Effects of Combination Dobutamine and Vasopressin Therapy on Microcirculatory Blood Flow in a Porcine Model of Severe Endotoxic Shock

A clinical approach to acute mesenteric ischemia

PURPOSE OF REVIEW

To summarize current evidence on acute mesenteric ischemia (AMI) in critically ill patients, addressing pathophysiology, definition, diagnosis and management.

RECENT FINDINGS

A few recent studies showed that a multidiscipliary approach in specialized centers can improve the outcome of AMI. Such approach incorporates current knowledge in pathophysiology, early diagnosis with triphasic computed tomography (CT)-angiography, immediate endovascular or surgical restoration of mesenteric perfusion, and damage control surgery if transmural bowel infarction is present. No specific biomarkers are available to detect early mucosal injury in clinical setting. Nonocclusive mesenteric ischemia presents particular challenges, as the diagnosis based on CT-findings as well as vascular management is more difficult; some recent evidence suggests a possible role of potentially treatable stenosis of superior mesenteric artery and beneficial effect of vasodilator therapy (intravenous or local intra-arterial). Medical management of AMI is supportive, including aiming of euvolemia and balanced systemic oxygen demand/delivery. Enteral nutrition should be withheld during ongoing ischemia-reperfusion injury and be started at low rate after revascularization of the (remaining) bowel is convincingly achieved.

SUMMARY

Clinical suspicion leading to tri-phasic CT-angiography is a mainstay for diagnosis. Diagnosis of nonocclusive mesenteric ischemia and early intestinal injury remains challenging. Multidisciplinary team effort may improve the outcome of AMI.

The Neonatal and Juvenile Pig in Pediatric Drug Discovery and Development

Pharmacotherapy in pediatric patients is challenging in view of the maturation of organ systems and processes that affect pharmacokinetics and pharmacodynamics. Especially for the youngest age groups and for pediatric-only indications, neonatal and juvenile animal models can be useful to assess drug safety and to better understand the mechanisms of diseases or conditions. In this respect, the use of neonatal and juvenile pigs in the field of pediatric drug discovery and development is promising, although still limited at this point. This review summarizes the comparative postnatal development of pigs and humans and discusses the advantages of the juvenile pig in view of developmental pharmacology, pediatric diseases, drug discovery and drug safety testing. Furthermore, limitations and unexplored aspects of this large animal model are covered. At this point in time, the potential of the neonatal and juvenile pig as nonclinical safety models for pediatric drug development is underexplored.

ECMO with vasopressor use during early endotoxic shock: Can it improve circulatory support and regional microcirculatory blood flow?

Introduction

While extracorporeal membrane oxygenation (ECMO) is effective in preventing further hypoxemia and maintains blood flow in endotoxin-induced shock, ECMO alone does not reverse the hypotension. In this study, we tested whether concurrent vasopressor use with ECMO would provide increased circulatory support and blood flow, and characterized regional blood flow distribution to vital organs.

Methods

Endotoxic shock was induced in piglets to achieve a 30% decrease in mean arterial pressure (MAP). Measurements of untreated pigs were compared to pigs treated with ECMO alone or ECMO and vasopressors.

Results

ECMO provided cardiac support during vasodilatory endotoxic shock and improved oxygen delivery, but vasopressor therapy was required to return MAP to normotensive levels. Increased blood pressure with vasopressors did not alter oxygen consumption or extraction compared to ECMO alone. Regional microcirculatory blood flow (RBF) to the brain, kidney, and liver were maintained or increased during ECMO with and without vasopressors.

Conclusion

ECMO support and concurrent vasopressor use improve regional blood flow and oxygen delivery even in the absence of full blood pressure restoration. Vasopressor-induced selective distribution of blood flow to vital organs is retained when vasopressors are administered with ECMO.

ECMO Maintains Cerebral Blood Flow During Endotoxic Shock in Piglets

Cerebrovascular injury while on extracorporeal membrane oxygenation (ECMO) may be caused by excessive brain perfusion during hypoxemic reperfusion. Previous studies have postulated that the most vulnerable period of time for cerebrovascular injury is during the transfer period to ECMO. Therefore, our objective was to compare brain perfusion and hemodynamics in a piglet endotoxic shock ECMO model. The effect of ECMO flow on microcirculation of different brain regions was compared between 10 control pigs and six pigs (7–10 kg) administered IV endotoxin to achieve a drop in mean arterial blood pressure (MAP) of at least 30%. Cardiac output (CO), brain oxygen utilization, and microcirculatory blood flow (BF) were compared at baseline and 2 hours after ECMO stabilization. Matching ECMO delivery with baseline CO in control animals increased perfusion (p < 0.05) in all areas of the brain. In contrast, with endotoxin, ECMO returned perfusion closer to baseline levels in all regions of the brain and maintained brain tissue oxygen consumption. Both control and endotoxic pigs showed no evidence of acute neuronal necrosis in histologic cerebral cortical sections examined after 2 hours of ECMO. Results show that during endotoxic shock, transition to ECMO can maintain brain BF equally to all brain regions without causing overperfusion, and does not appear to cause brain tissue histopathologic changes (hemorrhage or necrosis) during the acute stabilization period after ECMO induction.

Bacterial translocation in critical illness

Bacterial translocation involves the passage of intestinal bacteria to extraintestinal sites and has been shown to increase morbidity and mortality in critical illness. This review outlines the pathophysiology of bacterial translocation, host defence mechanisms, and reviews the evidence for the clinical management of critically ill patients in order to minimise the negative outcomes associated with bacterial translocation.

Quantification of microcirculatory blood flow: a sensitive and clinically relevant prognostic marker in murine models of sepsis

Sepsis and sepsis-associated multiorgan failure represent the major cause of mortality in intensive care units worldwide. Cardiovascular dysfunction, a key component of sepsis pathogenesis, has received much research interest, although research translatability remains severely limited. There is a critical need for more comprehensive preclinical sepsis models, with more clinically relevant end points, such as microvascular perfusion. The purpose of this study was to compare microcirculatory blood flow measurements, using a novel application of laser speckle contrast imaging technology, with more traditional hemodynamic end points, as part of a multiparameter monitoring system in preclinical models of sepsis. Our aim, in measuring mesenteric blood flow, was to increase the prognostic sensitivity of preclinical studies. In two commonly used sepsis models (cecal ligation and puncture, and lipopolysaccharide), we demonstrate that blood pressure and cardiac output are compromised postsepsis, but subsequently stabilize over the 24-h recording period. In contrast, mesenteric blood flow continuously declines in a time-dependent manner and in parallel with the development of metabolic acidosis and organ dysfunction. Importantly, these microcirculatory perturbations are reversed by fluid resuscitation, a mainstay intervention associated with improved outcome in patients. These data suggest that global hemodynamics are maintained at the expense of the microcirculation and are, therefore, not sufficiently predictive of outcome. We demonstrate that microcirculatory blood flow is a more sensitive biomarker of sepsis syndrome progression and believe that incorporation of this biomarker into preclinical models will facilitate sophisticated proof-of-concept studies for novel sepsis interventions, providing more robust data on which to base future clinical trials.

Is arginine/asymetric dimethylarginine ratio depletion an indicator of insufficient resuscitation in a porcine model of hemorrhage-reperfusion?

BACKGROUND

Hemorrhagic shock leads to a complex cascade of metabolic and hormonal processes that may result in hypoperfusion, end organ damage, and death even when blood pressure is restored. Studies have shown that morbidity and mortality could be attributable to a diminished availability of endothelial-derived nitric oxide (eNO). It is unclear whether adequate levels of citrulline (CIT) and arginine (ARG)--the precursors of eNO synthesis--are available to sustain the eNO needed to maintain adequate perfusion in severe shock. An indirect measure of eNO is the ratio between the levels of ARG and its inhibitor asymmetric dimethylarginine (ARG/ADMA). The purpose of the study was to identify the temporal impact of the ARG/ADMA ratio, ARG, CIT, and ADMA in response to hemorrhage and crystalloid fluid resuscitation by the use of a porcine model of severe hemorrhagic shock.

METHODS

Hemorrhagic shock was induced in Yorkshire cross pigs by mimicking a bleeding pattern of rapid uncontrolled hemorrhage to achieve a shed volume of 30 mL/kg, a 50% decrease in mean arterial pressure, and an oxygen debt of >60 mL/kg. Normal saline, up to 2 times the shed blood volume, was started 1 hour after the start of hemorrhage with the goal of restoring mean arterial pressure to >50 mm Hg. Hemodynamics, blood gas measurements, and plasma samples were obtained at baseline, 1 hour after the start of hemorrhage, and 1 hour after resuscitation. Amino acids were measured by liquid chromatography coupled to mass spectrometry.

RESULTS

During hemorrhage, a distinct subset of pigs was better able to tolerate ischemia than the rest. These pigs required less resuscitation, had evidence of better organ perfusion, and exhibited less of an increase in interleukin-6 (IL-6) after resuscitation. Compared with their less-tolerant counterparts, this group had a greater increase in CIT above baseline (analysis of variance, P < .05) with hemorrhage. ARG levels were similar and remained stable with hemorrhage, which indicated the similar availability of substrate for eNO synthesis but differences in the quantity produced in response to the blood volume loss. With crystalloid fluid resuscitation, ARG levels and ARG/ADMA decreased (analysis of variance, P < .05), whereas CIT remained increased in the group less able to tolerate hemorrhage. ARG/ADMA decreased proportional to greater oxygen debt during hemorrhage and greater IL-6 levels with fluid resuscitation.

CONCLUSION

Our results suggest that a sufficient decrease in MAP during hemorrhagic shock is associated with a subsequent increase in IL-6, persisting impairment of end organ perfusion, and evidence of ongoing eNO deficit and an increase in ADMA despite resuscitation. The ARG/ADMA ratio reflects both of these parameters and corresponds to the increase in IL-6 and persistent ischemia after resuscitation. We propose that the mechanism of IL-6 increase in trauma derives from eNO deficiency, and the ARG/ADMA ratio more accurately depicts the pathologic mechanism responsible for increased morbidity and mortality in trauma.

Vasopressin Aggravates Cardiopulmonary Bypass-Induced Gastric Mucosal Ischemia

Background/Aim: Upper gastrointestinal bleeding (UGIB) is one of the most frequent gastrointestinal complications after cardiac surgery with cardiopulmonary bypass (CPB). Endothelin expression and microcirculatory dysfunction have been shown to be involved in UGIB. The aim of this study was to analyze the effect of vasopressin during CPB on the gastric mucosal microcirculation and the involvement of the endothelin system. Methods: Eighteen pigs were randomized into three groups (n = 6 each): group I = sham, group II = CPB (1-hour CPB) and group III = CPB + vasopressin (1-hour CPB and vasopressin administration during CPB to maintain baseline arterial pressure). All animals were observed for a further 90 min after termination of CPB. Systemic hemodynamics as well as blood flow and oxygen saturation of the gastric mucosa were measured continuously. At the end of the experiment, the gastric mucosal expressions of endothelin-1 (ET-1) and its receptor subtypes A (ETA) and B (ETB) were determined by polymerase chain reaction. Gastric mucosal injury, apoptotic cell death and leukocytic infiltration were determined by histology and immunohistochemical analyses of cleaved caspase-3 and myeloperoxidase. Results: CPB decreased gastric microvascular perfusion, which was associated with an increased expression of ET-1 and ETA. Vasopressin aggravated the CPB-associated malperfusion, whereas it completely abrogated the upregulation of ET-1 and ETA. Interestingly, vasopressin did not induce gastric mucosal morphologic injury, leukocytic infiltration or apoptotic cell death. Conclusion: Vasopressin aggravates CPB-associated microvascular malperfusion of the gastric mucosa but does not induce gastric mucosal injury.

Correlation between early sublingual small vessel density and late blood lactate level in critically ill surgical patients

BACKGROUND

Surgical stress may cause excessive inflammation and lead to microcirculatory dysfunction. The hypothesis of this study was that early microcirculatory dysfunction may result in anaerobic glycolysis and lead to elevated blood lactate levels in patients admitted to surgical intensive care units.

METHODS

This prospective observational study enrolled adult patients admitted to surgical intensive care units after general surgery or thoracic surgery. We measured blood lactate levels before the operation and at 1 h and 24 h after the operation. We obtained images of sublingual microcirculation using a sidestream dark field video microscope and analyzed them employing automated analysis software.

RESULTS

A total of 31 patients completed the study. Perioperative total and perfused small vessel densities were lower in patients with a blood lactate level ≥3 mmol/L. We observed a significant correlation between the total small vessel density at 1 h and the blood lactate level at 24 h (r = -0.573; P = 0.001). In addition, we saw a significant correlation between the perfused small vessel density at 1 h and the blood lactate level at 24 h (r = -0.476; P = 0.008).

CONCLUSIONS

Early total and perfused small vessel density may be used as an early predictor or therapeutic goal for critically ill surgical patients in further studies.

Bench-to-bedside review: Vasopressin in the management of septic shock

This review of vasopressin in septic shock differs from previous reviews by providing more information on the physiology and pathophysiology of vasopressin and vasopressin receptors, particularly because of recent interest in more specific AVPR1a agonists and new information from the Vasopressin and Septic Shock Trial (VASST), a randomized trial of vasopressin versus norepinephrine in septic shock. Relevant literature regarding vasopressin and other AVPR1a agonists was reviewed and synthesized. Vasopressin, a key stress hormone in response to hypotension, stimulates a family of receptors: AVPR1a, AVPR1b, AVPR2, oxytocin receptors and purinergic receptors. Rationales for use of vasopressin in septic shock are as follows: first, a deficiency of vasopressin in septic shock; second, low-dose vasopressin infusion improves blood pressure, decreases requirements for norepinephrine and improves renal function; and third, a recent randomized, controlled, concealed trial of vasopressin versus norepinephrine (VASST) suggests low-dose vasopressin may decrease mortality of less severe septic shock. Previous clinical studies of vasopressin in septic shock were small or not controlled. There was no difference in 28-day mortality between vasopressin-treated versus norepinephrine-treated patients (35% versus 39%, respectively) in VASST. There was potential benefit in the prospectively defined stratum of patients with less severe septic shock (5 to 14 μg/minute norepinephrine at randomization): vasopressin may have lowered mortality compared with norepinephrine (26% versus 36%, respectively, P = 0.04 within stratum). The result was robust: vasopressin also decreased mortality (compared with norepinephrine) if less severe septic shock was defined by the lowest quartile of arterial lactate or by use of one (versus more than one) vasopressor at baseline. Other investigators found greater hemodynamic effects of higher dose of vasopressin (0.06 units/minute) but also unique adverse effects (elevated liver enzymes and serum bilirubin). Use of higher dose vasopressin requires further evaluation of efficacy and safety. There are very few studies of interactions of therapies in critical care - or septic shock - and effects on mortality. Therefore, the interaction of vasopressin infusion, corticosteroid treatment and mortality of septic shock was evaluated in VASST. Low-dose vasopressin infusion plus corticosteroids significantly decreased 28-day mortality compared with corticosteroids plus norepinephrine (44% versus 35%, respectively, P = 0.03; P = 0.008 interaction statistic). Prospective randomized controlled trials would be necessary to confirm this interesting interaction. In conclusion, low-dose vasopressin may be effective in patients who have less severe septic shock already receiving norepinephrine (such as patients with modest norepinephrine infusion (5 to 15 μg/minute) or low serum lactate levels). The interaction of vasopressin infusion and corticosteroid treatment in septic shock requires further study.