Group 1 of PAH patients encompasses patients with a diverse underlying etiological condition, having histological modifications that can affect gas exchange across the alveolar-capillary membrane, as reflected by decreased DLCO. Values of DLCO did not identify the exact reason for their decrease, but they can provide insights into the underlying pathobiology and prognosis of PAH patients. Our aim was to explore whether PAH patients with low DLCO constitute a different subpopulation and describe their characteristics and response to treatment. A total of 69 PAH patients were studied retrospectively and divided into two groups: group 1: DLCO ≥ 45% and group 2: DLCO < 45%. IPAH and PAH-CTD mainly constituted our population. The proportion of IPAH to PAH-CTD was almost the same between the two groups. Patients in group 2 were older (66.83 ± 11.61 vs. 59.27 ± 111.90, p = 0.035), mostly male (47.5% vs. 11.5% p = 0.008), and ever smokers (59% vs. 22%, p = 0.049). They mainly had WHO-FC III (68% vs. 32%) and had received more advanced therapy (40% on triple combination therapy vs. 16%). The two groups had similar mean PAP (group 1 = 32 (22.00–38.00) vs. group 2 = 35 (28.50–48.50) mmHg), while PVR was higher in group 2 (6.49 (4.10–9.52) vs. 3.61 (2.95–5.22) WU). In group 2, neither IPAH nor PAH-CTD patients improved WHO-FC, 6MWD, or NT-proBNP after treatment. In our center, PAH patients with low DLCO had some distinct clinical characteristics, such as poor prognosis and poor treatment response to vasodilatory therapy. Understanding the role of DLCO in both phenotyping PAH patients and in treatment response would be useful in guiding therapeutic approaches, especially now that new therapeutic targets are involved in PAH treatment.

Group of PAH patients encompasses patients with a diverse underlying aetiological condition, having histological modifications that can affect gas exchange across the alveolar-capillary membrane as it is reflected by decreased DLCO. DLCO values did not identify the exact reason of its decrease, but can provide insights into the underlying pathobiology and prognosis of PAH patients. Our aim was to explore if PAH patients with low DLCO constitute a different subpopulation and describe their characteristics and response to treatment. 69 PAH patients were studied retrospectively and divided into group1: DLCO≥45% and group2: DLCO< 45%. IPAH and PAH-CTD constituted mainly our population. The proportion of IPAH and PAH-CTD was almost the same. Patients in group 2 were older (66.83± 11.61 vs 59.27±111.90, p=0.035), more often male (47.5% vs 11.5% p=0.008) and ever smokers (59% vs 22%, p=0.049). They had mainly WHO-FC III ((68%) vs 32%) and have received more advanced therapy (40% on triple combination therapy vs 16%). The two groups had the same mean PAP (group1=32 (22.00-38.00) vs group2=35 (28.50-48.50) mmHg) and PVR was higher in group 2 (6.49(4.10-9.52) vs 3.61 (2.95-5.22) WU). In group 2 neither IPAH nor PAH-CTD patients did improve WHO-FC, 6MWD, or NT-proBNP after treatment. In our centre PAH patients with low DLCO have some distinct clinical characteristics. poor prognosis and treatment response in vasodilatory therapy. Understanding the role of DLCO in phenotyping PAH patient and in treatment response would be useful in guide therapeutic approaches, especially now that new therapeutic targets are involved in the PAH treatment.