Effects of Lung Expansion on Global and Regional Pulmonary Blood Volume in a Sheep Model of Acute Lung Injury

Background: Pulmonary capillary blood volume is a major determinant of lung gas transport efficiency and also potentially related to ventilator-induced lung injury. However, knowledge on how lung expansion influences pulmonary blood volume in injured lungs is scant. The hypothesis was 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 of 5 to 20 cm H₂O), this study aimed to determine whole-lung and regional blood volume, their dynamic changes, and association with gas volume changes. Seven healthy sheep were subjected to 3 h of low-lung volume mechanical ventilation at a PEEP of 0 cm H₂O and systemic endotoxemia. PEEP values of 5 (low), 20 (high), and 12 (intermediate) cm H₂O were applied to produce distinct lung expansion. Respiratory-gated positron emission tomography with ¹¹C-labeled carbon monoxide and four-dimensional computed tomography were obtained to quantify blood volume and aeration. results: Transpulmonary pressures were lowest at a PEEP of 12 cm H₂O. Changes in whole-lung blood volume correlated with gas volume changes between PEEP of 5 and 12 cm H₂O at end expiration (P < 0.001) and end inspiration (P < 0.001) but not between 12 and 20 cm H₂O. Tissue-normalized blood volume (VB_tissue) was heterogeneously distributed, with mean values in nondependent regions (VB_tissue = 0.116 ± 0.055) approximately seven times smaller than those in mid-dependent regions (VB_tissue = 0.832 ± 0.132). A positive end-expiratory pressure of 12 cm H₂O resulted in the most homogeneous VB_tissue distribution, with the largest means in mid-dependent regions and inspiratory 10th percentile, a measure of lowest values, throughout the lung. VB_tissue increased with inspiration at PEEP of 5 and 12 cm H₂O but decreased with a PEEP of 20 cm H₂O 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 an intermediate PEEP, indicating its advantage to spatial distribution of blood volume.