Preface

Sepsis, shock and circulatory compromise are the leading conditions in perioperative and critical care medicine requiring urgent resuscitation. Resuscitation hemodynamic targets, the medications needed to achieve such targets as well as the tools to monitor the success of resuscitation, however, are an area of on-going controversy. Recently a Task Force from the European Society of Intensive Care Medicine published a consensus document on circulatory shock and hemodynamic monitoring . It defined the meaning of shock and resuscitation strategies and comprised a discussion of hemodynamic tools and resuscitation targets in resolving states of shock. Its understanding of shock and hemodynamic resuscitation is of direct relevance to the subject matter of this issue of Best Practice & Research Clinical Anaesthesiology. Two important statements in this regard are worth quoting. The first statement concerns the definition of shock, which is “a state in which the circulation is unable to deliver sufficient oxygen to meet the demands of the tissues, resulting in cellular dysfunction.” The second is the identification of the primary aim of resuscitation from a state of shock: “The aim of providing hemodynamic support in cases of acute circulatory failure is to increase cardiac output in order to improve tissue perfusion”. This definition of shock identifying comprised tissue (e.g. microcirculatory) perfusion and oxygenation, and the target of resuscitation as being the improvement of tissue perfusion are shared widely in the literature by key opinion leaders (e.g. ).

In achieving this aim of resuscitation, fluids, vasopressors, inotropic agents and blood products are administered targeting the correction of systemic hemodynamic variables, such as cardiac output and blood pressure. In doing so the expectation is that correction of systemic hemodynamic variables will result in a parallel improvement of the microcirculation and an optimized oxygen supply to the parenchymal cells to ultimately maintain their viability in supporting organ function. Indeed this occurs under normal physiologic conditions where intact vascular regulation ensures that oxygen rich blood is trafficked effectively to the various compartments to match oxygen need by the tissue cells. Thus, successful resuscitation requires a coupling between the systemic and microcirculation. We termed this condition needed for successful resuscitation Hemodynamic Coherence .

Since cellular and microvascular oxygenation are the main goals of hemodynamic resuscitation, monitoring of microvascular perfusion and diffusion would be of at least as much interest as macrohemodynamic variables. Nevertheless, monitoring of heart rate, blood pressure and even cardiac output have been globally installed since decades, whereas microvascular monitoring has been more difficult to establish. Some fifteen years ago we clinically introduced hand-held microscopes which allowed for the first time the direct observation of the microcirculation of human internal organs . More than 400 publications have in the meantime been generated using these hand-held microscopes. Applied sublingually at the bedside these hand-held video microscopes revealed the occurrence of persistent microcirculatory alterations despite the correction of systemic hemodynamic variables. Such a loss of coherence between the systemic and microcirculation was found to be associated with adverse outcome in critically septic shock patients. This observation was found to be much more sensitive and specific for predicting outcome than alterations in systemic hemodynamic variables. These conditions led us to define the clinical importance of identifying 4 types of microcirculatory conditions related to a loss of hemodynamic coherence at the bedside by monitoring the microcirculation .

Loss of hemodynamic coherence has been especially found in states of infection, inflammation and reperfusion injury such as in sepsis where endothelial and red blood cell alterations can cause obstructions in the microcirculation of various organ systems. Such a type 1 loss of hemodynamic coherence in perfusion of the microcirculation can result in a functional shunting of oxygen transport past the obstructed microcirculatory units which clinically manifests itself as a heterogeneous perfusion with reduced capacity of the tissues to extract oxygen . Excessive use of fluids or vasopressor agents causing decreased microvascular hematocrit (type 2), microcirculatory vasoconstriction (type 3) and increased diffusion distance (type 4), respectively, can also result in a reduced oxygen extraction capacity of the organ tissues despite normalized systemic circulation.

To highlight the concept of hemodynamic coherence and discuss the need to identify its loss by bedside monitoring of the response of the microcirculation to resuscitation procedures, we invited an impressive faculty of key opinion leaders in the field to contribute to this issue of Best Practice & Research Clinical Anaesthesiology. The issue is focused on hemodynamic coherence as it applies to different patient categories in perioperative and intensive care medicine. The issue covers methodological aspects of microcirculation monitoring as well as the impact of different therapeutic modalities on variables related to the systemic circulation and the microcirculation. In doing so, this issue covers patients suffering from sepsis, trauma, hypovolemia, burns and heart failure in perioperative, critically ill adult and pediatric patients. The impact of resuscitation strategies covers fluid, vasoactive drug and blood resuscitation. The methodologies for assessing the microcirculation are mainly focused on hand held microscopy (e.g. OPS, SDF and IDF/Cytocam imaging) since it has been these techniques which have mainly been used in the identification of hemodynamic coherence in the various clinical scenarios but also on NIRS and lactate as tools to identify microcirculatory dysfunction.

It is impressive to note that across the board of these independently written chapters, authors identify as practice points the importance of monitoring the microcirculation for identification of the presence or loss of hemodynamic coherence as a result of resuscitation. Of equal importance are their recommendations for the research agenda that despite significant technical improvements in hand held video microscopy in the last decade, it still requires significant technology developments especially related to automatic analysis of the microcirculation if it is to be used as point of care methodology. The need for interventional trials targeting the resuscitation of the microcirculation as a clinical end point are equally underscored.

Finally, the cumulative evidence presented in the diverse chapters suggests that microvascular dysfunction and its reaction to specific interventions show marked interindividual variability. Thus, an intervention that improves microvascular perfusion in one patient (e.g. red blood cell transfusion) may be neutral or even detrimental in other patients. The individual response to therapeutic measures is not predictable by conventional monitoring. This underscores the need for bedside microvascular monitoring to individualize therapeutic options. Maybe the lack of this type of monitoring is one reason for the series of negative studies on (in principle promising) macrohemodynamic interventions in critically ill patients.

We would like to thank all the authors for their excellent papers spanning the range of the relevance of hemodynamic coherence in perioperative and intensive care medicine. We are sure that with this issue of Best Practice & Research Clinical Anaesthesiology a standard text has been published in the appreciation to integrate systemic, peripheral and microcirculatory hemodynamics to achieve comprehensive information about the individual state of the cardiovascular system and success of resuscitation. We hope that this issue has emphasized that the ultimate goal of resuscitation is to achieve hemodynamic coherence between the different physiological compartments in terms of perfusion and oxygenation.