Background:
Pulmonary capillary blood volume is a major determinant of lung gas transport efficiency, and also potentially related to ventilator-induced lung injury. Yet, knowledge on how lung expansion influences pulmonary blood volume in injured lungs is scant. We hypothesize that lung expansion produced by positive end-expiratory pressure (PEEP) modulates the global and regional spatial distribution of pulmonary blood volume.
Methods:
In a lung injury model exposed to distinct lung expansion within clinical range (PEEP=5-20 cmH2O), we aimed to determine whole-lung and regional blood volume, their dynamic changes, and association with gas volume changes. Seven healthy sheep were subjected to 3h of low-lung volume mechanical ventilation at PEEP=0 cmH2O and systemic endotoxemia. PEEP=5 (low), 20 (high), and 12 (intermediate) cmH2O were applied to produce distinct lung expansion. Respiratory-gated positron emission tomography with 11C-labelled carbon monoxide and 4-dimensional computed tomography were obtained to quantify blood volume and aeration.
Results:
Transpulmonary pressures were lowest at PEEP=12 cmH2O. Changes in whole-lung blood volume correlated with gas volume changes between PEEP=5 and 12 cmH2O at end-expiration (P<0.001) and end-inspiration (P<0.001), but not between PEEP=12 and 20 cmH2O. Tissue-normalized blood volume (VBtissue) was heterogeneously distributed, with mean values in non-dependent regions (VBtissue=0.116±0.055) approximately 7-times smaller than those in mid-dependent regions (VBtissue=0.832±0.132). PEEP=12 cmH2O resulted in the most homogeneous VBtissue distribution, with largest means in mid-dependent regions and inspiratory 10th-percentile, a measure of lowest values, throughout the lung. VBtissue increased with inspiration at PEEP=5 and 12 cmH2O but decreased with PEEP=20 cmH2O in mid-nondependent regions.
Conclusions:
During low-volume mechanical ventilation and systemic endotoxemia, lung blood volume is markedly heterogeneously distributed, and modulated by PEEP. Nondependent regions are susceptible to low blood volume and capillary closure. Recruitment of pulmonary vascular blood volume with gas volume is nonlinear, limited at intermediate PEEP indicating its advantage to spatial distribution of blood volume.