Paul J. Troup

Unexplained Syncope Evaluated by Electrophysiologic Studies and Head-up Tilt Testing

OBJECTIVE To determine the clinical characteristics of subgroups of patients with unexplained syncope having electrophysiologic studies and head-up tilt testing and to assess the efficacy of various therapies. DESIGN Retrospective study. SETTING Inpatient services of a tertiary referral center. PATIENTS Eighty-six consecutively referred patients with unexplained syncope. MEASUREMENTS All patients had electrophysiologic examinations. Patients with negative results subsequently had head-up tilt testing. MAIN RESULTS Twenty-nine (34%) patients (group 1) had abnormal electrophysiologic results, with sustained monomorphic ventricular tachycardia induced in 72%. Thirty-four (40%) patients (group 2) had syncope provoked by head-up tilt testing. The cause of syncope remained unexplained in 23 (26%) patients (group 3). Structural heart disease was present in 76%, 6%, and 30% of groups 1, 2, and 3, respectively. In group 1, pharmacologic or nonpharmacologic therapy was recommended based on electrophysiologic evaluation. All group 2 patients had negative results on head-up tilt testing while receiving oral beta blockers (27 patients) or disopyramide (7 patients). Group 3 patients did not receive any specific therapy. During a median follow-up period of 18.5 months, syncope recurred in 9 (10%) patients. CONCLUSIONS The combination of electrophysiologic evaluation and head-up tilt testing can identify the underlying cause of syncope in as many as 74% of patients presenting with unexplained syncope. Therapeutic strategies formulated according to the results of these diagnostic tests appear to prevent syncope effectively in most patients.

The implanted defibrillator: relation of defibrillating lead configuration and clinical variables to defibrillation threshold

Forty-two defibrillating lead systems for the automatic implantable defibrillator were implanted and tested in 41 patients. Two basic lead configurations were used: 1) spring-patch, consisting of a transvenous superior vena cava spring electrode as the anode and an apical or left lateral ventricular patch electrode (either small [13.9 cm2] or large [27.9 cm2]) as the cathode; and 2) patch-patch, consisting of an anterior right ventricular patch as the anode and a posterior left ventricular patch as the cathode. Of the 42 lead systems, 10 were spring-patch and 32 were patch-patch combinations. The defibrillation threshold for the patch-patch combinations (9.8 +/- 6.5 J, mean +/- standard deviation) was significantly (p less than 0.01) lower than that for the spring-patch combinations (19.1 +/- 10.3 J). Subgroup analysis revealed the lowest defibrillation thresholds for patch-patch combinations with at least one large patch. Total surface area of defibrillating leads was strongly negatively correlated with the defibrillation threshold (p less than 0.005). Analysis of the relation of clinical variables to defibrillation threshold revealed that only amiodarone therapy was independently associated with a significantly (p less than 0.05) higher defibrillation threshold. Thus, surface area of the defibrillating leads is a critical determinant of the defibrillation threshold for the implanted defibrillator. Patch-patch lead systems with at least one large patch may provide an increased safety margin for defibrillation. Conversely, amiodarone therapy is associated with higher defibrillation thresholds and may decrease the margin of safety.

Use of intravenous esmolol to predict efficacy of oral beta-adrenergic blocker therapy in patients with neurocardiogenic syncope.

The usefulness of esmolol in predicting the efficacy of treatment with an oral beta-adrenergic blocking agent was evaluated in 27 consecutive patients with neurocardiogenic syncope. Seventeen patients had a positive head-up tilt test response at baseline and 10 patients required intravenous isoproterenol for provocation of hypotension. All patients were then given a continuous esmolol infusion (500 micrograms/kg per min loading dose for 3 min followed by 300 micrograms/kg per min maintenance dose) and rechallenged with a head-up tilt test at baseline or with isoproterenol. Of the 17 patients with a positive baseline tilt test response, 11 continued to have a positive response to esmolol challenge. Sixteen patients (including all 10 patients with a positive tilt test response with isoproterenol) exhibited a negative response to upright tilt during esmolol infusion. Irrespective of their response to esmolol infusion, all patients had a follow-up tilt test with oral metoprolol after an interval of greater than or equal to 5 half-lives of the drug. All 16 patients (100%) with a negative tilt test response during esmolol infusion had a negative tilt test response with oral metoprolol. Of the 11 patients with a positive tilt test response during esmolol infusion, 10 (90%) continued to have a positive response with oral metoprolol. It is concluded that in the electrophysiology laboratory, esmolol can accurately predict the outcome of a head-up tilt response to oral metoprolol. This information may be helpful in formulating a therapeutic strategy at the initial head-up tilt test in patients with neurocardiogenic syncope.

Comparative efficacy of monophasic and biphasic truncated exponential shocks for nonthoracotomy internal defibrillation in dogs.

Pentobarbital-anesthetized dogs were studied to determine the relative efficacy of monophasic and biphasic truncated exponential shocks employing a nonthoracotomy internal defibrillation pathway that consisted of a right ventricular catheter electrode (cathode) and a subcutaneous chest wall patch electrode (anode). In part 1 of the experiments, six dogs (19.6 +/- 1.1 kg) were utilized. Monophasic pulses of 5, 7.5, 10 and 12.5 ms duration were compared with biphasic pulses of the same total duration. The biphasic pulses had an initial positive phase (P1) followed by a terminal negative phase (P2) with the initial voltage equal for each phase. For each biphasic total pulse width, five relative P1 versus P2 durations were tested (50 and 50%, 75 and 25%, 90 and 10%, 25 and 75%, 10 and 90%). Ventricular fibrillation was induced by alternating current and pulse configurations were tested randomly to determine the minimal voltage and energy for defibrillation (threshold). Biphasic shocks with P1 longer than P2 were associated with significantly lower (p less than 0.01) energy thresholds than were monophasic shocks. Additionally, there was no significant relation between pulse width and voltage or energy thresholds. In part 2 of the experiments, six dogs (20.2 +/- 1.6 kg) were studied. Monophasic shocks were compared with biphasic shocks with P1 versus P2 durations of 75 and 25% and 90 and 10% for total pulse widths of 7.5, 10 and 12.5 ms. Threshold determinations were performed as in part 1. Subsequently, five initial voltages clustered about threshold were randomly tested four times and dose-response curves constructed for each pulse configuration with the use of stepwise logistic regression. Biphasic shocks resulted in significantly lower energy (p less than 0.0001) and voltage (p less than 0.001) requirements than did monophasic shocks.(ABSTRACT TRUNCATED AT 250 WORDS)

The automatic implantable cardioverter-defibrillator: Evaluating suspected inappropriate shocks

Two patients who received inappropriate shocks from an implanted defibrillator are presented. In one case, fracture of a sensing lead was responsible and in the other case, sensing of both pacemaker stimuli and the evoked ventricular electrogram resulted in inappropriate shocks. In both cases, phonograms recorded over the generator area with a magnet in place revealed audible tones synchronous with each sensed event which allowed noninvasive documentation of a sensing problem. This procedure appears to be a valuable step both in the confirmation of sensing problems, including pacemaker-defibrillator interactions, and in evaluating suspected inappropriate shocks.

Relationship of left ventricular mass to defibrillation threshold for the implantable defibrillator: A combined clinical and animal study

Defibrillation results when a critical mass of myocardium is depolarized. The relationship between echocardiographic determinations of left ventricular mass, volume, and cavity radius to wall thickness ratio and defibrillation threshold for the implantable defibrillator was examined. Ten patients with two large patch defibrillating lead systems were studied. Defibrillation threshold was determined intraoperatively as the lowest energy terminating ventricular fibrillation. Left ventricular mass, volume, and radius/posterior wall thickness ratio were calculated from two-dimensional echocardiograms. A significant correlation was found between left ventricular mass and defibrillation threshold (r = 0.78, p less than 0.01). The correlations between defibrillation threshold and left ventricular volume (r = 0.59) and radius/wall thickness ratio (r = 0.55) were not significant. Subsequently, 11 dogs undergoing defibrillation trials with a transvenous catheter and a chest wall patch were studied. Defibrillation threshold was defined as the lowest energy-terminating ventricular fibrillation (four separate attempts). Subsequently, the heart was dissected, and the left ventricle (including the septum) was weighed. The correlation between left ventricular weight and defibrillation threshold (r = 0.76) was significant (p less than 0.01). We conclude that noninvasive assessment of left ventricular mass and direct measurement of left ventricular weight are significantly correlated with defibrillation threshold and consistent with the critical mass hypothesis.

Sudden Cardiac Death: Management of High-Risk Patients

Sudden cardiac death remains a leading cause of death in the United States, accounting for more than 350,000 deaths each year, and the survival rate of victims remains low. Most survivors face a significant risk for recurrence. The typical substrate is chronic--abnormal myocardium with fibrosis (often from previous myocardial infarction) and left ventricular dysfunction. Acute triggers for sudden cardiac death are primarily electrical, ischemic, metabolic, neurohormonal, and pharmacologic. In most electrocardiographically documented cases of sudden cardiac death, the trigger-substrate interaction appears to result in ventricular tachycardia and fibrillation. After initial resuscitation, survivors need a thorough cardiovascular evaluation, including definition of coronary anatomy, left ventricular function, and wall-motion abnormalities, as well as an electrophysiologic evaluation. An attempt must be made to determine what each survivors correctable triggers are. Management should address all reversible triggers, such as acute ischemia and electrolyte abnormalities, and should include modifying or correcting the arrhythmogenic substrate. Empiric antiarrhythmic therapy offers no advantage in such modification. Pharmacologic therapy with antiarrhythmic drugs should be guided by an objective therapeutic endpoint, which is best accomplished through the use of programmed ventricular stimulation and serial electrophysiologic studies. Other therapeutic options include surgical suppression of ventricular tachycardia and implantation of a cardioverter defibrillator.

Strength‐Duration Curves of Fixed Pulse Width Variable Tilt Truncated Exponential Waveforms for Nonthoracomy Internal Defibrillation in Dogs

Six anesthetized dogs (wgt 19.6 + 1.1 kg) underwent defibrillation trials using truncated monophasic pulses of 2.5—20 msec in duration. The current pathway consisted of a 4 cm2 RV catheter electrode (cathode) and a 13.9 cm2 subcutaneous chest wall patch (anode). Fibrillation was induced by alternating current and defibrillation attemped 10 seconds later. Only one test shock was assessed for each fibrillation episode. The various durations were tested randomly, and the minimum peak voltage and energy resulting in defibrillation was determined for each. Shorter pulse durations were associated with lower energies with pulses of 2.5—15 msec having significantly lower energy thresholds than shocks of 20 msec (P < 0.05). The relationship between duration and voltage threshold is hyperbolic with minimum voltage between 7.5 and 12.5 msec while the shortest and longest pulses were associated with the highest voltage thresholds. Shocks of 5 to 15 msec were associated with significantly lower voltage threshold than 2.5 msec pulses (P < 0.05). The threshold average current (Iav) reached a nadir at 10 msec. Shocks in the midrange of those tested resulted in the best combination of low average current and energy requirements for defibrillation using this nonthoracotomy lead system.

Comparison of monophasic with single and dual capacitor biphasic waveforms for nonthoracotomy canine internal defibrillation.

Monophasic and single capacitor and dual capacitor biphasic truncated exponential shocks were tested in pentobarbital-anesthetized dogs with use of a nonthoracotomy internal defibrillation pathway consisting of a right ventricular catheter electrode and a subcutaneous chest wall patch electrode. Seven dogs weighing 20.2 +/- 0.5 kg were utilized. Monophasic pulses of 10 ms duration were compared with three biphasic pulses. All biphasic waveforms had an initial positive phase (P1) followed by a terminal negative phase (P2) and the total duration of P1 plus P2 was 10 ms. The dual capacitor biphasic waveform (P1 9 ms, P2 1 ms) had equal initial voltages of P1 and P2. Two simulated single capacitor biphasic waveforms were also tested, the first designed to minimize the magnitude of P2 (P1 9 ms, P2 1 ms with initial voltage of P2 equal to 0.3 of the initial voltage of P1) and the second to maximize P2 (P1 5 ms, P2 5 ms with initial voltage of P2 = 0.5 P1). Alternating current was used to induce ventricular fibrillation and four trials of eight initial voltages from 100 to 800 V were performed for each of the four waveforms. Stepwise logistic regression was utilized to construct curves relating probability of successful defibrillation and energy. In the logistic model, the dual capacitor biphasic and single capacitor biphasic waveforms that maximized P2 were associated with significantly (p less than 0.001) lower energy requirements for defibrillation than those of the monophasic waveform. The single capacitor biphasic waveform that minimized P2 was not significantly better than the monophasic waveform.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonthoracotomy internal defibrillation in dogs: Threshold reduction using a subcutaneous chest wall electrode with a transvenous catheter electrode

The efficacy of truncated exponential waveform shocks using a cardioverter-defibrillator catheter with and without a 13.9 cm2 subcutaneous thoracic patch electrode was examined in 10 pentobarbital-anesthetized dogs. The cardioverter-defibrillator catheter was positioned through the external jugular vein with the distal 4 cm2 shocking electrode located in the right ventricular apex and the 8 cm2 proximal electrode located in the superior vena cava. Four electrode configurations were tested: 1) distal electrode (cathode) to proximal electrode and chest wall patch (common anodes), 2) distal electrode (cathode) to chest wall patch (anode), 3) distal electrode (cathode) to proximal electrode (anode), and 4) chest wall patch (cathode) to proximal electrode (anode). The lowest randomized energy resulting in termination of alternating current-induced ventricular fibrillation on four trials at that energy was 20.2, 21.3, 27.4 and greater than 40 J, respectively, for configurations 1 through 4. The energy requirements for configurations 1, 2 and 3 were significantly lower than for configuration 4 (p less than 0.001). Additionally, configurations incorporating the distal electrode and the patch electrode (configurations 1 and 2) were significantly better than the catheter alone (configuration 3; p less than 0.05). There was no significant difference between configurations 1 and 2. In conclusion, the addition of a subcutaneous chest wall electrode to the cardioverter-defibrillator catheter significantly lowered energy requirements for defibrillation, suggesting that a nonthoracotomy approach for the automatic implantable cardioverter-defibrillator is feasible.