FIG. 16:  This Figure shows the sympathetic nervous drive in patients with OSA during wakefulness, during REM sleep, and during REM sleep while being treated with CPAP.^[5]  As shown, there are high levels of sympathetic drive during wakefulness.  However, because of the episodes of OSA and consequent hypoxemia and hypercapnia during  REM sleep, the chemoreflex activation caused by sympathetic vasoconstriction causes an increase in blood pressure from 130/60 mmHg (“normal”) to about 240/130 mmHg, which is a high blood pressure for the patient to experience during sleep. 

When the patient with sleep apnea is treated with CPAP, there is a reduction in blood pressure and a reduction in sympathetic activity consequent to the normalization of breathing.  However there is another phenomenon that happens here, and that is bradycardia.  Apnea is one of the few stimuli that will simultaneously signal sympathetic activation to the blood vessels and vagal activation to the heart, which is shown in this Figure as a progressive slowing of heart rate (which we will look at in more detail in the next few Figures).