To determine if a rhythm originates from the SA node, which is true?

• A. There is one P wave for every QRS complex with consistent P wave morphology
• B. P waves are absent
• C. P waves occur only after QRS
• D. There are multiple P waves per QRS complex

Answer: (A)

The key idea is that a rhythm coming from the SA node shows a normal atrial depolarization before every ventricular depolarization, with a consistent appearance of the atrial activity. This means you’ll see one P wave before each QRS complex, and all the P waves will have the same shape and size from beat to beat. That orderly one-to-one relationship—one P wave for each QRS—reflects the SA node acting as the natural pacemaker that sets the pace and structures the sequence of activation.

If P waves are absent, the rhythm isn’t SA node–originated because there’s no visible atrial depolarization preceding the ventricles. If P waves occur after the QRS, that suggests retrograde atrial activation or a rhythm driven from below the atria (such as a junctional rhythm), not sinus rhythm. If there are multiple P waves for a single QRS, that signals an abnormal atrial rhythm not produced by the SA node.

So the best indicator of SA node origin is the presence of one, identical P wave before every QRS complex.