Prognostic value of blood gas analyses in patients with idiopathic pulmonary arterial hypertension

Blood gas abnormalities in patients with idiopathic pulmonary arterial hypertension (IPAH) may be related to disease severity and prognosis. The present authors performed a 12-yr retrospective analysis assessing arterialised capillary blood gases, haemodynamics, exercise variables and survival in 101 patients with IPAH. At baseline, arterial oxygen tension (Pa,O-2) and carbon dioxide arterial tension (Pa,CO2) were 9.17+/-1.86 and 4.25+/-0.532 kPa, respectively. While Pa,O-2 was not associated with survival, a low Pa,CO2 was a strong and independent prognostic marker. When patients were divided according to a baseline Pa,CO2 value above or below 4.25 kPa, a cut-off value determined by receiver operating characteristics analysis, survival rates were 98 and 86% at 1 yr, 82 and 69% at 2 years, 80 and 51% at 3 yrs, 77 and 41% at 5 yrs, and 65 and 12% at 8 yrs, respectively. Pa,CO2 after 3 months of medical therapy was strongly associated with survival. Hypocapnia at rest and during exercise correlated with low cardiac output, low peak oxygen uptake and reduced ventilatory efficacy. Multiple regression analysis revealed that 6-min walking distance, right atrial pressure and Pa,CO2 were independently associated with survival. In patients with idiopathic pulmonary arterial hypertension, hypocapnia (carbon dioxide arterial tension <4.25 kPa) is an independent marker of mortality.