Pharmacologic Therapy of Ventricular Tachycardia Using Electrophysiologic Techniques

Differential effects of lidocaine on defibrillation threshold with monophasic versus biphasic shock waveforms

BACKGROUND

Defibrillation waveforms and antiarrhythmic drugs have disparate effects on myocardial excitability and refractoriness, making it likely that antiarrhythmic drugs will interact with one waveform differently than with another. The aim of the present study was to determine if the increase in defibrillation threshold (DFT) induced by lidocaine is similar for electrical shocks with monophasic and biphasic waveforms.

METHODS AND RESULTS

Twenty-six pentobarbital-anesthetized farm-raised pigs were instrumented with pacing catheters and epicardial defibrillation electrodes. Each pig was assigned to one of four groups: (1) monophasic shock waveform and placebo (5% dextrose in water [D5W]) (n = 7), (2) monophasic shock waveform and lidocaine (n = 7), (3) biphasic shock waveform and placebo (D5W) (n = 5), or (4) biphasic shock waveform and lidocaine (n = 7). DFT was measured at baseline and subsequently during treatment (D5W or lidocaine). In the monophasic waveform groups, DFT increased from baseline in response to lidocaine by 92% (P < .0001), whereas DFT values in response to D5W did not change. In the biphasic waveform groups, DFT values did not change from baseline in response to lidocaine (P = NS), whereas DFT values from baseline in response to D5W significantly decreased by 29% (P = .04). In the monophasic waveform groups, the change in DFT from baseline in response to lidocaine was significantly different than the change from baseline in response to D5W (92 +/- 29% versus -0.5 +/- 29%, respectively) (P < .0002). In the biphasic waveform groups, however, the change in DFT from baseline in response to lidocaine was similar to the change from baseline in response to D5W (-5.66 +/- 15% versus -29 +/- 17%, respectively) (P = .48). Furthermore, the change in DFT from baseline in response to lidocaine differed significantly between monophasic and biphasic waveform groups (92 +/- 29% versus -5.66 +/- 15%) (P < .0002), whereas the change from baseline in response to D5W did not differ between monophasic and biphasic waveforms (-0.5 +/- 29% versus -29 +/- 17%) (P = .34).

CONCLUSIONS

Compared with placebo groups, DFT values increased during lidocaine treatment to a much greater degree in the monophasic waveform group than in the biphasic waveform group receiving lidocaine. These data support our hypothesis that antiarrhythmic drugs can affect the defibrillation efficacy of monophasic waveforms differently than that of biphasic waveforms.

Evaluation of the need to modify antiarrhythmic therapy because of drug intolerance or inefficacy in patients evaluated by electrophysiology study

The incidence of recurrent ventricular tachycardia (VT) and drug intolerance after therapy guided by electrophysiology study (EPS) leading to a change in antiarrhythmic therapy is unknown. To explore this issue, we studied 138 patients undergoing EPS with a serial drug-testing protocol. Patients were followed for 24 +/- 21 months (range 1 to 91 months) for recurrent VT, drug intolerance, or both. At baseline EPS, sustained VT (76%), nonsustained VT (20%), and ventricular fibrillation (4%) were induced. On discharge antiarrhythmic therapy, 70 (51%) patients were noninducible (< 16 beats) and 68 (49%) patients remained inducible. Of the 138 patients, 56 (41%) required modification of antiarrhythmic therapy (change in drug or dose) for recurrent VT (n = 20) or drug intolerance (n = 36). These events occurred early during follow-up (8 +/- 12 months). Patients who were rendered noninducible had a greater likelihood of remaining free of antiarrhythmic modification than patients who remained inducible at 1-year (68% and 60%, respectively) and 2-year (63% and 52%, respectively) follow-up examinations (p < 0.05). Likewise, the noninducible patients had a greater likelihood of remaining free of recurrent VT than inducible patients at 1-year (95% and 80%, respectively) and 2-year (92% and 80%, respectively) follow-up testing (p < 0.025). In contrast, the likelihood of remaining free of drug intolerance was similar between inducible and noninducible patients at 1 year (72% and 75%, respectively) and 2 years (69% and 65%, respectively). We conclude that antiarrhythmic therapy guided by EPS requires early modification in more than 40% of patients during follow-up.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiologic evaluation of antiarrhythmic drugs

Electrophysiologic techniques are used to assess the properties of antiarrhythmic drugs and also to provide support for the selection of antiarrhythmic therapy for individual patients. Assessment of the antiarrhythmic efficacy of drugs requires that the arrhythmia can be induced by programmed electrical stimulation and is critically dependent on the stimulation protocol. Continued inducibility on drug therapy appears to be a strong predictor of recurrences and sudden death, although the predictive value of electrophysiologic testing remains controversial for some drugs. These techniques may also be useful for determining the proarrhythmic potential of antiarrhythmic agents, but the aggravation of tachycardia that occurs in 10-30% of patients during electrophysiologic testing is unpredictable and its significance is unknown. The electrophysiologic approach to drug therapy has limitations, but, nevertheless, it is useful and should maintain a prominent place in the evaluation of antiarrhythmic therapy.