Endothelin-1 receptor antagonist prevents deterioration of left ventricular function and coronary flow reserve in brain-dead canine heart

Brain stem death induces pro-inflammatory cytokine production and cardiac dysfunction in sheep model

Introduction

Organs procured following brain stem death (BSD) are the main source of organ grafts for transplantation. However, BSD is associated with inflammatory responses that may damage the organ and affect both the quantity and quality of organs available for transplant. Therefore, we aimed to investigate plasma and bronchoalveolar lavage (BAL) pro-inflammatory cytokine profiles and cardiovascular physiology in a clinically relevant 6-h ovine model of BSD.

Methods

Twelve healthy female sheep (37–42 Kg) were anaesthetized and mechanically ventilated prior to undergoing BSD induction and then monitored for 6 h. Plasma and BAL endothelin-1 and cytokines (IL-1β, 6, 8 and tumour necrosis factor alpha (TNF-α)) were assessed by ELISA. Differential white blood cell counts were performed. Cardiac function during BSD was also examined using echocardiography, and cardiac biomarkers (A-type natriuretic peptide and troponin I were measured in plasma.

Results

Plasma concentrations big ET-1, IL-6, IL-8, TNF-α and BAL IL-8 were significantly (p < 0.01) increased over baseline at 6 h post-BSD. Increased numbers of neutrophils were observed in the whole blood (3.1 × 10⁹ cells/L [95% confidence interval (CI) 2.06–4.14] vs. 6 × 10⁹ cells/L [95%CI 3.92–7.97]; p < 0.01) and BAL (4.5 × 10⁹ cells/L [95%CI 0.41–9.41] vs. 26 [95%CI 12.29–39.80]; p = 0.03) after 6 h of BSD induction vs baseline. A significant increase in ANP production (20.28 pM [95%CI 16.18–24.37] vs. 78.68 pM [95%CI 53.16–104.21]; p < 0.0001) and cTnI release (0.039 ng/mL vs. 4.26 [95%CI 2.69–5.83] ng/mL; p < 0.0001), associated with a significant reduction in heart contractile function, were observed between baseline and 6 h.

Conclusions

BSD induced systemic pro-inflammatory responses, characterized by increased neutrophil infiltration and cytokine production in the circulation and BAL fluid, and associated with reduced heart contractile function in ovine model of BSD.

Peritransplant Cardiometabolic and Mitochondrial Function: The Missing Piece in Donor Heart Dysfunction and Graft Failure

Primary graft dysfunction is an important cause of morbidity and mortality after cardiac transplantation. Donor brain stem death (BSD) is a significant contributor to donor heart dysfunction and primary graft dysfunction. There remain substantial gaps in the mechanistic understanding of peritransplant cardiac dysfunction. One of these gaps is cardiac metabolism and metabolic function. The healthy heart is an "omnivore," capable of utilizing multiple sources of nutrients to fuel its enormous energetic demand. When this fails, metabolic inflexibility leads to myocardial dysfunction. Data have hinted at metabolic disturbance in the BSD donor and subsequent heart transplantation; however, there is limited evidence demonstrating specific metabolic or mitochondrial dysfunction. This review will examine the literature surrounding cardiometabolic and mitochondrial function in the BSD donor, organ preservation, and subsequent cardiac transplantation. A more comprehensive understanding of this subject may then help to identify important cardioprotective strategies to improve the number and quality of donor hearts.

Endothelin receptor antagonist improves donor lung function in an ex vivo perfusion system

BACKGROUND

A lung transplant is the last resort treatment for many patients with advanced lung disease. The majority of donated lungs come from donors following brain death (BD). The endothelin axis is upregulated in the blood and lung of the donor after BD resulting in systemic inflammation, lung damage and poor lung graft outcomes in the recipient. Tezosentan (endothelin receptor blocker) improves the pulmonary haemodynamic profile; however, it induces adverse effects on other organs at high doses. Application of ex vivo lung perfusion (EVLP) allows the development of organ-specific hormone resuscitation, to maximise and optimise the donor pool. Therefore, we investigate whether the combination of EVLP and tezosentan administration could improve the quality of donor lungs in a clinically relevant 6-h ovine model of brain stem death (BSD).

METHODS

After 6 h of BSD, lungs obtained from 12 sheep were divided into two groups, control and tezosentan-treated group, and cannulated for EVLP. The lungs were monitored for 6 h and lung perfusate and tissue samples were processed and analysed. Blood gas variables were measured in perfusate samples as well as total proteins and pro-inflammatory biomarkers, IL-6 and IL-8. Lung tissues were collected at the end of EVLP experiments for histology analysis and wet-dry weight ratio (a measure of oedema).

RESULTS

Our results showed a significant improvement in gas exchange [elevated partial pressure of oxygen (P = 0.02) and reduced partial pressure of carbon dioxide (P = 0.03)] in tezosentan-treated lungs compared to controls. However, the lungs hematoxylin-eosin staining histology results showed minimum lung injuries and there was no difference between both control and tezosentan-treated lungs. Similarly, IL-6 and IL-8 levels in lung perfusate showed no difference between control and tezosentan-treated lungs throughout the EVLP. Histological and tissue analysis showed a non-significant reduction in wet/dry weight ratio in tezosentan-treated lung tissues (P = 0.09) when compared to control.

CONCLUSIONS

These data indicate that administration of tezosentan could improve pulmonary gas exchange during EVLP.

Novel 24-h ovine model of brain death to study the profile of the endothelin axis during cardiopulmonary injury

BACKGROUND

Upregulation of the endothelin axis has been observed in pulmonary tissue after brain death, contributing to primary graft dysfunction and ischaemia reperfusion injury. The current study aimed to develop a novel, 24-h, clinically relevant, ovine model of brain death to investigate the profile of the endothelin axis during brain death-associated cardiopulmonary injury. We hypothesised that brain death in sheep would also result in demonstrable injury to other transplantable organs.

METHODS

Twelve merino cross ewes were randomised into two groups. Following induction of general anaesthesia and placement of invasive monitoring, brain death was induced in six animals by inflation of an extradural catheter. All animals were supported in an intensive care unit environment for 24 h. Animal management reflected current human donor management, including administration of vasopressors, inotropes and hormone resuscitation therapy. Activation of the endothelin axis and transplantable organ injury were assessed using ELISA, immunohistochemistry and standard biochemical markers.

RESULTS

All animals were successfully supported for 24 h. ELISA suggested early endothelin-1 and big endothelin-1 release, peaking 1 and 6 h after BD, respectively, but there was no difference at 24 h. Immunohistochemistry confirmed the presence of the endothelin axis in pulmonary tissue. Brain dead animals demonstrated tachycardia and hypertension, followed by haemodynamic collapse, typified by a reduction in systemic vascular resistance to 46 ± 1 % of baseline. Mean pulmonary artery pressure rose to 186 ± 20 % of baseline at induction and remained elevated throughout the protocol, reaching 25 ± 2.2 mmHg at 24 h. Right ventricular stroke work increased 25.9 % above baseline by 24 h. Systemic markers of cardiac and hepatocellular injury were significantly elevated, with no evidence of renal dysfunction.

CONCLUSIONS

This novel, clinically relevant, ovine model of brain death demonstrated that increased pulmonary artery pressures are observed after brain death. This may contribute to right ventricular dysfunction and pulmonary injury. The development of this model will allow for further investigation of therapeutic strategies to minimise the deleterious effects of brain death on potentially transplantable organs.

Inflammatory signalling associated with brain dead organ donation: from brain injury to brain stem death and posttransplant ischaemia reperfusion injury

Brain death is associated with dramatic and serious pathophysiologic changes that adversely affect both the quantity and quality of organs available for transplant. To fully optimise the donor pool necessitates a more complete understanding of the underlying pathophysiology of organ dysfunction associated with transplantation. These injurious processes are initially triggered by catastrophic brain injury and are further enhanced during both brain death and graft transplantation. The activated inflammatory systems then contribute to graft dysfunction in the recipient. Inflammatory mediators drive this process in concert with the innate and adaptive immune systems. Activation of deleterious immunological pathways in organ grafts occurs, priming them for further inflammation after engraftment. Finally, posttransplantation ischaemia reperfusion injury leads to further generation of inflammatory mediators and consequent activation of the recipient's immune system. Ongoing research has identified key mediators that contribute to the inflammatory milieu inherent in brain dead organ donation. This has seen the development of novel therapies that directly target the inflammatory cascade.

Systemic and Myocardial Oxygen Transport Responses to Brain Death in Pigs

BACKGROUND

Brain death is associated with profound disturbances of systemic and myocardial oxygen transport, but little is known regarding the acute response of systemic oxygen consumption (VO(2)).

METHODS

Brain death was induced in 6 pigs (30.6 +/- 3.0 kg) by balloon inflation into the cranial cavity. VO(2) was continuously measured by respiratory mass spectrometry. Blood pressures and gases were measured from the aorta, superior vena cava, and coronary sinus, with arterial epinephrine and norepinephrine, prior to brain death, at 1, 10, and 90 minutes after brain death. Cardiac output (CO), systemic vascular resistance (SVR), oxygen delivery (DO(2)), oxygen extraction (EO(2)), and myocardial oxygen (mEO(2)) and lactate extractions (mE(1ac)) were calculated. Left ventricular contractility was assessed by micromanometer tipped catheters.

RESULTS

VO(2) increased from 4.8 +/- 0.9 to 6.3 +/- 0.9 mL/min/kg 1 minute after brain death (P < .001), and subsequently decreased to below baseline at 90 minutes (P < .001). Left ventricular contractility, CO, and DO(2) increased 1 minute after brain death (P < .001), followed by a rapid decrease to baseline within 10 minutes (P < .001). SVR and EO(2) decreased after brain death (P < .01) and remained low. Lactate remained unchanged. mE(1ac) decreased after brain death despite a decrease in mEO(2) (P < .01), and returned to baseline at 90 minutes.

CONCLUSIONS

The initial surge in VO(2) after brain death is offset by the greater increase in DO(2), thus tissue perfusion remains adequate. The lower than baseline VO(2) and SVR at the end of the study period may indicate general metabolic and hemodynamic compromise. The information regarding the profound metabolic alterations imposed by brain death may have implications for management of brain death donors.

L-arginine improves endothelial and myocardial function after brain death

BACKGROUND

Recently, we showed that brain death (BD) leads to a severe impairment of endothelial function.

METHODS

To test the hypothesis, that nitric oxide supply improves endothelial function, we infused L-arginine (40 mg/kg) in 6 dogs after BD induction (subdural balloon). Six vehicle-treated BD animals served as controls. Coronary blood flow (CBF), preload recruitable stroke work (PRSW), and plasma L-arginine and nitrite/nitrate levels were measured before and 6 hr after BD induction. In addition, endothelium-dependent vasodilatation after intracoronary application of acetylcholine (ACH) and endothelium-independent vasodilation after sodium nitroprusside (SNP) were assessed.

RESULTS

Six hours after BD, CBF decreased significantly in the control group (38.2+/-3.5 vs. 26.8+/-3.1 ml/min, P<0.05), whereas the decrease was less pronounced in the L-arginine group (41.8+/-6.9 vs. 36.0+/-1.2 ml/min, P<0.05 vs. control). Before BD, ACH led to a similar vasodilative response in both groups (81+/-6 vs. 75+/-7%). After BD, a paradox vasoconstriction occurred after ACH in the control group, while the vasodilative response did not change in the L-Arginine group (36+/-6 vs. 69+/-7%, P<0.05). The response to SNP did not differ between the groups and over the time. After BD PRSW decreased in both groups, however, it was still significantly higher in the L-arginine group (56+/-7 vs. 71+/-7 kerg, P<0.05). L-arginine (711+/-144 vs. 234+/-54 microM P<0.05) and nitrite/nitrate (39+/-3 vs. 27+/-3 microM P<0.05) levels were significantly higher in the L-arginine group.

CONCLUSION

L-arginine treatment prevents endothelial dysfunction and improves myocardial performance after BD via enhancement of endogenous nitric oxide synthesis.

Brain Death Does Not Change Epicardial Action Potentials and Their Response to Ischemia–Reperfusion in Open-chest Pigs

BACKGROUND

It is debated whether brain death (BD) causes transient functional ischemia. In this investigation we used monophasic action potential (AP) recording during BD as a sensitive means to assess: (i) whether ischemia was present; and (ii) the effect of BD on a subsequent ischemia-reperfusion challenge.

METHODS

In Period 1, BD was induced (BD group, 6 pigs) or not induced (sham maneuver, control [C] group, 6 pigs), and effects were followed for 3 hours. In Period 2, left anterior descending (LAD) coronary artery ligation ischemia was applied for 20 minutes to all hearts, followed by 60-minute reperfusion.

RESULTS

In Period 1, plasma norepinephrine was 3.1-, 6.3- and 5-fold greater in BD than in C at 1, 120 and 180 minutes, respectively, and systolic blood pressure was 26% greater at 1 minute and 35% at 120 minutes. The arteriovenous difference in lactate was similar or lower in BD than in C. In both groups, at all time-points, the action potential recording had a rectangular plateau shape and action potential duration (APD50) had a linear relationship to the cardiac inter-beat (RR) interval (R2 = 0.89 and 0.73, slope = 0.42 +/- 0.02 and 0.46 +/- 0.06 in BD and C, respectively). In Period 2, ischemia caused a similar (50%) APD shortening in BD and C. Restoration of the APD upon reperfusion was complete in both groups.

CONCLUSIONS

Our findings suggest that BD does not cause direct cardiac ischemia and does not change the response of the heart to subsequent ischemia-reperfusion challenge.