Sudden fast: supraventricular tachycardia
Supraventricular tachycardia (SVT) is a fast, regular rhythm that starts above the ventricles — usually a re-entry loop that races at 150–220 beats a minute. It typically switches on and off abruptly, like a light: a person is fine, then their heart suddenly pounds, then it stops just as suddenly. Because the impulse still travels down the normal wiring, the QRS is narrow. SVT is frightening but usually not dangerous in an otherwise healthy heart.
Because the AV node is part of most SVT loops, the rhythm can often be broken by briefly slowing that node — for instance with vagal maneuvers (bearing down, or a cold stimulus) or with medication that an emergency team gives. A special variant, Wolff–Parkinson–White syndrome, involves an extra accessory wire bypassing the AV node, which deserves specialist care because it occasionally allows dangerously fast conduction.
Too slow: when the pace or the wiring fails
Bradycardia means a resting heart rate below 60. In trained athletes this is normal and healthy. It only becomes a problem when the heart is too slow to meet the body's needs, causing fatigue, dizziness, or fainting. Two mechanisms drive abnormal slowing. In sick sinus syndrome, the SA node itself ages and fires unreliably. In heart block, the SA node is fine, but signals are delayed or blocked as they try to cross the AV node into the ventricles.
- First-degree (first-degree AV block): every signal gets through, just slowly — a long PR interval. Usually benign.
- Second-degree: some signals get through and some are dropped, so the ventricles skip beats.
- Third-degree (complete heart block): no signals cross. The atria and ventricles beat independently, and a slow backup pacemaker keeps the ventricles going — often too slow. This is serious.
The fix for a slow heart
When a slow rhythm causes symptoms or comes from complete block, no drug can reliably speed the heart for the long term. The definitive answer is a pacemaker: a small device implanted under the skin with a wire to the heart, which senses the rhythm and delivers a tiny electrical pulse whenever the heart falls too slow. It does nothing while the heart behaves and steps in only when needed — a reliable backstop against dangerous pauses.