Effect of posture and extracranial contamination on results of cerebral oximetry by near-infrared spectroscopy.

BACKGROUND: Near-infrared spectroscopy (NIRS)-based cerebral oximetry is a noninvasive technology used to estimate regional cerebral oxygen saturation (rSO2 ). Extracranial blood flow is known to substantially affect rSO2 values measured by most clinically available devices. Several studies have also reported that the Trendelenburg position and upright position have a larger effect on rSO2 measurements than the supine position. Therefore, we investigated the effect of these two positions (the Trendelenburg position versus the upright position) and extracranial contamination on rSO2 measurements obtained using two commercially available devices and one prototype device.

METHODS: Twelve healthy volunteers were enrolled in the study. They each had three cerebral oximetry devices applied to their forehead (FORE-SIGHT ELITE™, CAS Medical Systems Inc., Branford, CT, USA; INVOS 5100c™, Medtronic, Minneapolis, MN, USA; and NIRO-TRS, Hamamatsu Photonics, Hamamatsu, Japan). A circumferential pneumatic head cuff was positioned proximal to the NIRS cerebral oximetry sensors. We measured rSO2 , heart rate (HR), and blood pressure (BP) in six conditions (supine, Trendelenburg and upright positions, with and without scalp ischemia induced by head cuff inflation) every 5 min with each oximetry device. Total hemoglobin (tHb), which is associated with cerebral blood volume (CBV) as measured by positron emission tomography, was measured using the NIRO-TRS device to determine extracranial blood volume in each position.

RESULTS: Measurements of rSO2 with all the devices were affected by extracranial contamination. The percentage of extracranial contamination was highest with INVOS 5100c™ in the upright position (INVOS, 21.3%; FORE-SIGHT, 14.3%; NIRO-TRS, 3.6%). Measurements of rSO2 obtained in the upright position were significantly lower than those obtained in the supine position, using INVOS-5100c™ and FORE-SIGHT ELITE™ (71 vs. 74% and 67 vs. 72%, respectively), but not using NIRO-TRS (62 vs. 64%). A significant decrease in tHb was observed after head cuff inflation in the supine and Trendelenburg positions (supine, 0.132-0.123 μmol/l; Trendelenburg, 0.133-0.125 μmol/l).

CONCLUSIONS: Except when using NIRO-TRS, measurements of rSO2 in the forehead are significantly lower when measured in the upright position than in the supine position. All devices in this study were affected by extracranial contamination.