Direct assessment of microcirculation in shock: a randomized-controlled multicenter study

UNDERSTANDING HEMODYNAMIC INCOHERENCE: MECHANISMS, PHENOTYPES, AND IMPLICATIONS FOR TREATMENT

ABSTRACT

The reversal of microcirculation dysfunction is crucial for assessing the success of shock resuscitation and significantly influences patient prognosis. However, hemodynamic incoherence is observed when microcirculatory dysfunction persists despite the restoration of macrocirculatory function after resuscitation. Recent advancements in technology have enabled bedside assessment of microcirculation in shock patients, allowing for direct visualization of microcirculatory morphology and quantitative evaluation of its functional status. This article reviews the pathophysiological mechanisms that lead to hemodynamic incoherence. It also introduces the current understanding and classification framework for the different phenotypes of hemodynamic incoherence. Existing evidence indicates that the diverse mechanisms leading to microcirculatory disorders result in varied manifestations among patients experiencing hemodynamic incoherence, highlighting the heterogeneity of this population. Some classification frameworks have been proposed to enhance our understanding of these phenotypes. By integrating pathophysiological mechanisms, clinical symptoms, indicators of macrocirculation, microcirculation, tissue metabolism, and biomarkers, we can summarize certain clinical features of phenotypes in hemodynamic incoherence to form a conceptual framework. Additionally, strategies for creating targeted treatments based on different phenotypes require further validation.

The endothelium or mitochondrial level therapy: new frontiers in sepsis?

The host and microbes play complex roles in balancing the pro- and anti-inflammatory pathways that cause sepsis. It is now increasingly recognized as a disorder of the mitochondrial system intrinsically or as a consequence of microcirculatory abnormalities leading to hypoperfusion/hypoxia ("microcirculatory and mitochondrial distress syndrome"). It is expected that improvements in endothelium or mitochondrial level therapy will lower sepsis-related morbidity and mortality. This article aimed to clarify the mitochondrial and microcirculation abnormalities in patients with sepsis and the futuristic research agenda for the management of sepsis.

Shenfu injection ameliorates endotoxemia-associated endothelial dysfunction and organ injury via inhibiting PI3K/Akt-mediated glycolysis

ETHNOPHARMACOLOGICAL RELEVANCE

Microcirculatory dysfunction is one of the main characteristics of sepsis. Shenfu Injection (SFI) as a traditional Chinese medicine is widely applied in clinical severe conditions. Recent studies have shown that SFI has the ability to ameliorate sepsis-induced inflammation and to improve microcirculation perfusion.

AIM OF THE STUDY

This study aims to investigate the underlying mechanism of SFI for ameliorating sepsis-associated endothelial dysfunction and organ injury.

MATERIALS AND METHODS

Side-stream dark-field (SDF) imaging was used to monitor the sublingual microcirculation of septic patients treated with or without SFI. Septic mouse model was used to evaluate the effects of SFI in vivo. Metabolomics and transcriptomics were performed on endothelial cells to identify the underlying mechanism for SFI-related protective effect on endothelial cells.

RESULTS

SFI effectively abolished the disturbance and loss of sublingual microcirculation in septic patients. Twenty septic shock patients with or without SFI administration were enrolled and the data showed that SFI significantly improved the levels of total vessel density (TVD), perfused vessel density (PVD), microvascular flow index (MFI), and the proportion of perfused vessels (PPV). The administration of SFI significantly decreased the elevated plasma levels of Angiopoietin-2 (Ang2) and Syndecan-1, which are biomarkers indicative of endothelial damage in sepsis patients. In the mouse septic model in vivo, SFI inhibited the upregulation of endothelial adhesion molecules and Ly6G + neutrophil infiltration while restored the expression of VE-Cadherin in the vasculature of the lung, kidney, and liver tissue. Additionally, SFI reduced the plasma levels of Ang2, Monocyte Chemoattractant Protein-1(MCP1), and Interleukin-6 (IL6), and alleviated liver and kidney injury in septic mice. Moreover, SFI significantly inhibited the inflammatory activation and increased permeability of endothelial cells induced by endotoxins in vitro. By performing metabolomics and transcriptomics, we identified the activation of PI3K/Akt-mediated glycolysis as the underlying mechanism for SFI-related protective effect on endothelial cells.

CONCLUSIONS

Our findings revealed that SFI may improve microcirculation perfusion and endothelial function in sepsis via inhibiting PI3K/Akt-mediated glycolysis, providing theoretical evidence for the clinical application of SFI.

The microcirculation in cardiogenic shock

Cardiogenic shock is a life-threatening condition characterized by inadequate cardiac output, leading to end-organ hypoperfusion and associated mortality rates ranging between 40 and 50%. The critical role of microcirculatory impairments in the progression of organ failure during shock has been highlighted in several studies. Traditional therapies have often focused on stabilizing macrocirculation, neglecting microcirculatory dysfunction, which can result in persistent tissue hypoxia and poor outcomes. This review highlights the importance of assessing microcirculation in cardiogenic shock, including parameters such as skin perfusion, sublingual microcirculation, and lactate dynamics. Integrating microcirculatory assessments into clinical practice remains challenging due to the complexity of the methods and limited therapeutic options targeting microvascular perfusion. While advances in microcirculation-guided therapies hold promise for improving outcomes in cardiogenic shock, further research is needed to establish effective protocols.

Fluid management of acute kidney injury

PURPOSE OF REVIEW

Acute kidney injury (AKI) is commonly encountered in critical care medicine as is intravenous fluid therapy. It is accepted that there is interplay between fluid use and AKI, both potentially positive and negative. An understanding of the physiological rationale for fluid is important to help clinicians when considering fluid therapy in patients with, or at risk for AKI; this includes understanding choice of fluid, method of monitoring, administration and clinical sequelae.

RECENT FINDINGS

There is increasing interest in combining both static and dynamic measures to assess fluid balance, fluid responsiveness effects of fluid therapy, which are areas requiring ongoing study to translate this theory into clinically useful practice at the bedside. Whilst the debate of choice of crystalloid in ICU practice continues, further evidence for benefits for balanced solutions emerges in the form of international guidelines and patient data meta-analysis of previously performed trials.

SUMMARY

This review assesses the physiological rationale for fluid use in ICU cohorts with AKI of various types, as well as a systematic approach for choice of fluid therapy using a number of different variables, which aims to help guide clinicians in managing fluid use and fluid balance in critically ill patients with AKI.

Prognostic Value of Sublingual Microcirculation in Sepsis: A Systematic Review and Meta-analysis

Objectives: To investigate the relationship between sublingual microcirculation and the prognosis of sepsis. Data sources: The PubMed, Web of Science, Embase, and China National Knowledge Infrastructure (CNKI) databases were searched to identify studies published from January 2003 to November 2023. Study selection: Clinical studies examining sublingual microcirculation and the prognosis of sepsis were included. Data extraction: Sublingual microcirculation indices included the microvascular blood index (MFI), total vascular density (TVD), perfusion vascular density (PVD), perfusion vascular vessel (PPV), and heterogeneity index (HI). Prognostic outcomes included mortality and severity. Funnel plots and Egger's test were used to detect publication bias. The ability of the small vessel PPV (PPVs) to predict sepsis-related mortality was analyzed based on the summary receiver operating characteristic (SROC) curve, pooled sensitivity, and pooled specificity. Data synthesis: Twenty-five studies involving 1750 subjects were included. The TVD (95% CI 0.11-0.39), PVD (95% CI 0.42-0.88), PPV (95% CI 6.63-13.83), and MFI (95% CI 0.13-0.6) of the survival group were greater than those of the nonsurvival group. The HI in the survival group was lower than that in the nonsurvival group (95% CI -0.49 to -0.03). The TVD (95% CI 0.41-0.83), PVD (95% CI 0.83-1.17), PPV (95% CI 14.49-24.9), and MFI (95% CI 0.25-0.66) of the nonsevere group were greater than those of the severe group. Subgroup analysis revealed no significant difference in TVD between the survival group and the nonsurvival group in the small vessel subgroup. The area under the SROC curve (AUC) was 0.88. Conclusions: Sublingual microcirculation was worse among patients who died and patients with severe sepsis than among patients who survived and patients with nonsevere sepsis. PPV has a good predictive value for the mortality of sepsis patients. This study was recorded in PROSPERO (registration number: CRD42023486349).

Heliox ventilation in elderly, hypertensive ICU patients improves microcirculation: A randomized controlled study

BACKGROUND

Conventional mechanical ventilation has adverse impacts on the hemodynamics of elderly, hypertensive ICU patients. Limited studies have addressed ways to ameliorate these negative effects. This study aimed to determine whether heliox ventilation could improve the hemodynamics, especially microcirculation, of elderly, hypertensive patients undergoing mechanical ventilation.

METHODS

Thirty-eight patients, over the age of 65 with essential hypertension who underwent invasive mechanical ventilation treatment, were divided into two groups: a control group of nitrogen‑oxygen ventilation (n = 19) and an experimental group of heliox ventilation (n = 19). The control group received conventional room air ventilation and the experimental group adopted the innovative, closed heliox ventilation technique. Changes in blood pressure, heart rate (HR), peripheral oxygen saturation (SpO2), central venous oxygen saturation (ScvO2), regional cerebral oxygen saturation (rSO2), lactic acid (Lac) and airway pressure were measured at 0,1,2,3 h under volume-controlled ventilation (VCV) mode throughout the study. Sublingual microcirculation parameters were additionally measured at 0 h and 3 h of ventilation treatment.

RESULTS

SpO2 in both groups increased after 1 h of ventilation compared with 0 h (p < 0.001), subsequently remaining stable. Compared with the control group, the experimental group showed a decrease in airway pressure and Lac, while blood pressure, ScvO2, and rSO2 increased (p < 0.05). Moreover, the sublingual microcirculation indexes in the experimental group improved compared with the control group (p < 0.05).

CONCLUSIONS

Heliox ventilation improves blood pressure and microcirculation in elderly hypertensive patients and may resolve the limitations of traditional nitrogen‑oxygen ventilation.

TRIAL REGISTRATION

This trial was registered. The Chinese trial registration number is ChiCTR2100043945. The date of registration is 6-3-2021.

Management of cardiogenic shock: state-of-the-art

The management of cardiogenic shock is an ongoing challenge. Despite all efforts and tremendous use of resources, mortality remains high. Whilst reversing the underlying cause, restoring/maintaining organ perfusion and function are cornerstones of management. The presence of comorbidities and preexisting organ dysfunction increases management complexity, aiming to integrate the needs of vital organs in each individual patient. This review provides a comprehensive overview of contemporary literature regarding the definition and classification of cardiogenic shock, its pathophysiology, diagnosis, laboratory evaluation, and monitoring. Further, we distill the latest evidence in pharmacologic therapy and the use of mechanical circulatory support including recently published randomized-controlled trials as well as future directions of research, integrating this within an international group of authors to provide a global perspective. Finally, we explore the need for individualization, especially in the face of neutral randomized trials which may be related to a dilution of a potential benefit of an intervention (i.e., average effect) in this heterogeneous clinical syndrome, including the use of novel biomarkers, artificial intelligence, and machine learning approaches to identify specific endotypes of cardiogenic shock (i.e., subclasses with distinct underlying biological/molecular mechanisms) to support a more personalized medicine beyond the syndromic approach of cardiogenic shock.

Assessment of Microcirculatory Dysfunction by Measuring Subcutaneous Tissue Oxygen Saturation Using Near-Infrared Spectroscopy in Patients with Circulatory Failure

BACKGROUND

Patients with circulatory failure have high mortality rates and require prompt assessment of microcirculation. Despite the improvement in hemodynamic parameters, microcirculatory dysfunction persists. We measured subcutaneous regional tissue oxygen saturation (rSO2) with near-infrared spectroscopy (NIRS), which can assess microcirculation in patients with circulatory failure.

METHODS

A finger-worn oximeter with NIRS measured rSO2 in the forehead, thenar eminence, thumb, and knees. First, the rSO2 was measured in healthy adult volunteers (n = 10). Circulatory failure was defined as a systolic blood pressure ≤ 90 mmHg and lactate ≥ 2 mmol/L. The study included 35 patients without circulatory failure and SOFA score of 0 at ICU admission and 38 patients with circulatory failure at ICU admission. Both groups included a single-center prospective study of patients who were transported to the ICU of the Nihon University Hospital. The rSO2 was measured only on ICU admission in the non-circulatory failure group and later in the circulatory failure group.

RESULTS

In the volunteer group, rSO2 at each site was approximately 58%. The rSO2 was significantly lower in the circulatory failure group than in the non-circulatory failure group (knee, p < 0.01). In the circulatory failure group, knee rSO2 showed a significant negative correlation with SOFA score (Day 0, ρ = -0.37, p = 0.02; Day 1, ρ = -0.53, p < 0.01; Day 2, ρ = -0.60, p < 0.01).

CONCLUSIONS

Subcutaneous knee rSO2 was associated with SOFA score and was considered an indicator of microcirculatory dysfunction and organ damage.

Sublingual microcirculatory assessment on admission independently predicts the outcome of old intensive care patients suffering from shock

Shock is a life-threatening condition. This study evaluated if sublingual microcirculatory perfusion on admission is associated with 30-day mortality in older intensive care unit (ICU) shock patients. This trial prospectively recruited ICU patients (≥ 80 years old) with arterial lactate above 2 mmol/L, requiring vasopressors despite adequate fluid resuscitation, regardless of shock cause. All patients received sequential sublingual measurements on ICU admission (± 4 h) and 24 (± 4) hours later. The primary endpoint was 30-day mortality. From September 4th, 2022, to May 30th, 2023, 271 patients were screened, and 44 included. Patients were categorized based on the median percentage of perfused small vessels (sPPV) into those with impaired and sustained microcirculation. 71% of videos were of good or acceptable quality without safety issues. Patients with impaired microcirculation had significantly shorter ICU and hospital stays (p = 0.015 and p = 0.019) and higher 30-day mortality (90.0% vs. 62.5%, p = 0.036). Cox regression confirmed the independent association of impaired microcirculation with 30-day mortality (adjusted hazard ratio 3.245 (95% CI 1.178 to 8.943, p = 0.023). Measuring sublingual microcirculation in critically ill older patients with shock on ICU admission is safe, feasible, and provides independent prognostic information about outcomes.Trial registration NCT04169204.

Blood Pressure Management for Hypotensive Patients in Intensive Care and Perioperative Cardiovascular Settings

Blood pressure is a critical physiological parameter, particularly in the context of cardiac intensive care and perioperative settings. As a primary indicator of organ perfusion, the maintenance of adequate blood pressure is imperative for the assurance of sufficient tissue oxygen delivery. Among critically ill and major surgery patients, the continuous monitoring of blood pressure is performed as a standard practice for patients. Nonetheless, uncertainties remain regarding blood pressure goals, and there is no consensus regarding blood pressure targets. This review describes the determinants of blood pressure, examine the influence of blood pressure on organ perfusion, and synthesize the current clinical evidence from various intensive care and perioperative settings to provide a concise guidance for daily clinical practice.

Hemodynamic management of cardiogenic shock in the intensive care unit

Hemodynamic derangements are defining features of cardiogenic shock. Randomized clinical trials have examined the efficacy of various therapeutic interventions, from percutaneous coronary intervention to inotropes and mechanical circulatory support (MCS). However, hemodynamic management in cardiogenic shock has not been well-studied. This State-of-the-Art review will provide a framework for hemodynamic management in cardiogenic shock, including a description of the 4 therapeutic phases from initial 'Rescue' to 'Optimization', 'Stabilization' and 'de-Escalation or Exit therapy' (R-O-S-E), phenotyping and phenotype-guided tailoring of pharmacological and MCS support, to achieve hemodynamic and therapeutic goals. Finally, the premises that form the basis for clinical management and the hypotheses for randomized controlled trials will be discussed, with a view to the future direction of cardiogenic shock.

Microvascular Autoregulation in Skeletal Muscle Using Near-Infrared Spectroscopy and Derivation of Optimal Mean Arterial Pressure in the ICU: Pilot Study and Comparison With Cerebral Near-Infrared Spectroscopy

IMPORTANCE

Microvascular autoregulation (MA) maintains adequate tissue perfusion over a range of arterial blood pressure (ABP) and is frequently impaired in critical illness. MA has been studied in the brain to derive personalized hemodynamic targets after brain injury. The ability to measure MA in other organs is not known, which may inform individualized management during shock.

OBJECTIVES

This study determines the feasibility of measuring MA in skeletal muscle using near-infrared spectroscopy (NIRS) as a marker of tissue perfusion, the derivation of optimal mean arterial pressure (MAPopt), and comparison with indices from the brain.

DESIGN

Prospective observational study.

SETTING

Medical and surgical ICU in a tertiary academic hospital.

PARTICIPANTS

Adult critically ill patients requiring vasoactive support on the first day of ICU admission.

MAIN OUTCOMES AND MEASURES

Fifteen critically ill patients were enrolled. NIRS was applied simultaneously to skeletal muscle (brachioradialis) and brain (frontal cortex) while ABP was measured continuously via invasive catheter. MA correlation indices were calculated between ABP and NIRS from skeletal muscle total hemoglobin (MVx), muscle tissue saturation index (MOx), brain total hemoglobin (THx), and brain tissue saturation index (COx). Curve fitting algorithms derive the MAP with the lowest correlation index value, which is the MAPopt.

RESULTS

MAPopt values were successfully calculated for each correlation index for all patients and were frequently (77%) above 65 mm Hg. For all correlation indices, median time was substantially above impaired MA threshold (24.5-34.9%) and below target MAPopt (9.0-78.6%). Muscle and brain MAPopt show moderate correlation (MVx-THx r = 0.76, p < 0.001; MOx-COx r = 0.69, p = 0.005), with a median difference of -1.27 mm Hg (-9.85 to -0.18 mm Hg) and 0.05 mm Hg (-7.05 to 2.68 mm Hg).

CONCLUSIONS AND RELEVANCE

This study demonstrates, for the first time, the feasibility of calculating MA indices and MAPopt in skeletal muscle using NIRS. Future studies should explore the association between impaired skeletal muscle MA, ICU outcomes, and organ-specific differences in MA and MAPopt thresholds.

Resuscitating the macro- vs. microcirculation in septic shock

PURPOSE OF REVIEW

This review summarizes current literature about the relationships between macro and microcirculation and their practical clinical implications in children with septic shock.

RECENT FINDINGS

Current evidence from experimental and clinical observational studies in children and adults with septic shock reveals that the response to treatment and resuscitation is widely variable. Furthermore, there is a loss of hemodynamic coherence, as resuscitation-induced improvement in macrocirculation (systemic hemodynamic parameters) does not necessarily result in a parallel improvement in the microcirculation. Therefore, patient-tailored monitoring is essential in order to adjust treatment requirements during resuscitation in septic shock. Optimal monitoring must integrate macrocirculation (heart rate, blood pressure, cardiac output, and ultrasound images), microcirculation (videomicroscopy parameters and capillary refill time) and cellular metabolism (lactic acid, central venous blood oxygen saturation, and difference of central venous to arterial carbon dioxide partial pressure).

SUMMARY

There is a dire need for high-quality studies to assess the relationships between macrocirculation, microcirculation and tissue metabolism in children with septic shock. The development of reliable and readily available microcirculation and tissue perfusion biomarkers (other than lactic acid) is also necessary to improve monitoring and treatment adjustment in such patients.

The impact of ECPELLA on haemodynamics and global oxygen delivery: a comprehensive simulation of biventricular failure

BACKGROUND

ECPELLA, a combination of veno-arterial (VA) extracorporeal membrane oxygenation (ECMO) and Impella, a percutaneous left ventricular (LV) assist device, has emerged as a novel therapeutic option in patients with severe cardiogenic shock (CS). Since multiple cardiovascular and pump factors influence the haemodynamic effects of ECPELLA, optimising ECPELLA management remains challenging. In this study, we conducted a comprehensive simulation study of ECPELLA haemodynamics. We also simulated global oxygen delivery (DO2) under ECPELLA in severe CS and acute respiratory failure as a first step to incorporate global DO2 into our developed cardiovascular simulation.

METHODS AND RESULTS

Both the systemic and pulmonary circulations were modelled using a 5-element resistance‒capacitance network. The four ventricles were represented by time-varying elastances with unidirectional valves. In the scenarios of severe LV dysfunction, biventricular dysfunction with normal pulmonary vascular resistance (PVR, 0.8 Wood units), and biventricular dysfunction with high PVR (6.0 Wood units), we compared the changes in haemodynamics, pressure-volume relationship (PV loop), and global DO2 under different VA-ECMO flows and Impella support levels.

RESULTS

In the simulation, ECPELLA improved total systemic flow with a minimising biventricular pressure-volume loop, indicating biventricular unloading in normal PVR conditions. Meanwhile, increased Impella support level in high PVR conditions rendered the LV-PV loop smaller and induced LV suction in ECPELLA support conditions. The general trend of global DO2 was followed by the changes in total systemic flow. The addition of veno-venous ECMO (VV-ECMO) augmented the global DO2 increment under ECPELLA total support conditions.

CONCLUSIONS

The optimal ECPELLA support increased total systemic flow and achieved both biventricular unloading. The VV-ECMO effectively improves global DO2 in total ECPELLA support conditions.

Surviving Sepsis Campaign Research Priorities 2023

OBJECTIVES

To identify research priorities in the management, epidemiology, outcome, and pathophysiology of sepsis and septic shock.

DESIGN

Shortly after publication of the most recent Surviving Sepsis Campaign Guidelines, the Surviving Sepsis Research Committee, a multiprofessional group of 16 international experts representing the European Society of Intensive Care Medicine and the Society of Critical Care Medicine, convened virtually and iteratively developed the article and recommendations, which represents an update from the 2018 Surviving Sepsis Campaign Research Priorities.

METHODS

Each task force member submitted five research questions on any sepsis-related subject. Committee members then independently ranked their top three priorities from the list generated. The highest rated clinical and basic science questions were developed into the current article.

RESULTS

A total of 81 questions were submitted. After merging similar questions, there were 34 clinical and ten basic science research questions submitted for voting. The five top clinical priorities were as follows: 1) what is the best strategy for screening and identification of patients with sepsis, and can predictive modeling assist in real-time recognition of sepsis? 2) what causes organ injury and dysfunction in sepsis, how should it be defined, and how can it be detected? 3) how should fluid resuscitation be individualized initially and beyond? 4) what is the best vasopressor approach for treating the different phases of septic shock? and 5) can a personalized/precision medicine approach identify optimal therapies to improve patient outcomes? The five top basic science priorities were as follows: 1) How can we improve animal models so that they more closely resemble sepsis in humans? 2) What outcome variables maximize correlations between human sepsis and animal models and are therefore most appropriate to use in both? 3) How does sepsis affect the brain, and how do sepsis-induced brain alterations contribute to organ dysfunction? How does sepsis affect interactions between neural, endocrine, and immune systems? 4) How does the microbiome affect sepsis pathobiology? 5) How do genetics and epigenetics influence the development of sepsis, the course of sepsis and the response to treatments for sepsis?

CONCLUSIONS

Knowledge advances in multiple clinical domains have been incorporated in progressive iterations of the Surviving Sepsis Campaign guidelines, allowing for evidence-based recommendations for short- and long-term management of sepsis. However, the strength of existing evidence is modest with significant knowledge gaps and mortality from sepsis remains high. The priorities identified represent a roadmap for research in sepsis and septic shock.

Capillary leak and endothelial permeability in critically ill patients: a current overview

Capillary leak syndrome (CLS) represents a phenotype of increased fluid extravasation, resulting in intravascular hypovolemia, extravascular edema formation and ultimately hypoperfusion. While endothelial permeability is an evolutionary preserved physiological process needed to sustain life, excessive fluid leak-often caused by systemic inflammation-can have detrimental effects on patients' outcomes. This article delves into the current understanding of CLS pathophysiology, diagnosis and potential treatments. Systemic inflammation leading to a compromise of endothelial cell interactions through various signaling cues (e.g., the angiopoietin-Tie2 pathway), and shedding of the glycocalyx collectively contribute to the manifestation of CLS. Capillary permeability subsequently leads to the seepage of protein-rich fluid into the interstitial space. Recent insights into the importance of the sub-glycocalyx space and preserving lymphatic flow are highlighted for an in-depth understanding. While no established diagnostic criteria exist and CLS is frequently diagnosed by clinical characteristics only, we highlight more objective serological and (non)-invasive measurements that hint towards a CLS phenotype. While currently available treatment options are limited, we further review understanding of fluid resuscitation and experimental approaches to target endothelial permeability. Despite the improved understanding of CLS pathophysiology, efforts are needed to develop uniform diagnostic criteria, associate clinical consequences to these criteria, and delineate treatment options.

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.

Microcirculation-guided resuscitation in sepsis: the next frontier?

Microcirculatory dysfunction plays a key role in the pathogenesis of tissue dysoxia and organ failure in sepsis. Sublingual videomicroscopy techniques enable the real-time non-invasive assessment of microvascular blood flow. Alterations in sublingual microvascular perfusion were detected during sepsis and are associated with poor outcome. More importantly, sublingual videomicroscopy allowed to explore the effects of commonly applied resuscitative treatments in septic shock, such as fluids, vasopressors and inotropes, and showed that the optimization of macro-hemodynamic parameters may not be accompanied by an improvement in microvascular perfusion. This loss of "hemodynamic coherence," i.e., the concordance between the response of the macrocirculation and the microcirculation, advocates for the integration of microvascular monitoring in the management of septic patients. Nonetheless, important barriers remain for a widespread use of sublingual videomicroscopy in the clinical practice. In this review, we discuss the actual limitations of this technique and future developments that may allow an easier and faster evaluation of the microcirculation at the bedside, and propose a role for sublingual microvascular monitoring in guiding and titrating resuscitative therapies in sepsis.