Correlation between microcirculatory flow, density and heterogeneity scores in septic shock patients

Introduction

A recent conference recommended that microcirculatory images should be analyzed with several scores that evaluate density, flow and flow heterogeneity [1]. Following these recommendations, we analyzed sublingual microcirculatory images from septic shock patients to determine how these different scores were correlated with each other and with clinical hemodynamic and perfusion parameters.

Methods

Using side dark-field videomicroscopy (Microscan^®; Microvision Medical) we repeatedly evaluated sublingual microcirculation at different time points in septic shock patients. In total, we performed 17 microcirculatory single-time-point assessments (3–6 site images/time point), in parallel with hemodynamic and perfusion measurements (mean arterial pressure (MAP), nor-adrenaline dose (NA), cardiac index (CI), mixed venous O₂ saturation (SmvO₂), arterial lactate). Images were analyzed by semi-quantitative scores of flow (mean flow index (MFI) and proportion of perfused vessels (PPV)) and density (perfused vascular density (PVD)) of small vessels (<20 μm). Heterogeneity indexes (Het Index = maximum - minimum/mean) were calculated for the MFI and PPV. Correlations between parameters were determined by the Pearson coefficient and considered significant if P < 0.05.

Results

We found that PVD was correlated to PPV (r = 0.55), and negatively to Het Index PVD (r = -0.54) and Het index PPV (r = -0.43), but we found no correlation of PVD with any homodynamic or perfusion parameter. Flow indexes (PPV and MFI) were strongly correlated with each other (r = 0.81) and inversely with their respective heterogeneity indexes (PPV and Het Index PPV, r = -0.88; MFI and Het Index MFI, r = -0.83). In addition, PPV and MFI were correlated to SmvO₂ (r = 0.44 and 0.52), and CI (r = 0.49 and 0.47), and inversely to lactate levels (r = -0.46 and -0.4). Only the MFI was correlated to MAP (r = 0.5). Heterogeneity indexes were correlated to lactate (r = 0.40 with PPV and r = 0.44 with MFI), and inversely to MAP (r = -0.40 with Het Index PPV and r = -0.64 with Het index MFI). The Het Index MFI was also correlated to NA (r = 0.5).

Conclusion

Higher microcirculatory flow scores, but not density, are associated with higher CI and better systemic perfusion parameters. Both MFI and PPV seem equally effective to assess microcirculatory flow.