Measurement of Cerebral Blood Flow in Dogs with Near Infrared Spectroscopy in the Reflectance Mode Is Invalid

Near infrared spectroscopy (NIRS) is used to measure CBF (CBF NIRS ) in humans, based on Fick's principle, using oxygen as an intravascular tracer. We compared CBF NIRS with CBF measured by microspheres (CBFμ) and the venous outflow technique (CBFv) in 15 dogs, altering CBF with ventilation-induced changes in PaCO 2 . Five hundred forty-nine CBF NIRS measurements were attempted using an integration time of 2.5 s on the saturation signal from the tongue. One hundred ninety-eight (36.1%) of the measurements fulfilled predefined criteria. The coefficient of variation (CV) for six measurements under stable conditions was 29.1%. The CBF NIRS measurements correlated best with microsphere-measured blood flows in the cortical gray matter (median 0.43, range 0.16–0.93); the contributions of the skull and dura were variable. The CBFv varied by a median of 12% (range 0–67%) during the CBF NIRS measurements. The percentage of acceptable CBF NIRS measurements, the CV, and the correlation coefficients of the CBF NIRS were improved by using saturation signal directly from the artery and varying the integration time with an estimate of the minimum transit time. The current method of measuring CBF NIRS in the reflectance mode is inaccurate when compared with other accepted techniques.