328 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



A variable delay circuit in the stimulator made possible the application of test 

 stimuli at any point in the ventricular cycle, but such stimuli were delivered only 

 when a manually operated switch on the testing stimulator was closed. The test 

 stimuli were of constant 15 m. sec. duration and of variable voltage from to 150. 

 A 50,000 ohm resistor in series with the test electrode minimized the importance of 

 possible tissue resistance change in the course of an experiment for purposes of 

 computing current strength. Maximum stimulus intensities were thereby reduced 

 to 2.73 ma. Diastolic excitability was measured at that point in the cycle repre- 

 sented by the descending limb of the electrocardiographic T-wave. Threshold was 

 taken as the least intensity required to elicit a ventricular ectopic response. 



A. Controls {normothennia ) . To cool a dog to terminus by immersion in iced 

 water requires 2 to 3 hours. Thus both time and temperature may be factors affect- 

 ing ventricular excitability. To test whether fluctuations in diastolic thresholds 

 might occur independently of temperature change, seven dogs under pentobarbital 

 anesthesia were maintained at near normal temperature for 2.6 hours during which 



CONTROLS (PENTOBARBITAL) 



2.9- 

 q: 27- 



z 2.3h 

 Q 2.1 

 1.9- 

 1.7- 

 1.5- 



o 



X 

 C/) 

 UJ 



cr 



X 



.3- 



A 



^ 



B 



20 40 60 80 100 120 140 160 

 TIME IN MINUTES 



Fig. 1. — Ventricular thresholds vs. time under pentobarbital anesthesia. Solid vertical lines 

 represent measurable range. Dashed vertical lines are thresholds beyond intensity range of 

 apparatus employed. Curves A and B described in text. 



