168 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



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Fig. 4.— Effect of hypothermia on ventricular (upper curve), aortic (middle curve with 

 incisura), and left atrial (lower curve) pressure pulses in the same dog. A, control heart blood 

 temperature 40° C. ; B, 33.5° C; C, 33.5° C. with peripheral right vagus stimulation; D, 

 27.5° C; E, 26° C; F, 23° C; G, 22° C. ; H, 20.5° C. Ventricular and aortic pressures in 

 millimeters Hg. Left atrial pressure in millimeters saline. Time scale records A through F, 

 0.02 second; records G and H, 0.20 second. 



much an impedance to inflow in the coronary bed is caused by the slow ventricular 

 relaxation and how much by the prolonged systole. 



In figure 5 we have records of an aortic pressure curve with a phasic coronary 

 flow curve beneath. These records were obtained from the same dog at 36° C. 

 (No. I) and 23° C. (No. II). In normothermia we observe a sharp decrease in flow 

 with some backflow in early systole followed by a slight rise in late systole. In early 

 diastole there is a sharp increase in flow followed by a decrease that is associated 

 with the fall in aortic pressure. In hypothermia there is also a slight reversal of 

 flow during systole followed by a slight rise and fall. The latter corresponds to the 

 dip in pressure caused by the standing wave. However, in diastole there is a gradual 

 and continuous increase in coronary blood flow during ventricular relaxation 

 (line B-C). It is only shortly l)ef()re the next systole that flow follows the aortic 

 pressure curve. It appears that this rising coronary flow in the face of a falling 

 aortic pressure is due to the gradual reduction in extravascular compression as the 



