Accuracy of Physiologic Dead Space Measurements in Patients With Acute Respiratory Distress Syndrome Using Volumetric Capnography: Comparison With the Metabolic Monitor Method

Abstract

BACKGROUND:

Volumetric capnography is an alternative method of measuring expired carbon dioxide partial pressure (P_eCO2 ) and physiologic dead-space-to-tidal-volume ratio (V_D/V_T) during mechanical ventilation. In this method, P_eCO2 is measured at the Y-adapter of the ventilator circuit, thus eliminating the effects of compression volume contamination and the need to apply a correction factor. We investigated the accuracy of volumetric capnography in measuring V_D/V_T, compared to both uncorrected and corrected measurements, using a metabolic monitor in patients with acute respiratory distress syndrome (ARDS).

METHODS:

There were 90 measurements of V_D/V_T made in 23 patients with ARDS. The P_eCO2 was measured during a 5-min expired-gas collection period with a Delta-trac metabolic monitor, and was corrected for compression volume contamination using a standard formula. Simultaneous measurements of P_eCO2 and V_D/V_T were obtained using volumetric capnography.

RESULTS:

V_D/V_T measured by volumetric capnography was strongly correlated with both the uncorrected (r² = 0.93, p < 0.0001) and corrected (r² = 0.89, p < 0.0001) measurements of V_D/V_T made using the metabolic monitor technique. Measurements of V_D/V_T made with volumetric capnography had a bias of 0.02 and a precision of 0.05 when compared to the V_D/V_T corrected for estimated compression volume contamination.

CONCLUSION:

Volumetric capnography measurements of V_D/V_T in mechanically-ventilated patients with ARDS are as accurate as those obtained by metabolic monitor technique.