Medicine chapters
Heart block: first, second and third degree
Atrioventricular block from simple PR prolongation to Wenckebach, Mobitz II and complete heart block: ECG recognition, causes, which ones need a pacemaker, and the bundle branch blocks that can lead to them.
- First degree: every P conducts but the PR is over 0.20 s. Second degree: some P waves do not conduct; Mobitz I (Wenckebach) shows progressive PR lengthening then a dropped beat (AV node, benign), Mobitz II drops beats without warning (below the node, dangerous).
- Third degree (complete): no P wave conducts; P waves and QRS complexes march independently; slow escape rhythm, cannon waves, Stokes-Adams attacks.
- Pacing: Mobitz II and complete heart block usually need a permanent pacemaker; atropine and temporary pacing are bridges.
On this page
Overview
Heart block is a delay or failure of conduction somewhere in the conducting system. Block in the AV node or His bundle causes atrioventricular (AV) block; block lower down causes bundle branch block.
AV block is graded first, second and third degree. The grade and, above all, the level of the block (in the AV node or below it) decide the risk and whether a pacemaker is needed. MCQs test ECG recognition, bedside signs and indications for pacing.
Normal values to know
- PR interval: 0.12–0.20 s (3–5 small squares).
- QRS duration: under 0.12 s (3 small squares).
- Bradycardia: heart rate below 60/min by day (below 50/min at night).
Causes of AV block
| Group | Examples |
|---|---|
| Physiological | High vagal tone in athletes and during sleep (first degree or Wenckebach) |
| Ischaemic | Acute MI (inferior or anterior), ischaemic cardiomyopathy |
| Degenerative | Lev's disease (fibrosis and calcification of the distal conducting system in the elderly); Lenegre's disease (younger patients) |
| Drugs | Beta-blockers, verapamil and diltiazem, digoxin, amiodarone |
| Electrolytes | Hyperkalaemia |
| Structural | Calcific aortic stenosis, cardiomyopathy, infiltration (amyloid, sarcoid) |
| Infection | Endocarditis (aortic root abscess), Lyme disease, Chagas disease, diphtheria, rheumatic fever |
| Surgical | After aortic valve replacement, VSD repair, CABG; after AV node ablation |
| Congenital | Maternal anti-Ro antibodies (SLE, Sjögren); congenitally corrected transposition |
| Others | SLE, rheumatoid arthritis, hypothyroidism, myotonic and Duchenne dystrophy |
First-degree AV block
- ECG: PR interval longer than 0.20 s, constant. Every P wave is followed by a QRS.
- It is a delay, not a true block.
- Usually no symptoms; may be normal in athletes, or due to drugs, ischaemia, rheumatic fever (prolonged PR is a minor Jones criterion).
- Management: none needed; review drugs and electrolytes.
First-degree block is a delay, not a dropped beat: every P wave is still conducted.
Second-degree AV block
Some P waves conduct to the ventricles; others do not.
Mobitz type I (Wenckebach)
- ECG: PR interval lengthens progressively with each beat until one P wave is not followed by a QRS. The cycle then restarts. The PR after the dropped beat is the shortest.
- The pulse is "regularly irregular" (grouped beating).
- Site: the AV node. Escape rhythms are reliable.
- Causes: high vagal tone, sleep, inferior MI, AV nodal drugs.
- Management: usually observe; stop offending drugs; pace only if symptomatic.
Mobitz type II
- ECG: PR interval is constant (normal or long), and suddenly a P wave fails to conduct, with no warning lengthening. QRS is often broad.
- Site: below the AV node (His bundle or bundle branches).
- High risk of progressing to complete heart block with an unreliable escape rhythm.
- Management: permanent pacemaker is usually indicated.
2:1 and high-grade (advanced) block
- 2:1 block: every second P wave conducts. You cannot tell type I from type II, because there are never two conducted beats in a row to compare PR intervals.
- 3:1 or higher: called high-grade or advanced block; behaves like Mobitz II and usually needs pacing.
Wenckebach = PR gets longer, longer, longer, drop. Mobitz II = PR constant, then sudden drop. Do not call 2:1 block Mobitz II automatically.
"Longer, longer, longer, drop! Then you have a Wenckebach."
Third-degree (complete) heart block
No atrial impulse reaches the ventricles. Atria and ventricles beat independently: AV dissociation. Life depends on an escape rhythm from below the block.
ECG
- P waves at a normal regular rate; QRS complexes at a slower, regular rate.
- No fixed relation between P and QRS (P waves "march through" the QRS complexes).
- The QRS width tells you the level:
| Escape rhythm | Origin | Rate | Reliability | Site of block |
|---|---|---|---|---|
| Narrow QRS (under 0.12 s) | His bundle (junctional) | 50–60/min | Fairly reliable | AV node |
| Broad QRS (0.12 s or more) | Ventricles (below His) | 15–40/min | Unreliable | His-Purkinje system |
Paper speed 25 mm/s; rate = 300 ÷ number of large squares between beats
P to P = 4 large squares → atrial rate = 300 ÷ 4 = 75/min
R to R = 8 large squares → ventricular rate = 300 ÷ 8 = 37.5/min (about 38)
Two independent regular rates with no fixed PR → complete heart block with a slow (probably broad, ventricular) escape
Clinical features
- Tiredness, breathlessness, dizziness, heart failure.
- Stokes-Adams attacks: sudden loss of consciousness (from ventricular standstill or a very slow rate), pale during the attack, flushed on recovery, with no warning.
- Slow, regular pulse that does not speed up with exercise or atropine (if infranodal).
- Large-volume pulse and wide pulse pressure (large stroke volume).
- Intermittent cannon a waves in the JVP: the atrium contracts against a closed tricuspid valve.
- Variable intensity of the first heart sound.
Slow pulse + cannon waves + variable S1 + syncope in an elderly patient = complete heart block.
Heart block in myocardial infarction
| Feature | Inferior MI | Anterior MI |
|---|---|---|
| Artery | Usually right coronary (supplies the AV node) | Left anterior descending (septal branches) |
| Level of block | AV node | Below the node (bundle branches) |
| Escape rhythm | Narrow, reasonably fast | Broad, slow, unreliable |
| Course | Often transient, responds to atropine | Signals extensive infarction; poor outlook |
| Pacing | Monitor; temporary pacing only if unstable | Temporary then permanent pacing usually needed |
Investigations
- 12-lead ECG and rhythm strip; 24-hour Holter or event recorder for intermittent block.
- Electrolytes (potassium), thyroid function, digoxin level if relevant, troponin.
- Echocardiography for structural disease.
- Drug review is essential.
- Electrophysiological study in selected patients with syncope and bundle branch block.
Management
Acute symptomatic bradycardia or block
- ABCDE approach; oxygen if hypoxic; IV access; ECG monitoring.
- Look for adverse features: shock, syncope, myocardial ischaemia, heart failure.
- Atropine IV (for example 0.5 mg, repeated if needed). Works mainly for AV nodal block.
- If no response: isoprenaline or adrenaline infusion, and transcutaneous pacing.
- Arrange temporary transvenous pacing (wire to the right ventricular apex via a central vein), then a permanent pacemaker if the cause is not reversible.
- Treat reversible causes: stop beta-blockers, calcium channel blockers or digoxin (digoxin-specific antibody fragments for serious digoxin toxicity), correct hyperkalaemia, revascularise MI.
Atropine often fails in broad-complex (infranodal) block and in heart transplant patients. Do not delay pacing.
Indications for a permanent pacemaker
- Complete heart block (symptomatic or not, especially with a broad QRS).
- Mobitz II and high-grade block.
- Symptomatic Wenckebach or first-degree block (uncommon).
- Sick sinus syndrome with symptoms.
- Alternating bundle branch block, or bifascicular block with syncope.
- Congenital complete heart block with symptoms or slow rates.
Types of pacemaker
| Code | Meaning | Typical use |
|---|---|---|
| VVI | Paces and senses the ventricle, inhibited by native beats | AF with slow ventricular rate |
| AAI | Paces and senses the atrium | Sick sinus syndrome with intact AV conduction |
| DDD | Paces and senses both chambers | Complete heart block in sinus rhythm (keeps AV synchrony) |
Complications of pacing: pocket haematoma or infection, lead displacement, erosion, pneumothorax, pacemaker syndrome.
Bundle branch and fascicular block
| Block | ECG | Common causes |
|---|---|---|
| RBBB | QRS 0.12 s or more; RSR' ("M") in V1, wide slurred S in I and V6 | Normal variant, ASD, PE, cor pulmonale, ischaemia |
| LBBB | QRS 0.12 s or more; broad notched R ("M") in I, aVL, V6; deep QS in V1 | Ischaemic heart disease, hypertension, aortic stenosis, cardiomyopathy; always abnormal |
| Left anterior hemiblock | Left axis deviation | Ischaemia, fibrosis |
| Left posterior hemiblock | Right axis deviation | Rare |
| Bifascicular | RBBB plus left anterior hemiblock | Risk of complete block |
| Trifascicular | Bifascicular block plus first-degree AV block | Higher risk; pacing if syncope |
WiLLiaM MaRRoW: LBBB = W in V1, M in V6; RBBB = M in V1, W in V6.
New LBBB with typical chest pain is treated as a STEMI equivalent. RBBB gives wide physiological splitting of S2; LBBB gives reversed (paradoxical) splitting.
Expect: PR over 0.20 s in first-degree block; progressive PR lengthening with a dropped beat in Wenckebach; constant PR with sudden dropped beats in Mobitz II and its need for pacing; AV dissociation, cannon waves, variable S1 and Stokes-Adams attacks in complete heart block; narrow vs broad escape rates; inferior vs anterior MI block; drug causes (beta-blockers, verapamil, digoxin); maternal anti-Ro and congenital block; atropine then pacing; and DDD pacing for complete block.
Quick revision
- First degree: PR over 0.20 s, all conducted, no treatment.
- Wenckebach: PR lengthens then a beat drops; AV node; usually benign.
- Mobitz II: constant PR, sudden drop; infranodal; pace.
- Complete block: AV dissociation, slow escape, cannon waves, variable S1, wide pulse pressure.
- Narrow escape 50–60/min (nodal block); broad escape 15–40/min (infranodal block).
- Inferior MI: nodal, transient, atropine. Anterior MI: infranodal, pace.
- Acute: atropine, then isoprenaline or transcutaneous pacing, then transvenous wire.
- Permanent pacing for complete block and Mobitz II; DDD keeps AV synchrony.
Written by The House Job from standard references (Kumar and Clark's Clinical Medicine).
FAQ
Why is Mobitz II more dangerous than Wenckebach?
Mobitz II block lies below the AV node, so it can suddenly progress to complete heart block with a slow, unreliable ventricular escape rhythm. Wenckebach is usually in the AV node and has a dependable escape.
What are cannon waves?
Large a waves in the JVP that appear when the right atrium contracts against a closed tricuspid valve. Intermittent cannon waves suggest AV dissociation, as in complete heart block.
Does first-degree heart block need treatment?
Usually not. Look for and stop AV nodal blocking drugs, and check electrolytes.
Why does heart block after inferior MI usually recover?
The right coronary artery usually supplies the AV node, and ischaemia there causes reversible nodal block with a reliable junctional escape rhythm.