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 ( $$V_{{{\text{CO}}_{2} }}$$ V CO 2 ) and CO_2 exhaled fraction ( $$F_{{{\text{ECO}}_{2} }}$$ 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.