Expiratory capnography is an accurate non-volitional measurement of airway obstruction and changes in airway physiology

Purpose: Spirometry using the forced vital capacity manuever is the clinical gold standard for assessment of airway function. Some patients are unable to coordinate and sustain a maximal respiratory effort and thus produce inconsistent results. We have previously shown that tidal expiratory capnography can accurately predict airway obstruction, and have now tested it's utility in assessing acute changes in airway physiology. Methods: One hundred pairs of routine spirometry and expiratory capnograms were collected prospectively from a convenience sample of patients with both normal and obstructive physiology. Studies included screening spirometry, post-bronchodilator studies, and methacholine challenges. The phase slopes, expired volumes, and transitional parameters from the capnograms were used to develop a predictive model for percent predicted forced expiratory volume in one second (FEV₁), using commercially available neural net software. Data from 56 capnograms were used to train the model. The remaining 44 served as a validation test set. The range of percent predicted FEV₁ was similar in the train and test sets. Results: There was a direct linear relationship between the percent predicted FEV₁ calculated by the model and that obtained spirometrically (R value of 0.77). In eighty percent of the cases where the percent predicted FEV₁ calculated by the model differed from the spirometrically obtained value, the degree of obstruction would still have fallen into the same clinical category of mild, moderate, severe, or very severe obstruction. Those parameters which describe the transitional angle between Phase II and Phase III of the capnogram were most predictive of the degree of obstruction. In particular, this included the angle itself, as well as the volume at which the most acute angle between the two phases was noted. Conclusions: We conclude that the expiratory capnogram can accurately estimate airflow limitations in a variety of clinical situations. Clinical Implications: Expiratory capnography may become a very useful tool in the pulmonary evaluation of patients who are either unwilling or unable to perform a forced vital capacity maneuver - the very young, the elderly, and the patient who presents with an acute exacerbation of chronic obstructive lung disease.