Heterogeneity of regional inflection points from pressure-volume curves assessed by electrical impedance tomography

Background: The pressure-volume (P-V) curve has been suggested as a bedside tool to set mechanical ventilation; however, it reflects a global behavior of the lung without giving information on the regional mechanical properties. Regional P-V (PVr) curves derived from electrical impedance tomography (EIT) could provide valuable clinical information at bedside, being able to explore the regional mechanics of the lung. In the present study, we hypothesized that regional P-V curves would provide different information from those obtained from global P-V curves, both in terms of upper and lower inflection points. Therefore, we constructed pressure-volume curves for each pixel row from non-dependent to dependent lung regions of patients affected by acute hypoxemic respiratory failure (AHRF) and acute respiratory distress syndrome (ARDS). Methods: We analyzed slow-inflation P-V maneuvers data from 12 mechanically ventilated patients. During the inflation, the pneumotachograph was used to record flow and airway pressure while the EIT signals were recorded digitally. From each maneuver, global respiratory system P-V curve (PVg) and PVr curves were obtained, each one corresponding to a pixel row within the EIT image. PVg and PVr curves were fitted using a sigmoidal equation, and the upper (UIP) and lower (LIP) inflection points for each curve were mathematically identified; LIP and UIP from PVg were respectively called LIPg and UIPg. From each measurement, the highest regional LIP (LIPrMAX) and the lowest regional UIP (UIPrMIN) were identified and the pressure difference between those two points was defined as linear driving pressure (Delta P-LIN). Results: A significant difference (p<0.001) was found between LIPrMAX (15.8 [9.2-21.1] cmH(2)O) and LIPg (2.9 [2.2-8.9] cmH(2)O); in all measurements, the LIPrMAX was higher than the corresponding LIPg. We found a significant difference (p<0.005) between UIPrMIN (30.1 [23.5-37.6] cmH(2)O) and UIPg (40.5 [34.2-45] cmH(2)O), the UIPrMIN always being lower than the corresponding UIPg. Median Delta P-LIN was 12.6 [7.4-20.8] cmH(2)O and in 56% of cases was <14cmH(2)O. Conclusions: Regional inflection points derived by EIT show high variability reflecting lung heterogeneity. Regional P-V curves obtained by EIT could convey more sensitive information than global lung mechanics on the pressures within which all lung regions express linear compliance.