cardiotonic /kär′dē·ōton″ik/ [Gk, kardia + tonos, tone] , 1. pertaining to a substance that tends to increase the efficiency of contractions of the heart muscle. 2. a pharmacological agent that increases the force of myocardial contractions. Cardiac glycosides, derived from certain plant alkaloids, exert a tonic effect by altering the transport of electrolytes across the myocardial membrane, causing a decreased efflux of sodium and calcium and a decreased influx of potassium. Digitoxin and digoxin, widely used cardiac glycosides obtained from leaves of a species of foxglove, increase the force of myocardial contractions, extend the refractory period of the atrioventricular node, and, to a lesser degree, affect the sinoatrial node and the heart’s conduction system.
