Sinus arrhythmia is the variation in the spontaneous rate of the sinus node (natural pacemaker of the heart), resulting in variation of heart rate. Sinus arrhythmia can be classified into respiratory (related to breathing) and non-respiratory. Respiratory sinus arrhythmia is the commonest. Heart rate increases in inspiration (breathing in) and decreases in expiration (breathing out). In inspiration the left ventricular output decreases and the baroreceptors (pressure sensors) trigger a rise in heart rate. The reverse occurs in expiration.
An important variety of non-respiratory sinus arrhythmia is ventriculophasic sinus arrhythmia. This is often seen in complete heart block. The PP interval enclosing a QRS complex (waves in the ECG caused by depolarization of the ventricles) is shorter than a PP interval (interval between the P waves due to atrial depolarization) 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.