ECG Rhythm Interpretation

Rhythm identification comes down to two questions — regular or irregular, narrow or wide? Answer them and the diagnosis falls out.

The two-question approach

Before naming a rhythm, answer two things:

	Narrow QRS	Wide QRS
Regular	Sinus, SVT, atrial flutter	VT, SVT + aberrancy
Irregular	Atrial fibrillation, MAT	AFib + aberrancy, polymorphic VT

Then confirm with the P waves and rate.

The rhythms on strips

Key distinguishing features

Rhythm	Give-away
Atrial fibrillation	Irregularly irregular, no discernible P waves
Atrial flutter	Sawtooth flutter waves, often 2:1 or 4:1
SVT	Narrow, regular, fast (150–250), P waves hidden
VT	Wide, regular, fast — treat as VT until proven otherwise
VFib	Chaotic, no complexes, pulseless → defibrillate
Complete heart block	P waves and QRS march independently

Don't-miss emergencies

• VFib / pulseless VT → defibrillate + CPR.
• Asystole / PEA → CPR + epinephrine, not a shock.
• Unstable SVT or VT → synchronised cardioversion.

Summary

• Ask regular/irregular and narrow/wide first.
• Confirm with P waves and rate.
• Learn the give-away of each rhythm from real strips.
• Know which rhythms are shockable.

Frequently asked questions

How do you identify an ECG rhythm?

Ask two questions first — is it regular or irregular, and is the QRS narrow or wide? Then confirm with the P waves and rate to name the rhythm.

What is the difference between VT and SVT?

VT is a wide-complex (≥0.12 s) regular tachycardia; SVT is narrow-complex. Treat a wide regular tachycardia as VT until proven otherwise.

How do you recognise atrial fibrillation?

An irregularly irregular rhythm with no discernible P waves and a fibrillatory baseline.

Which rhythms are shockable?

Ventricular fibrillation and pulseless ventricular tachycardia. Asystole and PEA are not shockable.