bradycardia
Meaning of bradycardia: A slow pulse is a problem only when it fails to deliver enough blood; in a trained athlete the same reading is fitness.
Definition of bradycardia
Bradycardia is a heart rate slower than the expected resting range, conventionally counted below sixty beats per minute in an adult. It may be entirely normal, as in trained athletes and during sleep, or it may reflect disease of the heart's electrical system, the effect of a medicine, or another condition that slows the heart. Whether it matters depends on the blood flow it produces.
How slow is too slow?
The usual line drawn for adults is a resting rate below sixty beats per minute, but the number means little by itself. Endurance athletes commonly rest in the forties, because a powerful heart moves more blood with each beat and needs fewer of them. Rates fall during sleep in almost everyone. What counts is whether the slower rate still supplies the body: one person holds a steady output and notices nothing, while another at the identical rate becomes dizzy, exhausted or breathless because the circulation cannot keep pace.
What causes it?
| Source | Mechanism |
|---|---|
| Physical conditioning | A larger volume per beat needs fewer beats |
| Sinus node disease | The heart's own pacemaker fires slowly or pauses |
| Heart block | Signals from the upper chambers reach the lower ones late or not at all |
| Medicines | Beta blockers, some calcium channel blockers, digoxin |
| Other conditions | Underactive thyroid, low body temperature, high potassium, raised pressure inside the skull |
Vagal responses form a group of their own. Straining, vomiting, swallowing, pain or the sight of blood can slow the heart sharply for a few seconds through the vagus nerve, occasionally far enough to cause a faint, and the rate then recovers by itself. Drug causes are the ones most often overlooked, because a rate slowed deliberately by a beta blocker looks the same on a tracing as one slowed by conduction disease.
When does a slow rate become dangerous?
When the beat stops supplying the brain and the rest of the body. Dizziness, confusion, unusual lethargy, chest pain, breathlessness and fainting mark the point at which output has become inadequate, and that combination is time critical because the rhythm can deteriorate further. Complete heart block, where no signal at all crosses from the upper chambers to the lower ones, is the clearest example. Where the slowing is permanent and causing symptoms, a pacemaker supplies the missing impulses electrically; where a medicine or a treatable condition is responsible, the cause is addressed instead. The opposite finding, an unusually fast rate, is tachycardia.
Also known as
Used in a sentence
Bradycardia was noted on the overnight monitor without associated symptoms.
Good to know
The rate alone does not establish illness, since a figure that represents fitness in a sleeping athlete can represent conduction disease in an older adult with dizziness; symptoms and the electrical pattern together determine what it means.
Word origin
Where “bradycardia” comes from
From the Greek bradys, meaning slow, and kardia, meaning heart.