Expiratory ventilation and carbon dioxide production measured with a thermistor flow-through system

Abstract

A thermistor flow-through system for measuring expiratory volume without a mouthpiece and a nose clip was developed. First, a thermostat and a large syringe were connected to a box used to stimulate a subject's head. A carbon dioxide (CO2) gas mixture was driven through the box, while the output of a thermistor sensor of the thermistor flow-through system was recorded. The correlation between the area under the temperature-time curve and the actual volume of gas driven through the box was computed. Second, the effects of driving time, gas temperature, and room temperature on the area under the temperature-time curve were measured. Third, corrections for expiratory time and for the temperature of exhaled gas were derived from regression analysis of the relation between the time taken to drive the CO2 gas mixture and the area under the temperature-time curve, and between the temperature of the CO2 gas mixture and the area under the temperature-time curve, respectively. Fourth, CO2 production was computed from the area under the CO2 concentration-time curve (obtained at the same time as the temperature-time curve). To measure the temperature-time curve and the CO2-time curve for the simulator, the box was placed under the transparent hood of the thermistor flow-through system. To measure the temperature-time and CO2-time curves for a subject, the head was placed in the hood while the subject was supine. The subject breathed with the mouth held slightly open, and the mixture of room air and expired gas was continuously drawn at a constant flow through an outlet at the top of the hood. The outlet was connected to a flow meter and to a constant-speed blower. The CO2 concentration and the temperature in the hood exhaust were measured at the outlet, and were continuously recorded with a chart recorder. To measure the actual volume of CO2, a Douglas bag was also used, and was connected to the blower. Increases in the driving time and in gas temperature caused increases in the area under the temperature-time curve of 9%/sec and 6%/degrees C, respectively; increases in room temperature caused it to decrease at 7%/degrees C. After the thermistor-derived expiratory volume was corrected for expiratory time and temperature, it correlated significantly with expiratory volume as measured with a respiratory inductance plethysmograph. The correlation coefficients were +0.904 for expiratory volume and +0.881 for tidal volume. A significant correlation (r = +0.992) was also found between the volume of CO2 computed from the area under the CO2-time curve and the actual volume of CO2 delivered by the simulator. A similar correlation was found between CO2 production computed from the area under the CO2-time curve and the volume of CO2 collected in the Douglas bag during breathing. With corrections for expiratory time and temperature, and with the development of more advanced thermistor sensors, a thermistor flow-through method such as the one described here may be used to measure expired volume. Such a system may also be used to measure CO2 production in the clinical pulmonary function laboratory.