Sinus arrhythmia

Sinus arrhythmia

Sinus arrhythmia is the variation in the spontaneous rate of the sinus node, resulting in variation of heart rate. Sinus arrhythmia can be classified into respiratory and non-respiratory. Respiratory variety is the commonest. Heart rate increases in inspiration and decreases in expiration. In inspiration the left ventricular output decreases and the baroreceptors trigger a rise in heart rate. The reverse occurs in expiration.

An important non-respiratory  variety is ventriculophasic sinus arrhythmia. This is often seen in complete heart block. The PP interval enclosing a QRS complex is shorter than a PP interval not enclosing a QRS. This is supposed to be the mechanical influence of the systolic blood flow into the sinus node shortening the sinus cycle length.