CARDIOVASCULAR FUNCTIONS— D'AMATO 



147 



Figure 2 demonstrates the occurrence of cardioacceleratory reflexes in the early 

 stages of hypothermia. The first point on the curve represents the pulse rate prior 

 to immersion into an iced hath. l'[)on immersion, a marked increase in rate occurs, 

 apparently referal)]e t(» rellex s\ini)athetic discharge in response to the noxious 

 stimulus. At suhsecjuent points on the curve the observed pulse rate results from 

 the modification of tlic control rate by two opposing factors, reflex stimulation and 

 direct depression by the cold. As we shall see, this situation obtains in several other 

 cardiodynamic functions. 



The greater part of this curve represents the average of nineteen experiments. 

 The term "pulse rate" includes both sinus beats and effective beats of ectopic origin. 

 In three of the nineteen dogs, the hearts were observed to beat down to a heart 

 temperature of 14° C. In all three dogs, a slight increase in pulse rate occurred be- 

 tween 16° C. and 14° C. perhaps due to an increase in ectopic activity just before 

 terminus. 



In the same series of dogs, pressure in the femoral artery was simultaneously 

 measured. Apparently systemic arterial pressure is to some degree independent of 

 pulse rate down to blood temperatures of 24° C. to 23° C. However, at this point it 

 becomes completely dependent on pulse rate. The exact cause of the abrupt change 

 in the curve is questionable. It could represent vasomotor paralysis, or could result 

 from complete cessation of shivering which invariably occurred at or before these 

 temperatures. This curve is in essential agreement with those observed bv other 

 workers.^' ^' ■*' ^ 



Nature of bradycardia. Bradycardia induced by hypothermia differs from that 

 induced at normal temperature by stimulation of the vagus nerve.- Typical tracings 

 of left intraventricular pressure in the dog are shown in figure 3. One may, for the 

 sake of convenience, consider systole to be represented by that part of the pressure 



< 



UJ 

 CO 



_J 

 Z5 

 CL 



a; 

 CL 



CD 



2 00_ 



I 80 _ 



160 



140 



I 20 



I00_ 

 80 _ 

 60- 

 40_ 

 20_ 



From: Hegnauer, Schriber, and 

 Haterius 

 "v. Am. J. Physiol. 161 •• 455 J950 







\ 



c— o Blood Pressure 

 •-• Pulse Rote 



\ 



S>_c 



\ 



^o .0. 



\ 



*N.. 



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38 36 f4' 32 30 ^8 ^6 ^4 tz iO fS f6 h 

 RIGHT HEART BLOOD TEMPERATURE— C. 



Fig. 2. — Blood pressure and pulse rate in hypothermia. 



CD 



