Noninvasive In Vivo Measurements of Hematocrit

Objective. To develop a clinically applicable method for noninvasive acoustic determination of hematocrit values in vivo. Methods. The value of hematocrit was determined initially in vitro from the pulse‐echo measurements of acoustic attenuation. The testing was carried out in a laboratory setup with an ultrasonic transducer operating at 20 MHz and with the use of human blood samples at 37°C. The attenuation coefficient measurements in blood in vivo were implemented by multigated, 20‐MHz pulsed Doppler insonation. The Doppler signal was recorded in the brachial and radial arteries. Both in vitro and in vivo hematocrit data were compared with those obtained by the centrifuge method. Results. The attenuation coefficient in vitro was determined from the measurements of 168 samples with hematocrit values varying between 23.9% and 51.6%. The attenuation from 20‐MHz data was equal to 3.66 + 0.089 · hematocrit (decibels per centimeter). The uncertainty of in vivo measurements in the brachial artery was determined to be within ±5% hematocrit. However, the measurements in the radial artery resulted in a clinically unacceptable uncertainty of ±20% hematocrit. Conclusions. The method proposed appears to be promising for in vivo determination of hematocrit, because 5% hematocrit error is adequate for monitoring changes in patients in shock or during dialysis. It was found that the multigate system largely simplified placement of an ultrasonic probing beam in the center of the blood vessel. Current work focuses on enhancing the method's applicability to arbitrarily selected vessels and to reducing the hematocrit measurement error to much less than 5% hematocrit.