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The R-on-T phenomenon was observed in two patients with narrow QRS complexes and repetitive supraventricular tachyarrhythmias. Case 1 had a typical Lown-Ganong-Levine syndrome in which the shortening of the P-R interval was due to a reduction of the A-H interval. In Case 2 the P-R interval (140 msec) was not short but the A-H interval was at the lower limits of normal (50 msec) and the P-A interval at the upper limits of normal (40 msec). Both patients showed the R-on-T phenomenon during periods of 1:1 A-V conduction occurring during atrial stimulation at a rate of 200/min (Case 1) and atrial flutter with a rate of 300/min (Case 2). Although total A-V conduction time was prolonged, the magnitude of this prolongation appeared to have been less than that occurring in normal subjects. This was attributed to the presence of an accessory pathway bypassing the area where the most significant delay occurred at the A-V node. The findings in Case 2 suggest, but do not prove, that a similar bypass might be operative in patients with atrial flutter with periods of 1:1 conduction, regardless as to whether the R-on-T phenomenon is, or is not, present.

The electrophysiologic effects of the antiarrhythmic agent disopyramide phosphate given intravenously were studied in 10 patients with cardiac disease. Studies included determinations of sinus recovery time and refractoriness of the atria, the atrioventricular (A-V) node and the His-Purkinje system. Measurements were performed at rest and 15 and 30 minutes after administration of disopyramide. Serum drug levels were measured at these times. Sinus recovery time was shortened at both 15 and 30 minutes, with an average decrease of 39.5 and 146.2 msec, respectively (P less than 0.01). Atrial refractoriness was not altered significantly, but tended to be reduced; the mean effective refractory period was 289.5 msec before administration of disopyramide and 259 and 270 msec 15 and 30 minutes, respectively, after administration. The functional refractory period of the atrioventricular (A-V) node was definitely prolonged in seven patients 15 minutes after administration of disopyramide. The relative refractory period of the His-Purkinje System was not altered. Although this study does not elucidate the mechanism by which disopyramide acheives its antiarrhythmic effects, animal work has shown that it is similar to that of quinidine. In the doses used the drug does not seem to cause first, second or third degree A-V block or fascicular or bundle branch block; it did not increase the severity of first degree A-V block in the three patients with this disturbance. The drug may be particularly useful when arrhythmias are associated with slow sinus rates.

Proper placement of pacemaker catheters can be facilitated by recording endocardial electrograms from the catheters. These electrograms are also helpful in the investigation of pacemaker failure. Endocardial contact is indicated by S-T segment elevation. When bipolar catheters are used, these "contact currents" may be visible in electrograms from the tip or ring electrodes, or both. Right ventricular cavity electrograms are characterized by their high voltage and are influenced by intraventricular conduction defects and acute myocardial infarction. Serial changes have been described indicating myocardial injury, infarction, and healing in the area underlying the catheter tip.