Feasibility of non-invasive measurement of tissue pH using near-infrared reflectance spectroscopy

Objective, Tissue pH measurement has a number of clinical applications, including the monitoring of both muscle pH and organ pH as an indicator of compromised blood flow and anaerobic metabolism. The objective of this work was to demonstrate the feasibility of a noninvasive measurement of deep tissue pH using near-infrared reflectance spectroscopy and multivariate calibration techniques. Methods. Six studies were done on five New Zealand white rabbits. Two pH electrodes were implanted in the teres major muscle and a vascular clamp placed across the single artery feeding the muscle. Reflected light was collected through the skin from a site between the two electrodes as the pH was lowered by closing the clamp and raised by opening the clamp. Partial least squares analysis with cross-validation techniques was used to relate pH to light absorption at 201 evenly spaced wavelengths between 700 and 1100 nm. Results, On average, the tissue pH started at 7.13 +/- 0.09 and decreased to 6.74 +/- 0.09, returning to 7.13 +/- 0.09 after reperfusion. Calibration models fit for each rabbit had an average of nine factors with an R-2 of 0.98 and a prediction error of 0.016 +/- 0.002 pH units. Conclusions. We believe this to be the first in vivo demonstration of a noninvasive method for measuring tissue pH in skin-covered muscle using near-infrared reflectance spectroscopy and multivariate calibration techniques.