Inspiratory vs. expiratory pressure-volume curves to set end-expiratory pressure in acute lung injury
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To study the effects of two levels of positive end-expiratory pressure (PEEP), 2 cmH2O 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.
Eight patients with early acute lung injury.
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 cmH2O above the inspiratory lower inflection point (15.5±3.1 cmH2O) and a PEEP equal to the expiratory point of maximum curvature (23.5±4.1 cmH2O).
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 PaCO2, airway pressures, and hyperaerated lung volume.
High PEEP levels according to the point of maximum curvature of the deflation limb of the pressure-volume curve have both benefits and drawbacks.
KeywordsAcute lung injury Positive end-expiratory pressure Pressure-volume curves Computed tomography Mechanical ventilation
The authors thank all the nursing and technician personnel in the Intensive Care Unit and CT Ward of the Hospital Universitario Central de Asturias for their help. We also thank Ana Villagrá for her suggestions about the manuscript.
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