Third degree atrioventricular block (complete heart block)

Third degree atrioventricular block (complete heart block)

Definition

Third degree heart block is complete electrical dissociation of the atria from the ventricles.

Aetiology

Acute third degree heart block is almost always as a result of inferior myocardial infarction due to occlusion of the right coronary artery, which supplies the AV node and bundle of His. It may also occur following a massive anterior myocardial infarction and is a sign of poor prognosis.

Chronic complete heart block is most commonly due to fibrosis of both bundle branches in the elderly. Rare causes include drugs, post-surgery, rheumatic fever and myocarditis.

Pathophysiology

With AV dissociation, an ectopic ventricular pacemaker is responsible for maintaining ventricular contractions. Depending on the site of this pacemaker the QRS complexes may be either narrow or wide:

Narrow complex disease is due to disease of the AV node or proximal bundle of His. The ectopic pace-maker within the specialised conducting fibres distal to the lesion gives a reliable rate of 50–60 bpm and is associated with congenital heart disease, inferior infarction, rheumatic fever and cardiac drugs, e.g. Î²-blockers.

Broad complex disease is due to more distal disease of the Purkinje system. The pacing thus arises within the myocardium giving an unreliable 15–40 bpm rate. In the elderly causes include fibrosis of the central bundle branches (Lenegre’s disease). It may also be associated with ischaemic heart disease.

Clinical features

Severity of symptoms is dependent on the rate and reliability of the ectopic pacemaker, and whether or not the myocardium can compensate for the bradycardia. Patients with underlying ischaemic heart disease, particularly recent myocardial infarction are most at risk of complications. Symptoms include those of cardiac failure, dizziness and Stokes–Adams attacks (syncopal episodes lasting 5–30 seconds due to failure of ventricular activity).

On examination, there are occasional cannon waves in the JVP due to the atria contracting on a closed tricuspid valve, with a variable intensity of the first heart sound.

Investigations

The ECG is diagnostic revealing independent, unrelated atrial and ventricular activity.

Complications

Cardiac failure, Stokes–Adams attacks, asystole, sudden cardiac death.

Management

In acute complete heart block, intravenous isopre-naline or a temporary pacing wire may be used. Post-MI it often resolves within a week.

Identification and removal of any cause.

Chronic complete AV block requires permanent pacing even in asymptomatic cases as this reduces mortality.

Prognosis

Untreated chronic AV block with Stokes–Adams episodes has a 1-year mortality of 35–50%.