tachycardia episodes in four patients (5.6%); 2 episodes required external cardioversion. After defibrillator reprogramming, underdetection did not occur.nnnCONCLUSIONSnAlgorithms to enhance specificity cause underdetection of ventricular tachycardia in a significant minority of patients with tiered-therapy cardioverter-defibrillators. Optimal programming can minimize underdetection.

Upper limit of vulnerability reliably predicts the defibrillation threshold in humans.

BACKGROUNDnThe upper limit of vulnerability is the stimulus strength above which electrical stimulation cannot induce ventricular fibrillation even when the stimulus occurs during the vulnerable period of the cardiac cycle. The purpose of this study was to test the hypothesis that the upper limit of vulnerability can accurately predict the defibrillation threshold in patients undergoing implantable cardioverter-defibrillator (ICD) implantation using nonthoracotomy lead systems.nnnMETHODS AND RESULTSnWe studied 77 patients at the time of ICD implantation. Multiple endocardial-endocardial and endocardial-subcutaneous shock pathways were used. Two different protocols were used to test the upper limit of vulnerability. In protocol 1 (n = 17), the upper limit of vulnerability was tested with two shocks on the peak or the up-slope of the T wave of paced rhythm. The shocks were given randomly either at the peak and 20 milliseconds before the peak of T wave (n = 7) or at 20 and 40 milliseconds before the peak of T wave (n = 10). In protocol 2 (n = 60), the upper limit of vulnerability was tested with three shocks delivered at 0, 20, and 40 milliseconds before the peak of the T wave. The weakest shock that failed to induce ventricular fibrillation by a 5-J step-down or step-up method was defined as the upper limit of vulnerability. The defibrillation threshold was also determined by a 5-J step-down or step-up method. In protocol 1, the upper limit of vulnerability (9 +/- 6 J) was significantly lower than the defibrillation threshold (13 +/- 7 J) with a correlation coefficient of .87 and P < .001. In protocol 2, the upper limit of vulnerability (13 +/- 6 J) was not significantly different from the defibrillation threshold (13 +/- 6 J) with a correlation coefficient of .85 and P < .001. In 45 of the 60 patients, the upper limit of vulnerability was < or = 15 J; all had a defibrillation threshold of < or = 20 J. In 51 of the 60 patients, the upper limit of vulnerability was within 5 J of the defibrillation threshold. The upper limit of vulnerability overestimated the defibrillation threshold by > 10 J in 8 patients and underestimated the defibrillation threshold by > 10 J in only 1 patient. The overestimation and underestimation occurred only in patients with the upper limit of vulnerability > 15 J.nnnCONCLUSIONSnWhen tested with three shocks on and before the peak of the T wave, the upper limit of vulnerability accurately predicted the defibrillation threshold in patients undergoing ICD implantation using nonthoracotomy lead systems. This method required either one or no episodes of ventricular fibrillation in most patients.

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

BACKGROUNDnThe 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.nnnMETHODS AND RESULTSnThis 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.nnnCONCLUSIONSnIn 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.

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

BACKGROUNDnA 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.nnnMETHODS AND RESULTSnWe 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.nnnCONCLUSIONSnICD shocks can be programmed on the basis of the ULV, a measurement made in regular rhythm, without a direct measure of defibrillation efficacy.

Effects of voltage and respiration on impedance in nonthoracotomy defibrillation pathways

The effects of applied voltage and phase of respiration on impedance of pathways used by implantable cardioverter-defibrillators were investigated. Patients were studied at implantation of cardioverter-defibrillators using epicardial (n = 12) or transvenous and subcutaneous (SQ) (n = 30) electrodes. Transvenous-SQ pathways were right ventricular cathode to SQ anode and coronary sinus cathode to SQ anode. Transvenous-transvenous pathways were right ventricle to coronary sinus and right ventricle to superior vena cava. Patients with nonthoracotomy electrode systems were studied at end-expiration and end-inspiration. Five shocks of 65 to 745 V (0.2 to 34 J) were given in random order in sinus rhythm. Over this range, end-expiratory impedance decreased monotonically for all pathways. This effect was greatest for transvenous-SQ pathways (13 +/- 3% to 17 +/- 4%, p < 0.001), intermediate for transvenous-transvenous pathways (5 +/- 4% to 8 +/- 5%, p < 0.001), and least for epicardial pathways (3 +/- 3%, p = 0.006). Paired data in inspiration and expiration showed that inspiration increased impedance in transvenous-SQ pathways (p < 0.001) but not in transvenous-transvenous pathways. Further, the effects of respiration and voltage on impedance in transvenous-SQ pathways were interactive (p < 0.001): Inspiration increased voltage-dependence of impedance. The magnitude of the inverse relationship between voltage and impedance depends on type of defibrillation pathway. The effect of respiration on impedance suggests that voltage-dependence of impedance is greatest in the lungs. These findings have potential relevance for intraoperative testing of cardioverter-defibrillators and selection of pathways for low-energy cardioversion.

Short Biphasic Pulses from 90 Microfarad Capacitors Lower Defibrillation Threshold

For defibrillation between right ventricular and retropectoral patch electrodes using truncated exponential pulses, the stored energy defibrillation threshold (DFT) is lower for short pulses from small 60‐μF capacitors than for conventional pulses from 120‐μF capacitors, but 60‐μF pulses frequently require higher voltages than are currently used. The goal of this study was to determine if DFT could be reduced by intermediate size 90‐μF capacitors. This study compared biphasic waveform DFTs for 120μF‐65% tilt pulses, 90μF‐65% tilt pulses, and 90 μF‐50% tilt pulses in 20 patients at defibrillator implantation. The 90μF‐50% tilt pulses were selected because their duration is half that of 120μF‐65% tilt pulses. The stored energy DFT for 90 μF‐50% tilt pulses (9.1 ± 4.3 J) was less than both the DFT for 120 μF‐65% tilt pulses (12.0 ± 5.5 J, P < 0.005) and the DFT for 90μF‐65% tilt pulses (11.6 ± 5.8 J, P < 0.005). There was no significant difference between the latter two values. The voltage DFTs for 90 μF‐50% pulses (436 ± 113 V) and 120 μF‐65% tilt pulses (436 ± 104 V) were not statistically different; the voltage DFT for 90 μF‐65% tilt pulses was higher than for either of the other two pulses (490 ± 131, P < 0.005). The DFT was 20 } or greater in three patients for both 120 μF‐65% tilt pulses and 90 μF‐65% tilt pulses, but it was 16 J or less in all patients for 90 μF‐50% tilt pulses. When pathways were dichotomized by the median resistance of 71 Ω, 90 μF‐50% tilt pulses significantly reduced DFTs compared to 120 μF‐65% tilt pulses for higher resistance pathways (9.2 ± 4.0 J vs 13.0 ± 6.2 J, P = 0.002), but not lower resistance pathways (9.0 ± 4.8 J vs 10.9 ± 4.6 J, P = NS). For the electrode configuration tested, biphasic 90 μF‐50% tilt pulses reduce stored energy DFT in comparison with 120 μF‐65% tilt pulses without increasing voltage DFT. However, 90 μF‐65% tilt pulses provide no benefit.

Optimal electrode configuration for pectoral transvenous intplantable 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)

Wolff-Parkinson-White syndrome in a cardiac allograft

A 61-year-old man underwent orthotopic heart transplantation for end-stage ischemic cardiomyopathy. The donor presented with Wolff-Parkinson-White syndrome and the allograft was successfully transplanted. The accessory pathway was interrupted postoperatively by radiofrequency current catheter ablation, and the patient is clinically well and free of preexcitation 24 months later.

808-3 Reentrant Wavefronts During Wiggers’ Stage II Ventricular Fibrillation in Dogs

The mechanisms of ventricular fibrillation (VF) are unknown. Reentrant wavefronts have been shown to underlie the onset (Wiggers’ stage I) of electrically induced VF in intact canine ventricles. These reentrant wavefronts, however, have a limited lifespan (1–2xa0s) while VF persists. Using computerized mapping techniques, we studied the mechanism by which VF is maintained beyond the initial few seconds (Wiggers’ stage II), both in normal and subendocardium-ablated canine ventricles. Eleven open-chest dogs were studied. In 6 of the dogs, the RV subendocardium was ablated with lugols solution. A plaque electrode array with 317–509 bipolar recording electrodes was sutured on the RV epicardium. VF was induced by a strong premature stimulus (S 2 ). Starting 2.5 s after the onset of VF, 2–5 s of data were analyzed. The activation patterns were visualized via dynamic display. Conventional isochronal maps were also constructed. Of the 15 runs of VF in dogs with intact ventricles, 3 episodes of reentrant wavefronts were detected. The mean lifespan was 4.5xa0±xa02.1 rotations. The mean cycle length was 102.5xa0±xa05.5xa0msec. The incidence of reentry was 0.018xa0±xa00.048 rotations/sec-cm 2 . Of the 18 runs of VF in dogs with ablated ventricles, 8 episodes of reentry were detected. The mean lifespan was 3.6xa0±xa01.1 rotations (pxa0=xa00.39 compared with intact ventricles). The mean cycle length was 107.2xa0±xa09.6xa0msec (pxa0=xa00.16). The incidence of reentry was 0.075xa0±xa00.097 rotations/sec-cm 2 (pxa0=xa00.048). In both groups of dogs, dynamic displays of the activation patterns demonstrate that the reentrant wavefronts spiral rather than follow a simple circular pathway. Conclusions (a) reentrant wavefronts are consistently present during Wiggers’ stage II VF, (b) ablation of the subendocardium and Purkinje fibers results in an increased incidence of reentrant wavefronts on the epicardium, and (c) the reentrant wavefronts are compatible with spiral waves of excitation.