Heart Block: First, Second & Third Degree (AV Block)

Heart block — more precisely, AV block — is a delay or failure of the electrical signal passing from the atria to the ventricles. This guide covers all the degrees, how to spot each on the ECG, and which ones need a pacemaker.

What is heart block?

Heart block — more precisely, atrioventricular (AV) block — is a delay or interruption of the electrical signal as it travels from the atria to the ventricles. Normally, each impulse born in the SA node passes through the AV node and the His-Purkinje system to trigger a ventricular beat. In heart block, that hand-off is impaired: sometimes just slowed, sometimes intermittently failing, and sometimes blocked completely so the atria and ventricles beat independently.

The term covers a spectrum, graded from first-degree (every impulse gets through, but slowly) to third-degree or complete block (nothing gets through). Where the block sits in the conduction pathway — the AV node itself versus the lower His-Purkinje system — matters as much as the degree, because it predicts how stable the rhythm is and whether a pacemaker is needed. To follow the anatomy, see our cardiac conduction system guide.

How to analyse AV block on the ECG

Every AV block is worked out from the relationship between the P waves (atrial activity) and the QRS complexes (ventricular activity). Three questions crack almost any strip:

1. Is the PR interval normal, long, or changing? A fixed long PR points to first-degree; a lengthening PR points to Mobitz I.
2. Are any P waves not followed by a QRS? Dropped beats mean second- or third-degree block.
3. Do the P waves and QRS complexes relate to each other at all? If they are completely independent, it's complete block.

Ground these skills in the ECG interpretation guide, then practise on real strips.

First-degree AV block

First-degree AV block is a consistently prolonged PR interval (greater than 0.20 seconds, or one large box) with every single P wave still conducting to a QRS. No beats are dropped — the signal simply takes the scenic route through the AV node. It is common, usually benign, and often needs no treatment at all.

That said, a long PR can be a signpost. It may reflect high vagal tone in a fit young person, the effect of AV-nodal-blocking drugs, or early conduction-system disease. A markedly prolonged PR (well over 0.30 s) can occasionally cause symptoms by desynchronising the atria and ventricles, and rarely warrants pacing.

Second-degree AV block

In second-degree block, some P waves fail to conduct — beats are intermittently dropped. Distinguishing the two subtypes is one of the highest-yield skills in electrocardiography, because they carry very different risks.

Type	ECG hallmark	Usual site	Risk
Mobitz I (Wenckebach)	PR interval lengthens progressively until a P wave is dropped, then the cycle resets	AV node	Usually benign
Mobitz II	Constant PR interval, then a sudden non-conducted P wave with no warning	His-Purkinje (infranodal)	Dangerous — can progress abruptly to complete block

Mobitz I is typically a nodal problem, often from high vagal tone or medications, and is usually stable and benign. Mobitz II is a lower (infranodal) problem, tends to have a wide QRS, and is unstable — it can lurch into complete block without notice, which is why it usually earns a pacemaker even without symptoms. A special case, 2:1 block (every other P wave conducts), can be either type; the company it keeps — a narrow versus wide QRS, and the response to vagal maneuvers or atropine — helps sort it out.

Third-degree (complete) AV block

Third-degree AV block is complete failure of conduction: no atrial impulse reaches the ventricles, so the atria and ventricles beat entirely independently. On the ECG the P waves and QRS complexes have no relationship at all — classic AV dissociation — and the atrial rate is faster than the ventricular rate.

The ventricles are kept going only by an escape rhythm arising below the block. A junctional escape (40–60 bpm, narrow QRS) is relatively reliable; a ventricular escape (20–40 bpm, wide QRS) is slow and unstable. Complete block often causes fatigue, dizziness, or syncope, and it is a medical emergency that usually requires urgent pacing and then a permanent pacemaker.

Causes

Heart block can be acute and reversible or chronic and progressive. Common causes include:

• Ischaemia — especially an inferior myocardial infarction, because the right coronary artery usually supplies the AV node; block here is often transient. Block complicating an anterior MI signals extensive damage and is more ominous.
• Degenerative conduction disease — age-related fibrosis of the conduction system (Lenègre and Lev disease), the most common chronic cause.
• Medications — beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, and some antiarrhythmics.
• High vagal tone — in athletes or during sleep (usually benign, nodal).
• Electrolyte disturbance (notably hyperkalaemia), infections (Lyme carditis, endocarditis), infiltrative disease (sarcoidosis, amyloidosis), congenital block, and cardiac surgery or valve procedures.

Symptoms

The symptoms of heart block track its severity. First-degree and Mobitz I are usually silent. As beats are dropped and the heart rate falls, patients may notice fatigue, light-headedness, breathlessness, reduced exercise tolerance, or palpitations from the pauses. The most dramatic presentation is syncope — a sudden faint from a long ventricular pause, sometimes called a Stokes-Adams attack. Any syncope with high-grade AV block is a red flag for urgent evaluation.

Treatment

Management depends on the type, the symptoms, and whether a reversible cause is present.

Type	Management
First-degree	Usually none; review rate-slowing medications
Mobitz I	Observe if asymptomatic; treat only if symptomatic
Mobitz II	Permanent pacemaker (high risk of progression)
Third-degree	Urgent temporary pacing, then a permanent pacemaker

Acute symptomatic bradycardia is managed with atropine and, if that fails, transcutaneous or transvenous pacing. Always look for and correct reversible causes first — stop offending drugs, treat hyperkalaemia, and reperfuse ischaemia — because block from an inferior MI or drug effect often resolves. When the block is due to irreversible conduction disease or is high-grade and symptomatic, the definitive treatment is a permanent pacemaker.

Key takeaways

• AV block ranges from first-degree (slow) to third-degree (complete).
• Read it from the P-wave-to-QRS relationship and the PR interval.
• First-degree = long PR, all conducted; usually benign.
• Mobitz I lengthens the PR then drops a beat (usually benign); Mobitz II drops suddenly (dangerous).
• Third-degree = full AV dissociation with an escape rhythm — an emergency.
• Mobitz II and third-degree usually need a pacemaker; always correct reversible causes first.

Frequently asked questions

What is heart block?

A delay or interruption of the electrical impulse travelling from the atria to the ventricles through the AV node, ranging from a benign slowing (first-degree) to complete failure (third-degree) that needs a pacemaker.

What is first-degree heart block?

A consistently prolonged PR interval over 0.20 seconds where every P wave still conducts to a QRS; no beats are dropped and it is usually benign.

What is the difference between Mobitz I and Mobitz II?

Mobitz I (Wenckebach) shows progressive PR lengthening until a beat drops and is usually a benign AV-nodal problem; Mobitz II drops beats suddenly with a constant PR, is infranodal, and can progress abruptly to complete block.

What is third-degree heart block?

Complete AV block, where the atria and ventricles beat independently (AV dissociation) and a slow escape rhythm sustains the ventricles; it is an emergency that usually requires a pacemaker.

What causes heart block?

Ischaemia (especially inferior MI), age-related degenerative conduction disease, drugs like beta-blockers and calcium channel blockers, high vagal tone, hyperkalaemia, and infections such as Lyme carditis.

Which types of heart block need a pacemaker?

Mobitz II second-degree block and third-degree (complete) block typically need a permanent pacemaker; first-degree and Mobitz I are usually just observed.

Is heart block an emergency?

High-grade block — Mobitz II and third-degree — can be, especially with a slow ventricular rate or syncope; first-degree and Mobitz I are usually benign.

Can heart block be reversed?

Sometimes — block from an inferior heart attack, drug effect, high vagal tone, or hyperkalaemia often resolves once the cause is treated; block from degenerative conduction disease is permanent.