A fundamental understanding of cardiac electrophysiology allows the clinician to manipulate cardiac conduction in the clinical setting. The normal myocardium has the ability to generate, contract in response to, and propagate an action potential. Cardiac cells maintain an ion gradient with an overall negative intracellular charge. This transmembrane gradient allows for ion flow and the generation of the action potential, which is driving force for cardiac activity (Fig. 29-1). Cardiac cells maintain high intracellular potassium (140 mM) and low intracellular sodium (10 mM) concentrations as the major source of the transmembrane gradient. Low intra-cytoplasmic calcium levels (10−4 mM) also contribute to the resting equilibrium potential. These values have clinical implications, For example, a rise of the extracellular potassium concentration (hyperkalemia) decreases the cardiac transmembrane potential that, if severe, can ultimately lead to asystole. Conversely, bolus intravenous calcium administration augments the calcium component of the transmembrane gradient, and counteracts some of the negative electrophysiologic effects of hyperkalemia.
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