Cardiovascular · Topic 21 of 48
Bradyarrhythmias & AV Blocks
Cardiovascular · bradycardia and conduction
Bradyarrhythmias & AV Blocks
Slow rhythm with poor perfusion: atropine 1 mg, then pacing or epinephrine or dopamine. Mobitz II and complete heart block need a permanent pacemaker.
Key numbers
Symptomatic bradycardia: atropine 1 mg IV1 mg q3–5 min, max 3 mg
Atropine fails: pace, epinephrine or dopamineepi 2–10 mcg/min
First-degree block: PR above 200 msPR > 200 ms
01 · GiveSymptomatic bradycardia: atropine 1 mg IV
For bradycardia with hemodynamic compromise, give atropine 1 mg IV, repeated every 3–5 minutes to a total of 3 mg, while looking for and treating reversible causes.
02 · DoAtropine fails: pace, epinephrine or dopamine
If atropine fails, start transcutaneous pacing or an infusion of epinephrine 2–10 mcg/min or dopamine 5–20 mcg/kg/min, and arrange temporary transvenous pacing for refractory instability.
03 · The ruleMobitz II or complete block: pacemaker
Acquired Mobitz II, high-grade or third-degree AV block not caused by a reversible or physiologic condition needs a permanent pacemaker, regardless of symptoms.
01
What it is
Core ideaSick sinus syndrome: bradycardia, pauses, tachy-bradySinus node dysfunction includes sinus bradycardia < 50 bpm, pauses > 3 s, sinoatrial exit block, chronotropic incompetence and tachy-brady syndrome; symptomatic SND without a reversible cause gets a pacemaker.
02
Causes & risk factors
Why it happensLook for reversible causes firstReversible causes: AV-nodal drugs (beta-blockers, calcium channel blockers, digoxin, amiodarone), hyperkalemia, ischemia, hypoxemia, hypothyroidism, hypothermia, Lyme carditis, high vagal tone and sleep apnea.
03
Presentation
What you'll seeUnstable: hypotension, confusion, shock, ischemia, failureTreat bradycardia urgently when it causes hypotension, acutely altered mental status, shock, ischemic chest discomfort or acute heart failure; otherwise monitor and look for the cause.
Cannon a waves and variable S1Bedside signs of AV dissociation in complete heart block: cannon a waves in the jugular pulse, fluctuating blood pressure and a first heart sound of varying loudness.
04
Diagnosis & monitoring
Work-upMobitz I: PR lengthens; Mobitz II: PR fixed must knowMobitz I (Wenckebach): the PR lengthens until a QRS drops — usually AV nodal and benign. Mobitz II: constant PR with a suddenly dropped QRS — infranodal, often progressing abruptly to complete block.
First-degree block: PR above 200 msFirst-degree AV block: every P wave conducts but the PR exceeds 200 ms; it is rarely symptomatic and needs no treatment beyond reviewing AV-nodal drugs.
Third-degree: AV dissociation, atrial rate fasterComplete heart block: no P waves conduct and the atrial rate exceeds the ventricular rate; a narrow junctional escape runs 35–50 bpm, a wide ventricular escape 20–40 bpm and is unreliable.
05
Do right now
In this orderFirst moves
1
→Unstable: hypotension, confusion, shock, ischemia, failure
2
→Symptomatic bradycardia: atropine 1 mg IV1 mg q3–5 min, max 3 mg
3
→Atropine fails: pace, epinephrine or dopamineepi 2–10 mcg/min
4
Look for reversible causes first
Why each step
Unstable: hypotension, confusion, shock, ischemia, failureTreat bradycardia urgently when it causes hypotension, acutely altered mental status, shock, ischemic chest discomfort or acute heart failure; otherwise monitor and look for the cause.
Symptomatic bradycardia: atropine 1 mg IVFor bradycardia with hemodynamic compromise, give atropine 1 mg IV, repeated every 3–5 minutes to a total of 3 mg, while looking for and treating reversible causes.
Atropine fails: pace, epinephrine or dopamineIf atropine fails, start transcutaneous pacing or an infusion of epinephrine 2–10 mcg/min or dopamine 5–20 mcg/kg/min, and arrange temporary transvenous pacing for refractory instability.
Look for reversible causes firstReversible causes: AV-nodal drugs (beta-blockers, calcium channel blockers, digoxin, amiodarone), hyperkalemia, ischemia, hypoxemia, hypothyroidism, hypothermia, Lyme carditis, high vagal tone and sleep apnea.
06
Management
OrdersAtropine works at the sinus and AV nodeAtropine is reasonable for sinus node dysfunction and for AV block believed to be at the AV-nodal level; infranodal block (Mobitz II, wide-escape complete block) is managed with pacing.
07
Red flags & don'ts
Avoid harmNo atropine after heart transplant avoidAtropine should not be used for sinus bradycardia in a heart transplant recipient without autonomic reinnervation (Class 3: Harm); use aminophylline or theophylline, or pace.
08
The rules
Know thesePace when symptoms correlate with bradycardiaNo heart rate or pause length alone mandates pacing in sinus node dysfunction — document symptom–rhythm correlation; asymptomatic first-degree or AV-nodal Mobitz I block is not paced.
09
Mobitz I or Mobitz II?
Commonly confused| PR interval | Lengthens until a QRS drops |
|---|---|
| Level of block | AV node (when the QRS is narrow) |
| Course | Slow; reliable junctional escape |
| Atropine | Reasonable (AV-nodal level) |
| Permanent pacemaker | Only if symptoms correlate |
10
Commonly confused
Tell apartInferior-MI block resolves; anterior-MI block needs pacingAV block with inferior MI is usually nodal (vagal tone, adenosine), responds to atropine or aminophylline and resolves within days; block with anterior MI is infranodal and needs immediate transvenous pacing.
11
More detail
ReferenceShow 10 reference facts
The 2018 ACC/AHA/HRS guideline defines sinus bradycardia as a rate below 50 bpm; slow rates can be physiologic in trained athletes and during sleep.
Transcutaneous pacing is painful in a conscious patient (sedate when feasible) and is a bridge to transvenous pacing; confirm both electrical and mechanical capture.
When coronary ischemia is unlikely, isoproterenol (1–20 mcg/min infusion), dopamine, dobutamine or epinephrine may be considered to improve AV conduction and rate (Class IIb).
For second- or third-degree AV block with acute inferior MI, aminophylline 250 mg IV bolus may be considered; after heart transplant the dose is 6 mg/kg over 20–30 minutes.
Up to one-third of patients aged 18–60 with new Mobitz II or complete heart block and no evident structural heart disease are found to have cardiac sarcoidosis.
In sick sinus syndrome, an atrial or dual-chamber pacemaker lowers the risk of atrial fibrillation compared with ventricular-only pacing.
Nocturnal bradycardia should prompt screening for sleep apnea, whose treatment reduces these arrhythmias; nocturnal bradycardia alone is not an indication for pacing.
Mobitz I with a wide QRS is more often infranodal (60–70%) than nodal; 2:1 block cannot be labeled type I or II, and high-grade block means ≥ 2 consecutive nonconducted P waves.
Toxin-induced bradycardia needs the antidote alongside pacing preparation: calcium for calcium channel blocker overdose, digoxin immune Fab for digoxin toxicity (see the toxicology sheets).
AV block from a known reversible cause that resolves completely is not paced; if it persists despite treating the cause, a permanent pacemaker is recommended.
Wenckebach warns before it drops; Mobitz II drops without warning — and Mobitz II gets a pacemaker.
Sources · AHA 2025 ACLS algorithms poster set (Adult Tachyarrhythmia With a Pulse and Adult Bradycardia With a Pulse — doses/details), international edition · 2025 AHA Guidelines for CPR and ECC — Part 9: Adult Advanced Life Support (Circulation 2025; doi 10.1161/CIR.0000000000001376), cpr.heart.org text · AHA 2025 Interim Training Materials — ACLS Provider Manual changes (bradycardia algorithm doses) · Kusumoto FM et al. 2018 ACC/AHA/HRS Guideline on Bradycardia and Cardiac Conduction Delay: Executive Summary. Circulation 2019;140:e333–e381 (doi 10.1161/CIR.0000000000000627) · Merck Manual Professional — Atrioventricular Block · Merck Manual Professional — Sick Sinus Syndrome · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).