Sinus arrhythmia refers to a changing sinus node rate with the respiratory cycle, on inspiration and expiration. This is quite common in young, healthy individuals and has no clinical significance. The heart rate increases with inspiration, due to the Bainbridge reflex, and decreases with expiration.
The ECG criteria to diagnose sinus arrhythmia is a variation of the P-P interval, from one beat to the next, of at least 0.12 seconds, or 120 milliseconds. Sinus arrhythmia ― if not in a young person and not occurring with respiration may be a sign of ― sick sinus syndrome, or SSS.
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