CARDIOVASCULAR FUNCTIONS— D'AMATO 



149 



On the (ithcr hand, wlicn the heart is cooled to 23° C, to a heart rate of fifty heats 

 per minute, the total cycle also increases hy 300 per cent, hut the durations of hoth 

 systole and isometric relaxation increase hy 250 per cent. In other words, when the 

 heart is slowed hy hypothermia the resting period undergoes no great relative 

 increase in duration. 



This phenomenon is illustrated in figure 4. As heart temperature decreases, the 

 absolute time of systole and the absolute time of isometric relaxation increase. These 

 curves represent the averages obtained from a large series of dogs. The data are 

 from pressure contours which are "normal" for the given temperature. Those from 

 irregular cycles, ectopic beats, etc., are not included. 



In figure 5 the .same data are expressed as the per cent of the total cycle repre- 

 sented by the activity phase in relation to the pulse rate. At normal temperature, 

 there is a progressive decline in relative length of the activity phase throughout the 

 entire range of pulse rate. When the heart is slowed by cooling there is no great 

 change in the relative length of activity phase down to pulse rates of fifty to sixty 

 ])eats per minute, which occur at 23° C. With further cooling and further slowing of 

 the pulse rate, there is an abrupt decrease in the relative length of activity phase. At 

 a pulse rate of twenty beats per minute, at a temperature of about 18° C, the rela- 

 tive length of the activity phase of the hypothermic heart is still far greater than 

 that of the normothermic heart beating at a similar rate. Berne" has since corrobo- 

 rated these results. 



Coronary circulation. In view of these findings concerning the duration of 

 systole and isometric relaxation in hypothermia, two questions arise with regard to 

 the coronary circulation. Is the so-called "rest period" of sufficient length to insure 



O 



0.8- 



a7- 



Ofi- 

 05 



o 

 o 



UJ 



CO 0.4 



~ 0.3 



UJ 



1 0.2 



0.1 



OOL 



38 36 34 32 30 28 26 24 22 20 

 RIGHT HEART TEMPERATURE— ° C. 



.SYSTOLE 



ISOMETRIC 

 RELAXATION 



18 



16 



14 



Fig. 4. — Duration of systole and isometric relaxation in the hypothermic dog. 



