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Dysfunction of the heart’s pacemaker (sinus or sinoatrial node) may result in a persistently slow heartbeat (sinus bradycardia) or complete cessation of normal pacemaker activity (sinus arrest). When activity ceases, another area of the heart usually takes over the function of the pacemaker. This area, called an escape pacemaker, may be located lower in the atrium, in the atrioventricular node, in the conduction system, or even in the ventricle.
All types of pacemaker dysfunction are more common among older people. Some drugs and an underactive thyroid gland (hypothyroidism) can cause pacemaker dysfunction. However, the cause is usually unknown. When the cause is unknown, the disorder is called sick sinus syndrome.
An important subtype of the sick sinus syndrome is the bradycardia-tachycardia syndrome, in which periods of slow heart rhythms (bradycardia) alternate with periods of fast atrial arrhythmias (tachycardia), such as atrial fibrillation and atrial flutter.
Pacemaker dysfunction affects mainly older people, especially those with another heart disorder or with diabetes. The most common cause is formation of scar tissue (fibrosis) in the sinoatrial node. Other causes include drugs (for example, beta blockers and other antiarrhythmic drugs), excessive impulses from the vagus nerve, which inhibit the heartbeat (as may occur in well-trained athletes), and many disorders that limit blood flow (for example, coronary artery disease) or cause inflammation (for example, rheumatic fever) or myocarditis.
Many types of pacemaker dysfunction cause no symptoms. A persistent slow heart rate commonly causes weakness and tiredness. Fainting may occur if the rate becomes very slow. A fast heart rate is often perceived by the person as palpitations. When the fast heart rate stops, fainting may occur if the pacemaker is slow in restarting normal heart rhythm.
A slow pulse (especially an irregular one), a pulse that varies greatly without any change in the person’s activity, or a pulse that does not increase during exercise suggests pacemaker dysfunction. Doctors can usually diagnose pacemaker dysfunction based on symptoms and the results of electrocardiography (ECG—see Electrocardiography), particularly when heart rhythm is recorded over a 24-hour period with a Holter monitor.
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