Microcirculatory dysfunction and dead-space ventilation in early ARDS: a hypothesis-generating observational study

Background Ventilation/perfusion inequalities impair gas exchange in acute respiratory distress syndrome (ARDS). Although increased dead-space ventilation ( V D / V T ) has been described in ARDS, its mechanism is not clearly understood. We sought to evaluate the relationships between dynamic variations in V D / V T and extra-pulmonary microcirculatory blood flow detected at sublingual mucosa hypothesizing that an altered microcirculation, which is a generalized phenomenon during severe inflammatory conditions, could influence ventilation/perfusion mismatching manifested by increases in V D / V T fraction during early stages of ARDS. Methods Forty-two consecutive patients with early moderate and severe ARDS were included. PEEP was set targeting the best respiratory-system compliance after a PEEP-decremental recruitment maneuver. After 60 min of stabilization, hemodynamics and respiratory mechanics were recorded and blood gases collected. V D / V T was calculated from the CO 2 production ( V_CO_2 ) and CO 2 exhaled fraction ( F_ECO_2 ) measurements by volumetric capnography. Sublingual microcirculatory images were simultaneously acquired using a sidestream dark-field device for an ulterior blinded semi-quantitative analysis. All measurements were repeated 24 h after. Results Percentage of small vessels perfused (PPV) and microcirculatory flow index (MFI) were inverse and significantly related to V D / V T at baseline (Spearman’s rho = − 0.76 and − 0.63, p < 0.001; R 2 = 0.63, and 0.48, p < 0.001, respectively) and 24 h after (Spearman’s rho = − 0.71, and − 0.65; p < 0.001; R 2 = 0.66 and 0.60, p < 0.001, respectively). Other respiratory, macro-hemodynamic and oxygenation parameters did not correlate with V D / V T . Variations in PPV between baseline and 24 h were inverse and significantly related to simultaneous changes in V D / V T (Spearman’s rho = − 0.66, p < 0.001; R 2 = 0.67, p < 0.001). Conclusion Increased heterogeneity of microcirculatory blood flow evaluated at sublingual mucosa seems to be related to increases in V D / V T , while respiratory mechanics and oxygenation parameters do not. Whether there is a cause–effect relationship between microcirculatory dysfunction and dead-space ventilation in ARDS should be addressed in future research.