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It was recently shown that supernormal conduction in the diseased His-Purkinje system is more common than previously thought, and is always associated with prolongation of refractoriness. To assess whether supernormal conduction could also occur in the accessory pathway of patients with ventricular pre-excitation, 21 patients with manifestly prolonged refractoriness in the accessory pathway were studied. Under these conditions, programmed atrial stimulation revealed a phase of supernormal conduction in 16 (76%) of the 21. Therefore, what was believed to be a nonexistent or exceptional physiologic event was shown to be a rather common finding, at least under certain circumstances. Supernormal conduction occurred in all 7 patients with an anterograde refractory period of 480 to 980 ms, and in 5 of 10 patients with a refractory period greater than 1.0 second or with no anterograde conduction. Supernormal conduction could not be demonstrated in four patients with a refractory period less than or equal to 440 ms, but appeared in all four patients after the refractory period was prolonged by a rapid rate of stimulation or administration of ajmaline. The electrophysiologic changes underlying the occurrence of supernormal conduction in the accessory pathway are similar to those previously reported for the bundle branch system. The demonstration of supernormal conduction in the accessory pathway may uncover the presence of concealed ventricular pre-excitation. Supernormal conduction over the accessory pathway may facilitate a rapid ventricular response during atrial fibrillation, even if the refractory period is prolonged.

We studied 28 cases of chronic chagasic myocarditis (CCM) with frequent ventricular arrhythmias. Two-hundred and three conventional ECGs recorded during 3 months showed ventricular extrasystoles (VE) ranging between 0.2 and 6 per ten beats in 100%; multiform VE in 97.04%; couplets in 79.31%; ventricular tachycardia (VT) in 42.85%; and R on T in 21.67%. A 24-hour continuous recording showed that VE ranged between 3780 and 61733 (mean 16618 +/- 2627); multiform VE and couplets were present in 100% of patients, and VT was present in 78.5%. In 16 patients (group I) the frequency of VE was persistently high, without diurnal variation; 11 patients showed sustained reduction during sleeping hours and only one showed an increase during night sleep (group II). Even in group II, VE never disappeared for periods longer than 10 minutes. In five patients, four 24-hour recordings were obtained at weekly intervals, and in five other patients a second 24-hour recording was performed 10 to 24 months later. The remarkable frequency, persistence and low variability of ventricular arrhythmias in CCM suggest that such arrhythmias can be used as a most stable, reliable, but highly demanding model for testing the efficacy of antiarrhythmic drugs.

Twenty-one patients were studied in whom ventricular preexcitation (VP) had been recorded in the past and had later disappeared, indicating antegrade block in the accessory pathway (AP), either spontaneously (10 patients) or under the effect of chronic treatment with amiodarone (11 patients). VP reappeared in nine cases during vagal stimulation, and in five cases during an i.v. isoproterenol infusion. Retrograde conduction over the AP was studied in four of the remaining seven patients and was found to be present in three and absent in one. Although these patients differ from the ordinary patient with concealed AP in that antegrade preexcitation had been demonstrated in the past, this study suggests that concealed VP may result from the following mechanisms: 1) an extremely prolonged refractory period in the AP, causing a rate-dependent VP that can be identified during vagal stimulation; 2) a rate-independent depression of antegrade conduction that can be reversed by isoproterenol; 3) a depression of conduction that is apparently no longer reversible. Only in the latter case is a study of retrograde conduction needed to identify the concealed VP. These three mechanisms are likely to be a natural sequence of events leading to complete antegrade block in the AP.

Escape beats probably arising from the anomalous bundle were documented in 2 patients with the Wolff-Parkinson-White (WPW) syndrome. A third patient, in whom complete AV block developed both in the anomalous bundle and the normal pathway, showed the occurrence of escape beats (an escape-bigeminy pattern), as well as a regular idioventricular rhythm arising from the anomalous bundle. Phase 4 block in the anomalous bundle occurred in 7 other patients, in 4 of them spontaneously and in 3 only after the administration of ajmaline or amiodarone. Only 4 of 14 fully investigated patients (out of a total number of 23) showed absence of both escape beats and phase 4 block. The escape beats were considered as direct evidence, and the phase 4 block as indirect evidence, for the existence of automaticity in the anomalous bundle. Such evidence supports the view that the anomalous bundle, like the His bundle-branch system, may be composed of specialised tissue endowed with the property of automaticity.

SUMMARY An isoproterenol infusion (1.0-4.0 ,ug/min) was administered to 15 patients with intermittent bundle branch block (BBB) and two patients with apparently fixed BBB. Three main effects were documented: (1) In all patients with phase 3, or tachycardia-dependent, BBB, isoproterenol caused a pronounced shortening of refractoriness in the affected fascicle. (2) In patients showing phase 4, or bradycardia-dependent, BBB, isoproterenol prolonged the phase 4 block range, probably because of enhanced diastolic depolarization. In one patient (four studies) in whom phase 4 block was not present, isoproterenol caused the appearance of a phase 4 block range. (3) In the two patients with fixed BBB, isoproterenol restored conduction, probably as a result of a hyperpolarizing effect. This study shows that isoproterenol tends to restore or improve conduction related to tachycardia-dependent block, but may impair conduction related to bradycardia-dependent block. dependent BBB is favored or provoked, probably through an enhancement of diastolic depolarization.Materials and Methods Fifteen patients with intermittent BBB were studied. Carotid sinus massage was performed in every case during a continuous ECG recording to document the existence and measure the extension of a phase 3 and phase 4 block range. The 15 cases were selected from a much larger series of patients with intermittent BBB and entered the study only when the maneuvers were effective in uncovering these conduction ranges, or in separating a phase 3 block range from a normal conduction range. Two patients who apparently had permanent or rate-independent BBB, but who were known to have intermittent BBB a few weeks before, were also included. During the vagal stimulation, junctional and ventricular escapes were sometimes documented. After this control study, carotid sinus massage was repeated during and after the administration of an i.v. drip of isoproterenol, for which 1 mg of the drug was dissolved in 500 ml of isotonic glucose solution. The infusion was started at a rate of 1 ,ug/min of isoproterenol during 5-10 minutes; if no significant changes in conduction were observed, the rate of infusion was raised in successive steps of 1 gg/min during 5-10 minutes; the infusion was interrupted either when clear changes in conduction were seen or when changes did not occur after the infusion of 4 ,ug/min, or when ventricular arrhythmias developed at any time (the latter occurred only in one patient, who was then excluded from the study). When conduction changes were observed, the vagal stimulation was repeated after the infusion, until the recorded changes returned to the control condition. All the tracings were recorded on a Sanborn direct-writing two-channel machine and conventional measurements were made to determine the variations in cardiac rate and extension of the conduction ranges, as well as in the coupling interval of the escape beats. The statistical significance of the results was assessed using the paired t test. Thirteen patients were studied only 135...