[Values of the effective anterograde refractory period of the bundle of His]

The effective refractory period of the His bundle (ERP.H), the longest H1, H2 interval not followed by a V2 ventricular complex, was measured in 45 cases during the electrophysiological investigation of 500 Patients by premature atrial stimulation techniques. The patients were divided into two groups according to the result: Group I: 22 patients with syncope, a spontaneous HV interval greater than 60 ms, greater than 100 ms after Ajmaline or infrahisian block with atrial pacing at less than 150 bpm. Group II: 23 patients without these abnormalities. The ERP.H was significantly different (p less than 0,001) in the two groups with valves greater than 400 ms in Group I and less than 400 ms in Group II. It is suggested that in the absence of other electrophysiological abnormalities an ERP.H of 400 ms or over may be an indication for permanent pacing in patients with Stokes-Adams attacks. The finding of an ERP.H of over 400 ms is associated with severe infrahisian block. However, the ERP.H depends on the ERP of the AV node which must be shorter to calculate the refractory periods of the His bundle, and, above all, on the basal sinus cycle. The ERP.H decreases with shorter sinus cycles and cannot be calculated when the sinus cycle is less than 600 ms. Conversely, the critical value of 400 ms is not valid for cycles longer than 1000 ms as cycles of that length are associated with lengthening of the ERP.H. The regression of infrahisian block during programmed atrial pacing after Atropine does not seem to be a reliable method of distinguishing between physiological and pathological atrioventricular block.