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). It 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.
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