Novel method for measuring effects of gas compression on expiratory flow.

During forced vital capacity maneuvers in subjects with expiratory flow limitation, lung volume decreases during expiration both by air flowing out of the lung (i.e., exhaled volume) and by compression of gas within the thorax. As a result, a flow-volume loop generated by using exhaled volume is not representative of the actual flow-volume relationship. We present a novel method to take into account the effects of gas compression on flow and volume in the first second of a forced expiratory maneuver (FEV(1)). In addition to oral and esophageal pressures, we measured flow and volume simultaneously using a volume-displacement plethysmograph and a pneumotachograph in normal subjects and patients with expiratory flow limitation. Expiratory flow vs. plethysmograph volume signals was used to generate a flow-volume loop. Specialized software was developed to estimate FEV(1) corrected for gas compression (NFEV(1)). We measured reproducibility of NFEV(1) in repeated maneuvers within the same session and over a 6-mo interval in patients with chronic obstructive pulmonary disease. Our results demonstrate that NFEV(1) significantly correlated with FEV(1), peak expiratory flow, lung expiratory resistance, and total lung capacity. During intrasession, maneuvers with the highest and lowest FEV(1) showed significant statistical difference in mean FEV(1) (P < 0.005), whereas NFEV(1) from the same maneuvers were not significantly different from each other (P > 0.05). Furthermore, variability of NFEV(1) measurements over 6 mo was <5%. We concluded that our method reliably measures the effect of gas compression on expiratory flow.