Smaller Tidal Volumes Improve ARDS Tidal Volume Lower Than 6 mL/kg Enhances Lung Protection: Role of Extracorporeal Carbon Dioxide Removal

Background: ARDSnet data reaffirm the practice of limiting tidal volume in patients with lung injury to 6 to 8 mL/kg of predicted body weight and limiting plateau pressures to avoid additional lung injury and excess mortality. Objective: To determine whether tidal volume <6 mL/kg is protective to the lungs and whether consequent respiratory acidosis can be managed by extracorporeal carbon dioxide removal (ECCOR). Design: Prospective, clinical trial. Participants: Adult patients with acute respiratory distress syndrome (ARDS). Methods: After 3 days of mechanical ventilation administered using the ARDSnet criteria; if the plateau pressure was between 25 and 28 cm of H2O, the ARDSnet strategy was continued. If the plateau pressure was between 28 and 30 cm H2O, a stepwise reduction in tidal volume was performed until plateau pressures were between 25 and 28 cm H2O. Respiratory rate was increased and bicarbonate was infused. Extracorporeal carbon dioxide removal was utilized if a pH <7.25 occurred. CT scans of the lungs were performed before and after the study period. Bronchoalveolar lavage was performed before and after the study period to determine levels of a number of inflammatory mediators. Results: The average age of the patients was the mid 60s and most were men. Pneumonia, sepsis, and trauma were the causes of lung injury. The group with the higher plateau pressures required higher positive end-expiratory pressures, higher minute ventilation, and lower ratios of partial arterial oxygen tension to inspired O2 fraction (PaO2/FiO2) than the lower plateau pressure group. Tidal volume in the higher plateau pressure group was reduced to approximately 4.5 mL/kg to achieve the desired lower plateau pressure. Significantly higher arterial carbon dioxide partial pressure (PaCO2 ) and lower pH resulted despite bicarbonate infusion and a higher respiratory rate. All patients in the reduced tidal volume group met pH criteria for ECCOR. Within 2 hours of initiation of ECCOR, PaCO2 and pH were significantly improved and were nearly normal at the conclusion of the 72-hour study period. The reduction in tidal volume was associated with improvement in aeration patterns on CT scans and a reduction in calculated lung weight. Inflammatory cytokine levels were lower in the lower plateau pressure group, and the reduction in tidal volume produced a reduction in the cytokine levels compared with the higher plateau pressure levels. No significant complications occurred from the use of ECCOR. Conclusions: Further reduction in tidal volume minimizes hyperinflation and attenuates pulmonary inflammation in patients with ARDS who have plateau pressures between 28 and 30 cm H2O when ventilated with ARDSnet ventilation protocol. Reviewer's Comments: Whether the respiratory acidosis from the low tidal volume ventilation needs to be treated at all is one question that needs to be addressed in further studies. It appears that lower tidal volumes than those associated with the ARDSnet strategy can be tolerated and may be beneficial in the setting of acute lung injury. (Reviewer-Allen Miranda, MD).