Pneumotachograph calibration for inspiratory and expiratory flows during HeO2 breathing

Maximum expiratory flows during breathing of a 80% helium-20% oxygen mixture (HeO2) are commonly used to determine the site of airflow limitation. To do this test the flowmeter is usually calibrated with the inspired gases, and the airflows are measured during expiration. We tested the adequacy of such calibration maneuvers by using two identical flowmeters in series through a bag-in-box system. Different gases were flowed though the test pneumotachograph into a bag contained in a closed box connected to the second pneumotachograph. Distension of the bag caused air to flow from the box through this second pneumotachograph. Our results indicate that when breathing HeO2, the flowmeter correction for different gas viscosity, compared with air, should be 20% for inspired HeO2 and 12% for expired gases. Inspired gases therefore cannot be used to calibrate the flowmeters when assessing expiratory flows.