Opto-electronic Plethysmography

Abstract

Although the measurement of pulmonary ventilation by a spirometer or a pneumotachograph may appear to be a simple procedure, it is much more complicated than most realize. Temperature, humidity, pressure, viscosity, and density of gas influence the recording of its volume. Mouthpieces, face masks and noseclips may introduce leaks and therefore cause losses, are impractical for prolonged measurement, limit the subject’s mobility, introduce additional dead space, and thereby increase tidal volume. They also make the subject aware that his breathing is being measured and therefore interfere with the natural pattern of breathing and its neural control [1, 2]. Breathing through a mouthpiece and flowmeter or from a spirometer is extremely difficult in children or uncooperative adults; it cannot be used during sleep, to analyze phonation, and during weaning from mechanical ventilation may require excessive patient co-operation. During exercise, rebreathing from a spirometer or a bag-in-box system can only be done for short time periods, while integration of flow at the mouth suffers from integration drift, so that changes in absolute lung volume are not accurately recorded. A possible approach to solve this problem is to collect the expired gas, breath by breath, in a large spirometer (e.g., a Tissot spirometer) or in a large, gas-tight bag (e.g., a Douglas bag), which are then emptied through a precision gasometer. But even emptying the spirometer or the bag causes problems due to the gasometer, which may require intermittent calibration over time.