Capillary carbon dioxide and standardised oxygen partial pressure as markers of clinical outcome in pulmonary arterial hypertension
T Kramer, U Limper, M Hellmich, M KramerAbstract
Background/Introduction
Pulmonary arterial hypertension (PAH) is associated with impaired gas exchange and increased respiratory drive, frequently resulting in hypocapnia with only moderately reduced capillary oxygen partial pressure (pcO₂). This constellation may mask clinically relevant hypoxaemia and limit the interpretability of blood gas analyses during disease monitoring.
Purpose
To investigate whether capillary carbon dioxide partial pressure (pcCO₂) and standardised capillary oxygen partial pressure (pc-stdO₂) reflect breathing effort, treatment response and clinical outcome in patients with PAH.
Methods
Arterialised capillary blood gases were obtained in 173 newly diagnosed patients with PAH at baseline and after 6 and 12 months of targeted therapy. pc-stdO₂ was calculated as pcO₂ − 1.66 × (40 − pcCO₂). Clinical assessment included echocardiography, World Health Organization functional class, six-minute walk distance, NT-proBNP, ESC/ERS risk status and right heart catheterisation. Additional parameters comprised alveolo-capillary oxygen partial pressure difference, capillary oxygen content, oxygen delivery and the Hill coefficient derived from pc-stdO₂ and capillary oxygen saturation to assess oxygen-binding dynamics over time.
Results
At baseline, patients were hypocapnic (pcCO₂ 32.5 ± 5.6 mmHg) with moderately reduced pcO₂ (67.5 ± 15.1 mmHg), but markedly lower pc-stdO₂ (55.0 ± 16.1 mmHg). During therapy, pcCO₂ increased towards normocapnia, pcO₂ remained unchanged, while pc-stdO₂ and oxygen delivery improved significantly. The alveolo-capillary oxygen partial pressure difference decreased and the Hill coefficient increased, indicating enhanced haemoglobin oxygen-binding behaviour. Changes in pcCO₂ and pc-stdO₂ correlated with clinical and haemodynamic improvement. Baseline pcCO₂ and pc-stdO₂ predicted NT-proBNP levels and were independently associated with mortality (all p < 0.05).
Conclusions
pcCO₂ and pc-stdO₂ reflect breathing effort and are associated with treatment response and clinical outcome in PAH. These findings support their potential role in therapeutic monitoring and risk stratification using routinely available blood gas analyses.