Development of Ambulatory Oxygen Uptake Measurement System

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An apparatus for monitoring prolonged measurement of oxygen uptake without discomfort has been developed. The principle on which the system is based is the open flow-through system. The subject wears a hood through which air is drawn by an exhaust blower. Oxygen concentration is maintained constant by a servo-controlled blower, so that flow rate is essentially proportional to oxygen uptake. The flow rate of the main stream was measured by a time-of-flight flow-meter located in the exhaust hose. Oxygen concentration was measured by a limiting current oxygen sensor. The signals representing flow rate and oxygen concentration were digitized and stored in a memory. All of the equipment was assembled in a small package (wt= 3 kg) which the subject carries on his back. The memory chip was removed from the portable device after each experiment. Data were transferred from the memory to a microcomputer and then an oxygen uptake trend was estimated. Response of the system to a step input was about 30 s. Compared with a commercially available oxygen monitoring system (Magna 88, P. K. Morgan, England), the maximum error was 7.2% of full scale within the range of 0.25 to 2.1 1/min of oxygen uptake. The oxygen uptake of exercising human subjects measured with this system showed good agreement with that obtained by a conventional oxygen monitor (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, and also while driving a car, without disturbance.