How to read any ECG without missing the thing that kills

The trap is trusting your eyes. A dangerous rhythm hides in plain sight until you read every strip the same boring way, in the same order, every time.

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An ECG lands in your hands. Where does your eye go first?

Nowhere yet. If I go by eye, I pattern-match, and I miss things.

⚠ Trap

But you can see it's a fast rhythm. Just call it.

That's the trap. "Looks like SVT" is how people miss a dying patient.

So what instead?

A fixed order, every single time. Rate, rhythm, axis, intervals, then the ST segments.

Start with rate. How, fast?

Count the big squares between two R waves and divide 300 by it.

So five big squares?

300 over 5 is 60. One big square, 300. It becomes a reflex once you drill it.

rate = 300 / (big squares between R waves)
1 -> 300   2 -> 150   3 -> 100   4 -> 75   5 -> 60

Rhythm next. What are you asking?

Two things. Is it regular? And is there a P wave before every QRS?

Why the P wave?

A P before every QRS, upright in lead II, means the beat starts in the sinus node. Normal sinus rhythm.

And if there are no P waves?

Then the atria aren't driving it. Think atrial fibrillation, or a junctional rhythm.

Move to intervals. Which ones matter?

The PR, the QRS width, and the QT.

Take PR first.

Normal is under 200 milliseconds, one big square. Longer means the signal is dragging through the AV node.

Which is called?

First-degree AV block. Harmless on its own, but a clue.

Now QRS width. Why do you care so much about it?

Because width tells you where the beat is born. Narrow means it came from above the ventricles.

And wide?

Wide is the dangerous word. The ventricles are involved, or conduction is broken.

⚠ Trap

A fast, wide rhythm, then. Probably SVT with a little aberrancy?

No. A wide-complex tachycardia is ventricular tachycardia until you prove otherwise.

Why assume the worst?

Because treating VT as if it were SVT can kill. Here the safe default is the deadly one.

Last, the ST segment. What jumps out?

Elevation. ST elevation across a group of leads is a STEMI, a blocked coronary, until proven otherwise.

And you trust one lead?

Never. I look for reciprocal change, depression in the opposite leads. That's what confirms it's real.

One line to carry?

Read every ECG the same boring way, because the system is what catches the thing your eye skips.

↑ answer it in your head first ↑

the mistakes this catches

Traps

⚠ Pattern-matching the strip at a glance instead of reading it in a fixed order. That is how the dangerous finding gets skipped.
⚠ Calling a wide-complex tachycardia SVT with aberrancy. Treat it as ventricular tachycardia until proven otherwise.
⚠ Trusting ST elevation in a single lead. A real STEMI shows a territory of leads with reciprocal changes.

test yourself, tap to flip

Flash drills

1 What is the systematic order for reading any ECG? tap →

Rate, then rhythm, then axis, then intervals (PR, QRS, QT), then the ST segments and T waves. A fixed order stops you pattern-matching and missing findings.

2 How do you quickly estimate rate on a regular rhythm? tap →

Divide 300 by the number of large squares between two consecutive R waves. One to five squares gives 300, 150, 100, 75, 60.

3 What does QRS width tell you, and why does a wide-complex tachycardia matter? tap →

Width tells you where the beat starts. Narrow means supraventricular, wide means ventricular or abnormal conduction. A wide-complex tachycardia is ventricular tachycardia until proven otherwise, because mistaking it for SVT can be fatal.

4 How do you confirm ST elevation is a true STEMI? tap →

Look for elevation across a coherent territory of leads plus reciprocal ST depression in the opposite leads. Reciprocal change supports a genuine acute coronary occlusion.

5 What does a PR interval longer than 200 ms indicate? tap →

First-degree AV block, a delay in conduction through the AV node. Benign in isolation but a clue to conduction disease.

⟳ These drills come back on a spaced schedule. Review →

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