bradycardia
Bradycardia simply means the heart is beating slowly — by convention, fewer than 60 times a minute in an adult. On its own that is not a diagnosis but a finding, and whether it matters depends entirely on the person. A slow heart can be a sign of superb fitness or of a failing electrical system.
In well-trained athletes a resting rate in the 40s is common and healthy, because their hearts pump a large volume with each beat and their vagal tone is high. Bradycardia becomes a concern when the rate is too slow to meet the body's needs, causing fatigue, dizziness, breathlessness or fainting — or when it stems from disease of the sinus node or a block in the conduction pathway.
Causes include ageing of the conduction tissue, medications such as beta-blockers, an underactive thyroid, raised vagal tone, and damage from a heart attack. Treatment is reserved for symptomatic or dangerous cases: removing an offending drug, correcting the cause, and, when the slowness is intrinsic and persistent, implanting a pacemaker.
An elderly woman faints while gardening; her ECG shows a heart rate of 38 from complete heart block, and a permanent pacemaker abolishes both the slow rate and the fainting.
Symptomatic bradycardia from intrinsic conduction disease is the classic indication for a pacemaker.
A slow rate is only a problem if it fails to meet the body's demand. Asymptomatic bradycardia, especially in athletes, usually needs no treatment at all.