CARDIAC EXCITABILITY— BROOKS 



293 



ft ARP 



u 



RRP 



T 



T 



T 



T 



P 

 P 

 P' 



P' 

 P' 

 P' 

 P" 

 P' 



T 



P' 



50 100 



INTERVAL OF 



150 200 250 300 

 CYCLE-MSEC 



350 



Fig. 3. — Diagram of dog auricular electrogram showing boundaries of absolute and relative 

 refractory periods and the latency between application of testing stimuli (T) placed at various 

 periods of the cycle and the response (P^). (From Brooks et al., 1955.) 



critical phases of the refractory period hut at no other intervals of the cycle (fig. 

 2-B). It should be pointed out that threshold stimuli produce, during the refractory 

 or irresponsive period, an effect which ultimately gives rise to a single extrasystole. 

 Stronger stimuli placed at these specific intervals (the vulnerable periods) have an 

 effect which eventuates in fibrillation. The persisting excitatory processes in the 

 two cases may be different. 



Fibrillation as a response to excitation. Fibrillation can be considered one 

 type of response resulting from stimulation of the heart. Fibrillation thresholds 

 and the effects of lowering heart temperature on the tendency to fibrillate can be 

 determined by methods described. In hearts cooled to a critical temperature of 20°- 

 25° C. arrhythmias may develop spontaneously (Talbott, 1941; Alexander, 1946; 

 Hegnauer and Covino, 1955) but they are more likely to result if mechanical 

 (Hegnauer et al., 1951) or electrical (Pinkston et al., 1953) stimulation occurs. 



Fibrillation is a continuous disorganized activity of myocardial cells. A reasona- 

 ble assumption to make is that any influence which creates abnormally great dis- 

 similarities of cellular excitabilities, either throughout the entire heart or in a local 

 region, would favor establishment of fibrillation by favoring escape of certain cells 

 from dominance by the pacemaker or by permitting a saltatory type of conduction 

 over the heart. By saltatory conduction is meant stimulation of cells at some dis- 



