Heart muscle cell. Coloured scanning electron micrograph (SEM) of a heart muscle cell (cardiac myocyte). Cardiac muscle is a specialised type of striated muscle found in the heart. It is the constant contraction of cells like this that pump blood around the body with each beat of the heart. This cell has been grown in a cell culture. Magnification: x600 when printed at 10 centimetres wide.

Why does Digoxin toxicity result in increased automaticity?

Hey everyone!

Digitalis and other cardiac glycosides are known to cause an AV nodal delay.

Then why does too much Digoxin result in some arrhythmias that are due to increased automaticity?

Brady arrhythmias are explainable. But why tachy arrhythmias?

You see, cardiac glycosides reversibly inhibit the sodium-potassium-ATPase, causing an increase in intracellular sodium and a decrease in intracellular potassium. The increase in intracellular sodium prevents the sodium-calcium antiporter from expelling calcium from the myocyte, which increases intracellular calcium. The net increase in intracellular calcium augments inotropy.

Excessive intracellular calcium may cause delayed after-depolarizations, which may in turn lead to premature contractions and trigger arrhythmias. Cardiac glycosides shorten repolarization of the atria and ventricles, decreasing the refractory period of the myocardium, thereby increasing automaticity and the risk for arrhythmias.

That’s all!