Volume-dependent compliance in ARDS: proposal of a new diagnostic concept

Objective: Adaptation of ventilator settings to the individual's respiratory system mechanics requires information about the pressure-volume relationship and the change of compliance which is dependent on inflated volume. Unfortunately, established methods of obtaining this information are invasive and time-consuming, and, therefore, not well suited for clinical routine. We propose a new standardized diagnostic concept based on the recently developed slice method. This multiple linear regression method (MLR) determines volume-dependent respiratory system compliance (C-SLICE) within the tidal volume (V-T) during ongoing mechanical ventilation. The impact of a ventilator strategy, recommended by a consensus conference, on the course of compliance within V-T was investigated in patients with the acute respiratory distress syndrome (ARDS) or acute lung injury (ALI). Design: Prospective observational study. Setting: Intensive care unit of a university hospital. Patients: 14 ARDS patients, 2 patients with ALI. Interventions: None. Measurements and results: After measurement of flow and airway pressure and calculation of tracheal pressure, C-SLICE was determined. The resulting course of C-SLICE within V-T was estimated using a mathematical algorithm. C-SLICE data were compared to those obtained by standard MLR. We found decreasing C-SLICE mainly in the upper part of V-T in all patients. In 7 patients, we found an additional increasing C-SLICE mainly in the lower part of V-T. Conclusions: C-SLICE Was not constant in patients with ARDS/ALI whose lungs were ventilated according to consensus conference recommendations. The proposed diagnostic concept may serve as a new tool to obtain a standardized estimation of respiratory system compliance within V-T non-invasively without interfering with ongoing mechanical ventilation.