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.