Relation between Shunt, Aeration, and Perfusion in Experimental Acute Lung Injury

Abstract

Rationale: In a pulmonary process characterized by spatially heterogeneous loss of aeration, the impairment of gas exchange is expected to depend on the 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 [ 13 N]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 between regional shunt (FS) and gas (FG) fractions: FS 5 2m Á FG 1 FS 0 . This relation was similar among animals (m 5 1.25 6 0.14, FS 0 5 0.75 6 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 explained by the combined effect of this relation and the distribution of perfusion on regional shunt, rather than by differences in global aeration.

Bedside Estimation of Nonaerated Lung Tissue Using Blood Gas Analysis*

“…Pure oxygen ventilation for PaO 2 /Fio 2 measurement may reduce the impairment of the correlation between lnPaO 2 / Fio 2 and physiological shunt or CT shunt by regions with low ventilation-to-perfusion ratios as previously pointed out by others and reproduced in our study (5,12,18,19,27,35,37,47,48,(67)(68)(69). Additionally, pure oxygen ventilation may blunt hypoxic pulmonary vasoconstriction which may increase the sensitivity of lnPaO 2 /Fio 2 for assessing CT shunt (26,69).…”

“…The suitability of both pulmonary shunt and PaO 2 /Fio 2 for this purpose, however, is controversial because studies trying to correlate PaO 2 / Fio 2 to the pulmonary shunt or to the amount of nonaerated lung quantified by CT shunt yielded inconsistent results (5,12,(14)(15)(16)(17)(18)(19)(20)(21)(22)(23). Differences in regional perfusion (10,(24)(25)(26)(27), the nonlinear relationship between PaO 2 /Fio 2 and pulmonary shunt (28)(29)(30)(31), the use of different Fio 2 (12,(32)(33)(34)(35)(36)(37)(38)(39)(40), and the differences in CT methods (1,12,18,27,41,42) could help explain some of these inconsistencies.…”

Lung Imaging in Acute Lung Injury and Acute Respiratory Distress Syndrome with PET

“…13 N-N 2 [41], nicely demonstrating that improved regional shunt fraction occurs only in lung regions where improved aeration outweigh the detrimental effect of the PEEP to shift the regional perfusion to the dorsal lung regions.…”

Gas Exchange in the Respiratory Distress Syndromes

“…In addition to the isolated effects of alveolar filling and/or atelectasis is the role that regional perfusion distribution plays on the resulting shunt, V A /Q heterogeneity and A-aDO 2 (392).…”