358 PHYSIOLOGY OF INDUCED HYPOTHERA1IA 



Ion shifts in hypothermia, (lollaii found decreased K/Na ratio in hearts of 

 well-oxygenated hypotlierniic animals and in anoxic norniotherniic animals. 



Swan found positive myocardial K balances during coolrng. 



Rcnkin perfused skeletal muscle and did not lose K. 



Taylor perfused hypothermic heart and did not lose K. 



Hegnauer had varying results, notably, an uptake of Ca by cold hearts and loss 

 of K and H in fibrillators, and different electrolyte balances in non-fil)rillators. 



Other workers presented similarly contradictory results. 



All of the studies reported are open to many criticisms. In the first place, it is 

 not possible to accomplish any sort of electrolyte balance studies on tissues unless 

 careful studies of water shifts are also made (see Horvath and Spurr). This applies 

 to both intracellular and extracellular water. 



The bulk of available evidence shows that cooling of any tissue is associated with 

 a loss of intracellular K and uptake of Na. This change will be more marked in 

 aetive than in quiescent tissue. On the other hand, if perfusion of the immediate 

 extracellular space is poor, there will be two effects : 1 ) the local ion concentration 

 will change in such a direction that additional shifts of electrolyte across the mem- 

 brane will be minimized, and 2) the magnitude of transmeml)rane electrolyte shifts 

 will not be accurately mirrored by the concentration in the i)lasma or perfusion 

 medium. In addition, if there are significant water shifts, studies of perfusion fluid 

 concentration will be of still less meaning. 



These results are further complicated by two additional factors : ( 1 ) anoxia, and 

 (2) membrane potential. It is reasonably certain, in both myocardium and other 

 excitable tissues, that both anoxia and a decrease in resting potential alter the 

 permeability of the meml)rane to both Xa and K, so that Na is gained by the fiber 

 and K is lost. Furthermore, anoxia can result in a change in membrane potential. 



The results of Ca studies (Hegnauer, Lewis) seem reasonable. // the myocardial 

 fibers lose K, they will certainly become more sensitive to Ca, as has been shown 

 by many studies. 



The significance of pH and CO2 changes in hypothermia. The beneficial 

 effects of alkalosis due to hyperventilation (Hegnauer) are open to considerable 

 question (Lewis). Hegnauer states that too much alkalosis made fibrillation more 

 likely, and that institution of artificial ventilation after development of acidosis did 

 not prevent fibrillation. Lewis got his best results by preventing alkalosis with 10% 

 CO2, and others report the same findings. Hegnauer found that the blood pH had 

 no effect on the sensitivity of hypothermic hearts to calcium infusion, and that 

 coronary A-V potassium differences were not influenced by pH. Gollan states that 

 at 23-24° C. the heart can be completely deprived of circulation for periods up to 

 20-25 minutes without fibrillation, and under these conditions the tissue pC( )o 

 must be very high and pH low. 



All this makes one wonder if the various investigators have conducted studies of 

 sufficient breadth. Perhaps the necessary approach, at both 38° C and lower tem- 

 peratures, is to vary independently H ion concentration, pCOo, O... HCO;,, etc. This 

 has been done at normal temperature for certain hearts (Vaughn, Williams) and 

 results show that | H"^] and [CO2] have some quite different effects. 



Another line of work possibly indicated is to maintain some record of the CO2 



