Clinical Review ABC of clinical electrocardiography

Bradycardias and atrioventricular conduction block

BMJ 2002; 324 doi: http://dx.doi.org/10.1136/bmj.324.7336.535 (Published 02 March 2002) Cite this as: BMJ 2002;324:535
  1. David Da Costa,
  2. William J Brady,
  3. 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 …

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