A built-in metronome
Your heart does not wait for a signal from the brain to beat. It makes its own. Tucked into the top of the right atrium is a small patch of specialized cells called the sinoatrial (SA) node — the heart's natural pacemaker. These cells have a remarkable property called automaticity: they slowly leak electrical charge until they reach a threshold, fire, and reset, over and over, roughly 60 to 100 times a minute at rest. Each firing launches a wave of electricity that spreads across the heart and triggers a heartbeat.
When the SA node is in charge and everything follows in order, we call it sinus rhythm — the normal, healthy heartbeat. An arrhythmia is simply any rhythm that departs from this: too fast, too slow, irregular, or arising from the wrong place. Some arrhythmias are harmless quirks; others are emergencies. The whole field of rhythm medicine is about telling them apart.
The conduction highway
The electrical wave does not spread randomly — it travels a fixed route called the conduction system. After the SA node fires, the signal sweeps across both atria, squeezing them so they top up the ventricles with blood. Then it reaches a crucial relay station, the atrioventricular (AV) node, which deliberately slows the signal for a fraction of a second. This pause lets the atria finish emptying before the ventricles contract.
- SA node fires (the spark) — sets the pace in the right atrium.
- Wave spreads across both atria, which contract and fill the ventricles.
- AV node holds the signal briefly — the only normal electrical gateway from atria to ventricles.
- Signal races down the His-Purkinje wiring, and the ventricles squeeze, ejecting blood to the body and lungs.
Where rhythms break down
Almost every arrhythmia comes from one of three failures. First, the spark can misfire — an ectopic pacemaker elsewhere in the heart steals control from the SA node. Second, the wiring can fail — a block somewhere along the conduction system slows or stops signals from getting through. Third, a signal can start chasing its own tail, circling a loop of tissue endlessly (called re-entry), which drives many of the fast rhythms. Naming where in the heart the trouble sits — atria versus ventricles — is the single most useful first question.