Transducer for indirect measurement of blood pressure in small human subjects and animals

Rebecca Anne Rowe Roeder, Purdue University

Abstract

At present, it is not possible to obtain systolic, mean and diastolic pressure in very small human subjects noninvasively. An optical adaption of the oscillometric method was developed to obtain these three pressures. However instead of detecting the small-amplitude arterial pressure pulses in the extremity-encircling cuff, an optically transparent cuff was used to allow transillumination the extremity at the center of the cuff, thereby allowing detection of the pulses very efficiently. In addition, heart and respiratory rates can be determined from the same transducer. LEDs with a wavelength of 880 nm with a matched detector were chosen because the absence of ambient infrared light at this wavelength in the environment and because 880 nm is maximally transmitted through the extremity. The photodetector was placed on the opposite side of the cuff from the two LEDs. This arrangement minimizes blockage of the optical path by the bone in the extremity. With these specifications, an annular cuff was built and tested on pigs and humans. Data were collected for the development of this new type of noninvasive optical transducer designed for premature and neonatal infants and small animals. We identified unambiguously the correct cuff width was 40% of the extremity circumference for both animals and humans for agreement with direct mean pressure. The point of maximal optical oscillations coincides with mean pressure. Using the correct cuff size, it was determined that indirect mean pressure tracks direct mean pressure to below 50 mmHg and to 150 mmHg. We obtained data to identify systolic and diastolic pressures. The conventional pneumatic oscillometric method uses fixed ratio algorithms to obtain systolic and diastolic pressure. However, our optical method uses a fixed ratio for diastolic and an algorithm based on the prevailing mean pressure to determine systolic pressure. Using these algorithms, the lines of equal values for direct and indirect systolic and diastolic pressures were virtually coincident. Finally, we demonstrated the ability of device to attain heart rate and respiration rate and demonstrated the ability of the device to deflate the cuff at Yong's recommended rate of 3 mmHg per heart beat.

Degree

Ph.D.

Advisors

Geddes, Purdue University.

Subject Area

Biomedical engineering

COinS