a. The mechanisms of action of digitalis and related cardiac glycosides in the treatment of congestive heart failure are not fully understood. The main pharmacological property of these drugs is their ability to increase the force of myocardial contraction (the heart muscle's contraction) by a direct action on the ventricular heart muscles. Conduction is also slowed somehow between the SA node and the AV node, resulting in a decrease in heart rate. Because of the slower heart rate and increase in the force of the myocardial contraction, the heart has more time to adequately fill with venous blood. The secondary changes seen as a result of the first three mechanisms of action will be a decrease in heart size and a decrease in heart rate due to more efficient pumping of the heart. Because of the slower heart rate, cardiac glycosides are also used in the treatment of atrial flutter or atrial fibrillation.
b. Because of improved circulation to the kidneys, an increase in urinary output (diuresis) within 24 to 48 hours following administration of cardiac glycosides will also be seen. Digitalis toxicity is enhanced in patients who have low serum potassium (hypokalemia), so potassium supplements may be given based upon periodic blood test analysis.
c. Digitalis and related glycosides have very narrow therapeutic indices (the treatment dose is very close to the toxic dose) and many drug-drug interactions. The dose must also be adjusted in renal failure, which is common in CHF patients. For these reasons digitalis is reserved for acute symptomatic heart failure or in those patients with CHF and atrial fibrillation.
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