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

BACKGROUND: Volumetric capnography is an alternative method of measuring expired carbon dioxide partial pressure (P eCO 2 ) and physiologic dead-space-to-tidal-volume ratio (V D /V T ) during mechanical ventilation. In this method, P eCO 2 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 eCO 2 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 eCO 2 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 (r 2 = 0.93, p 2 = 0.89, p 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.