BACKGROUND:Lung-protective ventilation using tidal volume (VT) of 4–6 mL/kg (predicted body weight) reduces mortality (compared with traditional VT) in patients with acute respiratory distress syndrome and acute lung injury. Standardized use of lower VT can result in respiratory acidosis and has raised new concerns about the appropriate configuration of the ventilator circuit, especially in regard to the dead space (VD) of the apparatus. We hypothesized that, with a patient receiving lung-protective ventilation, the removal of all apparatus dead space from the circuit would reduce PaCO2 and allow a reduction in minute ventilation. METHODS:All the studied patients met the American-European consensus-conference criteria for acute respiratory distress syndrome/acute lung injury, were receiving a lung-protective ventilation strategy, were > 18 years of age, and were hemodynamically stable. We prospectively tested 3 different ventilator-circuit configurations, in random sequence, for 15 min each: (1) standard hygroscopic heat-and-moisture exchanger (HME) with 15-cm flexible tubing, (2) 15-cm flexible tubing only, (3) no HME or flexible tubing. VT, respiratory rate, positive end-expiratory pressure, and fraction of inspired oxygen were maintained constant throughout the study, and exhaled CO2 was measured continuously. Physiologic dead space (VD/VT) was calculated using the Enghoff modification of the Bohr equation. RESULTS:Seven patients were studied. Removal of the HME from the circuit significantly decreased VD/VT (by approximately 6%) and PaCO2 (by approximately 5 mm Hg). Removal of both the HME and flexible tubing from the circuit reduced VD/VT by an additional 5%, and PaCO2 by an additional 6 mm Hg. With both circuit-configuration changes, minute ventilation fell from a mean of 11.51 L/min to 10.35 L/min, and pH increased from 7.30 to 7.38. Carbondioxide production did not change significantly. CONCLUSION:In patients receiving lower-VT ventilation, removing all the apparatus VD from the ventilator circuit reduces PaCO2 and increases pH, at a lower minute ventilation. This information will help guide ventilator-circuit configuration for patients receiving lungprotective ventilation.
Hinkson et al. (Sun,) studied this question.