46 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



average tissue. Cardiac outputs do not indicate circulatory failure, and arterial 

 pressure is maintained in the absence of chemical anesthesia. 



Blood flows to specific organs need also to be considered. If flow should be 

 deficient in a critical area, the sufficient cardiac output would not avail. In anesthe- 

 tized hypothermic dogs the coronary venous blood was shown (Penrod, Berne) to 

 contain available oxygen. The cerebral blood flow was found (Rosomoff and Hola- 

 day) to be as adequate as in normothermia. 



jVIicroscopic observations of minute vessels in rat mesentery at 20° and in hamster 

 cheek pouch at 10° reveal that blood ceases to flow in at least half the capillaries and 

 venules without much visible constriction of those blood vessels (this laboratory). 

 These stoppages of flow reverse during rewarming. Therefore, general blood flow 

 in hypothermia diminishes partly l)y means of total cessation of flow through selected 

 capillaries and venules. Since the cessation is spotty for any given area of mesentery, 

 over-all oxygen supply may remain adequate. 



Oxygen transport in blood. When blood is exposed to low temperatures in vitro 

 the oxygen dissociation curve (fig. 3) moves far to the left (Brown and Hill, Pen- 

 rod). This means that oxygen easily coml)ines with the blood's hemoglobin, I)ut 

 dissociates only at unusually low tissue pressures of oxygen (pOo). Whether cold 

 tissues can operate when oxygen is supplied by the Ijlood at these low pressures 

 depends upon (a) how far the oxygen has to diffuse from blood capillaries to its 

 site, and (b) how low the pO^ is at which the oxidizing enzymes such as cytochrome 

 oxidase can hand it onward. The first factor is adequate since we find that enough 



1,00 



z 

 o 



I- 

 < 



«0 



(iJ 

 u 

 cr 

 o 

 u 



.80 



.60 



.40 



.20 



20 



!00 



120 



40 60 80 



MM. OXYGEN PR. 



Fig. 3. — Oxygen dis.sociatinn curves of hluod at (lit'i\i\iU tcnip(.Taturcs ; pC'O., 40 mm. Hg. 



