The Effect of Polarity of the Initial Phase of a Biphasic Shock Waveform on the Defibrillation Threshold of Pectorally Implanted Defibrillators

The effect of initial phase polarity on the DFT of two pectorally implanted biphasic ICDs was tested in a randomized, prospective manner at the time of implantation. Twenty-two consecutive patients with VT or VF who received either the Medtronic PCD 7219C fewel device (10 patients) or PCD 7219D fewel device (12 patients) were studied. DFT testing was performed in a standard step-down manner. Both initial phase polarities—initial defibrillation current flowing from active can/SVC coil (± subcutaneous patch) to the RV coil (RV-) or from RV coil to active can/SVC coil (RV+)—were tested in random order. The mean DFT achieved with RV+ compared with RV- was lower for the 7219C patient group (6.6 ±3.1 vs 10.8 ± 5.5 J; P = 0.007). A similar trend was observed forthe 7219D group, though the difference did not reach statistical significance (12.0 ± 4.0 vs 16.3 ± 7.3 J; P = 0.07). Seven of the 10 patients in the 7219C group had a lower DFT with RV+, while the initial phase polarity made no difference in 3. In the 7219D group, 7 patients had a lower DFT using RV+, 2 patients had a lower DFT using RV-, and the initial phase polarity made no difference in 3. In conclusion, this study demonstrates that changing the polarity of the initial phase of a biphasic shock wave form can have a significant impact on the DFT achieved at the time of ICD implantation.     

Long Runs of Non-Sustained Ventricular Tachycardia on 24-Hour Ambulatory Electrocardiogram Predict Major Arrhythmic Events in Patients with Idiopathic Dilated Cardiomyopathy

This study examined the prognostic significance of the rate and length of non-sustained (NS) ventricular tachycardia (VT) on 24-hour ambulatory electrocardiograms (ECG) recorded in 343 patients with idiopathic dilated cardiomyopathy (IDC) in the prospective Marburg Cardiomyopathy study. NSVT was defined as ≥3 consecutive ventricular premature beats at >120 bpm. During 52 ± 21 months of follow-up, major arrhythmic events defined as sustained VT, VF, or sudden cardiac death occurred in 46 of 343 patients (13%). Patients with 3–4 beat runs of NSVT had a similar arrhythmia-free survival as patients without NSVT on baseline 24-hour ambulatory ECG. The incidence of major arrhythmic events during follow-up increased significantly from 2% per year in patients without NSVT, to 5% per year in patients with 5–9 beat runs of NSVT, to 10% per year in patients with ≥10 beat runs of NSVT (P < 0.05). Unlike the length, the rate of NSVT was similar in patients with versus without subsequent major arrhythmic events (163 ± 23 vs 160 ± 24 bpm). Thus, the length but not the rate of NSVT on 24-hour ambulatory ECG was a predictor of major arrhythmic events in patients with IDC. The presence of NSVT with ≥10 beat runs on ambulatory ECG was associated with a particularly high risk of major arrhythmic events.     

Posterior Left Thoracic Cardiac Sympathectomy by Surgical Division of the Sympathetic Chain: An Alternative Approach to Treatment of the Long QT Syndrome

Although high thoracic left Sympathectomy via art anterior surgical approach is a highly efficacious treatment for refractory ventricular arrhythmias in patients with the long QT syndrome, the degree of sympathetic denervation has been variable, success of the operation is influenced by anatomical differences between patients, and Horner's syndrome may result. We hypothesized that interruption of sympathetic input to the heart could be accomplished using a posterior thoracic approach to this variable and often complex anatomy by division of the sympathetic chain rather than by direct destruction of the stellate and superior thoracic ganglia with the more conventional anterior, supraclavicular approach. In addition, the posterior approach should decrease the risk of Horner's syndrome by avoiding the ocular sympathetic efferent nerves. This posterior approach is described in five patients with the long QT syndrome and recurrent ventricular arrhythmias. After a mean follow-up of 18 ± 12 months, all are alive without Homer's syndrome.     

Cardiopulmonary Responses to Exercise in Children with Activity Sensing Rate Responsive Ventricular Pacemakers

The physiological benefits of activity sensing rate responsive ventricular pacing)VVIR) over fixed rate pacing)VVI) were investigated in 14 children during incremenlal cycle exercise. Based on their heart rhythm response during exercise, children were divided into two groups. Group I patients)13 ± 4 years) remained in a paced-only rhythm when exercised. Group II patients)16 ± 7 years) were paced at rest but converted to sinus rhythm with exercise. In Group I patients, the significant physioJogicol benefits of VVIR over VVI pacing were evidenced hy a 51% increase in peak heart rate)HRmax) and a 16% increase in exercise duration and maximum oxygen uptake)VO₂max). Additionally, a 27% reduction in peak oxygen pulse)O₂Pmax) was found, reflecting a similar decrease in stroke volume. The cardiorespiraiory responses of Group I and 11 patients were compared in terms of percent of predicted normal values. Although Group I patients in the VVIR mode attained a better exercise performance than in the VVI mode and a normal O₂Pmax)108% pred). their HRmax)62% pred) and VO₂max)70% pred) fell far below normal values. In comparison. Group II patients, who went into sinus rhythm, achieved normal values for HRmax)84% pred), VO₂max)90% pred), and O₂Pmax)97% pred). The higher pacing rates attained by Group I patients in the VVIR mode may have allowed them to reach not only a higher cardiac output but also a more normal stroke volume at peak exercise than in the VVI mode. However, the overall exercise performance of children paced in the VVI and VVIR modes were significantly diminished compared to the performance of children who went into sinus rhythm with exercise.)     

Toward Optimizing a Preshaped Catheter and System Parameters to Achieve Single Lead DDD Pacing

P wave electrogram amplitudes and atrial stimulation thresholds were determined in eight Hanford miniature swine using a preshaped catheter with an "S" curve in the SVC, and a major lobe in the atrium to enhance electrode contact with the atrial wall. The catheter was designed for pacing and sensing in the DDD mode. P wave amplitudes were also ascertained with two commercially available VDD leads and compared to the data from the experimental catheters. The preshaped catheter used two 6-mm² platinum iridium atrial electrodes with a 7-mm separation. Both atrial electrodes are on the same side of the catheter, facing outward on the major atrial lobe formed in the catheter. The P wave amplitudes were tested only in the differential bipolar configuration. For the eight preshaped catheters, the mean was 6.6 ± 3.8 mV while for the conventional leads it was 2.9 ± 1.6 mV. The mean atrial stimulation thresholds ranged from 1.1 t 0.2 V to 2.3 ± 1.2 V, with still lower thresholds of 0.9-1.3 V when using the parallel unipolar atrial electrode configuration, in which both parts of the bipolar atrial electrode are configured as a unipolar electrode. The data suggest that bipolar stimulation may be effective if sequential reverse polarity pulses are used to achieve cathodal stimulation from each electrode of the bipolar pair, on a beat-to-beat basis. (PACE 1997; 20[Pt. I]:1354-1358)     

The Safety and Efficacy of Chronic Ventricular Pacing at 1.6 Volts Using a Steroid Eluting Lead

Steroid eluting leads may allow for lower chronic pacing thresholds and therefore lower pacing outputs. Twenty-two patients (15 presenting with syncope) were implanted with VVI or VVIR pacemakers and transvenous steroid eluting leads and followed for a mean of 20.6 months while being paced at 1.6 V and 0.6 msec. Mean acute voltage pacing thresholds were 0.40 V at 0.5 msec and chronic pulse width thresholds were 0.21 msec at 0.8 V. Pacemaker function was documented with one to three 24-hour Holter monitors, attached during the 2-6 week postimplant period, bimonthly transtelephonic monitoring, and monthly pacemaker clinic visits. No patient developed recurrent symptoms and consistent capture was verified in all patients on every 24-hour Holter recording and transtelephonic monitor. Chronic ventricular pacing at an output of 1.6 V at 0.6 msec is safe and effective when using a steroid eluting lead and potentially has implications for pacemaker longevity.     

Use of Adenosine to Identify Patients at Risk for Recurrence of Accessory Pathway Conduction After Initially Successful Radiofrequency Catheter Ablation

Objectives: The use of adenosine after radiofrequency catheter ablation of accessory pathways was prospectively studied to determine its utility for identifying patients at risk for recurrence of accessory pathway conduction and to guide therapy that might reduce late recurrence in this group. Background: Accessory pathway conduction recursin 5%–12% of patients following initially "successful" radiofrequency catheter ablation. Adenosine may facilitate conduction over accessory pathways that have been modified by radiofrequency delivery, thus identifying patients at risk for recurrence. Methods: Radiofrequency catheter ablation was performed in 109 patients. Prior to ablation, 12–18 mg of adenosine was administered. After ablation, when all evidence of accessory pathway conduction remained absent for at least 30 minutes, adenosine 12–18 mg was again administered. Results: Adenosine given prior to radiofrequency catheter ablation did not block accessory pathway conduction in any patient. Adenosine given after elimination of accessory pathway conduction induced complete atrioventricular and ventriculoatrial block in 95 patients; 11 (11.6%) subsequently had recurrence of accessory pathway function. Accessory pathway conduction was unmasked by adenosine in 12 patients (11.2%). After further deliveries of radiofrequency energy, 7 of these 12 patients subsequently demonstrated adenosine induced atrioventricular and ventriculoatrial block; 1 of these 7 patients experienced recurrence of accessory pathway conduction. The remaining 5 patients demonstrated persistent accessory pathway conduction only with adenosine; all experienced clinical recurrence of accessory pathway function. Conclusion: The use of adenosine after presumed successful radiofrequency catheter ablation may reveal persistent accessory pathway conduction. Elimination of this latent accessory pathway conduction reduces the risk for recurrence.     

Implantation of an Automatic Defibrillator Using a New Nonthoracotomy Approach

Most current nonthoracotomy systems for defibrillator implantation use monophasic devices. To determine the safety and efficacy of a new nonthoracotomy lead configuration when used in conjunction with a device that used biphasic waveforms, 38 consecutive patients were taken to the operating room for implantation of a Cadence tiered therapy defibrillator system. The lead system consisted of a transvenous coil electrode positioned at the right atrial-superior vena caval junction, a bipolar endocardial right ventricular lead, and a large patch placed subcutaneously near the cardiac apex. Of the 38 nontboracotomy defibrillator implantations attempted, 36 (95%) were completed with adequate defibriliation thresholds. The mean defibriliation threshold in these 36 patients was ± 563 ± 10 V (± 20 ± 1 J). There was no perioperative mortality. Complications included coil lead migration (5). sensing lead migration (1), infection (3), pneumothorax (2), arterial embolism (1), and folding of the subcutaneous patch with an increase in defibriliation threshold (1). No patient died during a median follow-up period of 22 weeks. Fourteen patients (39%) had spontaneous sustained ventricular tachyarrhythmias, which were all successfully terminated by the implanted device. Shocks for nonsustained arrhythmias were aborted in eight patients (22%). Spurious discharges for sinus tachycardia or atrial fibrillation occurred in six patients (17%) and were readily diagnosed by examination of the stored electrograms. Thus, implantation of a biphasic tiered therapy defibrillator system using this nonthoracotomy approach is feasible in the majority of patients. The major complication associated with this procedure is lead dislodgment. The clinical course of these patients compares favorably with that of patients who have undergone defibrillator implantation via an epicardial approach.