Chest
Volume 125, Issue 6, June 2004, Pages 2061-2068
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Clinical Investigations
Pulmonary Function
Membrane and Capillary Blood Components of Diffusion Capacity of the Lung for Carbon Monoxide in Pulmonary Sarcoidosis: Relation to Exercise Gas Exchange

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Background

Resting pulmonary diffusing capacity of the lung for carbon monoxide (Dlco) is known to be the best predictor of arterial desaturation during exercise in patients with sarcoidosis. However, the relative contribution of each of the two components of Dlco—alveolar membrane diffusing capacity (Dm) and pulmonary capillary blood volume (Vc)—remains unclear.

Study objectives

To evaluate which component is responsible for the decrease of resting Dlco in patients with sarcoidosis, and to determine which resting pulmonary function test, including Dm and Vc, is the best predictor of gas exchange abnormalities during submaximal exercise.

Design

Prospective analysis of patients referred to our department of respiratory medicine.

Patients

Twenty four patients with pulmonary sarcoidosis were separated into two groups according to chest radiographic findings: group 1, stages 2 and 3 (n = 15); group 2, stage 4 (n = 9). All the patients completed pulmonary function tests (flows, volumes, single-breath Dlco, transfer coefficient [Ka], Dm, Vc) and submaximal exercise (two steady-state levels of mild and moderate exercise corresponding respectively to a target oxygen consumption of approximately 10 to 15 mL/min/kg).

Results

Dlco was reduced in the two groups (group 1, 63 ± 16% of predicted; group 2, 64 ± 16% of predicted). Dm was severely decreased (group 1, 58 ± 24% of predicted; group 2, 51 ± 15% of predicted), whereas Vc was unchanged or only mildly decreased (group 1, 81 ± 18% of predicted; group 2, 85 ± 28% of predicted). Whatever the group of patients and the exercise level, Dm and Dlco were the strongest predictors (p < 0.001) of gas exchange abnormalities. Ka or volumes were weak predictors, and Vc or flows were not related with exercise gas exchange.

Conclusions

This study demonstrates that a decrease in Dm mostly accounts for resting Dlco reduction, and that Dm as well as Dlco are highly predictive of gas exchange abnormalities at exercise in patients with sarcoidosis.

Section snippets

Patients

Twenty-four patients with pulmonary sarcoidosis histologically confirmed were prospectively studied in our department of physiology between February 1999 and June 2000. All the patients had pulmonary infiltration shown on chest radiography and were classified according to American Thoracic Society staging.18 Group 1 (n = 15) included patients with radiographic stages 2 or 3 (no evidence of pulmonary fibrosis), and group 2 (n = 9) included patients with radiographic stage 4 (pulmonary fibrosis).

Pulmonary Function

Results of function tests are summarized in Table 1. Both groups demonstrated a moderate reduction in lung volumes and airflows. Dlco was severely reduced in the two groups (63 ± 17% of predicted in group 1, p < 0.05; 64 ± 16% of predicted in group 2, p < 0.05). The actual value of FEV1/VC was smaller in group 2 than in group 1, but this difference disappeared when FEV1/VC was expressed in percentage of predicted value. Since FEV1/VC decreases with age, this finding likely reflects the age

DISCUSSION

The present study provides the first comprehensive analysis of the relation between the two components of Dlco—Dm and Vc—and exercise gas exchange in pulmonary sarcoidosis. It clearly shows that the reduction of Dm mostly accounts for the decrease of resting Dlco, and that this parameter is the best resting predictor of gas exchange abnormalities during exercise.

In sarcoidosis, alteration of Dlco may reflect changes in gas exchange area, barrier thickness, and ventilation-perfusion-diffusion

CONCLUSION

In conclusion, this study demonstrates that in sarcoidosis without or with pulmonary fibrosis, the decrease of Dlco is mainly related to its Dm component and that Dm is the best predictor of abnormal gas exchange at exercise. Vc is only mildly altered, probably because of vascular recruitment at rest. Vascular involvement in patients with sarcoidosis might be more accurately approached by analyzing Dlco and its two components at exercise.

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