The heart rate increases in inspiration (when you breathe in) and decreases in expiration (when you breath out). These respiratory changes in heart rate is known as respiratory sinus arrhythmia. Respiratory sinus arrhythmia can be exaggerated in children and may appear as an irregular heart rhythm. Normally when you breath in the blood is sucked into the thoracic cavity by the negative pressure so that the blood leaving the heart into the general circulation is lesser. This causes a reflex compensatory increase in the heart rate. The reverse occurs in expiration.