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Quantitative Assessment of Regional Alveolar Ventilation and Gas Volume Using ¹³N-N₂ Washout and PET

¹ Service de Réanimation Médicale et d’Assistance Respiratoire, Lyon, France
² Equipe d’Accueil Universitaire, Lyon, France
³ Hôpital Neuro-Cardiologique, Centre d’Exploration et de Recherche Médicales par Emission de Positons, Lyon, France
⁴ Hôpital Neuro-Cardiologique, Centre de Recherche et d’Application en Traitement de l’Image et du Signal (Unité Mixte de Recherche, Centre National de la Recherche Scientifique), Batiment Centre d’Etudes et de Recherche Médicales par Émission de Positons, Lyon, France

Measurement of alveolar volume (VA) and regional ventilation (A) is crucial to understanding the pathophysiology of acute lung injury and ventilator-induced lung injury. PET has previously been used as a noninvasive, quantitative method to assess A, but formal validation of this technique in experimental lung injury is lacking. This study aims to validate VA and A regional assessment with PET, using inhaled ¹³N-N₂ in pigs. Methods: Two normal and 2 oleic acid–injured pigs were tracheotomized, mechanically ventilated, and studied in 5 different levels of ventilation by changing respiratory rate. In each experimental condition, lungs were washed-in and then washed-out with ¹³N-N₂ through an open circuit in the ventilator. Using this method, multiframe images were acquired with a dedicated PET camera. Regions of interest (ROIs) were drawn on each lung. Regional time–activity curves during washout were generated for each ROI and fitted to a mono- and a bicompartmental model. Validation of this method was performed in 2 ways. First, regional values of predicted VA (VA_emission) were compared with regional volume obtained independently from density analysis on a transmission scan (VA_trans). Second, regional values of predicted A were summed in each animal during each experimental condition and compared with minute-ventilation values set on the ventilator. Results: The bicompartmental model best fitted the experimental values in normal (94.7% [62.2%–100.0%] (median [interquartile range]) of the ROIs) as well as in injured animals (90.7% [81.6%–97.4%] of the ROIs) (P = 0.49). VA_emission significantly correlated with VA_trans (R² = 0.89, P < 0.001) but exceeded VA_trans by 10%. Finally, A strongly and positively correlated with minute-ventilation in both normal (R² = 0.96, P < 0.001) and injured (R² = 0.96, P < 0.001) animals. Conclusion: Measurement of ¹³N-N₂ washout using PET is accurate to assess regional alveolar volume and ventilation during experimental acute lung injury.

Key Words: PET • ventilation • ¹³N • oleic acid • lung injury

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