The Effects of Apparatus Dead Space on P aCO 2 in Patients Receiving Lung-Protective Ventilation

Abstract

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 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-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 lungprotective ventilation.