In sinus arrhythmia, the sinus rhythm is irregular. the P wave and PR interval are normal and the PP interval VARIES BY MORE THAN APPROXIMATELY 0.16 sec. There are 3 types.
Figure 1 (Phasic sinus arrhythmia)
Phasic (respiratory) sinus arrhythmia is one of the 3 variants/kinds/type of sinus arrhythmia. The other 2 are nonphasic sinus arrhythmia and ventriculophasic sinus arrhythmia. With phasic sinus arrhythmia, the rate is dependent on the respiratory cycle, increasing during inspiration and decreasing during expiration. The pacemaker site within the SAN shifts with respiration. As the heart rate increases the pacemaker site moves higher to the sinoatrial node (SAN) and the P wave amplitude increases (II, III and aVF). As the heart rate decreases the pacemaker sites moves lower the SAN and the P wave amplitude decreases. The heart rate changes rhythmically and gradually which differentiates it from nonphasic sinus arrhythmia.
Fisch C and Knoebel SB. 2000. Electrocardiography of Clinical Arrhythmia. New York. Futura Publishing Co.