10.9: Heart Failure Drugs: Inotropic Agents
Positive inotropic agents are commonly used as the first line of treatment for heart failure. One such agent is digoxin, derived from the genus Digitalis, which has been known for centuries but effectively utilized since 1785. However, these cardiac glycosides can have potentially toxic effects due to their mechanism of action, which involves inhibiting Na+/K+-ATPase and increasing contractility. Digoxin is absorbed orally and distributed in various tissues, including the CNS. It has a long half-life and is not extensively metabolized, allowing for dosage adjustments in patients with renal impairment. Apart from its cardiac effects, digoxin may also have additional functions in organs such as the immune system, cell growth, differentiation, apoptosis, and carbohydrate metabolism. Cardiac glycosides exert positive inotropic effects by increasing calcium concentrations within cardiac cells and inhibiting Na+/K+-ATPase. However, they can also have electrical effects, such as altering action potentials and potentially leading to arrhythmias. Autonomically, they have parasympathomimetic effects and increase sympathetic outflow. Toxic manifestations of cardiac glycosides can occur in extracardiac tissues, including the gastrointestinal tract, central nervous system, and other excitable tissues. Potassium levels play a crucial role in their interactions, as low levels can inhibit the enzyme-inhibiting actions of cardiac glycosides and increase cardiac automaticity. Hypercalcemia increases the risk of digitalis-induced arrhythmias, while magnesium has the opposite effect.
In addition to digoxin, other positive inotropic agents treat heart failure. Bipyridines like milrinone beta-adrenoceptor agonists like dobutamine, istaroxime, levosimendan, and omecamtiv mecarbil all have different mechanisms of action and therapeutic effects. However, their use is carefully considered due to their potential adverse effects and narrow therapeutic windows. Careful monitoring of serum electrolytes is essential to minimize the risk of arrhythmias. In general, treatment for CG toxicity involves stopping the medication. Severe arrhythmias may need active treatments such as atropine or a temporary pacemaker for slow heart rate or potassium infusion for fast ventricular arrhythmias or hypokalemia. The antidote is anti-digoxin immunotherapy.
