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PHYSIOLOGY OF INDUCED HYPOTHERMIA 



respect to outside) and a subsequent slower phase of repolarization which restores 

 the normal resting potential (fig. 1). This sequence of changes is called the (trans- 

 membrane) action potential and differs in certain respects between fibers obtained 

 from various parts of the mammalian heart. 



Normal records. Typical records obtained from isolated preparations of the 

 dog auricle, papillary muscle and Purkinje system are shown in figure 2. Certain 

 differences between the transmembrane potentials of these three tissues are im- 

 mediately apparent. The resting potentials of both auricle and ventricle, amounting 

 to 85-90 mv,^*' are somewhat smaller than that of the dog Purkinje fiber (90 mv).^^ 

 Similarly, the magnitude of the reversal or overshoot is less in the unspecialized 

 fibers (15 mv) than in the specialized conducting system (30 mv). Most striking, 

 however, are the differences in the time-course of repolarization revealed by the 

 three types of fiber. In the case of the auricle, repolarization commences im- 

 mediately after the upstroke of the action potential and proceeds with a relatively 

 constant velocity to completion. Action potentials recorded from single ventricular 

 fibers, on the other hand, reveal an initial spike (lasting 10-15 msec.) followed by 

 a plateau during which the membrane potential remains close to zero. The plateau, 

 in turn, terminates in an abrupt phase of repolarization. In terms of total duration, 

 the action potential of the ventricular fiber is only 50 msec, longer than that of auric- 

 ular muscle. 



Fig. 2.— Transmembrane action potentials of single myocardial fibers. (A) Auricular action 

 potential —time marks on upper trace in 10 and 50 msec; —100 mv. calibration shown just 

 below upstroke of action potential. (P.) Purkinje fil)er action potential, —100 mv. calibration 

 shown as in A. (C) Simultaneous records of transmembrane action potential (top trace) and 

 unipolar electrogram (bottom trace) of ventricular muscle. (D) Ventricular action potential 

 from same heart as B. Action potential upstroke rctouclicd in all cases. 



