Mechanisms of critical illness--classifying microcirculatory flow abnormalities in distributive shock

Abstract

Over 30 years ago Weil and Shubin proposed a re-classification of shock states and identified hypovolemic, cardiogenic, obstructive and distributive shock. The first three categories have in common that they are associated with a fall in cardiac output. Distributive shock, such as occurs during sepsis and septic shock, however, is associated with an abnormal distribution of microvascular blood flow and metabolic distress in the presence of normal or even supranormal levels of cardiac output. This Bench-to-bedside review looks at the recent insights that have been gained into the nature of distributive shock. Its pathophysiology can best be described as a microcirculatory and mitochondrial distress syndrome, where time and therapy form an integral part of the definition. The clinical introduction of new microcirculatory imaging techniques, such as orthogonal polarization spectral and side-stream dark-field imaging, have allowed direct observation of the microcirculation at the bedside. Images of the sublingual microcirculation during septic shock and resuscitation have revealed that the distributive defect of blood flow occurs at the capillary level. In this paper, we classify the different types of heterogeneous flow patterns of microcirculatory abnormalities found during different types of distributive shock. Analysis of these patterns gave a five class classification system to define the types of microcirculatory abnormalities found in different types of distributive shock and indicated that distributive shock occurs in many other clinical conditions than just sepsis and septic shock. It is likely that different mechanisms defined by pathology and treatment underlie these abnormalities observed in the different classes. Functionally, however, they all cause a distributive defect resulting in microcirculatory shunting and regional dysoxia. It is hoped that this classification system will help in the identification of mechanisms underlying these abnormalities and indicate optimal therapies for resuscitating septic and other types of distributive shock.

Figure 1

Sidestream dark-field (SDF) imaging This imaging technique is an improved method of observing the human microcirculation at the bedside. SDF imaging consists of a light guide surrounded by green light-emitting diodes (LEDs; wavelength 530 nm) whose light penetrates the tissue and illuminates the microcirculation from within. The light is absorbed by hemoglobin of the red blood cells and scattered by leukocytes. A magnifying lens projects the image onto a video camera. Placed on organ surfaces, SDF imaging provides crisp images of the red blood cells and leukocytes flowing through the microcirculation. Reproduced with permission [1].

Figure 2

A classification system for categorizing sublingual microcirculatory flow abnormalities seen in distributive shock as observed by OPS/sidestream dark-field imaging. Each class consists of a venule with two capillaries. In this way, the heterogeneity of the capillary flow is described by showing the two most characteristic types of flow seen. Solid arrows depict normal flow whereas the striped arrows represent hyperdynamic flow. No arrow depicts stagnant flow (examples of real-time films of each class of abnormality can be downloaded from our web site [43]).

Figure 3

Examples of sidestream dark-field images of sublingual microcirculation from septic patients with distributive shock. (a) Image taken from a resuscitated septic patient with a class I type of microcirculatory abnormality, complete stasis in the capillaries. (b) An example of a patient with class IV abnormalities with some capillaries showing stasis and others showing high flow. (c) Image of a healthy volunteer with microcirculatory flow in all vessels.