Atrioventricular Blocks and Bradyarrhythmias
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Direct answer
Atrioventricular block is delayed or interrupted conduction from atrium to ventricle, classified as first, second or third degree by the proportion of atrial impulses that reach the ventricles. Mobitz type II block and complete heart block carry a risk of syncope and usually need a permanent pacemaker, whereas first-degree and Mobitz type I block are often benign and need only the underlying cause treated.
What you must remember
- First degree: PR interval longer than 200 ms with every P wave conducted; commonly due to vagal tone, beta blockers, digoxin or inferior ischaemia.
- Mobitz I (Wenckebach): the PR interval lengthens progressively until a P wave is dropped; the block lies in the AV node, the QRS is narrow, and pacing is usually unnecessary.
- Mobitz II: constant PR intervals with sudden dropped beats; the block is infranodal in the His-Purkinje system, the QRS is often wide, and pacing is indicated even when asymptomatic.
- Third degree: atrioventricular dissociation with P waves and QRS complexes unrelated; a narrow junctional escape at 40 to 60 per minute is more favourable than a wide ventricular escape at 20 to 40 per minute.
- Atropine: increases AV nodal conduction and suits nodal block with a narrow escape, but it is unreliable in infranodal block; up to 3 mg intravenously in total is the usual ceiling.
- Pacing triggers: symptomatic bradycardia, Mobitz II, complete block with symptoms or a wide escape, and sinus node dysfunction with pauses longer than three seconds.
- Drug and metabolic causes: beta blockers, digoxin, verapamil, diltiazem, amiodarone and hyperkalaemia precipitate AV block and are withdrawn first.
A bradycardic patient after an inferior infarct
A 70-year-old woman with an inferior STEMI develops heart rate 38 per minute, blood pressure 88 over 60 mmHg, dizziness and a narrow junctional escape rhythm at about 45 per minute. The first step is to treat the symptoms. Atropine is given intravenously, which will often work in a nodal block like this one. If it fails, transcutaneous pacing is started as a bridge, followed by transvenous temporary pacing if the bradycardia persists, since the escape rhythm is unreliable.
Inferior infarction typically blocks the AV node through its supply from the right coronary artery, so the block is often transient and resolves as reperfusion succeeds. The decision about permanent pacing is therefore delayed for several days, and the ECG is reviewed serially. Had the infarct been anterior, with new complete block and a wide ventricular escape, the prognosis would be worse, and temporary pacing would be the immediate priority. The key learning is that the site of the block and the width of the escape decide the urgency, more than the heart rate alone.
Where the exam tries to catch you
Examiners test whether candidates confuse Mobitz I with Mobitz II. In Wenckebach, the PR lengthens before the dropped beat, and the block is nodal and usually benign; in Mobitz II, the PR is constant, the block is infranodal, and the risk of progression to complete block justifies a pacemaker. A 2:1 block cannot be classified from the PR pattern alone, so the candidate should look at the QRS width and the response to exercise or atropine.
The second trap is atropine. It is a vagolytic that acts on the AV node, so it helps nodal block but has little effect in infranodal block with a wide escape. Candidates who expect atropine to rescue every bradycardia miss this distinction. In Indian district hospitals, digoxin toxicity and beta blocker overdose are common causes of AV block, and the viva answer names the drug before the pacemaker.
Frequently asked questions
Which ECG feature defines Mobitz type II block?
Mobitz type II block shows a constant PR interval with intermittent non-conducted P waves, with the block lying below the AV node. It usually requires a permanent pacemaker even when the patient is asymptomatic.
Where is the block located in Mobitz type I?
Mobitz type I, or Wenckebach block, lies in the AV node and is usually benign. The PR interval lengthens progressively before a P wave is dropped, and pacing is generally not required unless symptoms are present.
What is the maximum total dose of atropine for symptomatic bradycardia?
Intravenous atropine is given in repeated doses up to a total of 3 mg for symptomatic bradycardia, as advised in advanced life support guidance. Atropine is less effective when the block is infranodal, so pacing is needed if it fails.
Why is atropine unreliable in complete heart block with a wide escape rhythm?
Atropine acts on the AV node, but a wide ventricular escape arises below the block in the His-Purkinje system, where vagal effects are minimal. Transcutaneous or transvenous pacing is therefore needed rather than relying on atropine.