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1 Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA
2 Clinic of Anesthesiology and Intensive Care Medicine, University Clinic Carl Gustav Carus, Dresden University of Technology, Dresden, Germany
3 Department of Medicine (Pulmonary and Critical Care Unit), Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA
* To whom correspondence should be addressed. E-mail: mvidalmelo{at}partners.org.
A mathematical model was developed to estimate right-to-left shunt (Fs) and the volume of distribution of 13NN in alveolar gas (VA) and shunt tissue (Vs). The data obtained from this model is complementary to, and obtained simultaneously with, pulmonary functional positron emission tomography (PET). The model describes 13NN kinetics in four compartments: central mixing volume, gas exchanging lung, shunting compartment, and systemic recirculation. To validate the model, five normal prone (NP) and six surfactant-depleted sheep in the supine (LS) and prone (LP) positions were studied under general anesthesia. A central venous bolus of 13NN-labelled saline was injected at the onset of apnea as PET imaging and arterial 13NN sampling were initiated. The model fit the kinetics well (mean r2=0.93). Monte-Carlo simulations showed that parameters could be accurately identified in the presence of the expected experimental noise. Fs derived from the model correlated well with shunt estimates derived from O2 blood concentrations and from PET images. Fs was higher for LS=54±18% versus LP=5±4% and NP=1±1% (p<0.01). VA, as a fraction of PET measured lung gas volume, was lower for LS (0.18±0.09) versus LP (0.96±0.28, p<0.01) while Vs, as a fraction of PET measured lung tissue volume, was higher for LS (0.46±0.26) versus LP (0.05±0.08, p<0.01). The main conclusions are: 1) the model accurately described measured arterial 13NN kinetics and provided estimates of Fs; and 2) in this animal model of acute lung injury, the fraction of available gas volume participating in gas exchange is reduced in the supine position.
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