An apparatus for monitoring prolonged measurement of oxygen uptake with minimum discomfort to the subjects has been developed. The system is based on the open flow-through principle. The subject wears a hood through which air is drawn by an exhaust blower. Oxygen concentration is kept constant by a servo-controlled blower, so that flow rate is essentially proportional to oxygen uptake. The flow rate of the main stream is measured by a time-of-flight flowmeter located in the exhaust hose. Oxygen concentration is measured by a limiting-current oxygen sensor. The signals representing flow rate and oxygen concentration are digitized and stored on a memory chip. All of the equipment is assembled in a small package (
wt=3kg) which the subject carries on his back. The memory chip is removed from the portable device after each experiment. Data are transferred from the memory to a microcomputer and then an oxygen uptake trend is estimated. Response of the system to a step input is about 30s. Compared with a commercially available oxygen monitoring system (Magna 88, P. K. Morgan ltd., Rainham, U. K.), the maximum difference was 0.15
l/min within an oxygen uptake range of 0.25 to 2.3
l/min. The oxygen uptake of exercising human subjects measured with this portable device showed good agreement with that obtained by a conventional oxygen-uptake monitoring system (
r=0.973 for cycle ergometer,
r=0.932 for treadmill). This portable device was able to measure oxygen uptake in subjects during daily activities, such as driving a car, cycling, and mountain climbing, without any disturbance.
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