Mechanism of hypoxemia in acute lung injury evaluated by multidetector-row CT

Hypoxic pulmonary vasoconstriction (HPV) is a homeostatic mechanism causing pulmonary arterial constriction in response to local hypoxia, redistributing blood flow to lung regions with better oxygenation and ventilation. We present the use of computed tomographic (CT) volume and perfusion imaging to show differences in the mechanisms of hypoxemia from alterations in blood flow distribution within different animal models of acute lung injury (ALI).

Three anesthetized, instrumented, and ventilated sheep were studied, two with induced ALI and one with native pneumonia. One subject was injured by using intravenous infusion of lipopolysaccharide (LPS), and the other, by repetitive saline lavage. Subjects were imaged using multidetector-row CT (MDCT) before and after injury. Lung volume scans were gated to the respiratory cycle. Contrast injection perfusion images were electrocardiogram gated. Computer-based image analysis quantified regional blood flow and total lung, air, and tissue volumes.

Total lung air fraction was decreased in both ALI models. In lavage injury, there was a decrease in perfusion to dependent poorly aerated regions, with perfusion shifting to nondependent regions. Conversely, LPS injury greatly increased perfusion to dependent poorly aerated regions. In the subject with pneumonia, decreasing fraction of inspired oxygen redistributed blood flow into the injured regions.

MDCT techniques can be used to investigate regional lung perfusion and lung volume distributions to explain physiological mechanisms in ALI. Our findings suggest that after lavage injury, blood flow is redistributed, consistent with preserved HPV and resulting in better oxygenation despite greater lung volume loss compared with LPS injury. In native pneumonia, HPV inactivation can be localized to the injured regions.