Hypothermic renal perfusion during aortic surgery reduces the presence of lipocalin-2 and preserves renal extraction of dimethylarginines in rats

Using a Human Circulation Mathematical Model to Simulate the Effects of Hemodialysis and Therapeutic Hypothermia

Background: We developed a hemodynamic mathematical model of human circulation coupled to a virtual hemodialyzer. The model was used to explore mechanisms underlying our clinical observations involving hemodialysis. Methods: The model consists of whole body human circulation, baroreflex feedback control, and a hemodialyzer. Four model populations encompassing baseline, dialysed, therapeutic hypothermia treated, and simultaneous dialysed with hypothermia were generated. In all populations atrial fibrillation and renal failure as co-morbidities, and exercise as a treatment were simulated. Clinically relevant measurables were used to quantify the effects of each in silico experiment. Sensitivity analysis was used to uncover the most relevant parameters. Results: Relative to baseline, the modelled dialysis increased the population mean diastolic blood pressure by 5%, large vessel wall shear stress by 6%, and heart rate by 20%. Therapeutic hypothermia increased systolic blood pressure by 3%, reduced large vessel shear stress by 15%, and did not affect heart rate. Therapeutic hypothermia reduced wall shear stress by 15% in the aorta and 6% in the kidneys, suggesting a potential anti-inflammatory benefit. Therapeutic hypothermia reduced cardiac output under atrial fibrillation by 12% and under renal failure by 20%. Therapeutic hypothermia and exercise did not affect dialyser function, but increased water removal by approximately 40%. Conclusions: This study illuminates some mechanisms of the action of therapeutic hypothermia. It also suggests clinical measurables that may be used as surrogates to diagnose underlying diseases such as atrial fibrillation.

Cold Renal Perfusion During Simulation of Juxtarenal Aortic Aneurysm Repair Reduces Systemic Oxidative Stress and Sigmoid Damage in Rats

OBJECTIVES

Juxtarenal aortic surgery induces renal ischaemia reperfusion, which contributes to systemic inflammatory tissue injury and remote organ damage. Renal cooling during suprarenal cross clamping has been shown to reduce renal damage. It is hypothesised that renal cooling during suprarenal cross clamping also has systemic effects and could decrease damage to other organs, like the sigmoid colon.

METHODS

Open juxtarenal aortic aneurysm repair was simulated in 28 male Wistar rats with suprarenal cross clamping for 45 min, followed by 20 min of infrarenal aortic clamping. Four groups were created: sham, no, warm (37 °C saline), and cold (4 °C saline) renal perfusion during suprarenal cross clamping. Primary outcomes were renal damage and sigmoid damage. To assess renal damage, procedure completion serum creatinine rises were measured. Peri-operative microcirculatory flow ratios were determined in the sigmoid using laser Doppler flux. Semi-quantitative immunofluorescence microscopy was used to measure alterations in systemic inflammation parameters, including reactive oxygen species (ROS) production in circulating leukocytes and leukocyte infiltration in the sigmoid. Sigmoid damage was assessed using digestive enzyme (intestinal fatty acid binding protein - I-FABP) leakage, a marker of intestinal integrity.

RESULTS

Suprarenal cross clamping caused deterioration of all systemic parameters. Only cold renal perfusion protected against serum creatinine rise: 0.45 mg/dL without renal perfusion, 0.33 mg/dL, and 0.14 mg/dL (p = .009) with warm and cold perfusion, respectively. Microcirculation in the sigmoid was attenuated with warm (p = .002) and cold renal perfusion (p = .002). A smaller increase of ROS production (p = .034) was seen only after cold perfusion, while leukocyte infiltration in the sigmoid colon decreased after warm (p = .006) and cold perfusion (p = .018). Finally, digestive enzyme leakage increased more without (1.5AU) than with warm (1.3AU; p = .007) and cold renal perfusion (1.2AU; p = .002).

CONCLUSIONS

Renal ischaemia/reperfusion injury after suprarenal cross clamping decreased microcirculatory flow, increased systemic ROS production, leukocyte infiltration, and I-FABP leakage in the sigmoid colon. Cold renal perfusion was superior to warm perfusion and reduced renal damage and had beneficial systemic effects, reducing sigmoid damage in this experimental study.

Organ protection during aortic cross-clamping

Open surgical repair of an aortic aneurysm requires aortic cross-clamping, resulting in temporary ischemia of all organs and tissues supplied by the aorta distal to the clamp. Major complications of open aneurysm repair due to aortic cross-clamping include renal ischemia-reperfusion injury and postoperative colonic ischemia in case of supra- and infrarenal aortic aneurysm repair. Ischemia-reperfusion injury results in excessive production of reactive oxygen species and in oxidative stress, which can lead to multiple organ failure. Several perioperative protective strategies have been suggested to preserve renal function during aortic cross-clamping, such as pharmacotherapy and therapeutic hypothermia of the kidneys. In this chapter, we will briefly discuss the pathophysiology of ischemia-reperfusion injury and the preventative measures that can be taken to avoid abdominal organ injury. Finally, techniques to minimize the risk of complications during and after open aneurysm repair will be presented.

Reduced renal plasma clearance does not explain increased plasma asymmetric dimethylarginine in hypertensive subjects with mild to moderate renal insufficiency

Plasma concentrations of the nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA) increase already in the early stages of renal insufficiency. There is no agreement as to whether reduced renal plasma clearance (RPCL) contributes to this increase. Therefore, we investigated the relationship between estimated glomerular filtration rate (eGFR), RPCL, and plasma ADMA and SDMA in essential hypertensive patients with mild to moderate renal insufficiency. In 171 patients who underwent renal angiography, we drew blood samples from the aorta and both renal veins and measured mean renal blood flow (MRBF) using the (133)Xe washout technique. RPCL was calculated using arteriovenous concentration differences and MRBF. After correction for potential confounders, reduced eGFR was associated with higher plasma ADMA and SDMA [standardized regression coefficient (β) = -0.22 (95% confidence intervals: -0.41, -0.04) and β = -0.66 (95% confidence intervals: -0.83, -0.49), respectively]. However, eGFR was not independently associated with RPCL of ADMA. Moreover, reduced RPCL of ADMA was not associated with higher plasma ADMA. Contrary to ADMA, reduced eGFR was indeed associated with lower RPCL of SDMA [β = 0.21 (95% confidence intervals: 0.02, 0.40)]. In conclusion, our findings indicate that RPCL of ADMA is independent of renal function in hypertensive patients with mild to moderate renal insufficiency. Unlike the case for SDMA, reduced RPCL of ADMA is of minor importance for the increase in plasma ADMA in these patients, which indicates that increased plasma ADMA in this population is not a direct consequence of the kidneys failing as a plasma ADMA-regulating organ.