Expiratory capnography in asthma: evaluation of various shape indices

The shape of the capnogram is modified by airway obstruction, and the evaluation of this deformation, using measurable indices, could allow an indirect measurement of bronchial patency. A previous study undertaken in asthmatic subjects showed a good correlation between a capnographic index (end-tidal slope) and a spirometric parameter (forced expiratory volume in one second as a percentage of predicted (FEV1 %pred)) and suggested the study of other indices. The correlations between capnographic and spirometric indices were measured in 10 healthy subjects and 30 asthmatic patients. The usefulness of eight descriptive indices, analysing the successive phases of the capnogram, was assessed by measuring their reproducibility and their sensitivity to airway obstruction. The intraindividual and interindividual variabilities (Vi and VI) and the noise/signal ratio (Vi/VI) were measured by comparing the results of two successive capnographic measurements in 14 asthmatic subjects. The results show an increasing noise/signal ratio along the expiration (between 23 and 62%). Significant correlations between spirometry and capnography were found with all indices, but the strongest were observed with indices analysing the intermediate phase of the capnogram, that is the angle between the ascending phase (E2) and the alveolar plateau (E3). The correlations show that the analysis of the capnogram's shape is a quantitative method for evaluating the severity of bronchospasm. This ability, added to specific advantages (noninvasiveness, effort-independency, measurements during tidal breathing) opens new fields of application to capnography, such as measurement of bronchospasm in children and computerized monitoring of asthma.