In younger individuals, changes in right atrial filling associated with the changes in intrathoracic pressure that accompany inspiration and expiration result in reflexly mediated changes in vagal tone that in turn cause phasic changes in the rate of sinus node discharge. This is referred to as “sinus arrhythmia”. With inspiration, negative intrathoracic pressure sucks more blood into the right atrium. This decreases vagal tone leading to an increase in sinus rate. With expiration, the reverse occurs and sinus rate slows.
This ECG Illustrates sinus arrhythmia. It is from a 21 year old female with no known heart disease. The P-P interval varies in a phasic fashion from 0.72 to 1.2 seconds but that there is no change in the morphology of the P wave. This indicates that the location of the pacemaker cells within the sinus node is constant.