First Degree Heart Block

First degree atrioventricular block is a prolonged conduction time through the atrioventricular node, producing a PR interval longer than 200 ms (0.20 seconds) on ECG, with every atrial impulse still conducted to the ventricles. It is usually asymptomatic and is often an incidental finding.

• Causes: high vagal tone (athletes, sleep), ageing and degenerative conduction disease, drugs (beta blockers, verapamil, diltiazem, digoxin, amiodarone, donepezil, clonidine), inferior myocardial infarction, myocarditis (Lyme disease, rheumatic fever, diphtheria, viral), hyperkalaemia, hypothyroidism, sarcoidosis, amyloidosis, aortic valve disease with root abscess, congenital heart disease.
• Prevalence rises with age.
• A markedly prolonged PR interval (over 300 ms) can cause symptoms through loss of atrioventricular synchrony (pacemaker like syndrome).
• Progression to higher degree block is uncommon but possible, especially with bundle branch block or syncope.

• Usually no symptoms.
• Occasionally fatigue, exertional dyspnoea, light headedness or presyncope with markedly prolonged PR interval.
• Examination: regular pulse, soft first heart sound, intermittent cannon A waves.
• Red flags: syncope, bradycardia, bifascicular block, history of cardiac surgery or infective endocarditis, sudden fatigue, new PR prolongation with chest pain, fever, or tick bite.

• 12-lead ECG: constant PR interval over 200 ms with each P wave followed by a QRS. Check for bundle branch block, bifascicular block and ST changes.
• Medication review.
• Bloods: urea and electrolytes (potassium), thyroid function, troponin if ischaemic symptoms, Lyme serology if relevant exposure, inflammatory markers.
• Ambulatory ECG when there is syncope, symptoms, or concurrent bundle branch block, to exclude intermittent higher degree block.
• Echocardiography if structural disease is suspected. Exercise test if marked PR prolongation to confirm shortening with exertion.
• Differentials: second degree block (Mobitz I and II), sinus bradycardia with long PR, blocked atrial ectopics, third degree block.

Asymptomatic

• No specific treatment; reassure and give general cardiovascular risk advice.
• Review drugs prolonging PR: stop or reduce dose of causative drug where possible.
• Avoid combined atrioventricular nodal blocking agents (beta blocker with verapamil or diltiazem) without monitoring.

Treat underlying cause

• Correct hyperkalaemia and hypothyroidism.
• Lyme carditis: doxycycline 100 mg orally twice daily for 14 to 21 days, with specialist input if PR over 300 ms or higher block (intravenous ceftriaxone 2 g daily is used in hospital).
• Inferior infarction: urgent hospital admission for acute coronary care.

Symptomatic or high risk

• Pacemaker is indicated for symptomatic markedly prolonged PR (pacemaker syndrome like symptoms) or progression to Mobitz II, third degree block, or syncope with bifascicular block (specialist decision).

Special groups

• Athletes: benign if PR shortens with exercise; no restriction.
• Pregnancy: not harmful; monitor.
• Older adults: check for bifascicular block, which carries risk of progression.

Follow up and referral

• Annual ECG if PR over 240 ms, bundle branch block, or on blocking drugs; sooner if symptoms.
• Refer urgently for syncope, new bifascicular block, PR over 300 ms with symptoms, or progression to second or third degree block.