Diltiazem does not increase ventricular fibrillation threshold during hypothermia

This study was designed to investigate whether the calcium channel blocker diltiazem affects the threshold for ventricular fibrillation during hypothermia in dogs. Ten dogs were cooled from 37 to 25 degrees C and rewarmed to 37 degrees C. The threshold for ventricular fibrillation was determined at body temperatures 37, 34, 31, 28 and 25 degrees C by programmed electrical stimulation using a stimulation protocol which involved application of maximal five extrastimuli. At 25 degrees C, six dogs were given an i.v. bolus dose of 100 micrograms.kg-1 followed by a continuous infusion of 100 micrograms.kg-1.h-1 of diltiazem hydrochloride. The other four dogs, were given no drugs at 25 degrees C and served as a control group. The dogs were rewarmed, and the stimulus protocol was performed at the same temperatures as during cooling. Cooling from 37 to 25 degrees C reduced the threshold for ventricular fibrillation in both groups. Heart rate were reduced, monophasic action potential duration at the apex and base of the heart increased from 167 +/- 5 ms to 469 +/- 17 ms and from 164 +/- 5 ms to 466 +/- 17 ms, respectively, when the temperature was reduced. The ventricular effective refractory period increased from 176 +/- 9 ms at 37 degrees C to 472 +/- 15 ms at 25 degrees C. Cooling increased QRS time on the ECG from 55 +/- 4 ms to 138 +/- 13 ms. Addition of diltiazem at 25 degrees C did not affect the threshold for ventricular fibrillation during rewarming. Further, diltiazem at 25 degrees C did not affect the heart rate or refractoriness.(ABSTRACT TRUNCATED AT 250 WORDS)