Beyond hypocapnia: standardised capillary oxygen partial pressure predicts risk and treatment response in PAH
T Kramer, U Limper, M Hellmich, M KramerAbstract
Background
Pulmonary arterial hypertension (PAH) is characterised by impaired pulmonary gas exchange and increased ventilatory demand, frequently resulting in hypocapnia despite only moderately reduced capillary oxygen partial pressure (pcO₂). This dissociation may obscure clinically relevant impairment of oxygenation and limit the interpretability of conventional blood gas parameters during longitudinal disease assessment.
Purpose
To evaluate whether capillary carbon dioxide partial pressure (pcCO₂) and standardised capillary oxygen partial pressure (pc-stdO₂) are associated with clinical status, treatment response, and outcome in patients with PAH.
Methods
Arterialised capillary blood gas analyses were performed in 173 newly diagnosed PAH patients at baseline and after 6 and 12 months of guideline-directed 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 the alveolo-capillary oxygen partial pressure difference, capillary oxygen content, oxygen delivery, and the Hill coefficient derived from pc-stdO₂ and capillary oxygen saturation to characterise longitudinal changes in haemoglobin oxygen-binding dynamics.
Results
At baseline, patients were hypocapnic (pcCO₂ 32.5 ± 5.6 mmHg) with moderately reduced pcO₂ (67.5 ± 15.1 mmHg), whereas pc-stdO₂ was markedly lower (55.0 ± 16.1 mmHg). During therapy, pcCO₂ increased towards normocapnia, pcO₂ remained unchanged, and both pc-stdO₂ and oxygen delivery improved significantly. The alveolo-capillary oxygen partial pressure difference decreased, while the Hill coefficient increased, consistent with improved 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 are closely associated with clinical status, treatment response, and outcome in PAH. These parameters may capture relevant pathophysiological alterations in pulmonary gas exchange beyond conventional oxygen measurements and could support therapeutic monitoring and risk assessment using routinely available blood gas analyses.