Inspiratory vs. expiratory pressure-volume curves to set end-expiratory pressure in acute lung injury.

ABSTRACT

OBJECTIVE: To study the effects of two levels of positive end-expiratory pressure (PEEP), 2 cm H(2)O above the lower inflection point of the inspiratory limb and equal to the point of maximum curvature on the expiratory limb of the pressure-volume curve, in gas exchange, respiratory mechanics, and lung aeration. DESIGN AND SETTING: Prospective clinical study in the intensive care unit and computed tomography ward of a university hospital. PATIENTS Eight patients with early acute lung injury. INTERVENTIONS: Both limbs of the static pressure-volume curve were traced and inflection points calculated using a sigmoid model. During ventilation with a tidal volume of 6 ml/kg we sequentially applied a PEEP 2 cm H(2)O above the inspiratory lower inflection point (15.5+/-3.1 cm H(2)O) and a PEEP equal to the expiratory point of maximum curvature (23.5+/-4.1 cmH(2)O). MEASUREMENTS AND RESULTS: Arterial blood gases, respiratory system compliance and resistance and changes in lung aeration (measured on three computed tomography slices during end-expiratory and end-inspiratory pauses) were measured at each PEEP level. PEEP according to the expiratory point of maximum curvature was related to an improvement in oxygenation, increase in normally aerated, decrease in nonaerated lung volumes, and greater alveolar stability. There was also an increase in PaCO(2), airway pressures, and hyperaerated lung volume. CONCLUSIONS: High PEEP levels according to the point of maximum curvature of the deflation limb of the pressure-volume curve have both benefits and drawbacks.