What is the difference between two ECG lines

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At the hospital, I’ve seen ECG lines that are very different despite a similar heart rate. One is « hilly » with clear peaks and hollows. The other one is quite flat with peaks and hollows very close to the centre line.

In: Biology
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The different lines you see are different views of the heart. Different things can be seen from different directions, so every line adds additional information to help decode the ECG.

Explaining exactly how to read an ECG is more complex than is possible to explain in ELI5.

Understanding ECGs is a lot more complex than a lot of laypeople realise, and requires a good understanding of cardiac electrophysiology, but i’ll try and explain it as simply as I can…

At the top of your heart is a group of cells called the Sino atrial node, that generates electric pulses. The pulses travel down your heart, causing the heart muscle to contract as it goes past, and this is what regulates your heartbeat.

A 12 lead ECG is what you’ll most often get.
Each line you see is a “lead”. A lead is a view of the electrical activity of your heart from a certain orientation/perspective.
On each lead you should see a repeating pattern (the zigzag you see on ambulances and stuff). Each zigzag (complex) is the electric activity from one heartbeat, and each part of the complex is caused by a different part of the heart contracting or relaxing in response to the pulse sent from the Sino atrial node. The first little bump (p wave) is caused by the top of your heart contracting. The second big bump (QRS complex) is the bottom of your heart contracting. The last bump (T wave) is the bottom half recharging. This pattern is the same in every lead.

However, much like a mountain or a building will look very different when observed from different angles, the same is true with the heartbeats on an ECG, and this is why the complexes look so different, despite them representing the same heartbeat.

The pretty lead you see everywhere (lead II) takes a view down the side of your heart, so it “sees” everything. Other leads take a view from “directly below” or “directly from the right” and as such dont really “see” all of the electrical activity going equally, so some parts of the pattern become smaller or larger.

Edit:
Thanks for Silver 😉