Normal Sinus Rhythm & Basic ECG Interpretation

Every dysrhythmia on the NCLEX is identified by what it deviates from — if you can't rapidly define normal sinus rhythm, you can't recognize anything abnormal.

Normal sinus rhythm (NSR) means the SA node is firing at 60–100 bpm with consistent conduction through the heart. On the ECG strip, you verify five criteria: rate 60–100, regular rhythm (R-R intervals equal), one upright P wave before every QRS, a consistent PR interval of 0.12–0.20 seconds, and a narrow QRS duration of 0.06–0.12 seconds. The P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization. To calculate heart rate on a 6-second strip, count the number of QRS complexes in 30 large boxes and multiply by 10. For regularity, compare at least three consecutive R-R intervals. A PR interval longer than 0.20 seconds signals a conduction delay — that content belongs in heart blocks, but recognizing the normal cutoff is your job here. The ST segment should be isoelectric (flat at baseline); elevation or depression suggests ischemia or injury, which changes clinical urgency even when the rhythm itself is sinus.

Key Distinctions

Sinus bradycardia and sinus tachycardia are NOT dysrhythmias in the traditional sense — they meet every NSR criterion except rate. Students confuse the PR interval (start of P to start of QRS, 0.12–0.20 sec) with the QRS duration (0.06–0.12 sec); mixing these up causes misidentification of blocks versus bundle branch issues. An artifact on the monitor is not a rhythm change — always assess the patient before the strip.

Clinical Pearl

P, PR, QRS — check three things in order. Upright P present? PR 0.12–0.20? QRS under 0.12? If all three check out, it's sinus. Everything else is a deviation from this baseline.