Complete Heart Block

Complete (third-degree) heart block is a total failure of conduction between the atria and the ventricles, resulting in regular atrial activity with an independent ventricular escape rhythm — atrioventricular dissociation.

The escape rhythm is usually ventricular in origin, with a wide QRS complex and a rate of about 30–40 beats per minute.

A nodal or junctional escape with a narrow QRS implies the anatomical level of block is higher — within the AV node or bundle of His.

The lower the origin of the escape rhythm, the less specialised and therefore the less reliable the conduction tissue — this determines both the rate and the risk of asystole.

Idiopathic age-related fibrosis of the conduction system (Lev’s and Lenègre’s disease) is the commonest cause in the elderly.

Ischaemic heart disease — acute inferior MI (usually reversible, nodal level) or anterior MI (infranodal, poor prognosis).

Drugs — beta blockers, non-dihydropyridine calcium channel blockers (verapamil, diltiazem), digoxin, and antiarrhythmics. Concomitant use of diltiazem or verapamil with beta blockers increases the risk of bradycardia, heart block and asystole, particularly in older people.

Infiltrative disease — cardiac sarcoidosis, amyloidosis.

Neuromuscular disease — myotonic dystrophy type 1, Kearns-Sayre syndrome, lamin A/C mutations.

Iatrogenic — following cardiac surgery, TAVI, or catheter ablation of the AV junction.

Infective — infective endocarditis with aortic root abscess, Lyme carditis; also hyperkalaemia and hypothyroidism as reversible causes.

Syncope (Stokes-Adams attack), presyncope, dizziness, fatigue, exertional breathlessness, and worsening heart failure.

Examination: slow regular pulse, cannon ‘a’ waves in the JVP, variable intensity of the first heart sound, and a wide pulse pressure.

In frail older patients the presentation may be falls, confusion, or functional decline rather than classical syncope.

Pacing should be undertaken in acquired complete heart block not attributable to reversible or physiologic causes, regardless of symptoms — Class I recommendation.

Complete heart block due to neuromuscular disease with conduction disorders (myotonic dystrophy type 1, Kearns-Sayre syndrome) — Class I.

Symptomatic complete heart block arising from clinically necessary guideline-directed therapy (for example a beta blocker that cannot be stopped) — Class I.

Infiltrative cardiomyopathy such as cardiac sarcoidosis with Mobitz II block — Class IIa.

DDD pacing is recommended over AAI or VVI in those requiring permanent pacing for AV block (Class I), because it preserves AV synchrony.

Single-chamber ventricular (VVI) pacing is effective where frequent ventricular pacing is not expected, or where significant comorbidities are likely to determine outcome and limit the benefit of dual-chamber pacing (Class I).

Active patients over 70 benefit from DDD pacing in terms of quality of life; UKPACE (n=2021, mean age 80) found no difference in all-cause mortality, AF or heart failure between single- and dual-chamber pacing for AV block.

Physiological pacing (CRT or His bundle pacing) should be considered when LVEF <50% and >40% ventricular pacing is expected (Class IIa). His bundle pacing may be considered for AV nodal-level block (Class IIb).

BLOCK-HF showed CRT reduced the combined endpoint of urgent heart failure admission, death, or ≥15% increase in LV volume index, with about a 30% reduction in heart failure hospitalisation, though no mortality benefit.

In permanent or persistent AF where rhythm control is not planned, an atrial lead should not be implanted (Class III).

Complications: pneumothorax (1–2% subclavian, <0.1% cephalic/axillary), bleeding, myocardial perforation and tamponade (<0.2%), lead dislodgement, infection (<1%), and late device erosion or lead fracture. Early complications occur in about 7% within two weeks; late complications in up to 10%.

Increasing age was not associated with a higher risk of complications (FOLLOWPACE) — female sex and low BMI were.

In cachectic patients with thin skin or skeletal deformity from osteoarthritis or osteoporosis, the device may need to be placed submuscularly to avoid erosion.

All patients should undergo transthoracic echocardiography before implantation to identify pre-existing LV dysfunction and guide mode selection.