Background: Pulmonary gas exchange efficiency, determined by the alveolar-to-arterial PO2 difference (A-aDO2), progressively worsens during exercise at sea-level; this response is further augmented during exercise in hypoxia. Traditionally, pulmonary gas exchange efficiency is assessed through measurements of ventilation and end-tidal gases paired with direct arterial blood gas (ABG) sampling. Since these measures have a number of caveats, particularly invasive blood sampling, the development of new approaches for the non-invasive assessment of pulmonary gas exchange is needed. Research Question: Is a non-invasive method of assessing pulmonary gas exchange valid during rest and exercise in acute hypoxia? Study Design and Methods: Twenty-five healthy participants (10 female) completed a staged maximal exercise test on a cycle ergometer in a hypoxic chamber (FIO2=0.11). Simultaneous ABGs via a radial arterial catheter and non-invasive gas exchange measurements (AGM100) were obtained in two-minute intervals. Non-invasive gas exchange efficiency, termed the O2 deficit, was calculated from the difference between the end-tidal and the calculated PaO2 (via pulse oximetry and corrected for the Bohr effect by using the end-tidal PCO2). Non-invasive O2 deficit was compared to the traditional alveolar to arterial oxygen difference (A-aDO2) using the traditional Riley analysis. Results: Under conditions of rest at room air, hypoxic rest and hypoxic exercise, strong correlations between the calculated gPaO2 and directly measured PaO2 (R²=0.97; p<0.001; mean bias =1.70 mmHg) were observed. At hypoxic rest and exercise, strong relationships between the calculated and directly measured PaO2 (R²=0.68; p<0.001; mean bias =1.01mmHg) and O2 deficit with the traditional A-aDO2 (R²=0.71; p<0.001; mean bias =5.24mmHg) remained. Interpretations: Our findings support the use of a non-invasive measure of gas exchange during acute hypoxic exercise in heathy humans. Further studies are required to determine if this approach can be used clinically as a tool during normoxic exercise in patients with pre-existing impairments in gas exchange efficiency.
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