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The microcirculation is a central part of the cardiovascular system consisting of blood vessels with a diameter below 150 lm. Its main function is to exchange oxygen, carbon dioxide, nutrients, and other metabolites with the cells. The microcirculation of a healthy individual is characterized by a dense network of perfused capillaries with minimal heterogeneity, most of the capillaries being perfused even though flow in the various capillaries varies according to metabolic needs of the surrounding tissues. Adaptation to metabolic needs occurs by opening and closing capillaries, i.e. by adapting microcirculatory blood flow [1]. In recent years, a large body of knowledge pinpoints towards a pivotal pathophysiological role of the microcirculation in the development of organ failure [2]. Microcirculatory alterations are more severe in non-survivors than in survivors [3–5]. Improvements in microvascular perfusion in response to goal directed therapies are associated with subsequent improvements in organ function, while organ function further deteriorated when microvascular perfusion failed to improve or even worsened [6, 7]. Thus, the importance of monitoring the microcirculation may not be restricted to its prognostic value, but also opens up new possibilities to personalize strategies to attenuate the devastating consequences of circulatory failure in critically ill individuals. For many years, the direct study of the microcirculation was restricted to animal experimentation. Recent introduction of video microscopy using hand-held microscopes, including orthogonal polarization spectral (OPS) imaging and side-stream dark field (SDF) imaging, allows for noninvasive visualization of the microcirculation in patients [8, 9]. A recommended off-line quantitative assessment contains a minimum set of five microvascular parameters, derived from 3 to 5 video clips, Such analysis includes parameters of vessel density (total vessel density (TVD) and perfused vessel density (PVD)), perfusion (proportion of perfused vessels (PPV) and microvascular flow index (MFI)), and heterogeneity [10]. This leads to a considerable workload, since it requires a manual analysis. Moreover, the proposed parameters fail to quantify the exact red blood cell velocity in each vessel. Both practical and theoretical aspects hold back the development of the available imaging devices from a pure off-line research tool into a real-time clinical monitoring device at the bedside, and have fuelled a ‘gold rush’ for automated software analysis. The most commonly used software package to automatically analyse the video clips obtained with these devices is Automated Vascular Analysis (AVA). A disadvantage of AVA is the need for time consumingmanual correction after automated vessel detection, in order to provide reliable parameters of vessel density. In addition, space–time frames, to quantify red blood cell velocity, can only be constructed for single vessels, with sufficient length in one direction. Thus, measurements of red blood cell velocity are selective and time consuming. Despite these limitations, Dobbe and co-workers, responsible for the development of AVA, worked along the line of solid scientific reasoning [11]. External validation with simulation videos preceded & E. Christiaan Boerma e.boerma@chello.nl

Digging into the microcirculation: the rush for gold may excavate apples and oranges