Clinical review: Clinical imaging of the sublingual microcirculation in the critically ill - where do we stand?

A growing body of evidence exists associating depressed microcirculatory function and morbidity and mortality in a wide array of clinical scenarios. It has been suggested that volume replacement therapy using fluids and/or blood in combination with vasoactive agents to modulate macro- and microvascular perfusion might be essential for resuscitation of severely septic patients. Even after interventions effectively optimizing macrocirculatory hemodynamics, however, high mortality rates still persist in critically ill and especially in septic patients. Therefore, rather than limiting therapy to macrocirculatory targets alone, microcirculatory targets could be incorporated to potentially reduce mortality rates in these critically ill patients. In the present review we first provide a brief history of clinical imaging of the microcirculation and describe how microcirculatory imaging has been of prognostic value in intensive care patients. We then give an overview of therapies potentially improving the microcirculation in critically ill patients and propose a clinical trial aimed at demonstrating that therapy targeting improvement of the microcirculation results in improved organ function in patients with severe sepsis and septic shock. We end with some recent technological advances in clinical microcirculatory image acquisition and analysis.


Background
Th e microcirculation may have a key role in the development of (multiple) organ failure in the critically ill and the main aim of hemodynamic resuscitation in these patients is to restore microcirculatory perfusion and tissue oxygenation to prevent organ hypoxia and maintain organ function [1][2][3]. It has been recognized that therapeutic interventions should be delivered as early as possible [3,4] and early protocol-driven resus citation strategies (for example, early goal-directed therapy) targeting global hemodynamic parameters have been associated with the best clinical outcome in random ized controlled clinical trials [4,5]. However, even after interven tions eff ectively optimizing macrocircu la tory hemodynamics (for example, cardiac fi lling pressure, cardiac output, blood pressure, and central or mixed venous oxygen saturation), high mortality rates still persist [6]. In this light, it has been shown that improve ment of macrocirculatory hemodynamics does not guaran tee (suffi cient) improvement of the microcircu la tion [2].
In critical illness, and especially in sepsis and shock, microcirculatory dysfunction may arise as a result of several factors, such as endothelial dysfunction, leukocyte-endothelium interactions, coagulation and infl amma tory disorders, hemorheological abnormalities, and a disturbed balance between oxygen delivery and oxygen consumption [7]. Th is microcirculatory dysfunc tion is characterized by heterogeneous abnormalities in blood fl ow with some capillaries being non-or hypo-perfused while others are normally or even hyper-perfused. Due to the dysregulated heterogeneous fl ow distribution, weak microcirculatory units may become hypoxic. Th is is the main reason why monitoring systemic hemodynamicderived and oxygen-derived variables is not able to sense such microcirculatory dysfunction. Th erefore, rather than limiting (early) goal-directed therapy to macrocirculatory targets alone, microcirculatory targets could be incorporated to more eff ectively resuscitate the microcirculation and thereby potentially reduce mortality rates in these critically ill patients [8][9][10][11]. However, no such clinical study exists to date.
In the present review we fi rst provide a brief history of clinical imaging of the microcirculation and describe how microcirculatory images can be analyzed for measures of microvascular density and perfusion and Abstract A growing body of evidence exists associating depressed microcirculatory function and morbidity and mortality in a wide array of clinical scenarios. It has been suggested that volume replacement therapy using fl uids and/or blood in combination with vasoactive agents to modulate macro-and microvascular perfusion might be essential for resuscitation of severely septic patients. Even after interventions eff ectively optimizing macrocirculatory hemodynamics, however, high mortality rates still persist in critically ill and especially in septic patients. Therefore, rather than limiting therapy to macrocirculatory targets alone, microcirculatory targets could be incorporated to potentially reduce mortality rates in these critically ill patients. In the present review we fi rst provide a brief history of clinical imaging of the microcirculation and describe how microcirculatory imaging has been of prognostic value in intensive care patients. We then give an overview of therapies potentially improving the microcirculation in critically ill patients and propose a clinical trial aimed at demonstrating that therapy targeting improvement of the microcirculation results in improved organ function in patients with severe sepsis and septic shock. We end with some recent technological advances in clinical microcirculatory image acquisition and analysis.
how microcirculatory imaging has been of prognostic value in intensive care patients. Th en, we give an overview of therapies potentially improving the microcirculation in critically ill patients (fl uid resuscitation, blood transfusion, and vasoactive agents) and propose a clinical trial aimed at demonstrating that therapy targeting improvement of the microcirculation results in improved organ function in patients with severe sepsis and septic shock. Finally, some recent technological advances in clinical microcirculatory image acquisition (image acquisition stabilization) and analysis (automated image analysis) might allow such microcirculationtargeted resuscitation by providing instant feedback on the effi cacy of the applied therapeutic strategies at the microcirculatory level.

Brief history of clinical imaging of the microcirculation
After Van Leeuwenhoek's introduction of in vivo microcirculatory microscopy in 1688 [12,13], this technique was long limited to semi-transparent tissue that could be transilluminated to avoid image contamination by tissue surface refl ections and thereby obtain suffi cient image contrast [14][15][16]. Later, use has been made of incident light directed at an oblique angle to the studied tissue [17]. Such a setup, however, required very careful alignment of the light source and the microscopic lens system and still suff ered from tissue surface refl ections. It was not until 1971 that Sherman and colleagues [18] introduced a new method for studying the microcirculation: incident dark fi eld illumination microscopy. In their setup, dark fi eld illumination was provided through a circular prismatic lens surrounding the objective lens, which created a halo of light around and beyond the objective focal point. Th is type of illumination gave 'an unusual depth of fi eld and a three-dimensional quality to the tissue observed' and permitted visualization of micro circulatory structures beneath the surface of organs as dark red blood cell columns on a bright background. Th e authors visualized and photographed the circulation of the cat brain, lung, kidney, liver, mesentery, and intestine successfully.
Freedlander and Lenhart [19] were in 1922 the fi rst to visualize capillaries in living humans and to investigate the eff ects of infection. In 1987, Slaaf and colleagues [20] developed an alternative way of eliminating tissue surface refl ections for imaging subsurface microcirculatory networks that was inspired by fl uorescence microscopy. In fl uorescence microscopy, image contrast is created by spectral separation of the refl ected illumination light and the imaging light by application of an excitation and an emission fi lter in combination with a dichroic mirror. Similarly, Slaaf and colleagues proposed to separate the refl ected illumination light from the imaging light by application of a polarizer and an analyzer (that is, a polarizer oriented orthogonally to the orientation of the polarizer) in combination with a 50% refl ection mirror. Due to its orthogonal orientation with respect to the polarized illumination light, the analyzer blocked directly refl ected (undepolarized) light and allowed backscattered (depolarized) light to pass. Th is setting provided images of the microcirculation at suffi cient contrast, similar to those obtained using dark fi eld imaging.
Several years later, Groner and colleagues combined the methods developed by Sherman and colleagues and Slaaf and colleagues and added a spectral component for further optimization of image contrast. In 1999, they introduced orthogonal polarization spectral (OPS) imaging, incorporated into a hand-held, clinically applicable device [21]. Using OPS imaging we were the fi rst to image the human brain microcirculation during surgery [21]. Since then, numerous studies have been undertaken in various clinical scenarios where cardiovascular function is at risk (for example, [1][2][3]7,8,10,11]).
Despite the major contribution OPS imaging has made in the fi eld of intravital microcirculatory imaging, several shortcomings were still present [22,23]. Th ese include suboptimal imaging of the capillaries due to motioninduced image blurring by movement of the OPS device, the tissue, and/or fl owing red blood cells. Th is introduces diffi culties in measuring blood fl ow velocities in these vessels. Th us, driven by the success of OPS imaging and the drawbacks it has, Goedhart and colleagues [24] have developed a second generation device for clinical imaging of the microcirculation, which was termed sidestream dark fi eld (SDF) imaging. Typical OPS and SDF images obtained at the same sublingual microcirculatory area are presented in Figure 1.
For evaluation of the eff ects of interventions and (drug) therapy, microcirculatory images can be analyzed to assess (alterations in) microvascular density and perfusion. To assess microcirculatory perfusion, a semiquanti tative scoring method (that is, the microcirculatory fl ow index; MFI) has been developed to characterize microcirculatory fl ow as 'no fl ow' , 'intermittent fl ow' , 'sluggish fl ow' , and 'continuous fl ow' [25]. Micro circulatory density can be assessed as the total vessel density (TVD), including perfused and non-perfused microvessels, and perfused vessel density (PVD), including perfused microvessels only. Th e ratio PVD/TVD is used to express the proportion of perfused vessels (PPV). When only vessels with a diameter <20 μm are included in the analysis, the PVD represents the functional capillary density (FCD), which is considered the main determinant of microcirculatory blood supply.
To date, many studies have investigated the microcirculation using OPS and SDF imaging under various pathophysiological conditions, such as in surgery, emer gency medicine, and intensive care medicine. Both OPS and SDF imaging have had an important clinical impact by observation of the sublingual microcirculation under various pathophysiological conditions and especially during sepsis and shock (for example, [1][2][3]). Results from several medical centers have shown that alterations in the sublingual microcirculation might provide information with respect to patient outcome from sepsis and shock.

Prognostic value of the microcirculation
Microcirculatory failure has been shown to be of prognostic value in septic patients. Microcirculatory disorders before resuscitation and their persistence after have been associated with increased risk of morbidity and mortality [1][2][3]26,27]. De Backer and colleagues [1] found that the microcirculatory alterations in non-surviving septic patients were more severe compared to those in surviving patients. Th is was later confi rmed by Sakr and colleagues and Trzeciak and colleagues, who, furthermore, showed that a lack of improvement of microcirculatory fl ow after resuscitation was associated with organ failure and death [2] and that non-surviving patients had a signifi cantly higher microcirculatory fl ow heterogeneity compared to surviving patients [27]. In a later study, Trzeciak and colleagues [3] demonstrated that early increases in micro circulatory perfusion during protocol-directed resuscitation were associated with reduced severity of organ failure as assessed by the Sequential Organ Failure Assessment (SOFA) score in patients with sepsis.
Besides septic patients, microcirculatory disorders have also been shown to predict mortality in patients with acute severe heart failure and cardiogenic shock [28], and impaired microvascular fl ow was associated with the development of post-operative complications in patients who underwent major abdominal surgery [29].
Hence, a growing body of evidence exists associating depressed microcirculatory function with morbidity and mortality in a wide array of clinical scenarios.
Although many studies have found that microcirculatory dysfunction is a common complication of prognostic value in critically ill patients, most of these studies were single-center investigations only including specifi c patient populations. To date, therefore, no information on the overall prevalence of microcirculatory dysfunction in intensive care patients is available. To obtain such insight, a large multi-center international observational study has been conducted by Boerma and co-workers to investigate the prevalence of microcirculatory alterations in intensive care patients, regardless of their underlying disease. Th is is, in fact, the largest microcirculatory study ever performed in the critically ill (>400 patients). Because the study has been designed similarly to the well known multi-center Sepsis Occurrence in Acutely ill Patients (SOAP) studies in which clinical measurements and patient characteristics were recorded at a single time point in many intensive care units throughout the world (for example, [30][31][32][33]) but focused on the sublingual micro circulation, it was named the microSOAP study (Microcirculatory Shock Occurrence in Acutely ill Patients registered at ClinicalTrials.gov: NCT01179243). In the microSOAP study, the prevalence of microcirculatory alterations in intensive care patients and the relationship of microcirculatory alterations with the severity of disease in an epidemiological survey were investigated. In one week, the microcirculatory status of all intensive care patients in 40 participating intensive care units worldwide was assessed and patient characteristics were recorded. Th e patients were followed until death, hospital discharge, or for 60 days. Th e relationships between microvascular parameters and disease states were analyzed. Once published, this study might provide valuable information regarding the prevalence of microcirculatory disturbances in intensive care patients and their relationship to the underlying pathophysiology. Furthermore, it is expected that this study will provide a basis for future interventional studies, targeting resuscitation of the microcirculation.

Resuscitation of the microcirculation
In their key study, Rivers and colleagues [4] have developed an early goal-directed therapeutic protocol in which fl uid resuscitation was performed until central venous pressure was 8 to 12 mmHg, vasopressor agents were added to maintain the mean arterial pressure above 65 mmHg, and red blood cell transfusions and/or inotropic agents were used to increase central venous oxygen saturation to above 70%. With this protocol, Rivers and colleagues signifi cantly reduced the mortality rate in patients with septic shock (31% versus 47% for standard therapy). Th is demonstrates that volume replacement therapy using fl uids and/or blood in combination with vasoactive agents is essential for resuscitation of severely septic patients. A summary of the eff ects of various interventions on the sublingual microcirculation is provided in Table 1.

Fluid resuscitation
Fluid resuscitation is probably the major therapy aimed at restoring circulating volume and consequently increasing cardiac output and arterial blood pressure in (septic) shock patients. Pottecher and colleagues [34] showed that the sublingual microcirculatory perfusion in severely septic and septic shock patients was signifi cantly improved following fl uid loading. As the changes in microcirculation did not correlate to changes in macro circulation, however, the authors suggested that the macro-and microcirculation do not have the same dose-response to fl uid loading. Th is was also observed by Ospina-Tascon and colleagues [35] investigating the response of the macro-and microcirculation to fl uid loading in the early (within 24 hours after diagnosis) or late (more than 48 hours after diagnosis) phases of septic shock. Th e authors found that the microcirculation did increase after fl uid loading in the early phase of septic shock but not in the late phase despite signifi cant increases in cardiac output and arterial blood pressure. In patients undergoing major abdominal surgery, Jhanji and colleagues [36] compared stroke volume-guided versus central venous pressure-guided fl uid therapy with respect to their eff ects on microcirculatory perfusion and renal function. Th e main result was that perfused microvascular density remained normal in the stroke volumeguided therapy group, but decreased in the central venous pressure-guided therapy group. Acute kidney injury was also found more frequently in the central venous pressure-guided therapy group. However, this fi nding was a post hoc analysis after pooling data from both protocol groups, and other outcome parameters, such as complication rates, mortality, critical care-free days and mortality, were identical in both protocol groups and the control group, despite the improvement in microcirculation.
Hence, these studies indicate that fl uid loading is an eff ective fi rst step in the resuscitation of the microcirculation. In addition, Dubin and colleagues [37] demonstrated in a randomized controlled study in septic patients that a 6% HES/0.4 solution had superior microcirculatory recruitment power compared to a saline solution in early goal-directed therapy. In this study, however, baseline microcirculation was not assessed, making it diffi cult to understand whether diff erences at 24 hours result from diff erences at baseline or from specifi c eff ects of diff erent types of fl uids. Moreover, no outcome data are yet available showing benefi t from synthetic colloids over crystalloids.

Blood transfusion
Both OPS and SDF imaging have been used to investigate the direct eff ects of red blood cell (RBC) transfusions on the microcirculation [38,39]. Sakr and colleagues [38] studied sublingual microcirculation in 35 septic patients using orthogonal polarization spectral imaging. Th ey performed the measurements just before RBC unit transfusion and one hour after transfusion of one or two leukoreduced RBC units with a mean age of 24 days. Th ey found that although mean arterial pressure and oxygen delivery increased following RBC transfusion, oxygen uptake and microcirculatory parameters did not. It must be noted, however, that there was interindividual variability with an increase in sub lingual capillary perfusion in patients with depressed perfusion at baseline and a decrease in perfusion in patients with normal baseline perfusion [38]. In contrast, our group has demonstrated an increased sublingual microcirculatory density and tissue oxygenation after transfusion of one to three RBC units with a mean age of 18 days in cardiac surgery patients [39]. In this study we were able to verify that the transfused blood is eff ective in improving oxygen transport to the tissue by promoting RBC delivery to the microcirculation and identifi ed the mechanism by which this is accomplished: that is, not by increasing microcirculatory fl ow velocity but rather by fi lling empty capillaries, thereby reducing the oxygen diff usion distances to the tissue cells. However, whether this leads to improved oxygen consumption remains to be investigated. Parallel to the fi ndings by Sakr and colleagues, we have recently conducted a pilot study to investigate the effi cacy of RBC transfusions to improve microcirculatory density in adult septic patients and also found no improvement in the microcirculation after blood transfusion in these patients [40]. A potential explanation for this is that, in sepsis, hemorheological alterations and damaged host microcirculation (for example, endothelium and glycocalyx) could diminish the effi cacy of RBC transfusions to correct anemia at the microcirculatory level. However, this warrants further study.

Vasoactive agents
Vasoactive agents such as norepinephrine, epinephrine, dopamine, dopexamine, and dobutamine are often used in hypotensive (septic) shock patients to increase blood pressure and restore the systemic hemodynamic state. Th ese agents also have an impact on the microcirculation, as reviewed by Boerma and Ince [41]. Th e general fi nding is that while being eff ective at increasing blood pressure, vasopressors can have various eff ects on the microcirculation. Jhanji and colleagues [42] found in septic shock patients that norepinepherine, while increas ing blood pressure, was completely ineff ective at promoting microcirculatory blood fl ow. In another study by Jhanji and colleagues [36] it was found that a treatment algorithm incorporating stroke volume-guided fl uid therapy and a low-dose dopexamine infusion increased global oxygen delivery and central venous oxygen saturation in association with signifi cant improvements in sublingual and cutaneous microvascular fl ow, while stroke volumeguided fl uid therapy alone was associated with more modest improvements in global hemodynamics and micro vascular fl ow. In a similar study, Dubin and colleagues [43] found that norepinephrine in hypotensive patients with low microcirculation was able to increase microvascular fl ow, but in equally hypotensive patients with a normal microcirculation norepinephrine actually decreased microvascular fl ow. Th ese studies emphasize that using a fi xed target of blood pressure alone to guide resuscitation does not guarantee improvement of the microcirculation. Although in an earlier study De Backer and colleagues had showed that the proportion of perfused vessels was similar in patients treated with or without adrenergic agents [1], they later showed in septic shock patients that dobutamine infusion (5 μg/kg/ minute) markedly reduced the proportion of non-perfused capillaries [44]. Th e authors furthermore showed in a subset of patients that topical application of acetylcholine could further improve microcirculatory perfu sion, which suggests that the dobutamine infusion, although recruit ing some capillaries, did not fully open the microcirculation.
As mentioned above, the vasodilatory action of acetylcholine was able to recruit the capillaries of the sublingual microcirculation in patients with severe sepsis [44]. In line with this, Spronk and colleagues [25] found that intravenous infusion of nitroglycerin improved microcirculatory perfusion in septic shock patients. In a placebo-controlled randomized trial in septic patients, however, Boerma and colleagues [45] did not fi nd such benefi cial eff ects of intravenous infusion of nitroglycerin after fulfi ll ment of protocol-driven resuscitation endpoints. Th e authors showed an equal change in microcirculatory fl ow in all groups over the fi rst 24 hours of intensive care with no signifi cant eff ects of nitroglycerin. During cardiogenic shock, in contrast, Den Uil and colleagues [46,47] found that nitroglycerin improved the sublingual microcirculation in a dose-dependent fashion. Interest ingly, the observed improvement of the microcirculation was not correlated with changes in cardiac output or arterial blood pressure and disappeared after cessation of nitroglycerin infusion. Alternative routes for nitric oxide administration (for example, inhaled nitric oxide) are being explored to improve the microcirculation without worsening the macrocirculation, as extensively discussed by Trzeciak and colleagues [9].
Another agent with potential for improving microvascular function in critically ill patients is recombinant activated protein C (APC), which decreases the uncontrolled cascades of infl ammation and coagulation and impaired fi brinolysis in sepsis [48,49]. Bernard and colleagues [50] have shown that exogenous APC administration signifi cantly reduced organ failure and improved survival in severely septic patients, although this was later questioned by Silva and colleagues [51]. De Backer and colleagues [52] reported that severely septic patients had an increased proportion of perfused microvessels while receiving continuous infusion of APC. Once APC infusion stopped, microvascular perfusion transiently decreased. Th e authors furthermore showed that the improved microvascular perfusion was associated with more rapid resolution of hyperlactatemia.

Targeting the microcirculation
Even after interventions eff ectively optimizing macrocircu latory hemodynamics, high mortality rates still persist in critically ill and especially in septic patients. Th erefore, rather than limiting therapy to macro circulatory targets alone, microcirculatory targets could be incorporated to potentially reduce mortality rates in these critically ill patients [8][9][10][11]. Although an association between an abnormal microcirculation and adverse outcome may be confi rmed world-wide, this does not imply that improving the microcirculation in these condi tions will improve outcome of these patients. A randomized study should be conducted to prove that using microcirculatory parameters as end-points of resuscitation indeed improves outcome of the patients. However, no such clinical study yet exists.
Such a trial would, for the fi rst time, implement a resuscitation strategy based on resolving microcirculatory disorders known to be associated with increased morbidity and mortality in the intensive care unit. Th is novel goal-directed therapeutic strategy might, if successful, have a large impact on the care of intensive care patients. If not (or less) successful, this could be due either to the wrong choice of drug or to the secondary rather than primary role of microcirculatory failure in morbidity and mortality in the critically ill. With such a trial, microcirculatory diagnostics will be taken to the next level where the microcirculation will be used as a therapeutic target in the treatment of septic patients.

Image acquisition stabilization
Optimizing microcirculatory density and perfusion has become the focus of new clinical studies and microcirculatory images are therefore gaining a more prominent role in clinical research. Proper interpretation of microcirculatory images is essential and relies on the quality of the images with respect to axial and lateral stability. Since both OPS and SDF imaging technologies are incorporated into hand-held microscopes, operational issues may arise in terms of axial and lateral instability of the microscope probes, potentially causing pressure artifacts and image drifting, respectively. Th e current guidelines for microcirculatory image acquisition dictate that three to fi ve microcirculatory sites should be measured per time point with a minimal recording time of 20 s per site to allow reliable analysis of micro circulatory density and perfusion [53]. Image drifting, however, makes this particularly diffi cult in both sedated and awake patients. Pressure artifacts, in addition, can alter mucosal capillary blood fl ow, thereby limiting the use of the captured images for determination of microcirculatory perfusion.
To improve microcirculatory image acquisition, Balestra and colleagues [54] have developed, evaluated, and validated an image acquisition stabilizer for the SDF imaging device. Th e stabilizer was based on application of negative pressure to the periphery of the microscopic fi eld of view to create adherence of the microscope probe to the tissue of interest. Th e authors found that the stabilizer did not aff ect microcirculatory perfusion in the SDF imaging fi eld of view and prevented pressure artifacts up to a signifi cantly greater force applied by the SDF probe onto the tissue. Furthermore, the duration of maintaining a stable image sequence was signifi cantly increased with the stabilizer (8 ± 2 s without versus 42 ± 8 s with the stabilizer). Ultimately, the authors described that, using the stabilizer and a mechanical arm, it was possible to perform microcirculatory measurements without the need for an operator. Hence, instead of multiple measurements to determine the microcircula tory state at a certain time point, continuous measurements of microcirculatory perfusion and density could be made during a clinical maneuver or intervention.

Rapid automated image analysis
For evaluation of the eff ects of interventions and (drug) therapy, SDF images are analyzed to assess (alterations in) microvascular density and perfusion. To reduce the time required for SDF image analysis for microvascular density and perfusion, Dobbe and colleagues [55] have developed and validated a method that has been commer cialized into a software package termed Automated Vascular Analysis. However, the semi-automatic offl ine analysis of the SDF images is still a time consuming endeavor requiring a signifi cant amount of user interaction. Th is severely limits the bedside use of SDF imaging as a diagnostic tool.
Our group has recently developed a rapid and fully automatic method for the assessment of microvascular density and perfusion in SDF images [56]. We improved the algorithms for microvascular density assessment incorporated in the Automated Vascular Analysis software and introduced a new method for microvascular perfusion assessment. We showed that the new method was very rapid (<30 s per clip) and adequately recovered total vessel density. With video simulations, we showed that the detection of perfusion using the new method was possible, but was limited at high cell densities and velocities at a 25 Hz imaging rate. In high quality SDF video clips, however, the new method was able to discrimi nate between perfused and non-perfused microvasculature. With video simulations it was furthermore shown that the limitations of the new method were mainly hardware-related and could be overcome by implementing more advanced camera technology in SDF imaging (that is, higher spatial and temporal resolution).
For future SDF imaging research, the automatic microvascular density assessment can be combined with manually assigning a fl ow score to each quadrant of the image as proposed by Spronk and colleagues [25], evaluated by others [57,58], and included in the standard operating procedures as dictated by a consensus on microcirculatory image acquisition and analysis [53]. Although this introduces some user interaction, it allows analysis of microvascular density and perfusion in SDF video clips within a few minutes and may allow assessment of microcirculation at the bedside.

Novel video microscopy technology
As described above, current OPS and SDF imaging devices can be regarded as fi rst and second generation devices, respectively, employing relatively low resolution analogue camera technology. Braedius Scientifi c is currently in the process of introducing a potential third generation device as an improved imaging modality for more comprehensive clinical observation of the microcirculation. A computer-controlled digital camera incorpor ated in the device will have a much higher spatial (14 megapixels versus 1.3 megapixels) and temporal (60 versus 25 frames per second) resolution as well as shorter camera exposure times compared to the previous generation devices. Th is device, with increased spatial and temporal resolution in combination with a sensor attached to a powerful computer, might provide the needed hardware requirements to allow instant online analysis of microcirculatory images needed at the bedside for clinical decision making for guidance of microcirculatory-targeted therapies.

Conclusion
A growing body of evidence exists underlining that depressed microcirculatory function is associated with morbidity and mortality in a wide array of clinical scenarios and that even after interventions eff ectively optimizing macrocirculatory hemodynamics, high mortality rates still persist in critically ill and especially in septic patients. Th erefore, rather than limiting therapy to macrocirculatory targets alone, microcirculatory targets could be incorporated to potentially reduce mortality rates in these critically ill patients. To date, no such clinical study yet exists due to the unavailability of bedside technology scoring microvascular density and perfusion in real time. However, recent technological advances in the fi eld of microcirculatory image acquisition and analysis might allow such microcirculationtargeted resuscitation by providing instant feedback on the effi cacy of the applied therapeutic strategies at the microcirculatory level.