Myocardial conduction time and antiarrhythmic drugs

Complete a-v block was induced in anesthetized mongrel dogs by direct electrocoagulation of the a-v node. The ventricles were paced by steady stimulation (S1) at a rate of 100/min. and by test stimuli (S2) with varying post S1 delay. Right ventricular myocardial tension was measured from the S2 stimulation site of a specially designed miniature strain gage and from a different site by a Walton-Brodie strain gage. A reproducible time lag between the two sites could be measured by comparing the differences in mechanical response to S2 stimuli. This time difference was called delta IT. delta IT varied markedly (from 10-60 msec) when measured at different sites but no linear relationship between delta IT and the inter-gage distance could be observed. Increasing the S2 current intensity induced shortening of delta IT from 37 +/- 13 msec (mean +/- S.D.) at the threshold current to 16 +/- 10 msec (mean +/- S.D.) with 10 mA. A strength-delta IT curve could be constructed and was found to be remarkably reproducible during the experiment. Quinidine and disopyramide induced upward displacement of the curve, lidocaine did not change it while verapamil lowered the delta IT values. We suggest that delta IT can be used as a reliable indicator of myocardial conduction rate. The possible reasoning for this suggestion has been discussed.