# How an ECG Works

Follow one heartbeat from the spark of the SA node to the P, QRS and T waves an ECG draws\.

By virtualkelvin

Topic: Human Body & Medicine

[Open the interactive Jig](https://ask.jig.com/j/CeKotNVn4rVb)

## Transcript

### 1. Your heart's electric signature

Every heartbeat starts with a tiny electrical signal\. An ECG, short for electrocardiogram, picks up that signal through stickers on the skin and draws it as a wavy line\. It records electricity, not sound or movement\.

A resting heart usually beats about 60 to 100 times a minute\.

### 2. The spark: the SA node

High in the upper right chamber sits a tiny patch of special cells called the SA node\. It is the heart's natural pacemaker: it fires all by itself and starts every beat\. Watch for its flash as each beat begins\.

The heartbeat is slowed to a quarter of its speed here so you can follow the signal\.

### 3. P wave: the top chambers fire

The signal spreads like a ripple across the two upper chambers, called the atria\. They squeeze and push blood down into the lower chambers\. On the screen this makes a small blue bump: the P wave\.

The P wave lasts only about 0\.08 seconds\.

### 4. A clever pause

Next the signal reaches the AV node, a gatekeeper between the upper and lower chambers\. It holds the signal back for a moment, so the lower chambers can finish filling with blood\. On the screen the pause is a short flat line\.

From the start of the P wave to the big spike usually takes about 0\.12 to 0\.20 seconds\.

### 5. QRS: the big spike

Then the signal races down the heart's own special wiring, called the bundle of His and the Purkinje fibres, into the walls of the lower chambers, the ventricles\. These strong chambers fire almost together and squeeze, pumping blood to the lungs and body\. Their big signal draws the tall orange QRS spike\.

The whole spike normally takes less than about 0\.12 seconds\.

### 6. T wave: recharging

After firing, the ventricle cells reset so they are ready for the next beat\. This recharging draws a gentle rounded bump: the purple T wave\. Then the line rests flat until the SA node sparks again\.

The upper chambers recharge too, but their small signal hides inside the big spike\.

### 7. Stickers that listen

By the time the signal reaches your skin, it is tiny: about one thousandth of a volt\. Sticky pads called electrodes pick it up\. Comparing pairs of pads gives different views of the same beat, like cameras around a stage\.

A standard ECG uses 10 stickers to make 12 views, called leads\. Nothing goes into your body: the machine only listens\.

### 8. Reading the rhythm

The grid works like a clock: each small square is 0\.04 seconds and each big square is 0\.2 seconds\. Counting squares between spikes shows how fast the heart beats and whether every beat looks the same\. Slide the heart rate and watch the spikes crowd together or spread apart\.

Five big squares make one second\.