AV blocks: first, second (Mobitz I and II) and third degree

What it is

Atrioventricular (AV) block is a delay or failure of conduction from the atria to the ventricles, at the AV node or lower down in the His–Purkinje system. Its degree is read from the relationship between P waves and QRS complexes. In first-degree block, every P is conducted but the PR interval is longer than 200 ms. In second-degree block, some P waves are not conducted: in Mobitz type I (Wenckebach) the PR lengthens beat by beat until a P is dropped, a pattern of the AV node that is often benign; in Mobitz type II the PR is constant and a P is dropped without warning, which usually means disease below the node and a high risk of sudden complete block. In third-degree (complete) block no P wave is conducted: the atria and ventricles beat independently, and the ventricular rate depends on the escape rhythm, narrow at 40–60 bpm if junctional, wide at 20–40 bpm if ventricular. Causes include increased vagal tone, drugs that slow the AV node, ischaemia (an inferior infarct affects the node; an anterior one the bundles), degenerative fibrosis of the conducting system, hyperkalaemia, Lyme and Chagas disease, infiltrative disease such as sarcoidosis, endocarditis with a root abscess, and cardiac surgery or valve procedures.

How to recognise it

• First-degree AV block: PR above 200 ms (more than one large square), identical on every beat, no dropped beats.
• Mobitz I (Wenckebach): progressive PR prolongation, then a non-conducted P; the R–R shortens before the pause; beats come in groups.
• Mobitz II: PR constant (normal or long) before and after a non-conducted P; the P waves march out on time; QRS often wide.
• 2:1 block: every second P is non-conducted; look for the hidden P in the T wave; longer strips may reveal the type.
• High-grade (advanced) block: two or more consecutive P waves not conducted (for example 3:1), with some conducted beats.
• Third-degree block: P–P regular and R–R regular but independent; PR varies at random; escape QRS narrow (40–60 bpm) or wide (20–40 bpm).

Why it happens

• The AV node normally delays the impulse (most of the PR interval) so the ventricles fill after atrial contraction.
• Vagal tone, AV-nodal drugs (beta-blockers, verapamil, diltiazem, digoxin, amiodarone) and inferior ischaemia slow the node: first degree and Mobitz I.
• In Mobitz I the node fatigues progressively until one impulse fails; the pause lets it recover and the cycle repeats.
• Mobitz II and most wide-escape complete blocks come from disease in the His–Purkinje system: fibrosis, anterior infarction, infiltration or surgery.
• In complete block a subsidiary pacemaker takes over: the lower it sits, the wider the QRS and the slower and less reliable the rate.

Step by step

1. Find every P wave, including those hidden in T waves — march them out with calipers or a paper edge.
2. Check whether each P is followed by a QRS — dropped P waves mean second- or third-degree block.
3. Measure the PR on every beat — long and constant is first degree; lengthening then a drop is Mobitz I.
4. If a P drops with a constant PR before and after, call it Mobitz II — infranodal and dangerous.
5. If P–P and R–R are each regular but unrelated, call it complete block — then note the escape QRS width and rate.
6. Look at QRS width and the axis — a coexisting bundle branch or fascicular block points to disease below the node.
7. Check the patient and the drug chart — haemodynamic state and reversible causes decide the next step.

Telling the AV blocks apart

Differential and pitfalls

• Blocked premature atrial beats: the non-conducted P comes early and has a different shape; in Mobitz II the P arrives on time.
• AV dissociation without block (for example, accelerated junctional rhythm or ventricular tachycardia) has a ventricular rate faster than the atrial rate.
• Sinus arrhythmia can make PR and R–R look variable; measure the PR, not the R–R, to diagnose a block.
• 2:1 block: atropine or exercise improves nodal block and may worsen infranodal block; a narrow QRS and long PR favour the node.
• Inferior infarction often gives transient nodal block that responds to atropine; block in anterior infarction signals extensive necrosis and a poor prognosis.
• First-degree block with a new long PR in endocarditis suggests an aortic root abscess; in a young patient from a rural area consider Chagas or Lyme carditis.

Next step

• Mobitz II, high-grade or complete block: continuous monitoring, defibrillator pads on, call the senior and cardiology at once.
• Adverse features (shock, syncope, ischaemia, heart failure): atropine by protocol, then transcutaneous pacing or an isoprenaline infusion while transvenous pacing is arranged.
• Stop AV-nodal drugs where possible; check potassium, digoxin level if relevant, troponin and an echocardiogram.
• Ask about tick bites, travel and Chagas exposure, and request serology when the history fits.
• Asymptomatic first-degree or Mobitz I block: no acute treatment; review drugs and repeat the ECG; persistent or symptomatic block needs cardiology follow-up.

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Elsewhere in DurgiPedia

• ECG basics: waves, intervals and leads
• The cardiac conduction system
• The ECG in potassium and calcium disorders

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Atrioventricular Block (StatPearls (NCBI Bookshelf), 2024); First-Degree Heart Block (StatPearls (NCBI Bookshelf), 2026); Second-Degree Atrioventricular Block (StatPearls (NCBI Bookshelf), 2023); Third-Degree Atrioventricular Block (StatPearls (NCBI Bookshelf), 2023).

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