The repetitive extrasystole as an index of vulnerability to ventricular fibrillation during myocardial ischemia in the canine heart

Transient coronary stenosis associated with arrhythmia and ischemia 24 hours after reperfusion

We examined the electrophysiologic sequelae of transient reductions in coronary blood flow (CBF) in conscious dogs. Animals were instrumented to measure arterial pressure, heart rate, repetitive extrasystole threshold (RET), and CBF before coronary stenosis, during 90 minutes of a 50% reduction in CBF, and at 1, 24, 48, and 72 hours after reperfusion. RET was decreased at 24 and 48 hours but returned to baseline by 72 hours after reperfusion. Frequent ventricular ectopic activity (VEA), absent during the control period, was noted during stenosis and at 1, 24, and 48 hours after reperfusion. beta-Adrenergic blockade (metoprolol) normalized the decreased RET at 24 hours. Addition of alpha 1-adrenergic blockade (prazosin) abolished VEA. Endocardial blood flow in the posterior papillary muscle of the left ventricle was reduced by 52 +/- 9% during stenosis, was similar to baseline levels by 1 hour after reperfusion, and decreased by 39 +/- 5% at 24 hours. We conclude that episodes of modest coronary artery stenosis may be followed by electrophysiologic changes indicative of increased vulnerability to malignant ventricular arrhythmias.

Influence of beta 2-adrenoceptor stimulation and blockade on cardiac electrophysiologic properties and serum potassium concentration in the anesthetized dog

The influence of selective beta 2-adrenoceptor stimulation and blockade on cardiac electrophysiologic properties was studied in 18 anesthetized dogs. Selective beta 2-adrenoceptor activation by salbutamol failed to alter myocardia excitability but significantly lowered serum potassium concentration. Excitability, refractoriness, and ventricular fibrillation threshold were also not changed after administration of 100 and 200 micrograms/kg of the selective beta 2-antagonist ICI 118,551. However, at a dose of 500 micrograms/kg, refractoriness was prolonged and ventricular fibrillation threshold increased. These changes appear to be due to blockade of beta 1-adrenoceptors rather than to membrane stabilizing effects, since in catecholamine-depleted animals even the highest dose of ICI 118,551 did not alter myocardial electrical properties. It is concluded that beta 2-adrenoceptors do not influence electrophysiologic properties of the canine myocardium.

Effect of hypokalemia on susceptibility to ventricular fibrillation in the normal and ischemic canine heart

To examine the impact of hypokalemia on cardiac electrical stability, the thresholds for repetitive extrasystole and ventricular fibrillation were determined before and after potassium depletion in 15 chloralose-anesthetized dogs. A reduction in serum potassium concentration from 3.6 to 2.1 mEq/L induced by hemodialysis decreased the repetitive extrasystole threshold by 30% and the ventricular fibrillation threshold by 25% (p less than 0.01). The increase in ventricular vulnerability following acute coronary occlusion was markedly augmented by concomitant potassium depletion. Thus, hypokalemia enhances the propensity for ventricular fibrillation in the normal as well as in the ischemic canine heart. These findings shed light on clinical observations of enhanced susceptibility to life-threatening arrhythmias during acute myocardial ischemia in hypokalemic patients.

Can potentially significant polymorphic ventricular arrhythmias initiated by programmed stimulation be distinguished from those that are nonspecific?

Polymorphic ventricular arrhythmias (PVAs) initiated by programmed electrical stimulation may be a nonspecific response or evidence of ventricular electrical instability. To determine if PVAs initiated in patients with spontaneous sustained ventricular tachycardia or fibrillation differ from those which are clearly a nonspecific response in structurally normal hearts, the initiation, characteristics, and relationship to ventricular repolarization of PVAs greater than five beats in duration were evaluated in 32 patients without structural heart disease and in 36 patients with spontaneous sustained ventricular arrhythmias more than 9 days after myocardial infarction. Patients received one to four extrastimuli during sinus rhythm and right ventricular pacing. In a comparison with patients who completed the same steps (defined by the basic drive cycle length and number of extrastimuli) in the stimulation protocol, there was no difference in the cumulative risk of initiation of a PVA between the patients with and those without heart disease at any step. This risk was 51% vs 38% for patients who received two or fewer extrastimuli at four basic cycle lengths (p = NS). PVAs were initiated by the same mean number of extrastimuli (2.3 +/- 0.5 vs 2.6 +/- 0.9 p NS) with the same degree of prematurity in both groups. Forty-four percent of the PVAs in the myocardial infarction group had a cycle length greater than 250 msec or a coupling interval of the first tachycardia beat to its initiating stimulus greater than 320 msec as opposed to only one (6%) in the group without heart disease (p less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Arrhythmia inducibility and ventricular vulnerability in a chronic feline infarction model

Ventricular tachyarrhythmias are the cause of sudden cardiac death in ischemic heart disease. Reliable animal models are necessary to study techniques for identifying individuals at risk and to develop effective modes of therapy. The purpose of the present study was to evaluate the inducibility of ventricular tachyarrhythmias and vulnerability to ventricular fibrillation and to correlate these findings with changes in ventricular refractoriness in a chronic feline model. Twelve conditioned cats were randomly divided into two groups: group A, sham-operated controls (n = 5); or group B, permanent occlusion of the left anterior descending coronary artery (n = 7). Two weeks later, the following measurements were made: (1) assessment of refractory periods at several ventricular sites; (2) inducibility to ventricular tachyarrhythmias; and (3) determination of ventricular fibrillation threshold. After electrophysiologic testing, the animals were killed and the hearts were studied histologically. Ventricular fibrillation thresholds were significantly lower in group B compared with group A (13 +/- 3 vs 46 +/- 9 mA; p less than 0.01). One of the sham-operated controls had induction of nonsustained ventricular tachycardia, while six of the group B animals had reproducible, inducible ventricular tachyarrhythmias (p less than 0.01). There was a significant dispersion in effective refractory periods between normal and infarcted sites in group B (46 +/- 6 msec) not seen in group A (12 +/- 2 msec, p less than 0.01). The group A cats demonstrated minimal damage to the myocardium or cardiac architecture. Group B cats demonstrated extensive, transmural, homogeneous infarcts of approximately 30% of the anterior wall of the left ventricle.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of the effects of bethanidine, meobentine and quinidine on the electrical activity of rat hearts in vivo and in vitro

Glass microelectrodes were used to record transmembrane electrical activity from cells located just beneath the endocardial surface of segments of the right ventricular free wall of the rat heart during superfusion and electrical stimulation in vitro at 37 degrees C. The sulphates of bethanidine, meobentine or quinidine (4 to 20 microM) applied in vitro caused a prolongation of action potential duration and a delayed and slowed return of electrical excitability following an action potential. Intracardiac electrical stimulation of the urethane-anaesthetized rat heart in situ was used to measure ventricular refractory periods from the electrocardiogram. Intravenous administration of bethanidine, meobentine or quinidine (10 to 20 mg kg-1) caused a prolongation of ventricular refractory periods. Quinidine had a briefer duration of action than either of the other two drugs tested. Urethane-anaesthetized open-chested rats which were subjected to left coronary artery occlusion displayed ventricular tachyarrhythmias in their electrocardiogram. These arrhythmias occurred during the period of occlusion and even more prominently after release of the occlusion. Intravenous administration of bethanidine, meobentine or quinidine (1 to 20 mg kg-1) protected rats against these arrhythmias. The protective effect of quinidine was briefer than that of either of the other two drugs tested.

Vulnerability to ventricular fibrillation in patients with clinically manifest ventricular tachycardia

Ventricular vulnerability may be assessed by measuring the threshold current for the induction of ventricular fibrillation (VF). This technique has been widely utilized in animal experimentation and has been safely applied in a small number of clinical studies. We measured the VF threshold (VFT), using the single stimulus technique in 10 patients with coronary artery disease just prior to the institution of cardiopulmonary bypass. There were no adverse effects of VFT measurement. Three patients had nonsustained ventricular tachycardia (VT) on 24-hour ambulatory monitoring and had VFTs of 10, 14, and 16 mA. In this group VF was induced without any preceding repetitive ventricular responses. Seven patients had no repetitive forms on ambulatory monitoring. Their VFTs ranged from 30 to greater than 40 mA (mean greater than 37). Repetitive extrasystoles were regularly observed in this group at current intensities which ranged from 53% to 80% of the VFT. Thus patients with manifest VT appear to have an enhanced vulnerability to VF. Single or multiple responses were not observed in these patients but appeared to be present in patients with coronary disease and no demonstrable rhythm disorder.

Influence of site of stimulation on measurement of ventricular vulnerability in the normal and ischemic feline heart

Site of stimulation is an important variable in the inducibility of ventricular tachycardia in both non-human animal infarction models and in humans; that is, proximity of the stimulating electrode to the site of reentry facilitates the induction of sustained arrhythmia. Whether site of stimulation is decisive in measurement of vulnerability to ventricular fibrillation (VF) during acute coronary occlusion has not been fully evaluated. We measured VF thresholds in 9 chloralose-anesthetized cats at 2 right ventricular and 3 left ventricular sites (2 endocardial, 3 intramural) before and after abrupt occlusion of the anterior descending coronary artery. VF thresholds were measured using a single stimulus of increasing intensity delivered during ventricular drive. Although VF thresholds were lower at endocardial sites, there were no significant differences in VF threshold among any of the sites tested at control. After occlusion, VF thresholds fell to a similar extent at all 5 sites tested. The percent reduction in VF threshold at any site was not influenced by the sequence of testing. VF may be precipitated from multiple sites and, unlike ventricular tachycardia, does not represent an isolated focus of arrhythmogenicity.