Bradycardias and atrioventricular conduction block

doi:10.1136/bmj.324.7336.535

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David Da Costa,
William J Brady,
June Edhouse

By arbitrary definition, a bradycardia is a heart rate of <60 beats/min. A bradycardia may be a normal physiological phenomenon or result from a cardiac or non-cardiac disorder.

Many patients tolerate heart rates of 40 beats/min surprisingly well, but at lower rates symptoms are likely to include dizziness, near syncope, syncope, ischaemic chest pain, Stokes-Adams attacks, and hypoxic seizures

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.

Sick sinus syndrome

Sick sinus syndrome is the result of dysfunction of the sinoatrial node, with impairment of its ability to generate and conduct impulses. It usually results from idiopathic fibrosis of the node but is also associated with myocardial ischaemia, digoxin, and cardiac surgery.

Conditions associated with sinoatrial node dysfunction

Age
Idiopathic fibrosis
Ischaemia, including myocardial infarction
High vagal tone
Myocarditis
Digoxin toxicity

The possible electrocardiographic features include persistent sinus bradycardia, periods of sinoatrial block, sinus arrest, junctional or ventricular escape rhythms, tachycardia-bradycardia syndrome, paroxysmal atrial flutter, and atrial fibrillation. The commonest electrocardiographic feature is an inappropriate, persistent, and often severe sinus bradycardia.

Severe sinus bradycardia

Sinoatrial block is characterised by a transient failure of impulse conduction to the atrial myocardium, resulting in intermittent pauses between P waves. The pauses are the length of two …