Fig. 1.
(Top) Calculation of regional ventilation delay inhomogeneity, schematic description. (Left) Functional image (ventilation map) recorded by electrical impedance tomography during a slow inflation breath of 12 ml/kg ideal body weight. (Middle) Normalized regional impedance/time curves of three exemplary pixels. All curves are normalized to the beginning (tmin) and end (tmax) of the slow inflation maneuver: blue, right ventral pixel; red, right dorsal pixel; yellow, left dorsal pixel. Ventilation delay time (ΔtRVD) when the regional impedance time curve reaches a threshold of 40% of the maximal local impedance change. (Right) Delay map to visualize all regional ventilation delay indices. Regional ventilation delay inhomogeneity expressed as SD of all single pixel regional ventilation delay-values. (Middle) Exemplary delay maps from a representative animal (pig No. 13) during decremental titration of positive end-expiratory pressure (PEEP). In each map, dark red pixels indicate lung regions that are lately ventilated during a slow inflation maneuver. Very early ventilated lung regions appear as light blue pixels. Regional ventilation delay inhomogeneity indicates heterogeneity of ventilation time courses that is well correlated to the amount of tidal recruitment.22 (Bottom) Courses of oxygen partial pressure (Pao2, dark gray, left y-axis) and regional ventilation delay inhomogeneity (light gray, right y-axis) during decremental PEEP trial. At a PEEP of 22 cm H2O (dotted arrow marked with *) Pao2 decreased more than 5% below the maximal Pao2 observed during PEEP titration. Hence, a PEEP of 24 cm H2O was defined as “maximal oxygenation PEEP.” At a PEEP of 18 cm H2O (dotted arrow marked with **) regional ventilation delay inhomogeneity increased suggesting an increase in tidal recruitment with decreasing PEEP. Thus, a PEEP of 20 cm H2O was chosen as “minimal tidal recruitment PEEP.”