Expiratory washout versus optimization of mechanical ventilation during permissive hypercapnia in patients with severe acute respiratory distress syndrome

The aim of this study was to compare three ventilatory techniques for reduc ing Pa-CO2 in patients with severe acute respiratory distress syndrome trea ted with permissive hypercapnia: (1) expiratory washout alone at a flow of 15 L/min, (2) optimized mechanical ventilation defined as an increase in th e respiratory frequency to the maximal rate possible without development of intrinsic positive end-expiratory pressure (PEEP) combined with a reductio n of the instrumental dead space, and (3) the combination of both methods. Tidal volume was set according to the pressure-volume curve in order to obt ain an inspiratory plateau airway pressure equal to the upper inflection po int minus 2 cm H2O after setting the PEEP at 2 cm H2O above the lower infle ction point and was kept constant throughout the study. The three modalitie s were compared at the same inspiratory plateau airway pressure through an adjustment of the extrinsic PEEP. During conventional mechanical ventilatio n using a respiratory frequency of 18 breaths/min, respiratory acidosis (Pa -CO2 = 84 +/- 24 mm Hg and pH = 7.21 +/- 0.12) was observed. Expiratory was hout and optimized mechanical ventilation (respiratory frequency of 30 +/- 4 breaths/min) had similar effects on CO2 elimination (Delta Pa-CO2 = -28 /- 11% versus -27 +/- 12%). A further decrease in Pa-CO2 was observed when both methods were combined (Delta Pa-CO2 = -46 +/- 7%). Extrinsic PEEP had to be reduced by 5.3 +/- 2.1 cm H2O during expiratory washout and by 7.3 +/ - 1.3 cm H2O during the combination of the two modes, whereas it remained u nchanged during optimized mechanical ventilation alone. In conclusion, incr easing respiratory rate and reducing instrumental dead space during convent ional mechanical ventilation is as efficient as expiratory washout to reduc e Pa-CO2 in patients with severe ARDS and permissive hypercapnia. When used in combination, both techniques have additive effects and result in Pa-CO2 levels close to normal values.