MYOCARDIAL IRRITABILITY— HEGNAUER and COVINO 



329 



time diastolic thresholds were measured at 20 minute intervals. The results are 

 given in figure 1. Immediately after chest closure and resumption of spontaneous 

 respiration thresholds are found to vary over a considerahle range (2.1 to >2.73 

 ma.) in six of the dogs. After 1.25 hours, however, the thresholds of all have risen 

 to a value greater than 2.73 nia. and (with one exception) remain there. The 

 exception (curve A, fig. 1) remains unexplained. The data for curve B are from a 

 dog which possessed ventricular arrhythmias prior to, as well as after, thoracotomy, 

 and may therefore legitimately he ruled out as a normal control animal. A similar 

 series of control studies under thiopental anesthesia yielded practically identical 

 results. Thus after 1.0 hour all thresholds were greater than 2.73 ma. and remained 

 so for the duration of the observations. 



It appears, therefore, that time after completion of required surgery tends to 

 raise the diastolic thresholds to higher values than obtained initially. 



B. Hypothermia. Typical of the eflfects wrought by hypothermia are those rep- 

 resented by the data plotted in figure 2. The 15 dogs of this series were subjected 

 to the same experimental procedures as those of the normothermic controls except 



HYPOTHERMIA (PENTOBARBITAL) 



o 



X 

 CO 



u 

 q: 



X 



36 34 32 30 28 26 24 22 20 

 HEART TEMP. (^0) 



Fig. 2. — Ventricular thresholds (with standard deviations) of hypothermic dogs under pento- 

 barbital anesthesia. See text for basis of separation into subgroups A and B. 



