What is the positive inotropic effect of cardiac glycosides?
Not until the 1920s was it understood that cardiac glycosides increase the force of contraction of cardiac muscle — positive inotropic effect — resulting in a shift upward and to the left of the ventricular function curve.
The effect in the intact heart is illustrated by the two pressure-volume loops shown here, in which an increase in contractility due to digoxin results in a shift upward and to the left of the pressure-volume relation at the end of systole (A’-B’). When stroke volume remains constant, as shown here, or increases, as is often the case clinically in patients with heart failure, the end-diastolic pressure (EDP) and volume (EDV) also decline. (A-B).
This results in decreased congestive symptoms and increased cardiac output. The resulting decrease in ventricular chamber dimensions also reduces wall tension, a major determinant in myocardial oxygen consumption. Therefore, although the initial increase in myocardial contractility necessarily results in an increase in cardiac muscle cell adenosine triphosphate and oxygen consumption, the final net effect of the drug may be to lower cardiac oxygen consumption.
Source: Colucci WS, ed, Braunwald E, series ed. Atlas of Heart Failure: Cardiac Function and Dysfunction, 2nd ed. Philadelphia: Current Medicine Inc.; 1999. Used with permission.
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