Near infrared determined cerebral transit time and oxy- and deoxyhemoglobin relationships during hemorrhagic hypotension in the dog.

The goal of the present study was to investigate the relationships among CBF, CMRO2, MTT and tissue/superior sagittal sinus hemoglobin saturations over a wide range of CPP using a canine model of hypotension. Microsphere-determined CBF was autoregulated in all tested cerebral regions over the 40-130 mmHg CPP range, but it decreased 50% around 30 mmHg. MTT, calculated by an indicator-dilution technique, progressively and proportionately increased over the 40-130 mmHg CPP range. The HbO2 content, measured in the same area by an optical method, parallely decreased. Around 30 mmHg CPP, MTT disproportionately lengthened (700% of control). CMRO2 maintenance was accompanied by a progressive decrease of tissue HbO2 and sagittal sinus hemoglobin saturation. In this framework, MTT measurement and tissue HbO2 monitoring by NIRS might give a sensitive evaluation of the hemodynamic reserve than CBF measurement in anesthesia and intensive care.