The relationship between the repetitive extrasystole threshold and the ventricular fibrillation threshold in the dog. Non-parallel changes following pharmacological intervention.

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Abstract
The relationship between the repetitive (2 or more) extrasystole [RE] threshold (RET) and ventricular fibrillation [FV] threshold (VFT) in 38 pentobarbital anesthetized dogs. A 100 Hz train of 16 4 ms stimuli was delivered to the right ventricle during the T wave of every 12th paced supraventricular beat. Current was increased in 1 mA steps until ventricular fibrillation occurred. Five measurements were made in each dog at 15 min intervals. In 10.3% of the VFT determinations, VF was not preceded by a RE activity. For the remainder regression analysis of the correlation between RET and VFT revealed an r [correlation coefficient] = 0.37 (P < 0.001). The average RET was 54.6% of the VFT (5.4 .+-. 1.6 vs. 10.2 .+-. 2.4 mA); but it ranged from 5-98%. Five animals received a 90 min lidocaine infusion (0.3 mg/kg per min). Lidocaine significantly increased both RET and VFT as a linear function of log lidocaine plasma concentration (Cp), the mean slope of the VFT vs. log lidocaine Cp regression line was significantly different from that of RET vs. log lidocaine Cp regression line (P < 0.05). Nine dogs infused with the .beta.-blocker acebutolol (0.5 mg/kg in 5 and 2 mg/kg in 4 dogs) showed a concentration-dependent increase of VFT without significant increase of RET, and the mean slopes of the response vs. log Cp regression lines were significantly different (P < 0.01). In the control state the RET may be related to the VFT. During pharmacological studies, VFT and RET do not necessarily change in parallel.