ANALYSIS OF FACTORS WHICH AFFECT THE RELATIONSHIP BETWEEN INSPIRED OXYGEN PARTIAL PRESSURE AND ARTERIAL OXYGEN SATURATION

The adoption of pulse oximetry in anaesthesia and intensive care has resulted in oxyhaemoglobin saturation (Sa₀₂) becoming an important and widely used clinical descriptor of gas exchange, although its full potential has not been realized. We have explored the effect of impaired gas exchange on the relationship between Pl₀₂ and Sa₀₂ A curve of oxygen content vs Pl₀₂ for an ideal lung was constructed using the oxygen dissociation curve and allowing for the effect of dissolved oxygen in plasma. A plot of Pl₀₂ vs Sa₀₂ was derived from this content curve. The effect of impaired gas exchange (shunt and V/Q effects) on this relationship was then modelled using for shunt the equation Qs/Qt = (CC₀₂-Ca₀₂)/(Cc₀₂-C$V¯$₀₂) and for V/Q, a graphical method using the equation V/Q = K(Cc₀₂-C$V¯$₀₂) and the oxygen content curve to model the effect of a spread of V/Q ratios. A total of nine compartments were used to model the scatter of V/Q, and the size of these compartments were determined by their relative blood supply. Plots of Pl₀₂ vs Sa₀₂ were derived for different values of shunt and V/Q. The Pl₀₂ vs Sa₀₂ curve reflected the shape of the oxygen dissociation curve but lay to the right by Pl₀₂-PA₀₂ Shunt caused a depression of the plateau of this curve with relatively little lateral movement, whereas a low mean V/Q ratio caused a marked rightwards lateral displacement. Increased log_SD Q caused a flattening of the relationship. The combined effect of shunt and a spread of V/Q ratios caused both lateral and vertical displacements. Thus a series of simple measurements of Pl₀₂ and Sa₀₂ gave information about both shunt and V/Q abnormality. (Br. J. Anaesth. 1993; 71: 488–494)