Cardiac Glycosides

Cardiac glycosides are plant-derived compounds — digoxin is the prototype — that increase the force of cardiac contraction by inhibiting the sodium-potassium ATPase. Among the oldest cardiovascular drugs, they occupy a distinctive place because they raise contractility without raising cyclic AMP, and they also slow conduction through the atrioventricular node by enhancing vagal tone.

Definition

Cardiac glycosides are steroidal compounds bearing one or more sugar (glycoside) groups that inhibit the membrane sodium-potassium ATPase, indirectly increasing intracellular calcium and so the contractile force of the myocardium, while also exerting vagal effects that slow atrioventricular conduction.

Scope

This topic covers the pharmacology of the cardiac glycoside class: their chemical nature as steroidal glycosides, the sodium-potassium ATPase mechanism, the resulting inotropic and electrophysiological effects, and the narrow margin between effect and toxicity that defines the class. It is a reference pharmacology entry and provides no dosing or individualized treatment guidance.

Core questions

• How does inhibition of the sodium-potassium ATPase translate into increased contractile force?
• Why do cardiac glycosides have such a narrow therapeutic index, and what makes their toxicity characteristic?
• What is the basis for their dual action — positive inotropy combined with slowing of atrioventricular conduction?

Key concepts

• Sodium-potassium ATPase inhibition
• Sodium-calcium exchanger and raised intracellular calcium
• Positive inotropy without raised cyclic AMP
• Vagal enhancement and slowed atrioventricular conduction
• Narrow therapeutic index and glycoside toxicity
• Steroidal glycoside (cardenolide) structure

Mechanisms

Cardiac glycosides bind and inhibit the sarcolemmal sodium-potassium ATPase. With the pump inhibited, intracellular sodium rises, which reduces the gradient driving the sodium-calcium exchanger; the exchanger then extrudes less calcium, so intracellular calcium accumulates and is taken up by the sarcoplasmic reticulum. The larger calcium store released with each beat increases contractile force — a positive inotropic effect achieved without elevating cyclic AMP. Separately, cardiac glycosides increase vagal (parasympathetic) tone and slow conduction through the atrioventricular node, which underlies their rate-controlling action in some atrial arrhythmias. Because the same pump inhibition that produces inotropy can, when excessive, destabilize cellular ion gradients and provoke arrhythmia, the margin between therapeutic and toxic effect is narrow, and toxicity is a defining feature of the class.

Clinical relevance

Cardiac glycosides are a classic teaching example of an ion-pump-based inotrope and of a drug with a narrow therapeutic index, and the digoxin mortality trial is a standard reference in heart-failure evidence appraisal. This entry explains the mechanism and the shape of the evidence; it is not guidance for individual diagnosis or treatment and contains no dosing information.

Evidence & guidelines

The Digitalis Investigation Group trial (1997) found that digoxin reduced hospitalization for heart failure but did not reduce overall mortality, a result that shaped its subsequent positioning. The 2021 ESC heart-failure guidelines reflect this limited, selective role. Both sources are cited here for orientation, not as recommendations.

History

William Withering's 1785 monograph on the foxglove (Digitalis purpurea) for dropsy is the founding text of cardiac-glycoside pharmacology and one of the earliest systematic drug evaluations, documenting both benefit and toxicity. Digitalis preparations remained central to cardiovascular therapeutics for two centuries; the elucidation of the sodium-potassium ATPase as their molecular target in the twentieth century, and the neutral mortality result of the 1997 digoxin trial, together moved the class from a mainstay to a narrowly used agent.

Debates

What is the contemporary role of digoxin given a neutral mortality result?

Because digoxin reduced heart-failure hospitalizations without improving survival, and because of its narrow therapeutic index, its place is debated and generally selective; the question is presented here as an evidence-appraisal matter rather than as advice.

Key figures

• William Withering

Seminal works

• withering-1785
• dig-1997

Frequently asked questions

How do cardiac glycosides increase the heart's contractile force?

They inhibit the sodium-potassium ATPase, which raises intracellular sodium. This reduces calcium extrusion by the sodium-calcium exchanger, so intracellular calcium builds up and more is released with each beat, increasing contractile force — all without raising cyclic AMP.

Why are cardiac glycosides considered hard to dose safely?

They have a narrow therapeutic index: the gap between an effective effect and a toxic one is small, and the same ion-pump inhibition that strengthens contraction can, in excess, trigger dangerous arrhythmias. This entry does not provide dosing; that is a matter for clinical management.