The medical term for the heart "skipping a beat" is extrasystole, though many people refer to this as a heart palpitation. While these episodes may be worrisome, they are generally harmless, and the organ quickly regains a healthy rhythm. A variety of health conditions and minor factors can contribute to extrasystoles.
The medulla in the brain releases adrenaline into the blood in response to danger. This causes the blood vessels to contract, the palms to sweat, and the heart to race. The idea is to give the body a power boost, so it can be ready to fight or run. However, sometimes the body produces excessive amounts of the hormone for other reasons -- often a primary medical condition -- and this can lead to a cascade of problems. Without any way to expend that extra energy, excess adrenaline causes anxiety and spikes in blood pressure and heartbeat, triggering an extrasystole.
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