Atrioventricular block

Definition

Atrioventricular (AV) block is a partial or complete interruption of impulse conduction from the atria to the ventricles. The block may be located in the AV node or below it in the His-Purkinje system (His bundle, bundle branches). First- and second-degree AV blocks are partial; third-degree AV block is complete.

Classification

Degrees of AV block

• First-degree AV block: conduction delay without dropped beats; every P wave is conducted, but the PR interval is longer than 0.20 s.
• Second-degree AV block, Mobitz type I (Wenckebach): the PR interval lengthens with each beat until a P wave is not conducted and a QRS complex is dropped; the next conducted beat has the shortest PR interval, and the sequence repeats. With a narrow QRS the block is almost always in the AV node.
• Second-degree AV block, Mobitz type II: constant PR interval with intermittently nonconducted beats, often in a fixed ratio (e.g. 3:1 or 4:1); the RR interval spanning the pause equals twice the normal RR interval. The block is located in the His bundle in about 25 % and in the bundle branches in the rest; Mobitz II is always pathological.
• High-grade second-degree AV block: every second or more P waves are blocked; because two P waves are never conducted in a row, distinguishing type I from type II is difficult.
• Third-degree (complete) AV block: no P wave is conducted; atria and ventricles beat independently (AV dissociation), and the atrial rate is higher than the ventricular rate.

Occurrence & epidemiology

First-degree AV block and Mobitz I may be physiological in younger people with high vagal tone and in well-trained athletes. In people aged 18 to 60 years with new-onset Mobitz II or third-degree AV block without evident structural heart disease, up to one third are diagnosed with cardiac sarcoidosis after targeted investigation.

Aetiopathogenesis

• Idiopathic fibrosis and sclerosis of the conduction system: most common cause (about 40 %)
• Ischemic heart disease: about 20 %. In acute inferior infarction the block is usually located in the AV node (increased vagal tone, local adenosine release) and often resolves within a few days; in anterior infarction it indicates extensive necrosis involving the His-Purkinje system.
• Drugs that slow AV conduction, e.g. certain antiarrhythmic and rate-slowing cardiac drugs
• Other causes: increased vagal tone, cardiac sarcoidosis, valvular heart disease, congenital and genetic disorders
• Transient or reversible causes: Lyme carditis and neurally (vagally) mediated block

Clinical features

Symptoms by degree

• First-degree AV block: rarely symptomatic; with a very long PR interval (over 0.30 s), loss of AV synchrony can cause exertional dyspnea and fatigue.
• Mobitz I: often asymptomatic; usually progresses slowly.
• Mobitz II: asymptomatic or with light-headedness, presyncope and syncope, depending on the ratio of conducted to blocked beats; progression to complete block is typical and may be sudden.
• Third-degree AV block: an escape rhythm above the bifurcation of the His bundle (junctional) has narrow QRS complexes, a rate of about 35–50/min and is relatively reliable; symptoms tend to be mild (fatigue, postural light-headedness, effort intolerance). An escape rhythm below the bifurcation (ventricular) has wide QRS complexes, only 20–40/min, is unreliable and causes presyncope, syncope and heart failure.
• Adams-Stokes attack: sudden loss of consciousness due to asystole when no escape rhythm takes over; the risk is greatest with slow escape rhythms.
• Signs of AV dissociation: cannon a waves in the jugular venous pulse, fluctuating blood pressure, varying loudness of the first heart sound.

Diagnosis

ECG

• 12-lead ECG with rhythm strip: the ratio of P waves to QRS complexes, behavior of the PR interval, QRS width and escape rate indicate the degree and likely level of the block.
• Clues to the level of block: a narrow QRS in Mobitz I indicates block in the AV node; with a wide QRS (over 0.12 s) it lies in the His-Purkinje system in about 60–70 %.
• In Wenckebach block the RR interval spanning the pause is shorter than two normal RR intervals; in Mobitz II it equals exactly two RR intervals.

Further tests

• 24-hour ambulatory ECG and exercise testing: detection of intermittent block and assessment of AV nodal and infranodal conduction
• Electrophysiological study: localization of the block (nodal or infranodal), e.g. after spontaneous resolution of a block
• Search for the cause: drug history, signs of acute infarction, Borrelia serology if Lyme carditis is suspected, echocardiography if structural heart disease is suspected; in younger adults with unexplained high-grade block, targeted evaluation for cardiac sarcoidosis

Keep learning in the app

Further reading (open access)

1. MSD Manual Professional: Atrioventricular Block
2. StatPearls: Atrioventricular Block
3. StatPearls: Third-Degree Atrioventricular Block
4. MSD Manual Professional: Lyme Disease

Cross-references