Non-invasive Measurement of Pulmonary Gas Exchange Efficiency: The Oxygen Deficit

The efficiency of pulmonary gas exchange has long been assessed using the alveolar-arterial difference in PO₂, the A-aDO₂, a construct developed by Richard Riley ~70years ago. However, this measurement is invasive (requiring an arterial blood sample), time consuming, expensive, uncomfortable for the patients, and as such not ideal for serial measurements. Recent advances in the technology now provide for portable and rapidly responding measurement of the PO₂ and PCO₂ in expired gas, which combined with the well-established measurement of arterial oxygen saturation via pulse oximetry (SpO₂) make practical a non-invasive surrogate measurement of the A-aDO₂, the oxygen deficit. The oxygen deficit is the difference between the end-tidal PO₂ and the calculated arterial PO₂ derived from the SpO₂ and taking into account the PCO₂, also measured from end-tidal gas. The oxygen deficit shares the underlying basis of the measurement of gas exchange efficiency that the A-aDO₂ uses, and thus the two measurements are well-correlated (r ²~0.72). Studies have shown that the new approach is sensitive and can detect the age-related decline in gas exchange efficiency associated with healthy aging. In patients with lung disease the oxygen deficit is greatly elevated compared to normal subjects. The portable and non-invasive nature of the approach suggests potential uses in first responders, in military applications, and in underserved areas. Further, the completely non-invasive and rapid nature of the measurement makes it ideally suited to serial measurements of acutely ill patients including those with COVID-19, allowing patients to be closely monitored if required.