Conduction system disease from AV node to His-Purkinje. Know which blocks are benign (Mobitz I) vs which need pacing (Mobitz II, 3rd degree). Atropine is your bridge while you call for the pacer.
๐ Overview
AV Block Classification
Type
ECG Pattern
Level of Block
Clinical Significance
Needs Pacing?
1st Degree
PR > 200 ms, every P followed by QRS
AV node delay
Benign. No treatment needed. Common in athletes, vagal tone, beta-blockers.
Usually benign. Common in inferior MI (RCA supplies AV node), athletes, digoxin. Often transient.
Rarely. Only if symptomatic (bradycardia with hemodynamic compromise).
2nd Degree -Mobitz II
Constant PR interval โ sudden dropped QRS. No progressive prolongation.
Below His bundle (infranodal)
DANGEROUS. High risk of progressing to complete heart block without warning. Often associated with anterior MI, structural disease.
Yes -pacemaker indicated even if asymptomatic.
3rd Degree (Complete)
Complete AV dissociation. P waves march through at their own rate. QRS at escape rate (junctional 40โ60 or ventricular 20โ40).
Complete block at any level
EMERGENCY if symptomatic. May be stable if junctional escape with narrow QRS. Wide QRS escape = unstable, unreliable.
Yes -permanent pacemaker.
Mobitz I = safe (AV node, usually transient). Mobitz II = dangerous (infranodal, can drop to complete block suddenly). The key distinction: does the PR progressively lengthen before the drop? Yes = Mobitz I. No = Mobitz II.
Common Causes
Medications -beta-blockers, CCBs (verapamil/diltiazem), digoxin, amiodarone (most common reversible cause)
Ischemia -inferior MI (AV node block, often transient), anterior MI (His-Purkinje, often permanent)
Degenerative -fibrosis of conduction system (Lenegre disease, Lev disease) -most common in elderly
Infiltrative -sarcoidosis (think cardiac sarcoid in young patient with unexplained heart block), amyloidosis
Infectious -Lyme disease (early disseminated โ AV block, usually reversible with antibiotics), endocarditis with abscess
Post-surgical -TAVR, septal myectomy, congenital heart surgery
๐จ Management
Acute Symptomatic Bradycardia
Step 1
Atropine 0.5 mg IV q3โ5 min (max 3 mg). Works for AV nodal block (1st degree, Mobitz I, junctional). Does NOT work for infranodal block (Mobitz II, 3rd degree with wide escape) -the block is below where atropine acts.
๐ Updated Practice: Old teaching: atropine works for all types of heart block. Current practice: atropine only works for NODAL blocks (1st degree AV block, Mobitz Type I/Wenckebach) by increasing AV node conduction via vagolytic effect. It does NOT work for INFRANODAL blocks (Mobitz Type II, complete/3rd degree heart block), and can paradoxically worsen the block by increasing atrial rate without improving ventricular conduction. Infranodal blocks require transcutaneous or transvenous pacing.
Step 2
If atropine fails: transcutaneous pacing (external pads). Set rate 60โ80, increase mA until capture. Sedate the patient -it's painful. This is a bridge.
Step 3
Transvenous pacing (temporary pacing wire via IJ/subclavian/femoral). Definitive temporary bridge until permanent pacer.
Bridge agents
While awaiting pacing: dopamine 5โ20 mcg/kg/min or epinephrine 2โ10 mcg/min or isoproterenol 2โ20 mcg/min (ฮฒโ chronotropy).
Permanent Pacemaker Indications
3rd degree AV block (symptomatic or asymptomatic) ACC/AHA/HRS Bradycardia Guidelines, Kusumoto 2019
Mobitz II (symptomatic or asymptomatic -high progression risk)
Always rule out reversible causes before permanent pacing: medication effect (hold offending drugs and reassess), Lyme disease (treat with IV ceftriaxone), inferior MI (AV block often resolves in 5โ7 days), hyperkalemia, hypothyroidism. ACC/AHA/HRS, 2019
๐งช Workup
Workup
ECG -degree and level of block
BMP + Mgยฒโบ -hyperkalemia
Medication review -BB, CCB, digoxin, amiodarone
TSH -hypothyroidism
Lyme serology -AV block in Lyme carditis
Troponin -inferior MI โ AV node ischemia
Echo -structural/infiltrative disease
๐ On Rounds
Why doesn't atropine work in Mobitz II or infranodal block?
Atropine is a parasympatholytic (anticholinergic) -it blocks vagal tone at the AV node, increasing conduction through the node. In Mobitz II and 3rd degree heart block with wide QRS, the block is below the AV node (in the His bundle or bundle branches) where there is no parasympathetic innervation. Atropine has no target to act on.
A 28-year-old presents with new-onset 3rd degree heart block. What diagnosis should you consider?
Lyme disease (Lyme carditis) and cardiac sarcoidosis. Lyme is the most important to catch -it's fully reversible with IV ceftriaxone (2g daily ร 14โ21 days). Occurs in early disseminated Lyme (weeksโmonths after tick bite). Get Lyme serologies in any young patient with unexplained heart block, especially in endemic areas.
What medications cause heart block, and which are reversible?
Reversible (hold and observe): beta-blockers (metoprolol, atenolol, carvedilol), non-DHP calcium channel blockers (diltiazem, verapamil), digoxin, amiodarone, clonidine. Metabolic causes (correct the cause): hyperkalemia (most dangerous -can cause any degree of block โ sine wave โ arrest), hypothyroidism, hypothermia.
An inferior STEMI patient develops complete heart block. Is this the same as chronic CHB?
No -and the management is different. Inferior MI causes AV node ischemia (supplied by RCA in 85%) โ typically a junctional escape rhythm at 40-60 bpm (narrow QRS, relatively stable). This is usually transient (resolves in 2-7 days) as the AV node recovers. Management: temporary pacing if hemodynamically compromised, atropine may help (vagal-mediated). Does NOT routinely need a permanent pacer -monitor and reassess.
Case 1: Symptomatic Mobitz Type II
Presentation: 70M with recurrent syncope over 2 weeks, baseline HTN and prior anterior MI. Arrives after witnessed syncopal episode. HR 38, BP 88/54. ECG shows 2:1 AV block with wide QRS (RBBB morphology) and constant PR intervals on conducted beats, consistent with Mobitz Type II.
Management: Transcutaneous pacing pads placed immediately. Atropine 0.5 mg IV given, no response (expected: infranodal block has no vagal innervation to antagonize). Capture achieved with transcutaneous pacing at 70 mA. Dopamine drip started at 5 mcg/kg/min for BP support. EP consulted, permanent dual-chamber pacemaker (DDD) placed next day.
Teaching Point: Mobitz II with wide QRS = infranodal block. Atropine will not work. Do not delay pacing. Even asymptomatic Mobitz II is a Class I indication for permanent pacemaker.
Case 2: Complete Heart Block Post-Inferior STEMI
Presentation: 58F with acute inferior STEMI (RCA occlusion) taken for PCI. Post-cath, develops new 3rd degree AV block with narrow QRS junctional escape rhythm at 42 bpm. BP 96/60, mildly symptomatic with lightheadedness but alert.
Management:Atropine 0.5 mg IV, HR improves transiently to 56 bpm (AV nodal block, so atropine has partial effect). Temporary transvenous pacing wire placed via RIJ as backup. Monitored on telemetry. By day 3, AV conduction returns, 1st degree AV block only. Pacing wire removed day 4.
Teaching Point: Inferior MI causes AV node ischemia (RCA supplies AV node in 85%). Heart block is usually transient (resolves in 2โ7 days) with a stable junctional escape. Does NOT routinely require permanent pacing, monitor and reassess. Contrast with anterior MI, where heart block is infranodal and often permanent.
Case 3: Drug-Induced Bradycardia & High-Grade AV Block
Presentation: 82M with HFrEF and atrial fibrillation, on Metoprolol Succinate (Toprol-XL) 100 mg daily + Diltiazem (Cardizem) 120 mg TID + Digoxin (Lanoxin) 0.125 mg daily. Admitted with fatigue and near-syncope. HR 32, BP 84/50. ECG shows 1st degree AV block progressing to high-grade AV block (3:1 and 4:1 conduction) with narrow QRS.
Management: All three AV-nodal blocking agents held immediately. Atropine 1 mg IV, transient improvement to HR 48. Glucagon 3 mg IV bolus then 3 mg/hr drip (beta-blocker reversal). Digoxin level drawn, 2.4 ng/mL (elevated). Transcutaneous pads placed prophylactically. Over 36 hours, metoprolol and diltiazem cleared, HR improved to 64, normal sinus rhythm with 1st degree AV block only.
Teaching Point: Triple AV-nodal blockade (beta-blocker + non-DHP CCB + digoxin) is a common iatrogenic cause of high-grade AV block. First step: hold all offending agents and check digoxin level. Glucagon is the specific antidote for beta-blocker toxicity. Most drug-induced heart block resolves after drug washout, avoid permanent pacing until reversible causes are excluded.
๐ฃ Sample Presentation
One-Liner
"Mr. Davis is a 76-year-old on metoprolol 200 mg daily presenting with dizziness and HR 34. ECG shows complete (third-degree) AV block with a ventricular escape rate of 34 bpm."
Key Points to Cover on Rounds
Third-degree AV block. Hemodynamically stable but symptomatic. Metoprolol held (reversible cause). Temporary transcutaneous pacing pads placed. Atropine 0.5 mg given with transient improvement to HR 48. Lyme screen and TSH sent. Troponin negative. EP consulted for permanent pacemaker. Plan: if no improvement after holding BB ร24h, transvenous pacing wire โ permanent pacer.
๐ Medications
Key Medications -Heart Block & Bradycardia
Drug
Dose
Route
Notes
Atropine 1ST LINE
0.5-1 mg IV, repeat q3-5 min (max 3 mg)
IV push
Vagolytic -works for nodal block only (1st degree, Mobitz I). Will NOT work for Mobitz II or 3rd degree (infranodal) -no vagal innervation below AV node. Can paradoxically worsen infranodal block.
Isoproterenol (Isuprel)
2-10 mcg/min IV drip
IV
Beta-1 + Beta-2 agonist. Increases HR and AV conduction. Bridge to pacing. Caution: increases myocardial Oโ demand.
Dopamine
5-20 mcg/kg/min IV drip
IV
Chronotropic at 5-10 mcg/kg/min (beta effect). Alternative to isoproterenol if hypotensive. Higher doses add alpha vasoconstriction.
Epinephrine
2-10 mcg/min IV drip
IV
For symptomatic bradycardia unresponsive to atropine. Potent chronotrope and vasopressor.
Transcutaneous pacing
Start 60-80 mA, rate 60-80 bpm
External pads
Bridge to transvenous pacing. Painful -requires sedation. Verify mechanical capture (palpable pulse with each complex).
Transvenous pacing
Per EP/cardiology
Central venous
Temporary pacing wire via RIJ or femoral vein. For refractory symptomatic bradycardia awaiting permanent pacemaker.
First step: Review medication list -hold all AV-nodal blockers (beta-blockers, non-DHP CCBs, digoxin, amiodarone). Check Kโบ and Mgยฒโบ. Many cases of heart block are iatrogenic and reversible.
โก Summary
Summary
1st Degree
PR > 200ms. Benign. No treatment needed. Monitor if progressing.
2nd Degree Mobitz I
Progressive PR prolongation โ dropped beat. Usually AV nodal (narrow QRS). Often benign. Observe.