MYOCARDIAL IRRITABILITY— HEGNAUER and COVINO 333 



Once a critical set of conditions has been established by low pH and temperature 

 it does not appear readily reversible by simple restoration of a normal pH. In a 

 series of 10 acidotic hypothermic dogs (mean pH at 25° = 7.16) a normal pH was 

 re-established in 5 to 10 minutes by instituting artificial hyperventilation at the 25° 

 level. During further cooling 7 of the 10 succumbed to fibrillation within 30 to 90 

 minutes after the start of augmented respiration, i.e., between 23° and 18°. The 

 only beneficial effect of pH restoration thus appears to be a slight lowering of the 

 temperature at which fibrillation occurs. Miller et o/.^^ found that when acidosis 

 was induced in normothermic anesthetized dogs by COo inhalation, a sudden shift 

 back to normal pH was usually followed by ventricular fibrillation within 2 to 10 

 minutes. 



Hypothermia and pH on the ECG. Bigelow% Lindsey, and Greenwood^- were 

 the first to note a feature of the ECG which is common in simple hypothermia. 

 When present it is typified by a wave rising steeply from the S-wave, producing an 

 elevation of the early portion of the S-T segment. Osborn^° later showed that the 

 S-T elevation (termed "injury current" by him) was present in all but one of the 

 dogs in his experimental series which later succumbed to ventricular fibrillation, 

 and only rarely in dogs which cooled to low temperatures and terminal asystole. 

 Support for the view that it is a bad prognostic sign is an additional analysis of 

 ECG records of 49 hypothermic dogs cooled to terminus.^ The "injury current" 

 was present in 28 of the 36 fibrillators (78%) and in 6 of the 13 non-fibrillators. 

 Examination of the few extant ECG records of human victims of accidental 

 hypothermia reveals the presence of the abnormal wave in each.^'^' ^*' ^^ One of 

 these^^ survived a rectal temperature of 18° C. 



The term "injui*y" current or potential is perhaps a misnomer when applied in 

 hypothermia. In the first place it is not associated in these experiments with either 

 gross or microscopic injury. It is readily reversible by re warming, as seen in one 

 published record^*' and in many unpublished. According to Osborn^° it is reversible 

 at low temperature by raising blood pH to a normal value. In the experiments 

 described earlier in which pH reversal was carried out in 10 dogs at 25°, the seven 

 fibrillators all had S-T segment elevations which, however, persisted to terminus. 

 On the other hand, Altschule and Sulzbach^^ made note of the presence of a 

 similar wave in their unanesthetized human subjects breathing high concentrations 

 of COo, and which disappeared on resumption of room-air breathing. It is not clear, 

 therefore, whether the simple factor of temperature or the time that the wave is 

 permitted to persist determines the ECG reversibility. And in all probability the 

 same applies to the processes underlying excitability change and fibrillation as noted 

 above. 



Arterial and coronary venous electrolyte levels and differences. That 

 alterations in blood electrolyte levels might in some manner be influential in deter- 

 mining the course of events in hypothermic animals has been considered by a num- 

 l)er of investigators. h^Jliott and Crisman^^ measured both potassium and calcium 

 levels in the l)lood of anesthetized hypothermic rats, and noted a rise in concentra- 

 tion of both ions at low body temperatures. Bigelow^- too found an increase in the 

 serum potassium in hypothermic acidotic dogs. Fleming^" found no important de- 

 viations from normal with respect to potassium, calcium, sodium, or magnesium. 



