ACLS Bradycardia Algorithm Explained (2025 Guidelines)

ACLS Cards · Updated November 3, 2026 · Adult · 2025 guidelines · Clinical review pending

The bradycardia algorithm answers one question: is this slow heart rate causing the patient's problem? If not, you monitor and look for a cause. If it is, you climb a short ladder of treatments until perfusion improves.

When the algorithm applies

An adult with a pulse and a heart rate typically below 50/min^{pending review, ref B1}. The number alone isn't the trigger. Plenty of fit people, and people on rate-slowing medicines, are perfectly comfortable with a slow heart rate. What matters is whether the slow rate is producing signs of poor perfusion.

Is it causing instability?

Look for any of these, and decide whether the bradycardia is the likely reason:

• Hypotension^{pending review, ref B7}
• Acutely altered mental status^{pending review, ref B7}
• Signs of shock^{pending review, ref B7}
• Ischemic chest discomfort^{pending review, ref B7}
• Acute heart failure^{pending review, ref B7}

No signs of instability: monitor, get a 12-lead ECG, find the cause, and get expert input. Signs present: treat now.

The treatment ladder

1. Atropine first: 1 mg IV, repeat every 3–5 min, to a maximum of 3 mg^{pending review, ref B2}.
2. If atropine doesn't work, move to one of:
   • Transcutaneous pacing, or
   • Dopamine infusion 5–20 mcg/kg/min^{pending review, ref B3}, or
   • Epinephrine infusion 2–10 mcg/min^{pending review, ref B4}.
3. New in 2025: if the patient is still unstable despite medical therapy, temporary transvenous pacing is reasonable^{pending review, ref B6}. Bring in expert consultation early.

Pacing rate and current settings aren't listed here on purpose: they depend on your device and protocol.

When atropine is unlikely to work

Atropine acts on the vagus nerve's effect on the SA and AV nodes. It may fail when the block is below the AV node or the heart has no vagal connection. Watch for high-grade AV block with a wide-complex escape rhythm, or a heart transplant patient^{pending review, ref B5}. In those cases, move to pacing early^{pending review, ref B5} rather than waiting through repeated atropine doses.

AV block quick reference

• Normal PR interval: 0.12–0.20 s^{pending review, ref R1}.
• First-degree block: PR longer than 0.20 s^{pending review, ref R1}, every P followed by a QRS. Usually benign on its own.
• Second-degree type I: PR lengthens beat by beat until a QRS drops.
• Second-degree type II: constant PR with sudden dropped QRS complexes. Higher risk.
• Third-degree: P waves and QRS complexes march independently.

Look for the cause

While treating, ask why. Common culprits include ischemia, drugs, hyperkalemia, hypoxia, hypothermia, raised intracranial pressure, and hypothyroidism^{pending review, ref B8}. Fixing the cause often fixes the rhythm.

Common mistakes in megacode and real life

• Treating the number, not the patient. A slow but stable patient doesn't need atropine.
• Repeating atropine in a high-grade block instead of moving to pacing.
• Forgetting the cause. Name a reversible cause out loud; examiners and teammates both notice.
• Losing the pulse. If the patient loses a pulse, you're no longer in this algorithm. Switch to the cardiac arrest algorithm and start CPR.

Next: the tachycardia algorithm explained.

Source: 2025 Guidelines for CPR and Emergency Cardiovascular Care (Circulation, October 2025), as summarized in the pack's source checklist. Original wording and layout; no guideline figures reproduced.