Effects of early hemodynamic resuscitation on left ventricular performance and microcirculatory function during endotoxic shock

Modeling the relationship between arterial blood pressure and sublingual microcirculatory blood flow assessed by Sidestream Dark Field videomicroscopy: An experimental study in anesthetized piglets

Microcirculation is frequently assessed using videomicroscopy in the sublingual mucosa. However, limited research has been conducted on the existence of blood flow autoregulation in this region. We conducted a study in an experimental porcine model of pharmacologically induced hypotension to evaluate the relationship between mean arterial pressure (MAP) and the microvascular flow index (MFI). We hypothesized that this relationship would be linear in the absence of autoregulation or bilinear if autoregulation is present. Seven pigs underwent blood pressure changes induced by norepinephrine (hypertension) and sevoflurane (hypotension) administration. Sublingual microcirculation was assessed using a sidestream dark field device, and videos were recorded at different MAP levels ranging from 30 to 110 mmHg. MFI was calculated using the quadrant-based method. For our first hypothesis, we constructed a linear mixed model, while a bilinear model was used for the second hypothesis. The linear model demonstrated a statistically significant association (P = 0.03) described by the equation: MFI = 2.29 + 0.004 x MAP. The bilinear model identified a statistically significant inflection point at MAP = 99 mmHg (P = 0.01) with MFI = 2.7 AU (P < 0.0001). For MAP <99 mmHg, the relationship was: MFI = 2.26 + 0.004 x MAP, and for MAP >99 mmHg, MFI = 2.7. Despite statistical significance, neither model provided a satisfactory graphical fit due to high inter- and intra-individual variability. Consequently, this study did not allow us to draw conclusions regarding the presence of blood autoregulation.

Autoregulation of blood flow drives early hypotension in a rat model of systemic inflammation induced by bacterial lipopolysaccharide

Uncontrolled vasodilation is known to account for hypotension in the advanced stages of sepsis and other systemic inflammatory conditions, but the mechanisms of hypotension in earlier stages of such conditions are not clear. By monitoring hemodynamics with the highest temporal resolution in unanesthetized rats, in combination with ex-vivo assessment of vascular function, we found that early development of hypotension following injection of bacterial lipopolysaccharide is brought about by a fall in vascular resistance when arterioles are still fully responsive to vasoactive agents. This approach further uncovered that the early development of hypotension stabilized blood flow. We thus hypothesized that prioritization of the local mechanisms of blood flow regulation (tissue autoregulation) over the brain-driven mechanisms of pressure regulation (baroreflex) underscored the early development of hypotension in this model. Consistent with this hypothesis, an assessment of squared coherence and partial-directed coherence revealed that, at the onset of hypotension, the flow-pressure relationship was strengthened at frequencies (<0.2 Hz) known to be associated with autoregulation. The autoregulatory escape to phenylephrine-induced vasoconstriction, another proxy of autoregulation, was also strengthened in this phase. The competitive demand that drives prioritization of flow over pressure regulation could be edema-associated hypovolemia, as this became detectable at the onset of hypotension. Accordingly, blood transfusion aimed at preventing hypovolemia brought the autoregulation proxies back to normal and prevented the fall in vascular resistance. This novel hypothesis opens a new avenue of investigation into the mechanisms that can drive hypotension in systemic inflammation.

Deep learning and computer vision techniques for microcirculation analysis: A review

The analysis of microcirculation images has the potential to reveal early signs of life-threatening diseases such as sepsis. Quantifying the capillary density and the capillary distribution in microcirculation images can be used as a biological marker to assist critically ill patients. The quantification of these biological markers is labor intensive, time consuming, and subject to interobserver variability. Several computer vision techniques with varying performance can be used to automate the analysis of these microcirculation images in light of the stated challenges. In this paper, we present a survey of over 50 research papers and present the most relevant and promising computer vision algorithms to automate the analysis of microcirculation images. Furthermore, we present a survey of the methods currently used by other researchers to automate the analysis of microcirculation images. This survey is of high clinical relevance because it acts as a guidebook of techniques for other researchers to develop their microcirculation analysis systems and algorithms.

Effects of impaired microvascular flow regulation on metabolism-perfusion matching and organ function

Impaired tissue oxygen delivery is a major cause of organ damage and failure in critically ill patients, which can occur even when systemic parameters, including cardiac output and arterial hemoglobin saturation, are close to normal. This review addresses oxygen transport mechanisms at the microcirculatory scale, and how hypoxia may occur in spite of adequate convective oxygen supply. The structure of the microcirculation is intrinsically heterogeneous, with wide variations in vessel diameters and flow pathway lengths, and consequently also in blood flow rates and oxygen levels. The dynamic processes of structural adaptation and flow regulation continually adjust microvessel diameters to compensate for heterogeneity, redistributing flow according to metabolic needs to ensure adequate tissue oxygenation. A key role in flow regulation is played by conducted responses, which are generated and propagated by endothelial cells and signal upstream arterioles to dilate in response to local hypoxia. Several pathophysiological conditions can impair local flow regulation, causing hypoxia and tissue damage leading to organ failure. Therapeutic measures targeted to systemic parameters may not address or may even worsen tissue oxygenation at the microvascular level. Restoration of tissue oxygenation in critically ill patients may depend on restoration of endothelial cell function, including conducted responses.

Effects of volume resuscitation on the microcirculation in animal models of lipopolysaccharide sepsis: a systematic review

Background

Recent research has identified an increased rate of mortality associated with fluid bolus therapy for severe sepsis and septic shock, but the mechanisms are still not well understood. Fluid resuscitation therapy administered for sepsis and septic shock targets restoration of the macro-circulation, but the pathogenesis of sepsis is complex and includes microcirculatory dysfunction.

Objective

The objective of the study is to systematically review data comparing the effects of different types of fluid resuscitation on the microcirculation in clinically relevant animal models of lipopolysaccharide-induced sepsis.

Methods

A structured search of PubMed/MEDLINE and EMBASE for relevant publications from 1 January 1990 to 31 December 2015 was performed, in accordance with PRISMA guidelines.

Results

The number of published papers on sepsis and the microcirculation has increased steadily over the last 25 years. We identified 11 experimental animal studies comparing the effects of different fluid resuscitation regimens on the microcirculation. Heterogeneity precluded any meta-analysis.

Conclusions

Few animal model studies have been published comparing the microcirculatory effects of different types of fluid resuscitation for sepsis and septic shock. Biologically relevant animal model studies remain necessary to enhance understanding regarding the mechanisms by which fluid resuscitation affects the microcirculation and to facilitate the transfer of basic science discoveries to clinical applications.