Assessment of dead-space ventilation in patients with acute respiratory distress syndrome: a prospective observational study

BACKGROUND: Physiological dead space (V(D)/V(T)) represents the fraction of ventilation not participating in gas exchange. In patients with acute respiratory distress syndrome (ARDS), V(D)/V(T) has prognostic value and can be used to guide ventilator settings. However, V(D)/V(T) is rarely calculated in clinical practice, because its measurement is perceived as challenging. Recently, a novel technique to calculate partial pressure of carbon dioxide in alveolar air (PACO(2)) using volumetric capnography (VCap) was validated. The purpose of the present study was to evaluate how VCap and other available techniques to measure PACO(2) and partial pressure of carbon dioxide in mixed expired air (PeCO(2)) affect calculated V(D)/V(T). METHODS: In a prospective, observational study, 15 post-cardiac surgery patients and 15 patients with ARDS were included. PACO(2) was measured using VCap to calculate Bohr dead space or substituted with partial pressure of carbon dioxide in arterial blood (PaCO(2)) to calculate the Enghoff modification. PeCO(2) was measured in expired air using three techniques: Douglas bag (DBag), indirect calorimetry (InCal), and VCap. Subsequently, V(D)/V(T) was calculated using four methods: Enghoff-DBag, Enghoff-InCal, Enghoff-VCap, and Bohr-VCap. RESULTS: PaCO(2) was higher than PACO(2), particularly in patients with ARDS (post-cardiac surgery PACO(2) = 4.3 ± 0.6 kPa vs. PaCO(2) = 5.2 ± 0.5 kPa, P < 0.05; ARDS PACO(2) = 3.9 ± 0.8 kPa vs. PaCO(2) = 6.9 ± 1.7 kPa, P < 0.05). There was good agreement in PeCO(2) calculated with DBag vs. VCap (post-cardiac surgery bias = 0.04 ± 0.19 kPa; ARDS bias = 0.03 ± 0.27 kPa) and relatively low agreement with DBag vs. InCal (post-cardiac surgery bias = −1.17 ± 0.50 kPa; ARDS mean bias = −0.15 ± 0.53 kPa). These differences strongly affected calculated V(D)/V(T). For example, in patients with ARDS, V(D)/V(T)calculated with Enghoff-InCal was much higher than Bohr-VCap (V(D)/V(T)(Enghoff-InCal) = 66 ± 10 % vs. V(D)/V(T)(Bohr-VCap) = 45 ± 7 %; P < 0.05). CONCLUSIONS: Different techniques to measure PACO(2) and PeCO(2) result in clinically relevant mean and individual differences in calculated V(D)/V(T), particularly in patients with ARDS. Volumetric capnography is a promising technique to calculate true Bohr dead space. Our results demonstrate the challenges clinicians face in interpreting an apparently simple measurement such as V(D)/V(T). ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s13054-016-1311-8) contains supplementary material, which is available to authorized users.