Measuring gas exchange with step changes in inspired oxygen: an analysis of the assumption of oxygen steady state in patients suffering from COPD

Bedside estimation of pulmonary gas exchange efficiency may be possible from step changes in FiO₂ and subsequent measurement of arterial oxygenation at steady state conditions. However, a steady state may not be achieved quickly after a change in FiO₂, especially in patients with lung disease such as COPD, rendering this approach cumbersome. This paper investigates whether breath by breath measurement of respiratory gas and arterial oxygen levels as FiO₂ is changed can be used as a much more rapid alternative to collecting data from steady state conditions for measuring pulmonary gas exchange efficiency. Fourteen patients with COPD were studied using 4–5 step changes in FiO₂ in the range of 0.15–0.35. Values of expired respiratory gas and arterial oxygenation were used to calculate and compare the parameters of a mathematical model of pulmonary gas exchange in two cases: from data at steady state; and from breath by breath data prior to achievement of a steady state. For each patient, the breath by breath data were corrected for the delay in arterial oxygen saturation changes following each change in FiO₂. Calculated model parameters were shown to be similar for the two data sets, with Bland–Altman bias and limits of agreement of −0.4 and −3.0 to 2.2 % for calculation of pulmonary shunt and 0.17 and −0.47 to 0.81 kPa for alveolar to end-capillary PO₂, a measure of oxygen abnormality due to shunting plus regions of low \({\dot{\text{V}}}\) a/\({\dot{\text{Q}}}\) ratio. This study shows that steady state oxygen levels may not be necessary when estimating pulmonary gas exchange using changes in FiO₂. As such this technique may be applicable in patients with lung disease such as COPD.