Case B3. Sino Atrial Block. St Emlyn’s ECG Library

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History

The ECG was recorded from an 80 year old woman with a history of episodic loss of consciousness.

ECG

Sinus bradycardia is present at a rate of 52 per minute. Normal P waves are present in front of every QRS complex and every P wave is conducted. The PR interval is constant but prolonged at .24 seconds so first degree AV block is present. P wave and QRS complex morphology are constant. Pauses are noted when no P wave (nor QRS complex) occur at the expected time. No escape beat is seen and the pause is terminated by a sinus beat. The duration of each pause is double the R-R interval seen during normal rhythm. The underlying QRS morphology is normal.

Comment

Sino atrial block is present. In this condition the sinus node depolarizes normally but there is an intermittent failure of conduction of the impulse to the atrial myocardium (exit block) with failure of atrial depolarization. The subsequent discharge from the sinus node triggers atrial depolarizadon at the normal time so the pause resulting from the missed beat is double the basic cycle length. In the fourth and fifth row episodes are seen where sino atrial block occurs after alternate beats resulting in a rhythm that is half the normal rate. It is
possible for several consecutive sinus node discharges to be blocked which results in a prolonged pause (the duration of which is a multiple of the basic cycle length). Ambulatory monitoring showed such prolonged pauses in this patient and a permanent pacing system was implanted. Gauses of sino-atrial block include myocardial infarction and ischaemia but the condidon is very often a manifestadon of sino-atrial disease where an idiopathic degenerative process affects the SA node. First degree AV block is also present in this case implying that there is conduction delay in the AV node in addition to the problems that exist in the SA node. Ambulatory monitoring however did not show more advanced degrees of AV block in this patient.

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