Strength-interval relations in a chronic canine model of myocardial infarction. Implications for the interpretation of electrophysiologic studies.

Abstract

SUMMARY Fifteen adult mongrel dogs underwent two-stage occlusion of the mid-or distal left anterior descending coronary artery and then a reperfusion stage. The dogs were studied 3-30 days later to determine strength-interval relations in a canine model of chronic myocardial infarction. These dogs were susceptible to the initiation of sustained ventricular tachyarrhythmias with the introduction of one, two or three ventricular extrastimuli. Using unipolar cathodal stimuli with a pulse width of 2 msec, strength-interval curves were constructed from measurements made at multiple sites in the distribution of occluded and nonoccluded vessels during drive pacing at a cycle length of 300 msec. At 56 normal sites, ventricular refractory periods measured at twice-diastolic-excitability threshold approximated the relative refractory periods (mean absolute difference 3 msec), but were variably longer than effective refractory periods (mean difference 18 msec, range 4-29 msec). At 51 infarct sites, differences between ventricular refractory periods measured at twice-diastolic-excitability threshold and both relative refractory periods (mean difference 13 msec, range -60 to + 18 msec) and effective refractory periods (mean difference 28 msec, range 1-60 msec) were markedly disparate. These differences were further exaggerated after administration of i.v. procainamide. These findings suggest limitations in interpreting the results of programmed pacing studies performed using stimuli of twice-threshold intensity.PROGRAMMED ELECTRICAL STIMULA-TION has gained acceptance clinically as a means of initiating and terminating sustained tachyarrhythmias,'-8 studying arrhythmia mechanisms,7' 9-12 evaluating properties of excitability and refractoriness,5 13-17 and determining the electrophysiologic effects of various pharmacologic and antiarrhythmic interventions. 4,6,7,13,[18][19][20][21][22] In clinical practice, bipolar pacing via a multipolar catheter is initiated at a rate faster than the patient's intrinsic rate, and the milliamperage of a 1-2-msec pulse is increased until consistent capture is evident.The minimum milliamperage necessary for capture is then defined as the excitability threshold for that site.23 Routinely, clinical electrophysiologic studies have been performed at a milliamperage approximately twice the excitability threshold, and usually at less than 3 mA. 4' 7, 8, 24 Such methods have been considered adequate for obtaining necessary electrophysiologic information and safe enough to avoid inadvertent ventricular fibrillation.7 8,25 However, no specific data suggest that performing programmed electrical stimulation at twice-diastolic-threshold intensity is optimal. Even in the most experienced clin- Materials and Methods Studies were performed on 15 healthy adult mongrel dogs that weighed 8-16 kg. The dogs were anesthetized with i.v. sodium pentobarbital (30 mg/kg) and then ventilated with room air through a cuffed pharyngeotracheal tube using a volume-cycled positive-pressure respirator. Body temperature was ma...

Postrepolarization Refractoriness Versus Conduction Slowing Caused by Class I Antiarrhythmic Drugs

“…Stimulation at higher stimulus strengths was performed on the basis of the previously published observation that higher stimulus strengths can elicit action potentials at shorter coupling intervals, thereby creating more premature, and hence possibly more arrhythmogenic, excitations, and a wider range of PRR values. 19 (b) High-frequency burst pacing was used because it probes myocardial excitability in a more continuous fashion than intermittent extrastimulation and uncovers the rate-dependent relation between repolarization and refractoriness with greater resolution and sensitivity. 20 Burst pacing at twice diastolic threshold was applied for 10 seconds by one of the left ventricular MAP catheters (nϭ11) or through a hook electrode mounted in the apex (nϭ3) at burst cycle lengths of 10, 20, 30, and 40 ms in random order.…”

“…If a sustained arrhythmia was not induced at all burst frequencies, the process was repeated at 5 and, if necessary, at 10 times diastolic pacing threshold. 19 Selected burst pacing frequencies (nϭ5) were repeated after completion of the protocol to test reproducibility.…”