The effects of apparatus dead space on PaCO2 in patients receiving lung-protective ventilation

BACKGROUND: Lung-protective ventilation using tidal volume (V-T) of 4-6 mL/kg (predicted body weight) reduces mortality (compared with traditional V-T) in patients with acute respiratory distress syndrome and acute lung injury. Standardized use of lower V-T 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 (V-D) 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 P-aCO2 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. V-T, 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 (V-D/V-T) was calculated using the Enghoff modification of the Bohr equation. RESULTS: Seven patients were studied. Removal of the HME from the circuit significantly decreased V-D/V-T (by approximately 6%) and P-aCO2 (by approximately 5 mm Hg). Removal of both the HME and flexible tubing from the circuit reduced V-D/V-T by an additional 5%, and P-aCO2 by an additional 6 nun Hg. With both circuit-configuration changes, minute ventilation fell from a mean of 11.51 L/min to 1035 L/min, and pH increased from 7.30 to 7.38. Carbon-dioxide production did not change significantly. CONCLUSION: In patients receiving lower-V-T ventilation, removing all the apparatus V-D from the ventilator circuit reduces P-aCO2 and increases pH, at a lower minute ventilation. This information will help guide ventilator-circuit configuration for patients receiving lung-protective ventilation.