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Pulmonary Artery Occlusion Increases the Ratio of Diffusing Capacity for Nitric Oxide to Carbon Monoxide in Prone Sheep^*

^* From the Pulmonary and Critical Care Unit (Drs. Harris and Hadian) and Clinical Epidemiology Unit (Dr. Chang), Department of Medicine, and Departments of Respiratory Care (Dr. Hess) and Anesthesia and Critical Care (Dr. Venegas), Massachusetts General Hospital and Harvard Medical School, Boston, MA.

Correspondence to: R. Scott Harris, MD, FCCP, Pulmonary and Critical Care Unit, Bulfinch 148, Massachusetts General Hospital, 55 Fruit St, Boston, MA 02114; e-mail: rharris{at}partners.org

Objective: To test the hypothesis that the ratio of diffusing capacity of the lung for nitric oxide (DLNO) to diffusing capacity of the lung for carbon monoxide (DLCO) would be affected by occlusion of a fraction of the pulmonary vascular bed.

Design: Interventional physiologic study.

Setting: Animal laboratory of a university hospital.

Subjects: Thirteen sheep.

Interventions: We simultaneously measured single-breath DLNO and DLCO in anesthetized and mechanically ventilated sheep (fraction of inspired oxygen [FIO₂] of 1.0) before and after pulmonary artery occlusion by inflation of a balloon (n = 6), and by autologous clot embolism (n = 4). To see if the effect also occurred on FIO₂ of 0.21, four animals were studied during ventilation with room air, one of which was also in the FIO₂ of 1.0 group (14 total experiments with 13 sheep).

Results: On FIO₂ of 1.0, the mean DLNO/DLCO ratio rose by 35% from 4.76 ± 0.41 in control to 6.42 ± 0.82 after balloon occlusion (p = 0.002), and by 54% from 7.55 ± 2.09 to 11.6 ± 2.61 (p = 0.005) after autologous clot embolism (± SD). An equivalent relative increase of 27% took place during ventilation with room air, but the DLNO/DLCO ratio was lower (3.14 ± 0.22 in control and 3.98 ± 0.38 after balloon occlusion). Independent of the method of obstruction or FIO₂, the increase in DLNO/DLCO ratio was mostly due to a drop in DLCO. The DLNO/DLCO ratio reduced much of the intersubject variability of either DLNO or DLCO alone.

Conclusion: The DLNO/DLCO ratio increased after pulmonary artery occlusion regardless of the method of occlusion or FIO₂. This increase may be a result of a greater sensitivity of DLCO than DLNO to a regional reduction in capillary blood flow.

Key Words: carbon monoxide • diffusing capacity of the lung for CO • diffusing capacity of the lung for nitric oxide • nitric oxide • pulmonary embolism

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