Sinus bradycardia is characterised by regular P waves preceding every QRS complex, at a rate below 60 per minute. P wave originating from the sinus node is usually upright in inferior leads (II, III and aVF).
Important causes of sinus bradycardia
Sinus bradycardia can be seen in vagotonic states like athlete’s heart  and in raised intracranial tension. Another common cause is suppression of the sinus node by beta blockers. In early stages of inferior wall myocardial infarction, sinus bradycardia is often observed . Sinus bradycardia also occurs during vasovagal syncope. Hypothyroidism is another important cause of sinus bradycardia. Sinus bradycardia is also fairly common in the elderly, with one study noting it in around one fifth of those above 60 years . Sinus bradycardia is a classical feature in hypothermia .
Sinus bradycardia was noted in 51% of a group of 330 competitive rowers . Acute organophosphorus poisoning can present with sinus bradycardia . Sick sinus syndrome often manifests with severe and symptomatic sinus bradycardia, though other manifestations like long sinus pauses and tachyarrhythmias with tachy-brady syndrome are likely. Alcohol induced sinus bradycardia can present with recurrent syncope . Ictal sinus bradycardia can occur occasionally with temporal lobe epilepsy .
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