Sinus node dysfunction (SND), also called sick sinus syndrome (SSS), is a group of heart rhythm abnormalities caused by the sinus node (the heart’s natural pacemaker) failing to generate or conduct electrical impulses properly. The result can be a heart rate that is inappropriately slow, with or without intermittent fast rhythms alternating with slow ones (tachy-brady syndrome).
The role of the sinus node
The sinus node is a small cluster of specialised cells in the right atrium. It normally generates regular electrical impulses 60 to 100 times per minute at rest, setting the heart’s rhythm. When the sinus node fails, the heart rate falls, causing symptoms related to reduced cardiac output.
Causes
SND can result from ageing and degeneration of the sinus node, coronary artery disease, cardiomyopathy, inflammatory conditions, cardiac surgery, or certain medications (particularly beta-blockers and calcium channel blockers, which slow the sinus node as a side effect).
Symptoms
Symptoms depend on how slow the heart rate becomes and how abruptly it falls:
- Fatigue and low energy
- Dizziness and lightheadedness
- Near-fainting or fainting (syncope)
- Breathlessness on exertion
- In tachy-brady syndrome, palpitations alternating with symptoms of low heart rate
Treatment
The definitive treatment for symptomatic SND is implantation of a permanent pacemaker, which provides a guaranteed minimum heart rate. In tachy-brady syndrome, a pacemaker supports the slow phases while antiarrhythmic medications or ablation treat the fast phases. Medication causing SND should be reviewed and discontinued if possible.
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