Relation between shunt, aeration, and perfusion in experimental acute lung injury

Rationale: In a pulmonary process characterized by spatially heterogeneous loss of aeration, the impairment of gas exchange is expectedtodependonthe regional distribution of perfusion relative to that of aeration. Objectives: To investigate how regional aeration, shunt, and perfusion are interrelated at different levels of end-expiratory pressure and how their interplay relates to global shunt fraction in acute lung injury. Methods: Regional shunt and perfusion were assessed by imaging with positron emission tomography the pulmonary kinetics of [13N]nitrogen infused in saline solution in five sheep after lung lavage. The lung field was divided in six horizontal regions. Measurements and Main Results: Each animal showed an inverse relation betweenregional shunt (FS) and gas (FG) fractions: FS = -m·FG + FS 0. This relation was similar among animals (m = 1.25 ± 0.14, FS0 = 0.75 ± 0.15) and invariant with end-expiratory pressure, despite lack of correlation between global shunt and gas fractions and large interanimal variability in global shunt fraction. When this relation was used to estimate global shunt fraction as a perfusion-weighted average of the estimates of regional shunt fraction derived from regional gas fraction, 72% of the interanimal variability in global shunt fraction could be explained. Conclusions: Despite large interanimal variability in global shunt fraction, there was a consistent inverse relation between regional shunt and gas fractions, independent of end-expiratory pressure. Most of the interanimal variability in global shunt fraction could be explainedby thecombinedeffect of this relationandthe distribution of perfusion on regional shunt, rather than by differences in global aeration.