Sinus Node Disease

The term sinus node disease, sometimes referred to as sick sinus syndrome, is applied to a wide range of sinus node conduction abnormalities, including:

• Inappropriate sinus bradycardia — a slow heart rate that does not increase with exertion and which is not caused by drugs.
• Sinoatrial exit block — missed (or ‘dropped’) beats, where the underlying rhythm is retained. Each heartbeat is initiated within the sinus node, but fails to conduct to the atria.
• Tachy-brady syndrome — sudden changes between rapid and slow heart rates controlled by the sinus node.
• Sinus arrest and asystole — pauses in excess of three seconds.

Sinus node disease increases in incidence with age and is a strong indication for artificial pacemaker implantation.

The ECG trace below shows sinus rhythm with the expected position of QRS complexes arrowed. The P waves are highlighted to indicate a lack of atrial depolarisation where beats are missed.

Note that the final QRS complex is premature and the P wave morphology is different — this beat is an atrial ectopic.

The following example shows a delay between beats of over four seconds. The RR interval is highlighted.