Supraventricular Tachycardia

Electricity Spinning in a Circle

Imagine a relay race where runners pass a baton around a track. Normally, one runner (the SA node) starts at the line, passes to the next runner (the AV node), who slows down to let the baton holder fill up with water (the ventricles fill), then sprints ahead (bundle of His). But in SVT, two runners start on the same leg of the track at different speeds—one fast, one slow. A runner gets blocked on the fast lane, cuts to the slow lane, then when the fast lane is clear again, he runs back up it instead of forward. Now he is running in a tight loop, lap after lap, 3–4 times faster than normal, and the water carriers (ventricles) never get time to fill their cups between handoffs. The baton keeps spinning in a circle (re-entry loop) and the runners are exhausted (the heart muscle is starved of oxygen and blood pressure is falling).