- Variation in the P-P interval of more than 120 ms (3 small boxes).
- The P-P interval gradually lengthens and shortens in a cyclical fashion, usually corresponding to the phases of the respiratory cycle.
- Normal sinus P waves with a constant morphology (i.e. no evidence of premature atrial contractions).
- Constant P-R interval (i.e. no evidence of Mobitz I AV block).
- Sinus arrhythmia is a normal physiological phenomenon, most commnonly seen in young, healthy people.
- The heart rate varies due to reflex changes in vagal tone during the different stages of the respiratory cycle.
- Inspiration increases the heart rate by decreasing vagal tone.
- With the onset of expiration, vagal tone is restored, leading to a subsequent decrease in heart rate.
- The incidence of sinus arrhythmia decreases with age, presumably due to age-related decreases in carotid distensibility and baroreceptor reflex sensitivity.
NB. “Non-respiratory” sinus arrhythmia (not linked to the respiratory cycle) is less common, typically occurs in elderly patients and is more likely to be pathological (e.g. due to heart disease or digoxin toxicity).
There are several other entities that cause sinus rhythm with an irregular ventricular rate:
- Normal sinus P waves (upright in leads I and II) with a constant morphology — albeit with an appearance suggestive of left atrial enlargement.
- P-R interval is constant (no evidence of AV block).
- The P-P interval varies widely from 1.04 seconds (heart rate ~57 bpm) down to 0.60 seconds (heart rate ~100 bpm); a variability of over 400ms.
For irregular rhythms such as this, the ventricular rate is best estimated by multiplying the total number of complexes in the rhythm strip by 6. This gives an overall rate of 12 x 6 = 72 bpm.
- Hampton, JR. The ECG In Practice, 6e
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
- Wagner, GS. Marriott’s Practical Electrocardiography 12e
- Chan, TC. ECG in Emergency Medicine and Acute Care
- Mattu, A. ECG’s for the Emergency Physician
LITFL Further Reading
- ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis –alphabetical diagnostic approach to the ECG
- ECG CLINICAL CASES — ECG’s placed in clinical context with a challenging Q&A approach
- 100 ECG Quiz — Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS — the best of the rest
- LITFL ECG IMAGE Database — Searchable database of LITFL ECG’s
- ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
- ECG Exam Template — a framework for answering ECG exam questions.