ECG Cases 59 – ACLS Dysrhythmia Pitfalls Part 2: Stable Bradycardia? Or Dangerous Bradycardia Requiring Pacemaker?

In this ECG Cases blog we review 5 cases of seemingly stably bradycardia. Which require immediate intervention, and which require a pacemaker?

Written by Jesse McLaren; Peer Reviewed and edited by Anton Helman. December 2025

Five patients presented with bradycardia and normal blood pressure. How does ECG interpretation change management? Despite apparent stability, which require immediate treatment and which require admission for a pacemaker?

Case 1: 50yo with diarrhea and weakness

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Case 2: 50yo with epigastric pain

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Case 3: 60yo weak

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Case 4: 85yo syncope while sitting

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Case 5: 85yo, history of AF, with weakness and normal blood pressure

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Stable bradycardia? Or dangerous bradycardia requiring a pacemaker?

This is the second in a series of blog posts on the pitfalls of ACLS algorithms for adults with a pulse – and how a systematic approach to the 12-lead ECG can improve acquisition, interpretation, and application. The first looked at unstable bradycardia, and this will look at stable bradycardia.

The ACLS algorithm for stable bradycardia is simple: monitor and observe. But when do we need to consider immediate treatment, or admission after observation? If the patient becomes unstable do we just revert to the unstable bradycardia algorithm? There are a number of pitfalls:

ECG acquisition: artifact can mimic AF, but looking at unaffected leads and the regularity of the rhythm can help differentiate (unless its AF with a slow regular ventricular response from complete AV block)

ECG interpretation: ACLS puts all stable bradycardias in the same category: a slow QRS on the rhythm strip, ignoring atrial activity, AV blocks, and fascicular blocks. Computer interpretations often miss a second P wave, and label ‘sinus bradycardia’ what is actually normal sinus with AV block. Assessing the AV node and fascicles can anticipate deterioration, avoid ACLS medications that won’t work (or will make things worse), and identify patients requiring admission for a pacemaker no matter how stable they are.

For AV blocks, what’s clinically relevant (if there are no secondary causes) is the location of the AV block, which determines whether atropine will work or whether the patient needs a pacemaker.

1. Nodal block: atropine will work for bradycardia, if needed
   1. First degree: long PR (ie slow AV)
   2. Second degree type 1: prolonging PR (i.e slow AV) before drop
2. Infra-nodal block: atropine will not work, needs a pacemaker
   1. Second degree type 2: fixed PR (ie normal AV) then sudden drop; often with signs of infranodal disease (eg bundle branch block). By increasing conduction into a diseased system, atropine can make the block worse
   2. Third degree: variable PR (ie complete AV block)

ECG application: ACLS is designed for primary bradycardias from an electrical problem. But seemingly stable patients with a secondary bradycardia can harbor an acute medical emergency that might be revealed on the full 12-lead ECG. The BRADI mnemonic (BRASH/hyperkalemia, Reduced vitals, Acute coronary occlusion, Drugs, ICP/Infection) summarizes secondary bradycardias, and two stand out as causes of bradycardia that can present with a false reassurance of stability, where the initial lab test may be unreliable, where ACLS algorithms fail, and yet where the ECG can guide empiric treatment: severe hyperkalemia requiring empiric calcium (rather than waiting for serum potassium, which is often hemolyzed), and Occlusion MI requiring cath lab actitation (rather than waiting for troponin, the first of which is often normal and not predictive of the peak).

Back to the cases

After calcium: ectopic atrial rhythm and normal axis, but still non-specific wide QRS and peaked T waves

After potassium normalization: normal ECG