Non-invasive monitoring of brain oxygen sufficiency on cardiopulmonary bypass patients by near-infra-red laser spectrophotometry

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Abstract

A new portable high-performance apparatus for near-infra-red (NIR) laser spectrophotometry was developed to monitor the oxygenation state in the human brain. Three different wavelengths of 780,805 and 830 nm of the NIR light illuminated the head using a fibre-optic bundle, and the transmitted or reflected light was detected by a photodiode placed on the forehead. The oxygenated Hb (oxy Hb), deoxygenated Hb (deoxy Hb) content and the Hb (blood) volume changes in the brain were continuously monitored in cardiopulmonary bypass (CPB) patients in the reflectance mode. In hypothermic CPB with selective brain perfusion, the brain bypass flow rates of 0.5 L min−1 at 24–25°C and 0.3 L min−1 at 21–22°C were confirmed as the safe lower limits by our NIR monitoring. During deep hypothermic circulatory arrest, the brain oxy Hb and blood volume decreased significantly. The result indicates that brain Hb oxygenation decreases significantly during circulatory arrest. In the adult patients group, during moderate hypothermic CPB, the brain blood oxygenation level was maintained roughly constant at mean arterial perfusion pressure of over 60 mm Hg, whereas below 50–55 mmHg a progressive decline in the brain Hb oxygenation was observed. These findings led us to conclude that non-invasive monitoring of brain oxygenation using NIR light can provide valuable data at the bed-side regarding tissue metabolism, and it can allow for the proper management of critical patients.