THE SAFETY FACTOR FOR CONDUCTION IN CARDIAC MUSCLE

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Abstract
A method is described for measuring the safety factor for transmission of excitation in mammalian cardiac muscle. Dog trabecular muscle fibers were used. Urethane and cocaine were applied to the perfused fibers and the progressive changes in threshold (T) and action potential (A) recorded. The ratio between these values in their normal (n) and depressed state (b) just as block occurs was used to determine the safety factor: [image] = Safety Factor. The values thus obtained in trabecular muscle were 1.0 to 3.5. When anelectrotonus and catelectrotonus were used vary thresholds and action potential amplitude, the factor of safety found was somewhat higher: 2 to 7. It is concluded that the factor of safety varies in the same tissue and that in certain anatomical situations it is much less than in large diameter parallel fibers.

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