5.1 Symptomatic Bradycardia

Defining and Recognizing Bradycardia

Bradycardia is a heart rate below 50 beats per minute (some sources use under 60), but the ACLS Adult Bradycardia With a Pulse Algorithm only activates when the slow rate produces signs and symptoms of poor perfusion. A rate alone is never the trigger — well-conditioned athletes and sleeping adults routinely sit in the 40s without harm. The exam question almost always hands you a rate plus a clinical clue, and your job is to decide whether the bradycardia is symptomatic (unstable) or asymptomatic (stable).

The official AHA guidance is the 2020 ACLS Adult Bradycardia With a Pulse Algorithm, carried forward in current materials. The five instability signs you must memorize are: hypotension (systolic below ~90 mmHg), acutely altered mental status, signs of shock, ischemic chest discomfort, and acute heart failure. Any one of these, attributable to the rate, makes the patient unstable and pushes you down the treatment limb.

Bradycardia rhythm recognition

Sinus bradycardia vs. junctional vs. ventricular escape

Not every slow rhythm is sinus. In sinus bradycardia an upright P wave precedes every QRS in lead II at a slow rate. A junctional escape rhythm (rate ~40-60) shows absent, inverted, or retrograde P waves because the AV junction is pacing. A ventricular (idioventricular) escape rhythm (rate ~20-40) shows wide QRS complexes with no related P waves and is the dangerous escape seen in complete heart block. Recognizing the escape focus tells you how unstable the patient is likely to be: the lower the escape pacemaker, the slower and less reliable the rhythm, and the more urgently pacing is needed.

The Bradycardia Algorithm, Step by Step

Once you confirm symptomatic bradycardia, work the algorithm in order. Each escalation assumes the prior step failed or is inappropriate.

Step-by-step treatment

1. Maintain airway, give oxygen if hypoxic, attach monitor/pulse ox/BP, obtain IV access, and run a 12-lead without delaying urgent therapy.
2. Atropine 1 mg IV — the first-line drug. Repeat every 3-5 minutes to a maximum total dose of 3 mg. The 2020 AHA update raised the single dose from 0.5 mg to 1 mg; doses below 0.5 mg can paradoxically slow the heart and are wrong on the exam.
3. If atropine is ineffective, choose ONE of three co-equal options:
   • Transcutaneous pacing (TCP), or
   • Dopamine infusion 5-20 mcg/kg/min, or
   • Epinephrine infusion 2-10 mcg/min.
4. Treat reversible causes and arrange expert consultation / transvenous pacing for refractory or high-grade block.

Bradycardia drug table

When atropine is the wrong answer

Atropine acts at the AV node, so it works on sinus bradycardia, first-degree block, and Mobitz I. In Mobitz II and complete (third-degree) AV block the conduction failure is below the node (infranodal), where atropine has no target — and atropine can even worsen the situation by speeding atrial rate without conducting. For these high-grade blocks, begin transcutaneous pacing without waiting for atropine to fail, and arrange transvenous pacing as the definitive bridge.

Why the dose floor matters

The 2020 update's move to a 1 mg single dose was deliberate: sub-0.5 mg doses of atropine can produce a paradoxical bradycardia by central vagal stimulation before the peripheral vagolytic effect kicks in. So an exam answer offering "atropine 0.3 mg" or "atropine 0.5 mg" for adult symptomatic bradycardia is wrong on two counts — wrong dose and a real risk of slowing the heart further. The repeat interval of every 3-5 minutes and the hard 3 mg ceiling are the other two numbers you must reproduce exactly.

Transcutaneous Pacing and Reversible Causes

Transcutaneous pacing (TCP) delivers current through pads on the chest wall to drive the ventricles when the intrinsic rate is dangerously slow. Set the rate around 60-80/min, then increase the milliamps (mA) until electrical capture appears — a pacer spike followed by a widened QRS and a T wave. Then confirm mechanical capture by palpating a femoral pulse (NOT the carotid, where muscle twitch fools you). TCP is painful, so provide analgesia and sedation once perfusion is restored and time allows — but never delay pacing in a crashing patient to set up sedation first.

Reversible causes to hunt

• Hypoxia — the single most common reversible cause; oxygenate.
• Drug toxicity — beta-blocker, calcium-channel blocker, digoxin, organophosphates. Beta-blocker/CCB overdose may need glucagon or calcium; dig toxicity may need digoxin-immune Fab.
• Hyperkalemia — peaked T waves widening to a sine wave; give calcium.
• Inferior wall MI — RCA occlusion impairs the AV node; bradycardia is common and may need pacing plus reperfusion.
• Hypothermia and raised intracranial pressure (Cushing reflex).

Common exam traps

• Treating asymptomatic sinus bradycardia aggressively — observe instead.
• Pushing a fourth dose of atropine past the 3 mg ceiling instead of pacing.
• Forgetting that Mobitz II / complete block skips atropine and goes to pacing.
• Confirming capture on the carotid (twitch artifact) rather than the femoral pulse.

Scenario anchor: A 67-year-old has a rate of 38, BP 80/50, and is confused with chest pain. This is unstable symptomatic bradycardia: oxygen, IV, atropine 1 mg, and if no response, TCP or dopamine/epinephrine infusion while you look for an inferior MI or drug cause.