166 



PHYSIOLOGY OF INDUCED HYPOTHERAIIA 



Temp. 



30 



AOn 



30- 



ZO-" 



I 2 



Minutes 



blood entering L. Circumflex Art. 



Heart rate = 120 

 Aortic Pressure = 92/66 



Fig. 2. — Effect of changing temperature of blood perfusing the left circumflex coronary- 

 artery on coronary blood flow in the normothermic open chest dog. Perfusion pressure was 

 kept relatively constant by a pump-perfusion system. 



The heart rate was 120 beats per minute and the aortic pressure 92/66 millimeters of 

 mercury throughout the experiment. Perfusion pressure was kept relatively constant 

 during the experiment by means of a pump-perfusion system. Lowering the tem- 

 perature of the blood perfusing the left coronary artery produced an increase in flow 

 which returned to control levels when the blood temperature was increased to normal 

 values. 



Similar experiments were performed with the tibrillating dog heart and as can be 

 seen in figure 3 an inverse relationship exists between the temperature of the blood 

 reaching the left coronary and the rate of flow in this vessel. The increase in flow 

 that we observed with decrease in blood temperature did not occur immediately after 

 the temperature change. In fact, we missed it at first because we changed the tem- 

 perature too rapidly. However, if we waited one to two minutes after altering the 

 blood temperature, changes in flow would occur with great regularity. 



We do not know the mechanism by which this decrease in resistance in the coro- 

 nary bed is produced with cold but can only surmise, as did Anrep and his colleagues 

 a number of years ago, that it is a direct effect of cold on the vessel wall. 



In these experiments in which the left coronary arterial blood was cooled we meas- 

 ured the oxygen consumption of the myocardium and foiuid it to be essentially 

 constant. 



We made an interesting and rather puzzling observation on the ])lasnia j^otassium 

 levels across the heart during periods when the left coronary artery blood was 

 cooled. In each of these experiments the potassium concentration was lower in the 



