The resting potential on ventricular myocardium is about −84 mV at an extracellular K+ concentration of 5.4 mmol/L. Raising the K+ concentration to 16.2 mmol/L raises the resting potential to −60 mV, a level at which muscle fibers are inexcitable to ordinary stimuli. When the resting potential approaches −50 mV, sodium channels are inactivated, resulting in a diastolic arrest of cardiac activity.Hensley F, Martin D. A Practical Approach to Cardiac Anesthesia. 2nd Edition. Little, Brown and Company. 1995 Membrane inactivation gates, or h Na+ gates, are voltage dependent. The less negative the membrane voltage, the more h gates that tend to close. If partial depolarization is produced by a gradual process such as elevating the level of extracellular K+, then the gates have ample time to close and thereby inactivate some of the Na+ channels. When the cell is partially depolarized, many of the Na+ channels are already inactivated, and only a fraction of these channels is available to conduct the inward Na+ current during phase 0 depolarization.Berne R, Levy M. Physiology. 3rd Edition. Mosby St. Louis 1993.
