Cardiac Glycosides

Details

Cardiac glycosides are a class of plant-derived compounds (sourced from Digitalis purpurea and Digitalis lanata) that inhibit the Na⁺/K⁺-ATPase pump in cardiomyocytes. The two agents in clinical use are digoxin and digitoxin, which share the same pharmacodynamic mechanism but have markedly different pharmacokinetic profiles. They have been used for >200 years and remain the only positive inotropes with RCT evidence for reducing HF hospitalization without increasing mortality.

Key Facts

Mechanism of Action

• Inhibits Na⁺/K⁺-ATPase → intracellular Na⁺ accumulates → Na⁺/Ca²⁺ exchanger reduces Ca²⁺ extrusion → elevated intracellular Ca²⁺ → positive inotropy sources/cardiac-glycosides-hfrEF-fcvm-2026 (medium)
• Reduces heart rate via indirect vagotonic effect (indirect sympathetic tone suppression) — basis for rate control in AF sources/cardiac-glycosides-hfrEF-fcvm-2026 (medium)
• At toxic concentrations: intracellular Ca²⁺ overload → triggered activity (delayed afterdepolarizations) → arrhythmia sources/drug-arrhythmia-aha-2020 (very high)
• Also shortens cardiac action potential / QT interval (a class effect of digitalis glycosides) sources/drug-arrhythmia-aha-2020 (very high)

Pharmacokinetics: Digoxin vs Digitoxin

• Digoxin: hydrophilic; renal clearance ~85%; narrow therapeutic index; dose adjustment required in renal impairment; therapeutic window 0.5–0.9 ng/mL (DIG post-hoc: lower concentrations yield benefit with less harm; toxic >2 ng/mL) sources/cardiac-glycosides-hfrEF-fcvm-2026 (medium)
• Digitoxin: lipophilic → higher intestinal absorption; serum protein binding ~97%; predominantly hepatic clearance via enterohepatic circulation → effective even in severe renal impairment; more stable plasma concentrations; therapeutic drug monitoring (TDM) range 8–18 ng/mL used in DIGIT-HF sources/digitoxin-hfref-digithf-nejm-2025 (high)

Clinical Indication 1: HFrEF (symptom reduction / HF hospitalization)

• AHA/ACC/HFSA 2022 guideline: Digoxin COR IIb, Level B-R — may be considered in symptomatic HFrEF despite GDMT (or GDMT intolerance) to decrease HF hospitalizations; not a mortality-modifying agent sources/HF-AHA-2022 (very high)
• Pooled 6 RCTs (n=8,488): HF hospitalization HR=0.79 (95% CI 0.67–0.94) — significant benefit; all-cause mortality HR=0.98 (0.92–1.04) — no benefit sources/cardiac-glycosides-hfrEF-fcvm-2026 (medium)
• DIGIT-HF (2025): Digitoxin 0.07 mg OD vs placebo (n=1,212 on contemporary GDMT; ARNi 39.5%, SGLT2i 19.3%): primary composite (all-cause death or first HF hospitalisation) HR 0.82 (0.69–0.98; P=0.03); NNT=22; first cardiac glycoside RCT on modern quadruple GDMT; noninferiority for mortality confirmed; individual death and hospitalisation components each NS (underpowered) sources/digitoxin-hfref-digithf-nejm-2025 (high)
• Hospitalization benefit is DIG-sensitive: removing the DIG trial from the pooled analysis renders the hospitalization benefit non-significant (HR 0.71 [0.44–1.15]) — replication in a modern trial (DECISION trial, ongoing) is needed sources/cardiac-glycosides-hfrEF-fcvm-2026 (medium)
• Dosing principle: Target lower serum digoxin concentrations (0.5–0.9 ng/mL) — DIG post-hoc showed greater benefit and less harm at lower levels; TDM-guided dosing (used in DIGIT-HF for digitoxin) may be key to optimizing outcomes sources/cardiac-glycosides-hfrEF-fcvm-2026 (medium)

Clinical Indication 2: Rate Control in AF + HF

• In elderly HFpEF + AF: RATE-AF trial — digoxin vs bisoprolol showed similar primary QOL at 6 months; several secondary QOL and functional outcomes favoured digoxin at 12 months; more adverse events (dizziness, lethargy, hypotension) with bisoprolol sources/HF-AHA-2022 (very high)
• Vagotonic effect useful for ventricular rate control in AF, particularly in patients where beta-blockers/CCBs are not tolerated or are contraindicated due to haemodynamic compromise

Contraindications and Hazards

• Pre-excited AF (WPW): IV/oral digoxin is Class III Harm — shortens accessory pathway refractoriness → may accelerate ventricular rate over bypass tract → VF/SCD risk; absolutely avoided in AVRT or AF with pre-excitation sources/svt-aha-2015 (high)
• HCM with LVOTO: Digoxin may worsen LVOTO — discontinuation may be reasonable (Class IIb) sources/HCM-AHA-2024 (high)
• Renal impairment: Digoxin requires dose adjustment; digitoxin preferred in CKD (hepatic clearance) sources/digitoxin-hfref-digithf-nejm-2025 (high)
• Hypokalaemia / hypomagnesaemia: Electrolyte abnormalities potentiate digoxin toxicity — correct before initiating sources/drug-arrhythmia-aha-2020 (very high)

Drug Interactions

• Amiodarone: Increases serum digoxin concentration (P-gp inhibition); dose reduction and monitoring required sources/drug-arrhythmia-aha-2020 (very high)
• Dronedarone: 1.7–2.5-fold increase in serum digoxin concentration via P-glycoprotein-mediated renal interaction; requires frequent monitoring and dose reduction sources/dronedarone-circ-2009 (medium)
• Verapamil, quinidine: Also increase digoxin levels; monitor closely sources/drug-arrhythmia-aha-2020 (very high)

Toxicity

• Digoxin toxicity arrhythmias: Paroxysmal atrial tachycardia with AV block (pathognomonic); VT (bifascicular VT pattern also seen); sinus bradycardia; AV nodal block — all mediated by intracellular Ca²⁺ overload and vagotonic activity sources/drug-arrhythmia-aha-2020 (very high)
• Risk factors for toxicity: Digoxin level >2 ng/mL, renal disease, hypomagnesaemia, drug interactions (amiodarone, verapamil, quinidine) sources/drug-arrhythmia-aha-2020 (very high)
• Treatment of toxicity: Digoxin immune Fab (antibody fragments) — specific antidote; indicated for life-threatening arrhythmias or haemodynamic compromise sources/drug-arrhythmia-aha-2020 (very high)
• DIGIT-HF: serious adverse events 4.7% (digitoxin) vs 2.8% (placebo) — narrow therapeutic window requires monitoring sources/digitoxin-hfref-digithf-nejm-2025 (high)

Contradictions / Open Questions

• Digitoxin vs digoxin — superiority unproven: DIGIT-HF (2025) showed composite endpoint benefit for digitoxin on contemporary GDMT; indirect meta-analysis comparing digitoxin vs digoxin via placebo reference node shows no significant difference (mortality HR=0.93 NS; HF hospitalization HR=1.35 NS). The pharmacokinetic advantage of digitoxin (renal sparing, stable levels) has not translated to demonstrated clinical superiority; no head-to-head RCT exists. sources/cardiac-glycosides-hfrEF-fcvm-2026 sources/digitoxin-hfref-digithf-nejm-2025
• Mortality signal in observational data: Vamos 2015 meta-analysis suggested digoxin associated with increased mortality. This was driven by observational studies (confounding: sicker patients receive digoxin). RCT-only meta-analyses show no mortality increase (HR=0.98). The observational signal should not be extrapolated to appropriately selected, low-serum-concentration-guided therapy. sources/cardiac-glycosides-hfrEF-fcvm-2026 (medium)
• Hospitalization benefit stability: The pooled HR=0.79 is sensitive to DIG trial exclusion — crosses null without it. Entire hospitalization signal may rest on a single 1997 trial conducted on pre-modern GDMT. DECISION trial (ongoing: low-dose digoxin + contemporary GDMT) is needed to confirm. sources/cardiac-glycosides-hfrEF-fcvm-2026 (medium)
• Optimal serum concentration: DIG post-hoc analyses support 0.5–0.9 ng/mL for digoxin; DIGIT-HF used TDM for digitoxin at 8–18 ng/mL. No prospective RCT has formally evaluated concentration-guided dosing for digoxin. sources/digitoxin-hfref-digithf-nejm-2025 (high)
• Safety with SGLT2i-era GDMT: DIGIT-HF only had 19.3% SGLT2i uptake. Incremental benefit on full quadruple GDMT (ARNi + BB + MRA + SGLT2i) at complete adherence is untested. sources/digitoxin-hfref-digithf-nejm-2025 (high)

Connections

• Related to concepts/Cardiac-Glycosides-in-HFrEF — dedicated clinical evidence and guideline summary
• Related to entities/HFrEF — primary indication; fourth-line adjunct for symptom reduction
• Related to entities/Heart-Failure — broader HF syndrome context
• Related to entities/Atrial-Fibrillation — rate control indication
• Related to entities/Amiodarone — major drug interaction (P-gp inhibition); digoxin level monitoring required
• Related to entities/Dronedarone — major drug interaction; 1.7–2.5-fold digoxin concentration increase
• Related to concepts/Drug-Induced-Arrhythmia — digoxin toxicity arrhythmia patterns
• Related to entities/HCM — contraindicated with LVOTO

Sources

• sources/cardiac-glycosides-hfrEF-fcvm-2026 — 2026 systematic review + meta-analysis, 6 RCTs, n=8,488; pharmacokinetics, clinical outcomes, guideline context; medium quality
• sources/digitoxin-hfref-digithf-nejm-2025 — DIGIT-HF RCT 2025; only contemporary GDMT cardiac glycoside trial; high quality
• sources/HF-AHA-2022 — AHA/ACC/HFSA 2022 HF guidelines; COR IIb recommendation for digoxin; RATE-AF trial; very high quality
• sources/drug-arrhythmia-aha-2020 — AHA 2020: digoxin toxicity arrhythmias, drug interactions, antidote (Fab fragments); very high quality
• sources/svt-aha-2015 — AHA 2015 SVT guidelines: Class III Harm in pre-excited AF; digoxin in pediatric SVT; high quality
• sources/dronedarone-circ-2009 — Dronedarone pharmacology: P-gp-mediated digoxin interaction; medium quality
• sources/HCM-AHA-2024 — AHA 2024 HCM guidelines: digoxin/vasodilator discontinuation in LVOTO; high quality