To be classified as sinus, how should the P waves appear?

• A. No P waves permitted
• B. One P wave per QRS, upright, and all look the same in size and shape
• C. Multiple P waves before each QRS
• D. P waves inverted relative to the QRS

Answer: (B)

P waves show atrial depolarization, and in sinus rhythm the impulse starts in the sinoatrial node, producing a single P wave before every QRS complex with identical size and shape. That consistent, upright P wave pattern before each QRS is what indicates the rhythm is sinus. If there were no P waves, or if there were multiple P waves before a QRS, or if the P waves were inverted, it would point to a rhythm originating outside the SA node or with abnormal atrial activation, not a normal sinus rhythm.