The Prognostic Value of Simultaneous Assessment of Microcirculation Alteration With Sublingual Microcirculation and Near-infrared Spectroscopy on the Acute Respiratory Distress Syndrome of Various Phenotypes

Microcirculation consists of a branching network of small blood vessels (<100 μm diameter) that includes the arterioles, capillaries, and venules, and plays a pivotal role in the delivery of oxygen to tissue cells. Microcirculatory alterations are frequently observed in critically ill patients, and especially in patients with severe sepsis. These alterations are characterized by a decrease in capillary density that determines an increase in the diffusion distance of oxygen to tissues and an increase in heterogeneity of perfusion. This microcirculatory derangements involve the consequent presence of under or not perfused capillaries in close proximity to well perfused capillaries. Therefore, these functionally vulnerable microcirculatory areas may become hypoxic, resulting in an oxygen extraction deficit. This phenomenon has been termed "microcirculatory shunting" and plays a major role in the pathophysiology of sepsis and multi-organ failure.

Persistent and incomplete reversal of microcirculatory and regional hypoxia is considered to be a major factor contributing to the development and maintenance of multiple organ failure. Unfortunately, alterations in microvascular perfusion often persist after the correction of systemic hemodynamic abnormalities, and their severity is associated with a poor outcome. Such a loss of hemodynamic coherence between the macro- and microcirculation has been described in several clinical and experimental studies and has been found to be an independent predictor of adverse outcome and organ dysfunction. The microcirculation has been suggested to be the motor of sepsis.

Conceptually, current techniques that permit monitoring the microcirculation can be classified in two main groups: (1) Methods that allow evaluation of local tissue oxygenation as a surrogate of microcirculatory blood flow. (2) Methods that allow direct visualization of the microvascular network and microcirculatory blood flow.

Near-infrared spectroscopy (NIRS) has been used as a tool to monitor tissue oxygen saturation (StO2) in acutely ill patients and has been proposed as a tool to quantify microvascular dysfunction in patients with sepsis. According to Beer's law, the NIRS signal is limited to vessels that have a diameter less than 1 mm (arterioles, capillaries, and venules), but, as 75% of the blood in a skeletal muscle is venous, NIRS StO2 measurements mostly represent local venous hemoglobin O2 saturation. NIRS has been used in different clinical conditions such as severe trauma, hemorrhagic shock, septic shock, and cardiogenic shock or severe cardiac failure. The utility of and application of NIRS in intensive care mainly involved three main NIRS measurements: (1) continuous tissue oxygen saturation (StO2) measurement; 2) StO2 deoxygenation slope (DecStO2 slope) in response to vascular occlusion test (VOT test); and 3) StO2 recovery slope (RincStO2 slope) in response to VOT.

Tissue oxygen saturation (StO2) has been proposed as a marker of tissue perfusion depending on the device used and the location of the measurement. A systemic review and mea-analysis had showed that patients with severe sepsis or septic shock have lower levels of StO2, and RincStO2 slope. Moreover, survivors from severe sepsis or septic shock present higher levels of StO2 and RincStO2 compared with non-survivors. The VOT is a provocative test in which StO2 is measured on a distal site (such as the thenar eminence or forearm) whilst a transient rapid vascular occlusion is performed, using a sphygmomanometer, for either a defined time interval (for example, 3 minutes) or until the StO2 decreases to a defined minimal threshold. The ischemic tissue then induces vasodilation of surrounding arterioles, metarterioles and pre-capillary sphincters to decrease local vascular resistance and regain blood flow. Once this threshold is reached, the tourniquet is released and blood flow is restored. There is a reactive hyperemic response which represents the tissue's ability to auto-regulate blood flow and oxygenation. Several parameters arise from this technique, including: 1. the rate of deoxygenation (RdecStO2), thought to reflect the local metabolic rate, 2. the rate of reoxygenation (RincStO2), thought to reflects the time required to wash out stagnant blood and is thought to be determined by local cardiovascular reserve and microcirculatory flow, and the post-obstructive hyperemic response.