First Degree Heart Block

Definition

PR interval > 200ms (five small squares)
‘Marked’ first degree block if PR interval > 300ms
Marked first degree heart block (PR interval > 300 ms, P waves are buried in the preceding T wave)

Causes

Increased vagal tone
Athletic training
Inferior MI
Mitral valve surgery
Myocarditis (e.g. Lyme disease)
Electrolyte disturbances (e.g. Hyperkalaemia)
AV nodal blocking drugs (beta-blockers, calcium channel blockers, digoxin, amiodarone)
May be a normal variant

Clinical significance
Does not cause haemodynamic disturbance
No specific treatment is required.