Canine electrocardiographic and cardiac electrophysiologic changes induced by procainamide

Microelectrode technics were used to study the effects of procainamide (PA) on electrophysiologic properties of canine Purkinje fibers (PF) perfused in an extra-corporeal circuit with arterial blood of donor dogs. Donor electrocardiogram (ECG) and blood pressure (BP) and electrophysiologic properties of the isolated PF were simultaneously monitored. PA, 25 mg/kg, was administered as a single intravenous injection and plasma PA concentrations (PPAC) were determined spectrophotometrically. A decrease in PF automaticity was noted within 5-10 min of PA injection. Within 20 min, when PPAC was 10 µg/ml, amplitude and maximal rate of rise of phase 0 of the AP and conduction velocity (CV) decreased and alterations in AP duration and refractoriness were noted. BP and ECG rate were unchanged but the QRS complex had widened. At PPAC = 10-20 µg/ml (induced by further PA infusion) the aforementioned changes were accentuated. At PPAC > 20 µg/ml membrane responsiveness was significantly depressed. These experiments suggest that therapeutic PA levels rapidly decrease PF automaticity. Slowing of conduction commences later, simultaneously with changes in QRS duration. Within the same time span and range of plasma levels there are alterations in AP duration and refractoriness. Membrane responsiveness is not significantly depressed until PPAC is well into the clinically toxic range.

Effect of propranolol, procainamide, and lidocaine on ventricular automaticity and reentry in experimental myocardial infarction

The importance of two arrhythmogenic mechanisms, ventricular reentry and automaticity, was assessed after coronary ligation in 35 cats. Automaticity was estimated by the frequency of ventricular escape beats after intravenous acetylcholine, and reentry was estimated by the number of repetitive beats induced by premature ventricular stimuli. Increased automaticity only was present after coronary ligation in two studies; increased reentry only was noted in 24; and both increased automaticity and reentry were seen in nine studies. Incremental doses of lidocaine (2-4 mg/kg), propranolol (0.1-0.3 mg/kg), or procainamide (5-15 mg/kg) consistently returned automaticity to normal. Procainamide abolished repetitive beats in 12 of 13 studies, propranolol did so in eight of 11, but lidocaine did so in only six of 17. Repetitive beats actually increased in six studies after lidocaine, at a time when lidocaine blood levels were in the lower therapeutic range. A persistent arrhythmia appeared after coronary ligation in 12 studies; in 10 the disappearance or persistence of the arrhythmia after drug administration matched the disappearance or persistence of repetitive beats. We conclude that both reentry and increased ventricular automaticity appear during myocardial infarction but that reentry seems of greater importance in the genesis of arrhythmias. These drugs are more reliable in normalizing automaticity than in abolishing reentry; the failure of a drug may thus be related to its failure to abolish reentry. Moreover, lidocaine may accentuate ventricular reentry, especially at the lower blood levels.

Bretylium tosylate in the management of refractory ventricular fibrillation

Nine patients who had recurrent ventricular fibrillation following acute myocardial infarction or angina were given bretylium tosylate in a dose of 5 mg./kg. intramuscularly every eight hours after other measures had proved ineffective. Provided the patients were not in shock or in heart failure, there was a considerable reduction in the episodes of ventricular fibrillation.A second group of nine patients who developed recurrent ventricular fibrillation following open heart surgery were given bretylium intravenously, which controlled the arrhythmia in every instance.Bretylium did not completely abolish ventricular premature beats but the latter did not initiate ventricular fibrillation even when they occurred on the T wave.