Influence of breathing rate on airway resistance and compliance measurements

Airway resistance is commonly measured in a total body plethysmograph. However, measurements during naturally assumed breathing at frequencies below 2 breaths/sec are known to be a problem due to the presence of plethysmograph artefact. Specifically, breathing by the subject within the closed plethysmograph chamber produces undesired warming and wetting of chamber air and this in turn increases chamber pressure. In the present paper, a fundamental frequency method of analysis is used over a range of frequencies above and below 2 breaths/sec to extract values of airway resistance from data corrupted by artefact. Resistance values by this method were examined in seven normals and 17 subjects with obstructive lung disease. Within the limits of measurement accuracy, groups of normals and abnormais showed constant mean values of effective airway resistance over a range from 0.15 to 3.2 breaths/sec. A plethysmograph artefact occurred in response to changes in lung volume and appeared as a compliance-like effect on plethysmograph pressure. Compliance parameters, associated with the artefact, were nearly constant below 1.2 breaths/sec, and the means were the same for normals and abnormais. Above 1.2 breaths/sec the compliance parameter for normals tended to increase while that for abnormais decreased. These results suggest that if compliance measurements are derived from plethysmograph data both lung mechanical parameters and artefacts will contribute to computed values.