Fig 1.

(a) An inspiratory compliance curve (lung volume vs airway pressure) during OLV as the lung is slowly inflated by 100 ml increments in a patient with mild COPD. The lower inflection point of the curve (thought to represent FRC) is at 7 cm H₂O. During OLV, this patient developed intrinsic PEEP (measured by the end-expiratory airway occlusion plateau pressure ‘auto-PEEP’) of 6 cm H₂O. The addition of 5 cm H₂O of PEEP in this patient raised the end-expiratory lung volume above FRC, thus raising pulmonary vascular resistance in the ventilated lung and caused a deterioration in oxygenation. (b) The inspiratory compliance curve during OLV in a patient with normal pulmonary function. The lower inflection point of the curve is at 6 cm H₂O. During OLV, this patient developed intrinsic PEEP of 2 cm H₂O. The addition of 5 cm H₂O of PEEP raised the end-expiratory lung volume to FRC, thus decreasing pulmonary vascular resistance in the ventilated lung and caused an improvement in oxygenation. Based on data from Slinger and colleagues.³⁷