Abstract

Background: Right to left shunt and regional hypoventilation (reduced ventilation/perfusion ratio (V_A/Q)) have different effects on the curve relating inspired oxygen (P_IO₂) to oxygen saturation measured by pulse oximetry (SpO₂) and can be derived non-invasively from measurements of SpO₂ and inspired oxygen pressure (P_IO₂) using complex models of gas exchange. We developed a simpler computerised “slide-rule” method of making these derivations.

Aims: To describe the slide-rule method and determine agreement between measurements derived with this and a more complex algorithm.

Methods: Series of P_IO₂ versus SpO₂ data points obtained during 43 studies in 16 preterm infants with bronchopulmonary dysplasia were analysed. Percentage shunt and the degree of right shift (kPa) of the P_IO₂ versus SpO₂ curve compared with the oxyhaemoglobin dissociation curve (a measure of V_A/Q) were determined for each dataset with both methods, and the results were compared using the method of Bland and Altman.

Results: The computer slide-rule method produced results for all 43 datasets. The more complex model could derive results for 40/43 datasets. The mean differences (95% limits of agreement) between the two methods for measurements of shunt were −1.7% (−6.5 to +3.5%) and for measurements of right shift were 0.3 kPa (−2.9 to +3.6 kPa).

Conclusion: The slide-rule method was reliable for deriving shunt and right shift (reduced V_A/Q) of the P_IO₂ versus SpO₂ curve when compared with the more complex algorithm. The new method should enable wider clinical application of these measurements of oxygen exchange.