Diffusing Capacity for Nitric Oxide and Carbon Monoxide in Patients With Diffuse Parenchymal Lung Disease and Pulmonary Arterial Hypertension: Patients
Assuming that the Dlno is not affected or is less affected by impaired capillary filling, and thus represents the diffusing capacity of the pulmonary membrane, the Dlno/Dlco ratio has to differ between subjects with a pure alveolocapillary membrane disturbance and subjects with microvascular disease. Subjects with a decreased Dlco due to lowering of the Vcap component will have undisturbed Dlno; therefore, the Dlno/Dlco ratio will increase. Subjects with a decreased Dlco due to thickening of the membrane without disturbing the pulmonary capillaries will also have a lower Dlno, so the Dlno/Dlco ratio will not alter. The study by Harris et al7 demonstrated in sheep that the occlusion of one pulmonary artery increased the Dlno/ Dlco ratio by decreasing the Dlco while the Dlno remained constant. This effect is caused by the increase in CO backpressure in stagnant capillaries. The authors concluded that the Dlco has a much greater sensitivity than Dlno in detecting a regional reduction in capillary blood flow. The aim of this study was to test the hypothesis that the Dlno/ Dlco ratio significantly differs between patients with diffusion impairment due to fibrotic disease and patients with diffusion impairment due to pulmonary vascular disease.
Subjects were recruited from the pulmonary outpatient clinic of our hospital. This study was approved by the local ethics committee. Inclusion criteria consisted of a definitive diagnosis of diffuse parenchymal lung disease (DPLD) or pulmonary arterial hypertension (PAH) according to the Revised Clinical Classification of Pulmonary Hypertension. A control group of healthy nonsmoking subjects was recruited from among hospital personnel. All patients were extensively investigated by experienced pulmonologists; the standard procedure consisted of a medical history, a physical examination, laboratory investigations, a chest radiograph, a high-resolution CT scan of the lungs, spirometry, whole-body plethysmography (6200 Autobox DL; SensorMedics; Yorba Linda, CA), the measurement of Dlco (MasterLab Pro; Erich Jaeger GmbH; Wurzburg, Germany), the determination of subdivisions of the Dlco (ie, Dmco and Vcap, as described earlier), and ECG. Patients with (suspected) DPLD underwent bronchoscopy with bronchial lavage and transbronchial biopsies where indicated. Video-assisted thoracoscopic lung biopsy was performed in selected patients when the above-mentioned examinations did not lead to a definite diagnosis. In some patients, cardiologists or rheumatologists were consulted. Immunologic laboratory investigations and echocardiography were performed when indicated. The classification of the DPLD was based on the recommendations of the British Thoracic Society.