<title>Optical measurement of tissue pH for surgical and critical care monitoring</title>

Abstract

Tissue pH is an important physiological parameter which indicates both blood flow and cell metabolic state. Continuous monitoring of tissue pH can provide an assessment of the level of anaerobic metabolism and a measure of whether organs or muscles are revivable or have died. A noninvasive, optical technique for deep tissue pH determination has been demonstrated in-vivo using near infrared (NIR) spectroscopy and partial least-squares (PLS) multivariate calibration. NIR reflectance spectra (700 - 1100 nm) were collected from skin covered muscle in a rabbit, canine myocardium, and swine bowel along with reference pH values measured in the same tissue using microelectrodes. Muscle and myocardial pH were varied by controlling the blood supply through vessel occlusion; bowel pH was altered through hemorrhagic shock. PLS cross- validation techniques and data preprocessing methods were used to relate the tissue pH to spectra. The standard error of prediction for each of the multivariate calibrations was less than 13% of the average pH change in each of the animal models. Optically measured tissue pH promises to provide a noninvasive monitor for ischemia during heart and plastic surgery and an early indicator of shock in the ICU patient.