EFFECTS OF PACING RATE AND TIMING OF DEFIBRILLATION SHOCK ON THE RELATION BETWEEN THE DEFIBRILLATION THRESHOLD AND THE UPPER LIMIT OF VULNERABILITY IN OPEN CHEST DOGS

To test the relation between the defibrillation threshold and the upper limit of vulnerability, the shock strength associated with 50% probability of successful defibrillation (DFT50) and that associated with 50% probability of reaching the upper limit of vulnerability (ULV50) were determined in 20 open chest dogs with use of the delayed up-down method, with pacing drive cycle lengths of 150 to 500 ms and either single 6-ms shocks (10 dogs) or 12-ms biphasic shocks (10 dogs) given at the mid-upslope, peak and mid-downslope of the T wave of electrocardiographic lead II. The shocks were given by means of a patch-patch configuration on the anterior and posterior surfaces of the heart, which was paced from a stimulating electrode attached to the left ventricular apex. Analysis of variance showed no statistically significant differences in ULV50 as determined with different pacing cycle lengths. For monophasic shocks, DFT50 (331 +/- 66 V or 5.8 +/- 2.7 J) was not significantly different from ULV50 determined at the mid-upslope of the T wave (318 +/- 64 V or 5 +/- 2 J). The correlation coefficients between the two values were 0.74 (p = 0.014) for voltage and 0.67 (p = 0.034) for energy. In contrast, DFT50 was significantly higher than ULV50 as determined at the peak of the T wave (219 +/- 43 V or 2.3 +/- 1 J) and mid-downslope of the T wave (200 +/- 38 V or 1.9 +/- 0.9 J). In three dogs, ventricular fibrillation could not be induced at the mid-downslope of the T wave with any baseline pacing (S1) cycle length. For biphasic shocks, DFT50 (285 +/- 53 V or 4 +/- 1.5 J) was not significantly different from ULV50 as determined at the mid-upslope of the T wave (293 +/- 54 V or 3.9 +/- 1.2 J). The correlation coefficients between the two values were 0.90 (p < 0.001) for voltage and 0.82 (p = 0.004) for energy. In contrast, DFT50 Was significantly higher than ULV50 determined at the peak of the T wave (208 +/- 31 V or 1.9 +/- 0.5 J) and mid-downslope of the T wave (180 +/- 16 V or 1.7 +/- 0.8 J). In six dogs, ventricular fibrillation could not be induced at the mid-downslope of the T wave with any S1 cycle length. It is concluded that with either monophasic or biphasic shocks, DFT50 can be accurately estimated by ULV50 as determined at the mid-upslope of the T wave. Altering the baseline pacing cycle length does not affect this relation.