338 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



B 



Control ECG 



20min, after start of infusion 



25 m in. after start 



Fig. 5. — Progressive ECG changes in normothermic dogs during continuous infusion of isotonic 



calcium chloride. 



The electrocardiographic changes are similar in the two gronps, but distinct from 

 those for calcium. There occurs a progressive decrease in heart rate followed by a 

 widening of the QRS complex, inversion and increase of the T-wave, and shortening 

 of the S-T interval to complete extinction. The preterminal ECG complex con- 

 sists of a wide upward QRS passing into a similar wide deflection below the base 

 line. The terminal event is diastolic asystole in both hypothermic and normothermic 

 dogs. Thus hypothermia appears not to alter the sensitivity of the heart to exog- 

 enous potassium. 



With respect to the ECG phenomena associated with calcium infusion, the so- 

 called injury current differs only in magnitude from that which develops in acidotic 

 hypothermic dogs. It is tempting to think that the two have a common origin, 

 related to a disturbed calcium balance of the myocardium. That both are associated 

 with augmented excitability appears established. 



The question regarding the role of temperature in the establishment of lower 

 ventricular thresholds, the deviation of the S-T segment, and the electrolyte balance 

 of the myocardium is not fully answered. Certainly a temperature factor is not 

 conspicuous in relation to these phenomena during progressive hypothermia, being 

 overshadowed by the factor of pH. Without additional evidence one might conclude 

 tluit temperature per se was without influence. The observations which suggest that 

 temperature may not yet be ruled out are: (a) Reversibility of the conditions lead- 

 ing to fil)rillati()n, and the S-T segment change, are not readily achieved by a simple 



