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Rhythm

Normal sinus rhythm

Not medically reviewed · Content last updated · 1 min read ·

This is the date the draft was written, not a medical review date.

Normal sinus rhythm is the reference rhythm: impulses arise in the sinoatrial node and conduct normally through the atria, AV node and His-Purkinje system. Every QRS complex is preceded by a P wave.

ECG criteria

  • Rate 60 to 100 beats/min in adults (resting rate varies with age, fitness and sleep).
  • Regular rhythm, with only minor physiological variation (respiratory sinus arrhythmia).
  • P wave before every QRS, upright in I, II and aVF, negative in aVR.
  • Constant PR interval of 120 to 200 ms.
  • Narrow QRS (under 120 ms) with normal ST segment and T waves.

Mechanism

The sinoatrial node, the dominant pacemaker, depolarises spontaneously at the highest rate under autonomic control. The impulse spreads across the atria (P wave), pauses in the AV node (PR segment) and activates the ventricles rapidly through the His-Purkinje system (narrow QRS).

Common causes

  • Normal physiology: healthy heart under balanced sympathetic and vagal tone.
  • A normal ECG does not exclude structural or ischaemic heart disease.
  • Athletes and young people may show a lower rate or marked sinus arrhythmia that is still normal.

Management principles

No rhythm-specific treatment is needed. Interpret the tracing in its clinical context, and follow local guidelines for symptoms or risk factors that prompt further investigation.

High-level principles for study. Treatment decisions follow local guidelines and the clinical situation.

Pitfalls and mimics

  • Lead reversal (for example the arm electrodes) inverts the P wave and QRS in lead I and mimics dextrocardia or an ectopic atrial rhythm.
  • A normal rate does not exclude first-degree block or an abnormal PR interval; measure it.
  • A low-amplitude or hidden P wave can make a junctional or atrial rhythm look sinus.

Practise in the ECG trainer — Recognise this pattern on generated tracings.

References

  1. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, Parts I to VI. American Heart Association / American College of Cardiology Foundation / Heart Rhythm Society (2009) — Tier 2 (guidelines / primary literature)
  2. Wagner GS, Strauss DG. Marriott's Practical Electrocardiography. Wolters Kluwer, 12th edition (2013) — Tier 3 (textbook)
  3. Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. Elsevier, 14th edition (2020) — Tier 3 (textbook)

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