Correlation of near infrared reflectometry with intracranial pressure during intact and loss of vascular tone

A near infrared reflectometer was fabricated with a laser (820 nm) diode and a photodiode detector circuit. Because the isosbestic point of HbO₂ and Hb is approximately 810 nm, change in the reflected near infrared signal is approximately proportional to change of blood volume. With this instrument, recordings of local change of cerebral blood volume were obtained lateral to and along the sagittal sinus of piglets. Intracranial pressure was also recorded. A correlation analysis was employed to evaluate the strength of similarity between change in cerebral blood volume and change in intracranial pressure. During normal vascular tone, local changes of cerebral blood volume were found to have a high correlation with intracranial pressure. During loss of vascular tone, the two signals were found to be uncorrelated.