ECG findings in second-degree AV (type 2) include:
Rhythm: irregular (may be regularly irregular in 3:1 or 4:1 block)
P wave: present but there are more P waves than QRS complexes
PR interval: consistent normal PR interval duration with intermittently dropped QRS complexes
QRS complex: normal (<0.12 seconds) or broad (>0.12 seconds)
The QRS complex will be broad if the conduction failure is located distal to the bundle of His 3
Symptoms may include:
Clinical examination may detect a ‘regularly irregular’ pulse, where there is a pattern of how many atrial depolarisations (P waves) lead to ventricular depolarisation (QRS waves) such as 3:1 block.1
Because of the risk of progression to complete AV block, patients should be placed on a cardiac monitor as soon as possible.
The underlying cause of the AV block should be investigated.
Temporary pacing or isoprenaline may be required if the patient is haemodynamically compromised due to bradycardia.
A permanent pacemaker is usually inserted if there are no reversible causes identified.
Patients are at risk of progressing to symptomatic complete AV block, in which the escape rhythm is likely to be ventricular and thus too slow to maintain adequate systemic perfusion.
Patients are also at risk of developing asystole.3
Third-degree (complete) AV block
Third-degree (complete) AV block occurs when there is no electrical communication between the atria and ventricles due to a complete failure of conduction.4
Typical ECG findings include the presence of P waves and QRS complexes that have no association with each other, due to the atria and ventricles functioning independently.
Cardiac function is maintained by a junctional or ventricular pacemaker.4
Narrow-complex escape rhythms (QRS complexes of <0.12 seconds duration) originate above the bifurcation of the bundle of His. A typical heart rate would be >40bpm.
Broad-complex escape rhythms (QRS complexes >0.12 seconds duration) originate from below the bifurcation of the bundle of His. These escape rhythms produce slower, less reliable heart rates and more significant clinical features (e.g. heart failure, syncope).
Causes of third-degree (complete) AV block include:4
Congenital: structural heart disease (e.g transposition of the great vessels), autoimmune (e.g maternal SLE)
Idiopathic fibrosis: Lev’s disease (fibrosis of the distal His-Purkinje system in the elderly) and Lenegre’s disease (fibrosis of the proximal His-Purkinje system in younger individuals)