Comparison of different methods for the calculation of the microvascular flow index

The microvascular flow index (MFI) is commonly used to semiquantitatively characterize the velocity of microcirculatory perfusion as absent (0), intermittent (1), sluggish (2), or normal (3). There are three approaches to compute MFI: (1) the average of the predominant flow in each of the four quadrants (MFI(by quadrants)), (2) the direct assessment during the bedside video acquisition (MFI(point of care)), and (3) the mean value of the MFIs determined in each individual vessel (MFI(vessel by vessel)). We hypothesized that the agreement between the MFIs is poor and that the MFI(vessel by vessel) better reflects the microvascular perfusion. For this purpose, we analyzed 100 videos from septic patients. In 25 of them, red blood cell (RBC) velocity was also measured. There were wide 95% limits of agreement between MFI(by quadrants) and MFI(point of care) (1.46), between MFI(by quadrants) and MFI(vessel by vessel) (2.85), and between MFI(by point of care) and MFI(vessel by vessel) (2.56). The MFIs significantly correlated with the RBC velocity and with the fraction of perfused small vessels, but MFI(vessel by vessel) showed the best R(2). Although the different methods for the calculation of MFI reflect microvascular perfusion, they are not interchangeable and MFI(vessel by vessel) might be better.