A method for quantification of lung resistive and compliant properties for spirometry interpretation support—Tests on a virtual patient

The forced expiratory volume in one second (FEV1), the fundamental index in obstructive lung disease diagnosis, depends on both resistive (RP) and compliant (CP) properties of the respiratory system (RS). The study aim was to test initially a method that could differentiate their influence to aid spirometry interpretation during screening examinations. Tests were done on a virtual RS elaborated previously. After respiratory muscle relaxation, a part of air was exhaled passively to an added compliance (Cad) or through an added resistance (Rad). The CP and RP were estimated from mouth pressure changes under different conditions of RS and measurement (different obstruction severities, various Cad and Rad values, etc.). Measurements had to be performed after maximal inspiration to avoid dependence of results on the lung volume. The Cad maneuver enabled to estimate the CP properly. Inertances and bronchi collapse caused pressure fluctuations, whereas bronchi reopening modified pressure rise after airflow interruption. Rad > 0.8 kPa s/L eliminated these problems and made the RP estimation independent from the Rad value and the CP. The calculated value of resistance depended on both airway resistance and parenchyma viscosity (like FEV1) and viscosity of other tissues. Since collapse instantaneous observation in real patients is impossible, initial but extensive tests illustrating influence of the collapse on measurement could be done only on a virtual RS.