Figure 49-30 Four-part schematic diagram showing the effects of various differential lung management approaches. A, The one-lung ventilation situation. The down (dependent) lung is ventilated (Vent) but is compressed by the weight of the mediastinum (M) from above, by the pressure of the abdominal contents against the diaphragm (D), and by the positioning effects of rolls, packs, and shoulder supports (P). The up (nondependent) lung is nonventilated (Nonvent), and blood flow through this lung is shunt flow. B, The dependent lung has been selectively treated with positive end-expiratory pressure (PEEP), which improves V̇/ relationships in the dependent lung but also increases dependent lung vascular resistance; this situation diverts blood to and thereby increases shunt flow through the nonventilated lung. C, Selective application of continuous positive airway pressure (CPAP) to the nondependent lung permits oxygen uptake from this lung; even if CPAP causes an increase in vascular resistance and diverts blood flow to the dependent lung, the diverted blood flow can still participate in gas exchange in the ventilated dependent lung. Consequently, selective nondependent lung CPAP can greatly increase PaO₂ . D, With differential lung CPAP (nondependent lung)/PEEP (dependent lung), it does not matter where the blood flow goes because both lungs can participate in O₂ uptake. With this latter one-lung ventilation pattern, PaO₂ can be restored to levels near those achieved by two-lung ventilation.