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


OBJECTIVE
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.


RESULTS
Each animal showed an inverse relation between regional shunt (Fs) and gas (Fg) fractions: Fs = -m . Fg + Fs(0). This relation was similar among animals (m = 1.25 +/- 0.14, Fs(0) = 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 explained by the combined effect of this relation and the distribution of perfusion on regional shunt, rather than by differences in global aeration.
Musch
Guido
G
Department of Anesthesia and Critical Care, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114, USA. gmusch@partners.org
Bellani
Giacomo
G
Vidal Melo
Marcos F
MF
Harris
R Scott
RS
Winkler
Tilo
T
Schroeder
Tobias
T
Venegas
Jose G
JG
D000818:Animals; D004195:Disease Models, Animal; D011175:Positive-Pressure Respiration; D049268:Positron-Emission Tomography; D011659:Pulmonary Gas Exchange / Q000502:physiology*; D012128:Respiratory Distress Syndrome / Q000473:pathology* / Q000503:physiopathology; D012756:Sheep; D014057:Tomography, X-Ray Computed; D012122:Ventilators, Mechanical / Q000009:adverse effects*