Sinus Arrhythmia

Sinus Arrhythmia Overview

Sinus rhythm with a beat-to-beat variation in the P-P interval (the time between successive P waves), producing an irregular ventricular rate.

Characteristics

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).

Mechanism

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).

Differential Diagnosis

There are several other entities that cause sinus rhythm with an irregular ventricular rate:

ECG Examples

Sinus arrhythmia:

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.