A Novel Method to Calculate Compliance and Airway Resistance in Ventilated Patients

Abstract

Abstract Background. The respiratory system’s static compliance (Crs) and airway resistance (Rrs) are measured during a no-flow, end-inspiratory hold on volume-controlled ventilation (static method). A numerical method is presented to calculate Crs and Rrs in the presence of airflow and other ventilatory modes (dynamic method).Methods. The dynamic method combines a numerical solution of the equation of motion with frequency analysis of airway signals. The method was validated experimentally with a one-liter test lung and clinically using airway signals sampled at 32.25 hertz and stored as 131.1 seconds-long epochs in a database. 15 patients in the database had epochs on VC ventilation with breaths displaying end-inspiratory holds. Paired Crs and Rrs were calculated for all breaths within those epochs by both methods and their mean values compared in aggregate form and individually with Pearson’s R2 and Bland-Altman analysis.Results. Experimental method differences were 0.3[0.2,0.4] cmH2O·mL-1 for Crs and 0[-0.2,0.2] cmH2O·s· L-1 for Rrs. Clinical testing included 78,371 breaths found in 3,171 epochs meeting criteria. For aggregate data, Pearson’s R2 were 0.99 and 0.94 for Crs and Rrs, respectively. Bias ± 95% limits of agreement (LOA) were 0.2 ± 1.6 cmH2O·mL-1 for Crs and - 0.2 ± 1.5 cmH2O·s· L-1 for Rrs. Bias ± LOA median values for individual patients were 0.6[-0.2, 1.4] ± 0.9[0.8, 1.2] cmH2O·mL-1 for Crs and -0.1[-0.3, 0.2] ± 0.8[0.5, 1.2] cmH2O·s· L-1 for Rrs. Conclusions. Experimental and clinical testing produced equivalent values for Crs and Rrs by the dynamic and static methods under the conditions tested.