MYOCARDIAL I RRITAP.IMTV— TH^GNAUER and COVINO 



331 



2.7 



2.5 



Diastolic 

 Threshold 

 (m.Amps.) 



2.0 



1.5 



1.0 - 







-V Mean pH 



38° 36° 34° 32° 30° 28° 26° 24° 22° 

 HEART TEMPERATURE (*C) 



20' 



Fig. 3. — Diastolic threshold vs. heart temperature in hypothermic dogs when pH is permitted 

 to drop below 12. Subgroups A and B as for figure 2. leashed line is maximum stimulus 

 intensity available. 



decreased the incidence of ventricular fibrillation in dogs in which the circulation 

 was temporarily arrested for experimental cardiac stirgery, and the subsequent 

 demonstration by Osborn'° that maintenance of a normal (or slightly supernormal) 

 pH similarly decreased the number of fibrillary deaths, offered a promising lead. 

 For if altered thresholds and fibrillation were intimately related then pH control 

 should prevent the occurrence of both. The investigations based upon this hypoth- 

 esis established that ventricular excitability too is greatly influenced by pH.® 



In the experiments discussed to this point the dogs respired spontaneously to 24° 

 to 26° and were artificially respired during further cooling. The artificial ventila- 

 tion rate was made to approximate the immediately prior spontaneous rate and was 

 not again varied. At 25° the mean systemic pH approximates 7.15, and this may rise 

 slightly after artificial respiration is started. Prevention of a pH rise may be ac- 

 complished by substituting a 3 per cent CO^-Ou mixture for room air. In addition 

 to pH stabilization the mixture with a high O-. content assures adequate arterial 



