Shock Durations and Measurements of Cardiac Excitability

A turtle heart under vagus inhibition shows a complex of functional changes. Three of these changes will be referred to here. Consider the right vagus to be stimulated repeatedly so as to produce a constant condition of inhibition in the heart. Suppose that during this period of inhibition there is a slowing of heart rate, an increase of the time required for propagation of the cardiac impulse from sinus to atrium, and a decrease in the strength of the atrial beat. Suppose that now the sinus be stimulated by shocks such as to drive the heart at a rate faster than the above, slowed, spontaneous rate but slower than the normal rate. Unless partial or complete S-A block occurs, there will be a further increase in the S-A interval and the recorded beat of the atrium will be still further depressed. In other words, the slowed, vagus-inhibited heart appears as though the time required for recovery from a given contraction has been increased at least as much as has the interval intervening between beats. Nevertheless, by the use of induction shocks of from 2 to 5 times threshold strength, “the refractory period” of the dog's auricle is reported to be shortened markedly. 1 , 2

We have studied the threshold to electrical shocks of the turtle atrium in an attempt to discover if there exist measurable changes in electrical excitability during the later part of the recovery process. Induction shocks from a Porter coil and short condenser discharges were used. A shortened absolutely refractory period was always demonstrable when the tissue was under the effect of vagus inhibition. We then chose a fixed time interval (S-I) measured between the beginning of a normal systole (S) and the time of application of a shock to elicit an interpolated systole (I).