The Effects of Arterial Carbon Dioxide Partial Pressure and Sevoflurane on Capillary Venous Cerebral Blood Flow and Oxygen Saturation During Craniotomy

Abstract

Intraoperative routine monitoring of cerebral blood flow and oxygenation remains a technological challenge. Using the physiological principle of carbon dioxide reactivity of cerebral vasculature, we investigated a recently developed neuromonitoring device (oxygen-to-see, O2C device) for simultaneous measurements of regional cerebral blood flow (rvCBF), blood flow velocity (rvVelo), oxygen saturation (srvO2), and hemoglobin amount (rvHb) at the capillary venous level in patients subjected to craniotomy. Twenty-six neurosurgical patients were randomly assigned to anesthesia with 1.4% or 2.0% sevoflurane end-tidal concentration. After craniotomy, a fiberoptic probe was applied on a macroscopically healthy surface of cerebral tissue next to the site of surgery. Simultaneous measurements in 2 and 8 mm cerebral depth were performed in each patient during lower (35 mm Hg) and higher (45 mm Hg) levels (random order) of arterial carbon dioxide partial pressure (PaCO2). The principle of these measurements relies on the combination of laser-Doppler flowmetry (rvCBF, rvVelo) and photo-spectrometry (srvO2, rvHb). Linear models were fitted to test changes of end points (rvCBF, rvVelo, srvO2, rvHb) in response to lower and higher levels of PaCO2, 1.4% and 2.0% sevoflurane end-tidal concentration, and 2 and 8 mm cerebral depth. RvCBF and rvVelo were elevated by PaCO2 independent of sevoflurane concentration in 2 and 8 mm depth of cerebral tissue (P < 0.001). Higher PaCO2 induced an increase in mean srvO2 from 50% to 68% (P < 0.001). RvVelo (P < 0.001) and srvO2 (P = 0.007) were higher in 8 compared with 2 mm cerebral depth. RvHb was not influenced by alterations in PaCO2 but positively correlated to sevoflurane concentration (P = 0.005). Increases in rvCBF and rvVelo by PaCO2 suggest preserved hypercapnic vasodilation under anesthesia with sevoflurane 1.4% and 2.0% end-tidal concentration. A consecutive increase in srvO2 implies that cerebral arteriovenous difference in oxygen was decreased by elevated PaCO2. Unchanged levels of rvHb signify that there was no blood loss during measurements. Data suggest that the device allows detection of local changes in blood flow and oxygen saturation in response to different PaCO2 levels in predominant venous cerebral microvessels.