Figure 36-7 Hemoglobin-oxygen (OXYHb) saturation versus inspired partial pressure of oxygen (PO₂ ). The curves are plotted by changing inspired PO₂ in a stepwise fashion. A, A series of theoretical curves obtained by calculating the effect of different degrees of right-to-left shunt. Increasing shunt displaces the curves downward. B, The curve on the left of the graph (0%) is from a normal subject. The middle curve (30%) represented a 30% right-to-left shunt from the 30% curve seen in A. The curve on the right of this graph is from a patient undergoing thoracotomy for esophageal surgery. The points cannot be fitted by any of the shunt curve, but the fit is quite good when the 30% curve is shifted to the right. This implies a combination of shunt and V̇A/ mismatch. (Adapted from Jones JG, Jones SE: Discriminating between the effect of shunt and reduced VA/Q on arterial oxygen saturation is particularly useful in clinical practice. J Clin Monit Comput 16:337, 2000.)