Figure 71-9 Relationship between shunt fraction and arterial oxygen tension for varying inspired concentrations of oxygen (PaO₂ , 100 to 680). Patients with small shunt fractions have large increases in PaO₂ with increasing inspired oxygen, whereas patients with high shunt fractions have a small response to increasing oxygen concentration. (Redrawn from Pontoppidan H, Layer MB, Geffin B: Acute respiratory failure in the surgical patient. Adv Surg 4:163, 1970.