148 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



CYCLIC RELATIONS OF TRACINGS OF LEFT 

 VENTRICULAR PRESSURE CURVES 



Heart Rate 

 170/min. 



R.T-38.5*'C 



Heart Rate 

 50/mln. 



RI-38.5*C 



Heart Rate 

 50/min. 



R.T-2I.5*'C 



0.4 08 1.2 1.6 20 

 Seconds 



Fig. 3. — Duration of systole, isometric relaxation and total cardiac cycle at normal heart 



temperature and at 23° C. 



contour from the point at which the curve leaves the baseline to the point at which 

 the abrupt fall toward baseline begins. One may further consider isometric relaxa- 

 tion to endure from this abrupt fall in the contour to the point at which the curve 

 once again reaches the baseline. The two together constitute the "activity phase" 

 of the cardiac cycle. The remainder of the cycle represents the "resting phase." 



It will be noted that with a heart rate of 170 beats per minute at 2)7° C, systole 

 persists for a little more than fifty per cent of the total cycle. When the same heart 

 at the same temperature is slowed to fifty beats per minute by stimulation of the 

 vagus nerve, the total cardiac cycle length increases by over 300 per cent but the 

 duration of systole increases only 50 per cent. Isometric relaxation, meanwhile, does 

 not change appreciably. When the heart at normal temperature is slowed, the length 

 of the so-called resting period profits. 



