A novel method to calculate compliance and airway resistance in ventilated patients

Abstract

Background: The respiratory system’s static compliance (Crs) and airway resistance (Rrs) are measured during an end-inspiratory hold on volume-controlled ventilation (static method). A numerical algorithm is presented to calculate Crs and Rrs during volume-controlled ventilation on a breath-by-breath basis not requiring an end-inspiratory hold (dynamic method). Methods: The dynamic method combines a numerical solution of the equation of motion of the respiratory system with frequency analysis of airway signals. The method was validated experimentally with a one-liter test lung using 300 mL and 400 mL tidal volumes. It also was validated clinically using airway signals sampled at 32.25 Hz stored in a historical database as 131.1-s-long epochs. There were 15 patients in the database having epochs on volume-controlled ventilation with breaths displaying end-inspiratory holds. This allowed for the reliable calculation of paired Crs and Rrs values using both static and dynamic methods. Epoch mean values for Crs and Rrs were assessed by both methods and compared in aggregate form and individually for each patient in the study with Pearson’s R2 and Bland–Altman analysis. Figures are shown as median[IQR]. Results: Experimental method differences in 880 simulated breaths were 0.3[0.2,0.4] mL·cmH2O−1 for Crs and 0[− 0.2,0.2] cmH2O·s· L−1 for Rrs. Clinical testing included 78,371 breaths found in 3174 epochs meeting criteria with 24[21,30] breaths per epoch. For the 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 mL·cmH2O−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] mL·cmH2O−1 for Crs and − 0.1[− 0.3, 0.2] ± 0.8[0.5, 1.2] cmH2O·s· L−1 for Rrs. Discussion: Experimental and clinical testing produced equivalent paired measurements of Crs and Rrs by the dynamic and static methods under the conditions tested. Conclusions: These findings support to the possibility of using the dynamic method in continuously monitoring respiratory system mechanics in patients on ventilatory support with volume-controlled ventilation.