Key Facts

First degree heart block (first degree AV block) is characterised by delayed conduction through the AV node, resulting in a prolonged PR interval (>200 ms) on the ECG. All atrial impulses are conducted to the ventricles; no beats are 'dropped'.

Epidemiology

• Prevalence: 1-5% in young adults, higher in athletes (~8%)
• Increases with age: 5-10% in over-60s
• More common in men
• Found in ~2% of apparently healthy adults on routine ECG

Aetiology

• Physiological: increased vagal tone (athletes, sleep)
• Drugs: beta-blockers, rate-limiting CCBs (verapamil, diltiazem), digoxin, amiodarone
• Ischaemic heart disease: inferior MI (RCA supplies AV node in 90%)
• Degenerative: fibrosis of the conduction system (Lenegre/Lev disease)
• Inflammatory: myocarditis, rheumatic fever, endocarditis, sarcoidosis, Lyme disease
• Infiltrative: amyloidosis, haemochromatosis
• Electrolyte: hyperkalaemia
• Congenital: associated with congenital heart defects

Pathophysiology

• Delay in conduction through the AV node (most common), His bundle, or His-Purkinje system
• Intrinsic AV node delay is usually benign (narrow QRS)
• Infra-nodal delay (wide QRS) may indicate more diffuse conduction disease and risk of progression

Typical Presentation

• Usually asymptomatic — incidental finding on ECG
• No abnormal clinical signs in most cases

Symptomatic Cases

• Marked PR prolongation (>300 ms) may cause symptoms of pseudo-pacemaker syndrome: fatigue, exercise intolerance, dyspnoea
• This occurs due to atrial contraction against a closing or closed AV valve, reducing cardiac output

Red Flags

• Progressive PR prolongation on serial ECGs
• Association with broad QRS complex (infra-nodal disease)
• Symptoms of presyncope or syncope (may indicate progression)
• New first-degree block following inferior MI
• Lyme disease symptoms (erythema migrans, arthralgia) with new conduction abnormality

Bedside

• 12-lead ECG: PR interval measurement, QRS width, axis
• Rhythm strip: confirm constant PR interval with 1:1 conduction

Bloods

• U&Es: potassium (hyperkalaemia may impair conduction)
• Digoxin level: if on digoxin
• TFTs: hypothyroidism may contribute
• Lyme serology: if epidemiological risk
• Troponin: if acute presentation with concern for MI

Imaging

• Echocardiography: if structural heart disease suspected

Special Tests

• Holter monitor (24-48 hours): if intermittent symptoms or to assess for progression to higher-degree block
• Exercise ECG: first-degree block that resolves with exercise is likely nodal and benign; persistent or worsening block during exercise suggests infra-nodal disease

Non-pharmacological

• Reassurance in asymptomatic patients with isolated first-degree block and narrow QRS
• Review and adjust medications: reduce or stop AV nodal blocking drugs if clinically appropriate
• Serial ECGs/monitoring: if concern about progression

Pharmacological

• No specific pharmacological treatment for isolated first-degree block
• Treat underlying cause (e.g., antibiotics for Lyme carditis: doxycycline 100mg BD for 21 days, or IV ceftriaxone 2g OD if severe)

Surgical/Interventional

• Permanent pacemaker: rarely indicated; consider only if:
  • Marked PR prolongation (>300 ms) with symptoms (pseudo-pacemaker syndrome)
  • First-degree block with haemodynamic compromise demonstrated on echocardiography
  • Concomitant neuromuscular disease with progressive conduction abnormality

Referral Criteria

• Symptomatic first-degree block: refer to cardiology
• Evidence of progressive conduction disease: cardiology assessment
• New first-degree block post-MI: monitored setting

• Isolated first-degree block with narrow QRS: excellent prognosis, no increase in mortality
• In the Framingham study, isolated first-degree block was associated with a modestly increased risk of AF and pacemaker implantation over long-term follow-up
• First-degree block with wide QRS (infra-nodal): higher risk of progression to complete heart block
• Post-inferior MI: usually transient, resolves within days
• Lyme carditis: usually resolves completely with antibiotic treatment

PR Interval Reference Values

Key Causes by Category