Actions
The main pharmacological effects of digoxin are on the heart. Extracardiac effects are responsible for some of the therapeutic and many of the adverse effects (see above). It has mechanical effects as it increases myocardial contractility; however, the duration of the contractile response is just slightly increased. Overall, the heart rate is decreased, while blood pressure increases as the stroke volume is increased, leading to increased tissue perfusion. Myocardial efficiency is due to improved hemodynamics, and the ventricular function curve is improved.
Other, electrical effects are an initial brief increase in action potential, followed by a decrease as the K+ conductance increases due to an increased intracellular amounts of Ca2+ ions. The refractory period of the atria and ventricles is decreased, while it increases in the sinoatrial and AV nodes. A less negative resting membrane potential is made, leading to increased excitability. Other, more indirect effects are cholinomimetic because of vagal stimulation, giving rise to AV nodal delay.
The conduction velocity increases in the atria, but decreases in the AV node. The effect upon Purkinje fibers and ventricles is negligible. Automaticity is also increased, in the atria, AV node, Purkinje fibers and ventricles.
ECG changes are increased PR interval, due to decreased AV conduction, and a decreased QT interval because of the altered duration of decreased action potential. Also, the T wave is inverted, accompanied by ST depression. It may cause AV junctional rhythm and ectopic beats (bigeminy) resulting in ventricular tachycardia and fibrillation.
Slight vasodilation is seen in heart failure. This effect is contrary to effects that should be seen as a result of increased intracellular calcium levels, but this occurs since digoxin improves hemodynamics, which leads to restored angiotensin levels and decreased sympathetic discharge, causing indirect vasodilation.
Digoxin also affects the kidney by increased renal blood flow and increased glomerular filtration rate. A mild diuretic effect is seen only in heart failure.
Read more about this topic: Digoxin
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