318 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



range present in the immediate environment of poorly perfused cardiac filjers dur- 

 ing hypothermia. 



Effects of potassium and calcium. The effect of increased extracellular po- 

 tassium concentration on the resting transmembrane potential of single fibers is 

 similar for nerve,'* skeletal muscle/'" and cardiac muscle.-" The primary effect of 

 high potassium is to decrease the magnitude of the resting potential-'' roughly in 

 proportion to the change in K concentration. A direct result of the lowered resting 

 potential is a decreased action potential amplitude and rising velocity (fig. 12). 

 This effect, in turn, results in slower conduction. If the depolarization resulting 

 from potassium is excessive, conduction fails and excitability is lost. These changes 

 are similar in auricle, ventricle, and specialized conducting fibers although the 

 Purkinje system appears to be most sensitive to high extracellular K.^" In all three 



0. 



100 msec 



100 mv 



200 msec 



— -100 mv 



Fig. 12. — Top: Tracing of transmembrane action potentials of a single ventricular fiber 

 showing control (solid line) and progressive decrease in resting and action potential produced 

 by high extracellular potassium (dotted lines). Bottom: Tracing of control record from single 

 Purkinje fiber (solid line) and effects of high (dotted line) and low (dashed line) extracellular 

 potassium concentrations. Note similar changes in resting potential in both cases; increased 

 pacemaker activitj^ only with low potassium. 



