Non-invasive monitoring of brain oxygen metabolism during cardiopulmonary bypass by near-infrared spectrophotometry.

Abstract

A new portable high performance apparatus for near-infrared (NIR) laser spectrophotometry was developed to monitor the oxygenation state of the human brain. Three different wavelengths of the NIR laser beam were used, 780, 805 and 830 nm, to illuminate the head through a fiber optic bundle. The amount of light reflected by or transmitted from the tissue was detected by a photomultiplier or photodiode. Equations as explained here, were used to calculate the oxy and deoxy Hb content and blood volume changes non-invasively. The equations were verified in vivo with the rat head in order to confirm the reliability and acceptability of our methodology. NIR monitoring was applied to 15 cases of cardiopulmonary bypass (CPB) in reflectance mode. The results demonstrated that using current CPB technique, cerebral oxygenation levels during bypass were maintained within the physiological range and changes in brain blood volume corresponded well to the pump flow balance. That is, the brain oxygenation level was maintained roughly constant at a mean perfusion pressure of over 60 mmHg during CPB, whereas below 50 mmHg, apparent decremental changes in oxy Hb content were observed. These findings led us to conclude that non-invasive monitoring of cerebral oxygenation using NIR light can provide valuable bedside data about tissue metabolism and allow for the proper management of critical patients.