Ventricular excitability and refractoriness in the hypothermic dog.

ANGELAKOS, E. T., E. G. LAFORET AND A. H. HEGNAWER. Ve&hdar excitability and rejractminess in the hypothermic dog. Am. J. Physiol. 189(j) : SOI-5959 1957* -Measurements of ventricular exctiability through the entire cardiac cycle of the dog under progressive hypothermia show that the ventricular refractory period (measured as absolute, total or functional refractory period) is greatly prolonged. This effect is not secondary to the changes in heart rate since normothermic animals with surgical A-V block having heart rates similar to those obtained under hypothermia do not show any great prolongation in the ventricular refractory period. At heart temperatures ranging from 38” to 23OC there is no significant alteration in the diastolic excitability of the ventricle. The rate of ,recovery of diastolic excitability and the response latency were greatly prolonged as reflected in the marked increase in the duration of the relative and functional refractory periods. It is suggested that the increased susceptibility of the hypothermic myocardium to ventricular fibrillation may be related to the observed changes in the rate of recovery of excitability. T HE FUNDAMENTAL ALTERATIONS leading to the marked tendency for the development of ventricular fibrillation (VF) under hypothermia are still obscure. A study of the excitability and refractoriness of the hypothermic myocardium may be expected to provide some insight into the mechanism of hypothermic VF. Following the report by Hegnauer and Covino (I) in which previously reported results regarding cardiac excitability under hypothermia were found to be complicated by technical artefacts, a re-evaluation of the problem became necessary. In the experiments reported here the excitability and refractory period of the dog heart was measured at various temperatures using valid methods which were free of technical artefacts. In addition an experimental study of the influence of heart rate on the parameters measured was made in normothermic and hypothermic animals. Received for publication January 16, 1957. 1 This work was supported in part by the United States Air Force under contract no. AF 33(616)-3805 monitored by Aero Medical Laboratory Directorate of Research, Wright Air Development Center, Wright Patterson Air Force Base, Ohio, and in part by a grant from the Massachusetts Heart Association, Inc. MATERIALS AND METHODS All experiments were made in dogs anesthetized with pentobarbital 30-35 mg/kg. Hypothermia was induced by immersion in iced water as previously described (2). Under artificial respiration left thoracotomy was performed at the level of the fourth intercostal space and a bipolar electrode was attached to the left ventricular epicardial surface overlying an area relatively free of major blood vessels. The electrode consisted of a plastic disc 2 cm in diameter containing two Ag-AgCl electrodes approximately 0.5 cm long and 2 mm wide with an interelectrode distance of 0.5 cm. The plastic disc was attached to the myocardium with 4-6 stitches of nylon suture. Subsequently the chest was closed, the pneumothorax was reduced, and the animals were allowed to respire spontaneously for 15-30 minutes before any measurements were made. The stimulation apparatus consisted of two Grass square wave stimulators connected in parallel. One of the stimulators delivered the driving pulses and was synchronized with the second stimulator which delivered the testing pulses. An adjustable delay on the second stimulator determined the time interval between a driving and a testing pulse. Thus the strength and duration of the driving and testing pulses could be varied independently. A specially constructed switch was interposed in the synchronizing circuit between the driving and testing stimulators. When this switch was closed a single pulse from the synchronizer of the driving stimulator fired the testing stimulator; thereafter the circuit remained open until the switch was manually reset. Thus single testing stimuli could be delivered at will during any part of the cardiac cycle. Both driving and testing square wave pulses were of I msec. duration. Since Grass stimulators are so conby 10.2.32.247 on July 1, 2017 http://ajple.physiology.org/ D ow nladed fom 592 E. T. ANGELAKOS, E. G. LAFORET AND A. H. HEGNAUER strutted that the cathode is at ground potential, the only ground connection to the dog was through the cathode via a Ioo-ohm resistor. The voltage drop across this resistor was determined with an oscilloscope and served as a measure of the current passing between the stimulating electrodes. The effective resistance of the circuit was of the order of moo-2000 ohms in most experiments. The electrocardiogram was monitored on a DuMont oscilloscope with a long persistence (P7) screen and was also recorded on a Sanborn Visocardiette, In the majority of the experiments reported here both driving and testing stimuli were applied to the ventricle through the same pair of electrodes. In a few experiments (see table x) the ventricular diastolic threshold was determined while driving stimuli were delivered to the left atrium through a bipolar electrode attached to the auricle. In this case testing stimuli were delivered through a separate pair of electrodes attached to the left ventricle, and threshold current was measured in the ventricular circuit. In all cases measurements were made while the heart was driven at a constant rate slightly faster than its spontaneous rate. Thus it was not necessary to crush or otherwise inactivate the S-A node. Temperature measurements were made thermoelectrically through a catheter thermocouple placed into the right atrium via the right jugular vein. Surgical interruption of the A-V bundle was performed following the method described by Starzl and Gaertner (3). Following right auriculotomy the bundle