Control of Fibrillation in the Mammalian Heart. 319 



Any influence which depresses excitability without depressing — at least 

 proportionately — the function of conduction naturally tends to be in some 

 measure protective against the occurrence of fibrillation and favourable to 

 recovery from that condition when once it has been estabhshed. A diminution 

 of excitability opposes the attainment of acceleration sufficient to determine 

 fibrillation ; it also diminishes the responsiveness of the muscular fasciculi to 

 circulating excitations. (The control of auricular fibrillation which differs in 

 some respects from that of ventricular fibrillation will be dealt with elsewhere.) 

 Similarly any agency which improves conductivity without unduly exalting 

 excitability is inimical to the mechanism of circulating excitation. Obviously 

 a combination of a depressing influence on excitability with the maintenance 

 of a high level of conductivity would afford the most favourable condition for 

 protection or recovery. Concurrent depressions or elevations of excitability 

 and conductivity in proportionate degree naturally have no specific influence 

 on the question of fibrillation. The agencies which operate successfully in 

 opposing the development of fibrillation — either spontaneous {i.e. from 

 unknown causes) or excited artificially by drugs, electrical stimulation, etc. — 

 are often effective in restoring the normal action after fibrillation has been 

 established. Eemedies for fibrillation have commonly, in these experiments, 

 been injected into the cavity of the left ventricle through the apex by means 

 of a slender needle ; sometimes intravenous injection (external jugular, etc.) 

 was used, massage of the heart being done in both cases, while the artificial 

 respiration is of course maintained. Smaller doses were sufficient by the 

 intra-ventricular mode of injection. Approximately isotonic solutions were 

 used, warmed to body temperature. The doses stated are for cats, usi;ally 

 weighing 2-3 kilos, but sometimes more. 



Uretlmne. — Doses varying between 0*025 and 0-25 grm. injected into the 

 left ventricle were found effective in removing fibrillation in very numerous 

 experiments (fig. 10) ; 3 per cent, solutions were commonly used for 



Fio. 10. — The middle portion of the tracing .shows fibrillation caused by strong 

 faradisation (5000 units). After it had lasted for 2 minutes (with occasional 

 massage) 0"05 grm. urethane was injected into the L.V. The restored action is seen 

 in the right-hand portion. 



