A sound-speed sensor for the measurement of total protein concentration in disposable, blood-perfused tubes

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Abstract
The aim of this investigation was to develop a sensor containing a disposable measuring cell capable of measuring fluid shifts in blood-perfused circulation systems. Usually, the determination of fluid shifts in such systems is achieved by tracking the concentration of particles or solutes in the perfusate. Since acoustic properties of blood are closely related to the total protein concentration (TPC) of the sample, a new approach for the calculation of fluid shifts is made by the continuous measurements of sound speed. The measuring principle is based upon the evaluation of the propagation time of short acoustic pulses that are repeatedly transduced to the sample. The retransmission is triggered by means of a new type of delay line. The method stands out for its accuracy (better than 0.1 m/s) and for the short acoustic propagation path (5-10 mm). In order to develop the disposable measuring cell, the acoustic properties of porcine blood were determined at different hematocrits and TPC at temperatures ranging from 20-40.degree.C. The dependence of sound speed on temperature and on TPC is described by a polynomial. The new sensor was tested in vitro simulating mass shifts by adding a mass (Msaline) of isotonic saline solution to the circulating blood mass (M). The acoustic device containing the disposable cell proved capable of sensing mass fractions [Msaline/(M + Msaline)] in the order of 0.5 g/100 g.