近赤外光を用いる脳代謝無侵襲測定の新しい試み

A future technique-Non invasive optical methods for the monitoring of cerebral oxygen metabolism by infrared transmission spectrophotometry

　通常の光は，生体組織を透過し難い。これは主として組織内での光の散乱に起因する。この光の散乱は，波長に強く依存し長波長領域で急激に減少する。一方，赤外領域の光は，組織水分により大部分吸収されほとんど透過しない。著者らは，700〜1200nm付近の近赤外光の持つ良好な透過性^1〜3）を利用し脳組織酸素代謝の無侵襲，in situ測定を試みたので報告する。

Biological tissues are relatively transparent to light in near infrared region. Brain oxygen meta-bolism was non-invasively studied by the simulta-neous measurement of hemoglobin (Hb) oxygenation and cytochrome oxidase (Cyt. Aa₃) redox state in the rat brain using near infrared transmission spectro-photometer. The spectrophotometers used here were computer-controlled wavelengh scanning and 2-chan-nel dual wavelengh photometers operated with photon counting mode.

Wavelength pairs at 700-815 nm and 815-830 nm were used for the measurements of Hb oxygenation level and of the redox state of cyt. oxidase in situ. Change in the brain blood volume was also monitored by the absorption change at 815 nm.

With anesthetized and artificially ventilated rats, approximately 90% of Hb in the brain was in the oxygenated state at 95% O₂+5% CO₂ in the inspired gas, which was well coincident with the oxygen saturation of the jugular vein blood. At 20% O₂ in theinspired gas, about 70% of the brain Hb was still oxygenated.

Sharp reduction of the oxidase occured at the oxygen concentration below 15% in the inspired gas. But above 30% O₂, about 85% of the Cyt. oxidase was in the oxidized state which was almost constant up to 100% O₂.

Arteriovenous O₂ difference in the rat brain was constant at more than 20% O₂ (with 5% CO₂), but increased sharply between 10% and 5% 0₂ in in-spired gas, indicating the oxygen insufficieney in the brain.

Temporally clamping of carotid artery caused deoxygenation of the brain lib, reduction of Cyt. oxidase and a decrease in the blood volume, but completely recovered to the initial levels on the release of clamping.

At 30% O₂ respiration, nicardipine (Ca-Antagonist) infusion caused a dose-dependent increase in the brain Hb oxygenation which suggested that nicardi-pine improved the oxygen supply to the brain.

In conclusion, the simultaneous measurements of the Hb oxygenation and mitochondrial redox state can present the further informations about the oxygen balance and various vasodilating actions in the brain in situ.