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. .