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

Document Type

Journal Article

Publication Date



Intensive care medicine experimental








Acute respiratory failure; Airway resistance; Frequency analysis; Mechanical ventilation; Numerical analysis; Static compliance


BACKGROUND: The respiratory system's static compliance (C) and airway resistance (R) are measured during an end-inspiratory hold on volume-controlled ventilation (static method). A numerical algorithm is presented to calculate C and R 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 C and R values using both static and dynamic methods. Epoch mean values for C and R were assessed by both methods and compared in aggregate form and individually for each patient in the study with Pearson's R 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·cmHO for C and 0[- 0.2,0.2] cmHO·s· L for R. 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 R were 0.99 and 0.94 for C and R, respectively. Bias ± 95% limits of agreement (LOA) were 0.2 ± 1.6 mL·cmHO for C and - 0.2 ± 1.5 cmHO·s· L for R. Bias ± LOA median values for individual patients were 0.6[- 0.2, 1.4] ± 0.9[0.8, 1.2] mL·cmHO for C and - 0.1[- 0.3, 0.2] ± 0.8[0.5, 1.2] cmHO·s· L for R. DISCUSSION: Experimental and clinical testing produced equivalent paired measurements of C and R 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.