Sinus bradycardia Sinus bradycardia is common in normal individuals during sleep and in those with high vagal tone, such as athletes and young healthy adults. The electrocardiogram shows a P wave before every QRS complex, with a normal P wave axis (that is, upright P wave in lead II). The PR interval is at least 0.12 s. Pathological causes of sinus bradycardia Acute myocardial infarction Drugs—for example, β blockers, digoxin, amiodarone Obstructive jaundice Raised intracranial pressure Sick sinus syndrome Hypothermia Hypothyroidism The commonest pathological cause of sinus bradycardia is acute myocardial infarction. Sinus bradycardia is particularly associated with inferior myocardial infarction as the inferior myocardial wall and the sinoatrial and atrioventricular nodes are usually all supplied by the right coronary artery.