The present study has shown (1) that awake values for ICP are pathologically elevated in patients with severe OSA, (2) that the ICP increases further during sleep, especially NREM stages 2 to 3 and REM sleep related to the apneic episodes, and (3) strong correlations between durations of apnea and AP and ICP elevations and between AP variations and ICP elevations.
Elevations in ICP related to respiration, especially Cheyne-Stokes respiration, are well known. Elevations in ICP during OSA have been described by Sugita et al. The CSF pressure was measured via a lumbar cannula in two patients. In this study, significant associations between apnea, hypoxia, and CSF pressure were found; however, AP was not measured.
During apnea, at least three phases can be distinguished in ICP: an initial decrease, a slow increase, and a steep increase in ICP The observed slow increase in ICP during the apnea, associated with hypercapnia and hypoxia, indicates that carbon dioxide retention and hypoxia play a role for the initial ICP increase, possibly due to cerebral vasodilation; however, the steep increase in ICP that terminates the apneic episodes cannot be explained solely as induced by cerebral hypoxia or hypercapnia, but is possibly related to the steep simultaneous increase in the AP and central venous pressure, thereby increasing ICP (Fig 1). fully
Different mechanisms are suggested as important in developing the AP variations during apnea: the Muller maneuver; pulmonary blood pooling affecting cardiac stroke volume (Starling mechanism); barore-ceptor reflexes, hypercapnia, and hypoxia causing increased peripheral sympathetic outflow; increased TPR; and increased intrathoracic pressure at the termination of the apnea due to increased ventilation.
Elevations in ICP, hypoxia, and hypercapnia have been found to give rise to increased AP due to increased sympathetic activity.* The ICP elevations during apnea should increase sympathetic output and TPR; however, as far as we know, no study has measured TPR during apnea. Some studies have found increased sympathetic activity in patients with apnea, but this has not been found in other studies.