The Influence of Carbon Dioxide and Body Position on Near-Infrared Spectroscopic Assessment of Cerebral Hemoglobin Oxygen Saturation

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Abstract
Near-infrared spectroscopy may allow continuous and noninvasive monitoring of regional brain hemoglobin oxygen saturation by measuring the differential absorption of infrared light by oxyhemoglobin and deoxyhemoglobin. We have previously examined the correlation between the spectroscopic signal generated by a prototype cerebral oximeter (Invos 3100 Registered Trademark; Somanetics, Troy, MI), and global brain hemoglobin oxygen saturation calculated from arterial and jugular venous bulb oxygen saturations. Because the technology does not distinguish between arterial and venous hemoglobin saturation, changes in the proportion of cerebral arterial and venous blood volume, which may result from changes in blood flow or venous distending pressure, may confound measurements. In eight conscious volunteers breathing hypoxic oxygen mixtures, we examined the influence of supine, 20 degrees Trendeleburg, and 20 degrees reverse Trendelenburg positions on the correlation of the spectroscopic measurement of cerebral oxygen saturation in the field assessed by the probe (CSf O2) and the calculated brain hemoglobin oxygen saturation (CScomb O2), estimated as 0.25 times arterial saturation plus 0.75 times jugular venous bulb oxygen saturation. We found that changes in position did not influence the association between CSf O2 and CScomb O2 (r2 = 0.69-0.885) during hypoxic challenge. In a second set of eight volunteers, we studied the influence of hypercapnia and hypocapnia and body position on the association between CSf O2 and CScomb O2, and found that they were less well correlated (r2 = 0.366-0.976) in individual patients. Because changes in body position and PaCO2 confound the relationship between CSf O2 and CScomb O2, changes in CS (f) O2 can best be assessed if position and PaCO2 are constant. (Anesth Analg 1996;82:278-87)