Sensitivity of Breath-to-Breath Gas Exchange Measurements to Expiratory Flow Errors

During expiration, fluctuations in gas composition, water vapor, and temperature result in flowmeter errors when the flowmeter is calibrated for a given ambient inspiratory gas. In this paper, we indicate that alternative analytical methods of caleulating breath-to-breath gas exchange exhibit differing sensitivities to this error. A theoretical sensitivity analysis is verified by O2 consumption records from rest to exhausting exercise. We conclude that an error sensitivity of less than one is achieved by a method that incorporates measurements of nitrogen flow into and out of the lung, and includes the analysis of breath-to-breath changes in lung volume.