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Towards a Temperature Compensated Model for a Blood-pH Sensor in Extracorporeal Circulation

Under physiological conditions, the body maintains blood pH within the very narrow range [7.36, 7.44] pH. Small deviations from this range can reveal the onset of pathological states. In this work the performances of a real-time, non-invasive pH measuring sysem for extracorporeal circulation (ECC) are analyzed. In particular, this study focuses on the analysis of the effects that temperature of the measurand may have on the error in estimating blood pH. Indeed, the sensor is based on the analysis of the fluorescence produced by HPTS, which is known to vary with temperature. The extent of such a variation, however, depends on various factors, including the chemical environment. Blood temperature in ECC is often thermostated at 37 °C. Nevertheless, there are treatments in which the blood temperature is varied by a few Celsius degrees, generally reduced, from the physiological temperature of 37 °C. Therefore, the first objective of this study was to evaluate whether a modest reduction in temperature, that is a few Celsius degrees, introduce an error such as the measuring system no longer conforms to the maximum permissible measurement error of ±0.04 pH. Once verified that the temperature-induced error could exceed the limit of ±0.04 pH, a correction factor for temperature compensation was investigated and its robustness to unevenness in the sensor production was explored. The results obtained from this preliminary study performed using Phosphate Buffer Saline (PBS) showed how the addition to the measuring system of a temperature sensor can effectively allow to maintain the measurement error within the ±0.04 pH range, even when the temperature of the measurand decreases by a few degrees from the physiological temperature of 37 °C.