Single Fiber Recording of the Ventricular Response to Induced Hypothermia in the Anesthetized Dog

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Abstract
Ventricular membrane Potentials were recorded by the microelectrode technic in anesthetized, open-chest dogs. Concurrently, intraventricular pressure, electrocardiogram and esophageal temperature were recorded. Hypothermia was induced by an extracorporeal cooling method. Data on the parameters observed are presented for a temperature range from 38 to 22 C. The mean temperature for spontaneous ventricular fibrillation was approximately 22 C. Hypothermia increased single fiber action potential duration, primarily by prolonging the plateau phase. Depolarization and terminal repolarization were less affected. Action potential amplitude and resting potential were not changed. Change in action potential duration as a function of varying heart rate became more pronounced with cooling. A characteristic change in action potential shape during early repolarization became evident as hypothermia progressed. Hypothermic changes in the single fiber action potential are compared with changes in the electrocardiogram and the pressure curve. Recordings made from single fibers during hypothermic ventricular fibrillation showed a marked shortening in action potential duration.

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