John-Paul Tung

Optimal Management of the Critically Ill: Anaesthesia, Monitoring, Data Capture, and Point-of-Care Technological Practices in Ovine Models of Critical Care

Animal models of critical illness are vital in biomedical research. They provide possibilities for the investigation of pathophysiological processes that may not otherwise be possible in humans. In order to be clinically applicable, the model should simulate the critical care situation realistically, including anaesthesia, monitoring, sampling, utilising appropriate personnel skill mix, and therapeutic interventions. There are limited data documenting the constitution of ideal technologically advanced large animal critical care practices and all the processes of the animal model. In this paper, we describe the procedure of animal preparation, anaesthesia induction and maintenance, physiologic monitoring, data capture, point-of-care technology, and animal aftercare that has been successfully used to study several novel ovine models of critical illness. The relevant investigations are on respiratory failure due to smoke inhalation, transfusion related acute lung injury, endotoxin-induced proteogenomic alterations, haemorrhagic shock, septic shock, brain death, cerebral microcirculation, and artificial heart studies. We have demonstrated the functionality of monitoring practices during anaesthesia required to provide a platform for undertaking systematic investigations in complex ovine models of critical illness.

Donor Mannose-Binding Lectin Deficiency Increases the Likelihood of Clinically Significant Infection after Liver Transplantation

BACKGROUND Mannose-binding lectin (MBL) is an important mediator of innate immunity and is synthesized primarily by the liver. Low MBL levels are common, are due primarily to polymorphisms in the gene encoding MBL (MBL2), and are associated with an increased risk of infection, particularly when immunity is compromised. We report a large, retrospective study that examined the association between MBL status and clinically significant infection following orthotopic liver transplantation. METHODS One hundred two donor-recipient orthotopic liver transplantation pairs were studied. Five polymorphisms in the promoter and coding regions of MBL2 were examined. MBL levels were measured, using the mannan-binding and C4-deposition assays, in serum samples obtained before and after transplantation. Associations between MBL status, as assessed by serum MBL levels and MBL2 genotype, and time to first clinically significant infection (CSI) after transplantation were examined in survival analysis with consideration of competing risks. RESULTS The median duration of follow-up after orthotopic liver transplantation was 4 years. Thirty-six percent of recipients developed CSI after transplantation. The presence of MBL2 coding mutations in the donor was significantly associated with CSI in the recipient; the cumulative incidence function of infection was 55% in recipients of deficient livers, compared with 32% for recipients of wild-type livers (P = .002). Infection was not associated with recipient MBL2 genotype. Low MBL levels after orthotopic liver transplantation levels (mannan-binding <1 microg/mL or C4 deposition <0.2 C4 U/microL) were also associated with CSI (cumulative incidence function, 52% vs. 20%, P = .003; and cumulative incidence function, 54% vs. 24%, P = .007, respectively). In multivariate analysis, mutation in the MBL2 coding region of the donor (hazard ratio, 2.8; P = .005) and the use of cytomegalovirus prophylaxis (hazard ratio, 2.6; P = .005) were independently associated with CSI. CONCLUSIONS Recipients of MBL-deficient livers have almost a 3-fold greater likelihood of developing CSI and may benefit from MBL replacement.

Respiratory burst function of ovine neutrophils.

BackgroundRespiratory burst function resulting in the release of reactive oxygen species such as superoxide anion (O2-) from neutrophils is one of the key mechanisms of the innate immune system, and maladaptive control of this mechanism is thought to play a pivotal role in the development of pathologies such as acute lung injury and sepsis. Ovine models of these pathologies are limited by the poor understanding of ovine neutrophil respiratory burst function.ResultsAspects of ovine neutrophil respiratory burst function to be characterised were: i) the maximum rate of O2- generated (Vmax); ii) the time taken to reach Vmax; iii) the total amount of O2- generated during the reaction; and iv) the duration of the reaction. As well as for unstimulated neutrophils, these aspects were also characterised after incubation with a priming agonist (platelet activating factor [PAF], tumour necrosis factor alpha [TNF-α] and lipopolysaccharides [LPS]) activating agonists (N-formylmethionyl-leucyl-phenylalanine [fMLP] and phorbol 12-myristate 13-acetate [PMA]) or a combination of a priming and an activating agonist. In the absence of priming or activating agonists, ovine neutrophils displayed a low level of respiratory burst function which was not enhanced by either PAF, TNF-α, LPS or fMLP, but was significantly enhanced by PMA. The PMA-induced respiratory burst function was further enhanced by pre-incubation with PAF, but not with TNF-α or LPS. By varying the length of pre-incubation with PAF it was demonstrated that this effect decreased as the duration of pre-incubation with PAF increased, and that PAF was enhancing PMAs effects rather than PMA enhancing PAFs effects.ConclusionThis study successfully adapted a commonly used method of measuring human neutrophil respiratory burst function to characterise different aspects of ovine neutrophil respiratory burst function. This improved understanding of ovine neutrophils will facilitate the validitation of ovine biomedical models of human pathologies in which neutrophils have been implicated.

An Ovine Model of Hyperdynamic Endotoxemia and Vital Organ Metabolism

Background: Animal models of endotoxemia are frequently used to understand the pathophysiology of sepsis and test new therapies. However, important differences exist between commonly used experimental models of endotoxemia and clinical sepsis. Animal models of endotoxemia frequently produce hypodynamic shock in contrast to clinical hyperdynamic shock. This difference may exaggerate the importance of hypoperfusion as a causative factor in organ dysfunction. This study sought to develop an ovine model of hyperdynamic endotoxemia and assess if there is evidence of impaired oxidative metabolism in the vital organs. Methods: Eight sheep had microdialysis catheters implanted into the brain, heart, liver, kidney, and arterial circulation. Shock was induced with a 4 h escalating dose infusion of endotoxin. After 3 h vasopressor support was initiated with noradrenaline and vasopressin. Animals were monitored for 12 h after endotoxemia. Blood samples were recovered for hemoglobin, white blood cell count, creatinine, and proinflammatory cytokines (IL-1Beta, IL-6, and IL-8). Results: The endotoxin infusion was successful in producing distributive shock with the mean arterial pressure decreasing from 84.5 ± 12.8 mm Hg to 49 ± 8.03 mm Hg (P < 0.001). Cardiac index remained within the normal range decreasing from 3.33 ± 0.56 L/min/m2 to 2.89l ± 0.36 L/min/m2 (P = 0.0845). Lactate/pyruvate ratios were not significantly abnormal in the heart, brain, kidney, or arterial circulation. Liver microdialysis samples demonstrated persistently high lactate/pyruvate ratios (mean 37.9 ± 3.3). Conclusions: An escalating dose endotoxin infusion was successful in producing hyperdynamic shock. There was evidence of impaired oxidative metabolism in the liver suggesting impaired splanchnic perfusion. This may be a modifiable factor in the progression to multiple organ dysfunction and death.

Platelet Storage Lesions: What More Do We Know Now?

Platelet concentrate (PC) transfusions are a lifesaving adjunct to control and prevent bleeding in cancer, hematologic, surgical, and trauma patients. Platelet concentrate availability and safety are limited by the development of platelet storage lesions (PSLs) and risk of bacterial contamination. Platelet storage lesions are a series of biochemical, structural, and functional changes that occur from blood collection to transfusion. Understanding of PSLs is key for devising interventions that prolong PC shelf life to improve PC access and wastage. This article will review advancements in clinical and mechanistic PSL research. In brief, exposure to artificial surfaces and high centrifugation forces during PC preparation initiate PSLs by causing platelet activation, fragmentation, and biochemical release. During room temperature storage, enhanced glycolysis and reduced mitochondrial function lead to glucose depletion, lactate accumulation, and product acidification. Impaired adenosine triphosphate generation reduces platelet capacity to perform energetically demanding processes such as hypotonic stress responses and activation/aggregation. Storage-induced alterations in platelet surface proteins such as thrombin receptors and glycoproteins decrease platelet aggregation. During storage, there is an accumulation of immunoactive proteins such as leukocyte-derive cytokines (tumor necrosis factor α, interleukin (IL) 1α, IL-6, IL-8) and soluble CD40 ligand which can participate in transfusion-related acute lung injury and nonhemolytic transfusion reactions. Storage-induced microparticles have been linked to enhanced platelet aggregation and immune system modulation. Clinically, stored PCs have been correlated with reduced corrected count increment, posttransfusion platelet recovery, and survival across multiple meta-analyses. Fresh PC transfusions have been associated with superior platelet function in vivo; however, these differences were abrogated after a period of circulation. There is currently insufficient evidence to discern the effect of PSLs on transfusion safety. Various bag and storage media changes have been proposed to reduce glycolysis and platelet activation during room temperature storage. Moreover, cryopreservation and cold storage have been proposed as potential methods to prolong PC shelf life by reducing platelet metabolism and bacterial proliferation. However, further work is required to elucidate and manage the PSLs specific to these storage protocols before its implementation in blood banks.

Unintended Consequences: Fluid Resuscitation Worsens Shock in an Ovine Model of Endotoxemia

Rationale: Fluid resuscitation is widely considered a life‐saving intervention in septic shock; however, recent evidence has brought both its safety and efficacy in sepsis into question. Objectives: In this study, we sought to compare fluid resuscitation with vasopressors with the use of vasopressors alone in a hyperdynamic model of ovine endotoxemia. Methods: Endotoxemic shock was induced in 16 sheep, after which they received fluid resuscitation with 40 ml/kg of 0.9% saline or commenced hemodynamic support with protocolized noradrenaline and vasopressin. Microdialysis catheters were inserted into the arterial circulation, heart, brain, kidney, and liver to monitor local metabolism. Blood samples were recovered to measure serum inflammatory cytokines, creatinine, troponin, atrial natriuretic peptide, brain natriuretic peptide, and hyaluronan. All animals were monitored and supported for 12 hours after fluid resuscitation. Measurements and Main Results: After resuscitation, animals that received fluid resuscitation required significantly more noradrenaline to maintain the same mean arterial pressure in the subsequent 12 hours (68.9 mg vs. 39.6 mg; P = 0.04). Serum cytokines were similar between groups. Atrial natriuretic peptide increased significantly after fluid resuscitation compared with that observed in animals managed without fluid resuscitation (335 ng/ml [256‐382] vs. 233 ng/ml [144‐292]; P = 0.04). Cross‐sectional time‐series analysis showed that the rate of increase of the glycocalyx glycosaminoglycan hyaluronan was greater in the fluid‐resuscitated group over the course of the study (P = 0.02). Conclusions: Fluid resuscitation resulted in a paradoxical increase in vasopressor requirement. Additionally, it did not result in improvements in any of the measured microcirculatory‐ or organ‐specific markers measured. The increase in vasopressor requirement may have been due to endothelial/glycocalyx damage secondary to atrial natriuretic peptide‐mediated glycocalyx shedding.

Abstract P-559: FLUID RESUSCITATION WITH 0.9% SALINE IMPAIRS MYOCARDIAL CONTRACTILITY IN AN OVINE MODEL OF ENDOTOXEMIC SHOCK

During the 2 week audit period a total of 20 patients were included. We grouped patients with RACHS 1&2 scores (n=11), RACHS 3&4 scores (n=9) and RACHS 5&6 scores (n=0). Four patients required peritoneal dialysis; all survived to discharge. Daily fluid balance and cumulative balance (in ml/kg) is shown (table 1). In the RACHS 3&4 group, there was both a higher fluid balance on ‘day 2’ and in cumulative balance.

How different animal models help us understand TRALI

Transfusion‐related acute lung injury (TRALI) is still one of the leading causes of transfusion‐associated mortality, despite increased awareness and the implementation of various mitigation strategies. TRALI may be either antibody or non‐antibody mediated. Understanding the underlying mechanism through clinical cases has been challenging because of the low and unpredictable incidence. Thus, animal models provide an important tool for investigating the mechanism of TRALI in a controlled and systematic fashion. One of the earliest animal models, an ex vivo rabbit lung model, revealed the role of anti‐5b (anti‐HNA‐3a) and granulocytes. In vivo models with mice, rats and pig models have provided further insights into the mechanisms of antibody‐mediated TRALI. There is evidence for contributions from neutrophils, neutrophil extracellular traps, platelets, lymphocytes, monocytes and endothelial cells. Elevated levels of C‐reactive proteins and cardiopulmonary bypass may predispose to TRALI, while protective roles have been identified with lymphocytes, T‐regulatory cells and dendritic cells. Mouse models have shown that IL‐10 infusion protects from TRALI if administered prophylactically and successfully treats TRALI reaction if administered therapeutically. Silliman et al. introduced the non‐immune mechanism of TRALI with rat models, and since then, there have been sheep and pig models. Non‐immune animal models have demonstrated the importance of a priming event (e.g. endotoxin administration) and a role for the protein and lipid biological response modifiers (BRMs) that accumulate during routine blood product storage. This paper focuses on how the key ex vivo and in vivo animal models have progressed our understanding of the mechanisms leading to TRALI.

Transfusion of packed red blood cells at the end of shelf life is associated with increased risk of mortality – a pooled patient data analysis of 16 observational trials

Observational studies address packed red blood cell effects at the end of shelf life and have larger sample sizes compared to randomized control trials. Meta-analyses combining data from observational studies have been complicated by differences in aggregate transfused packed red blood cell age and outcome reporting. This study abrogated these issues by taking a pooled patient data approach. Observational studies reporting packed red blood cell age and clinical outcomes were identified and patient-level data sets were sought from investigators. Odds ratios and 95% confidence intervals for binary outcomes were calculated for each study, with mean packed red blood cell age or maximum packed red blood cell age acting as independent variables. The relationship between mean packed red blood cell age and hospital length of stay for each paper was analyzed using zero-inflated Poisson regression. Random effects models combined paper-level effect estimates. Extremes analyses were completed by comparing patients transfused with mean packed red blood cell aged less than ten days to those transfused with mean packed red blood cell aged at least 30 days. sixteen datasets were available for pooled patient data analysis. Mean packed red blood cell age of at least 30 days was associated with an increased risk of in-hospital mortality compared to mean packed red blood cell of less than ten days (odds ratio: 3.25, 95% confidence interval: 1.27–8.29). Packed red blood cell age was not correlated to increased risks of nosocomial infection or prolonged length of hospital stay.

Lessons from sheep models of transfusion

Animal models have been a valuable tool for research into blood products and outcomes of blood transfusions. Small animal models have been applied extensively and large animal models less frequently. This review describes the experience and details the findings from a recent series of in vivo sheep transfusion models.