How can you differentiate sinus tachycardia from atrial tachycardia on ECG?

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Multiple Choice

How can you differentiate sinus tachycardia from atrial tachycardia on ECG?

P waves reveal where the atrial depolarization starts. When tachycardia is sinus, the impulse comes from the SA node, so the atrial activation sequence is normal and you see P waves before every QRS with a normal P-wave axis. That consistent, normal axis is the clearest sign that the rhythm is sinus tachycardia. In atrial tachycardia, the atrial focus is ectopic, so the P waves reflect a different atrial activation pattern. This changes the P-wave morphology and axis, often making them differently angled or shaped compared with sinus P waves, even though a P wave still precedes each QRS. The other statements aren’t reliable: P waves absent during tachycardia would point to a rhythm without organized atrial activity, not typical for atrial tachycardia; the rate being "always faster" than sinus tachycardia isn’t true, since atrial tachycardia can have similar or even faster rates; and P-wave morphology being identical in both would defeat the distinguishing feature by design. So the best clue is a normal P-wave axis before each QRS, indicating sinus origin.

P waves reveal where the atrial depolarization starts. When tachycardia is sinus, the impulse comes from the SA node, so the atrial activation sequence is normal and you see P waves before every QRS with a normal P-wave axis. That consistent, normal axis is the clearest sign that the rhythm is sinus tachycardia.

In atrial tachycardia, the atrial focus is ectopic, so the P waves reflect a different atrial activation pattern. This changes the P-wave morphology and axis, often making them differently angled or shaped compared with sinus P waves, even though a P wave still precedes each QRS.

The other statements aren’t reliable: P waves absent during tachycardia would point to a rhythm without organized atrial activity, not typical for atrial tachycardia; the rate being "always faster" than sinus tachycardia isn’t true, since atrial tachycardia can have similar or even faster rates; and P-wave morphology being identical in both would defeat the distinguishing feature by design.

So the best clue is a normal P-wave axis before each QRS, indicating sinus origin.

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