Background/Purpose:   The Pulmonary Hypertension Assessment and Recognition of Outcomes in Scleroderma (PHAROS) cohort is a prospective longitudinal study of patients with systemic sclerosis (SSc) who are at risk for developing pulmonary arterial hypertension (PAH) or who have already been diagnosed with pulmonary hypertension (PH). Although PAH in SSc is usually associated with a very low diffusing capacity for carbon monoxide (DLCO) we identified some patients with a normal DLCO. The purpose of this study was to look at the subset of PAH patients with a normal DLCO defined as ³ 70% predicted at the time of diagnosis of PAH compared to those with expected low DLCO defined as < 70%. We compared demographics, BNP level, pulmonary function tests (PFT), echocardiogram, and right heart catheterization findings in these groups.

Methods:   Entry criteria into PHAROS for SSc patients with Group 1 PAH were a right heart catheterization (RHC) with a mean pulmonary artery pressure (mPAP) ≥ 25 mmHg and a pulmonary capillary wedge pressure (PCWP) ≤ 15 mmHg. We examined demographics (age, gender and ethnicity), echocardiogram variables (presence of pericardial effusion and left atrial size), PFT (FVC, FEV1/FVC, FVC%/DLCO%), BNP levels (<100 versus 100 pg/mL) and RHC findings and outcomes in PAH patients who had DLCO % predicted recorded near the time of RHC. We used Fisher’s exact test to examine associations between two categorical variables and Kolmogorov-Smirnov test for continuous variables. Significance was assigned at p< 0.05.

Results:   In the PAH patients (n=202), only 11 patients (5.4%) had a normal DLCO of > 70% (mean 78%) versus 191 patients (94.6%) who had a low DLCO <70% (mean 39%). There was no difference in demographics, other PFT results, echocardiogram findings, or RHC features between the normal DLCO and the low DLCO patients, The left atrium in the normal DLCO patients was slightly larger at 4.1cm than in the low DLCO group (3.7cm), but it was not statistically different, and the PCWP were the same on RHC. There were no differences in survival between these two groups. Finally, on follow up testing in10 of these11 patients, seven had subsequently developed a low DLCO. 

Conclusion:  Only 5% of this large cohort of SSc-PAH patients had a normal DLCO >70% predicted at the time of diagnosis. The majority of these fell below normal values on follow up testing. There were no identified differences in these patients to suggest why they maintained a normal DLCO compared to the vast majority of SSc –PAH with abnormal low DLCO. Although a low DLCO is almost always seen in SSc patients with PAH, PHAROS data suggests that in rare situations, a patient may develop PAH with a preserved DLCO.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-normal-pulmonary-diffusion-capacity-is-rare-in-pulmonary-artery-hypertension-in-systemic-sclerosis/