Influence of blood haemoglobin concentration on renal haemodynamics and oxygenation during experimental cardiopulmonary bypass in sheep

Urinary oxygen tension and its role in predicting acute kidney injury: A narrative review

Acute kidney injury occurs frequently in the perioperative setting. The renal medulla often endures hypoxia or hypoperfusion and is susceptible to the imbalance between oxygen supply and demand due to the nature of renal blood flow distribution and metabolic rate in the kidney. The current available evidence demonstrated that the urine oxygen pressure is proportional to the variations of renal medullary tissue oxygen pressure. Thus, urine oxygenation can be a candidate for reflecting the change of oxygen in the renal medulla. In this review, we discuss the basic physiology of acute kidney injury, as well as techniques for monitoring urine oxygen tension, confounding factors affecting the reliable measurement of urine oxygen tension, and its clinical use, highlighting its potential role in early detection and prevention of acute kidney injury.

Differential responses of cerebral and renal oxygenation to altered perfusion conditions during experimental cardiopulmonary bypass in sheep

We tested whether the brain and kidney respond differently to cardiopulmonary bypass (CPB) and to changes in perfusion conditions during CPB. Therefore, in ovine CPB, we assessed regional cerebral oxygen saturation (rSO2 ) by near-infrared spectroscopy and renal cortical and medullary tissue oxygen tension (PO2 ), and, in some protocols, brain tissue PO2 , by phosphorescence lifetime oximetry. During CPB, rSO2 correlated with mixed venous SO2 (r = 0.78) and brain tissue PO2 (r = 0.49) when arterial PO2 was varied. During the first 30 min of CPB, brain tissue PO2 , rSO2 and renal cortical tissue PO2 did not fall, but renal medullary tissue PO2 did. Nevertheless, compared with stable anaesthesia, during stable CPB, rSO2 (66.8 decreasing to 61.3%) and both renal cortical (90.8 decreasing to 43.5 mm Hg) and medullary (44.3 decreasing to 19.2 mm Hg) tissue PO2 were lower. Both rSO2 and renal PO2 increased when pump flow was increased from 60 to 100 mL kg-1  min-1 at a target arterial pressure of 70 mm Hg. They also both increased when pump flow and arterial pressure were increased simultaneously. Neither was significantly altered by partially pulsatile flow. The vasopressor, metaraminol, dose-dependently decreased rSO2 , but increased renal cortical and medullary PO2 . Increasing blood haemoglobin concentration increased rSO2 , but not renal PO2 . We conclude that both the brain and kidney are susceptible to hypoxia during CPB, which can be alleviated by increasing pump flow, even without increasing arterial pressure. However, increasing blood haemoglobin concentration increases brain, but not kidney oxygenation, whereas vasopressor support with metaraminol increases kidney, but not brain oxygenation.

Intraoperative hemodynamics and risk of cardiac surgery-associated acute kidney injury: An observation study and a feasibility clinical trial

Targeting greater pump flow and mean arterial pressure (MAP) during cardiopulmonary bypass (CPB) could potentially alleviate renal hypoxia and reduce the risk of postoperative acute kidney injury (AKI). Therefore, in an observational study of 93 patients undergoing on-pump cardiac surgery, we tested whether intraoperative hemodynamic management differed between patients who did and did not develop AKI. Then, in 20 patients, we assessed the feasibility of a larger-scale trial in which patients would be randomized to greater than normal target pump flow and MAP, or usual care, during CPB. In the observational cohort, MAP during hypothermic CPB averaged 68.8 ± 8.0 mmHg (mean ± SD) in the 36 patients who developed AKI and 68.9 ± 6.3 mmHg in the 57 patients who did not (p = 0.98). Pump flow averaged 2.4 ± 0.2 L/min/m2 in both groups. In the feasibility clinical trial, compared with usual care, those randomized to increased target pump flow and MAP had greater mean pump flow (2.70 ± 0.23 vs. 2.42 ± 0.09 L/min/m2 during the period before rewarming) and systemic oxygen delivery (363 ± 60 vs. 281 ± 45 mL/min/m2 ). Target MAP ≥80 mmHg was achieved in 66.6% of patients in the intervention group but in only 27.3% of patients in the usual care group. Nevertheless, MAP during CPB did not differ significantly between the two groups. We conclude that little insight was gained from our observational study regarding the impact of variations in pump flow and MAP on the risk of AKI. However, a clinical trial to assess the effects of greater target pump flow and MAP on the risk of AKI appears feasible.

Renal arterial infusion of tempol prevents medullary hypoperfusion, hypoxia, and acute kidney injury in ovine Gram-negative sepsis

AIM

Renal medullary hypoperfusion and hypoxia precede acute kidney injury (AKI) in ovine sepsis. Oxidative/nitrosative stress, inflammation, and impaired nitric oxide generation may contribute to such pathophysiology. We tested whether the antioxidant and anti-inflammatory drug, tempol, may modify these responses.

METHODS

Following unilateral nephrectomy, we inserted renal arterial catheters and laser-Doppler/oxygen-sensing probes in the renal cortex and medulla. Noanesthetized sheep were administered intravenous (IV) Escherichia coli and, at sepsis onset, IV tempol (IVT; 30 mg kg-1  h-1 ), renal arterial tempol (RAT; 3 mg kg-1  h-1 ), or vehicle.

RESULTS

Septic sheep receiving vehicle developed renal medullary hypoperfusion (76 ± 16% decrease in perfusion), hypoxia (70 ± 13% decrease in oxygenation), and AKI (87 ± 8% decrease in creatinine clearance) with similar changes during IVT. However, RAT preserved medullary perfusion (1072 ± 307 to 1005 ± 271 units), oxygenation (46 ± 8 to 43 ± 6 mmHg), and creatinine clearance (61 ± 10 to 66 ± 20 mL min-1 ). Plasma, renal medullary, and cortical tissue malonaldehyde and medullary 3-nitrotyrosine decreased significantly with sepsis but were unaffected by IVT or RAT. Consistent with decreased oxidative/nitrosative stress markers, cortical and medullary nuclear factor-erythroid-related factor-2 increased significantly and were unaffected by IVT or RAT. However, RAT prevented sepsis-induced overexpression of cortical tissue tumor necrosis factor alpha (TNF-α; 51 ± 16% decrease; p = 0.003) and medullary Thr-495 phosphorylation of endothelial nitric oxide synthase (eNOS; 63 ± 18% decrease; p = 0.015).

CONCLUSIONS

In ovine Gram-negative sepsis, renal arterial infusion of tempol prevented renal medullary hypoperfusion and hypoxia and AKI and decreased TNF-α expression and uncoupling of eNOS. However, it did not affect markers of oxidative/nitrosative stress, which were significantly decreased by Gram-negative sepsis.

Associations Between Systemic and Cerebral Inflammation in an Ovine Model of Cardiopulmonary Bypass

BACKGROUND

Intraoperative inflammation may contribute to postoperative neurocognitive disorders after cardiac surgery requiring cardiopulmonary bypass (CPB). However, the relative contributions of general anesthesia (GA), surgical site injury, and CPB are unclear.

METHODS

In adult female sheep, we investigated (1) the temporal profile of proinflammatory and anti-inflammatory cytokines and (2) the extent of microglia activation across major cerebral cortical regions during GA and surgical trauma with and without CPB (N = 5/group). Sheep were studied while conscious, during GA and surgical trauma, with and without CPB.

RESULTS

Plasma tumor necrosis factor-alpha (mean [95% confidence intervals], 3.7 [2.5-4.9] vs 1.6 [0.8-2.3] ng/mL; P = .0004) and interleukin-6 levels (4.4 [3.0-5.8] vs 1.6 [0.8-2.3] ng/mL; P = .029) were significantly higher at 1.5 hours, with a further increase in interleukin-6 at 3 hours (7.0 [3.7-10.3] vs 1.8 [1.1-2.6] ng/mL; P < .0001) in animals undergoing CPB compared with those that did not. Although cerebral oxygen saturation was preserved throughout CPB, there was pronounced neuroinflammation as characterized by greater microglia circularity within the frontal cortex of sheep that underwent CPB compared with those that did not (0.34 [0.32-0.37] vs 0.30 [0.29-0.32]; P = .029). Moreover, microglia had fewer branches within the parietal (7.7 [6.5-8.9] vs 10.9 [9.4-12.5]; P = .001) and temporal (7.8 [7.2-8.3] vs 9.9 [8.2-11.7]; P = .020) cortices in sheep that underwent CPB compared with those that did not.

CONCLUSIONS

CPB enhanced the release of proinflammatory cytokines beyond that initiated by GA and surgical trauma. This systemic inflammation was associated with microglial activation across 3 major cerebral cortical regions, with a phagocytic microglia phenotype within the frontal cortex, and an inflammatory microglia phenotype within the parietal and temporal cortices. These data provide direct histopathological evidence of CPB-induced neuroinflammation in a large animal model and provide further mechanistic data on how CPB-induced cerebral inflammation might drive postoperative neurocognitive disorders in humans.

Monitoring kidney size to interpret MRI-based assessment of renal oxygenation in acute pathophysiological scenarios

AIM

Tissue hypoxia is an early key feature of acute kidney injury. Assessment of renal oxygenation using magnetic resonance imaging (MRI) markers T₂ and T₂ * enables insights into renal pathophysiology. This assessment can be confounded by changes in the blood and tubular volume fractions, occurring upon pathological insults. These changes are mirrored by changes in kidney size (KS). Here, we used dynamic MRI to monitor KS for physiological interpretation of T₂ * and T₂ changes in acute pathophysiological scenarios.

METHODS

KS was determined from T₂ *, T₂ mapping in rats. Six interventions that acutely alter renal tissue oxygenation were performed directly within the scanner, including interventions that change the blood and/or tubular volume. A biophysical model was used to estimate changes in O₂ saturation of hemoglobin from changes in T₂ * and KS.

RESULTS

Upon aortic occlusion KS decreased; this correlated with a decrease in T₂ *, T₂ . Upon renal vein occlusion KS increased; this negatively correlated with a decrease in T₂ *, T₂ . Upon simultaneous occlusion of both vessels KS remained unchanged; there was no correlation with decreased T₂ *, T₂ . Hypoxemia induced mild reductions in KS and T₂ *, T₂ . Administration of an X-ray contrast medium induced sustained KS increase, with an initial increase in T₂ *, T₂ followed by a decrease. Furosemide caused T₂ *, T₂ elevation and a minor increase in KS. Model calculations yielded physiologically plausible calibration ratios for T₂ *.

CONCLUSION

Monitoring KS allows physiological interpretation of acute renal oxygenation changes obtained by T₂ *, T₂ . KS monitoring should accompany MRI-oximetry, for new insights into renal pathophysiology and swift translation into human studies.

Continuous bladder urinary oxygen tension as a new tool to monitor medullary oxygenation in the critically ill

Acute kidney injury (AKI) is common in the critically ill. Inadequate renal medullary tissue oxygenation has been linked to its pathogenesis. Moreover, renal medullary tissue hypoxia can be detected before biochemical evidence of AKI in large mammalian models of critical illness. This justifies medullary hypoxia as a pathophysiological biomarker for early detection of impending AKI, thereby providing an opportunity to avert its evolution. Evidence from both animal and human studies supports the view that non-invasively measured bladder urinary oxygen tension (PuO₂) can provide a reliable estimate of renal medullary tissue oxygen tension (tPO₂), which can only be measured invasively. Furthermore, therapies that modify medullary tPO₂ produce corresponding changes in bladder PuO₂. Clinical studies have shown that bladder PuO₂ correlates with cardiac output, and that it increases in response to elevated cardiopulmonary bypass (CPB) flow and mean arterial pressure. Clinical observational studies in patients undergoing cardiac surgery involving CPB have shown that bladder PuO₂ has prognostic value for subsequent AKI. Thus, continuous bladder PuO₂ holds promise as a new clinical tool for monitoring the adequacy of renal medullary oxygenation, with its implications for the recognition and prevention of medullary hypoxia and thus AKI.

Role of Renal Sympathetic Nerve Activity in Volatile Anesthesia's Effect on Renal Excretory Function

Regulation of fluid balance is pivotal during surgery and anesthesia and affects patient morbidity, mortality, and hospital length of stay. Retention of sodium and water is known to occur during surgery but the mechanisms are poorly defined. In this study, we explore how the volatile anesthetic sevoflurane influences renal function by affecting renal sympathetic nerve activity (RSNA). Our results demonstrate that sevoflurane induces renal sodium and water retention during pediatric anesthesia in association with elevated plasma concentration of renin but not arginine-vasopressin. The mechanisms are further explored in conscious and anesthetized ewes where we show that RSNA is increased by sevoflurane compared with when conscious. This is accompanied by renal sodium and water retention and decreased renal blood flow (RBF). Finally, we demonstrate that renal denervation normalizes renal excretory function and improves RBF during sevoflurane anesthesia in sheep. Taken together, this study describes a novel role of the renal sympathetic nerves in regulating renal function and blood flow during sevoflurane anesthesia.

Dynamic responses of renal oxygenation at the onset of cardiopulmonary bypass in sheep and man

INTRODUCTION

The renal medulla is susceptible to hypoxia during cardiopulmonary bypass (CPB), which may contribute to the development of acute kidney injury. But the speed of onset of renal medullary hypoxia remains unknown.

METHODS

We continuously measured renal medullary oxygen tension (MPO₂) in 24 sheep, and urinary PO₂ (UPO₂) as an index of MPO₂ in 92 patients, before and after induction of CPB.

RESULTS

In laterally recumbent sheep with a right thoracotomy (n = 20), even before CPB commenced MPO₂ fell from (mean ± SEM) 52 ± 4 to 41 ±5 mmHg simultaneously with reduced arterial pressure (from 108 ± 5 to 88 ± 5 mmHg). In dorsally recumbent sheep with a medial sternotomy (n = 4), MPO₂ was even more severely reduced (to 12 ± 12 mmHg) before CPB. In laterally recumbent sheep in which a crystalloid prime was used (n = 7), after commencing CPB, MPO₂ fell abruptly to 24 ±6 mmHg within 20-30 minutes. MPO₂ during CPB was not improved by adding donor blood to the prime (n = 13). In patients undergoing cardiac surgery, UPO₂ fell by 4 ± 1 mmHg and mean arterial pressure fell by 7 ± 1 mmHg during the 30 minutes before CPB. UPO₂ then fell by a further 12 ± 2 mmHg during the first 30 minutes of CPB but remained relatively stable for the remaining 24 minutes of observation.

CONCLUSIONS

Renal medullary hypoxia is an early event during CPB. It starts to develop even before CPB, presumably due to a pressure-dependent decrease in renal blood flow. Medullary hypoxia during CPB appears to be promoted by hypotension and is not ameliorated by increasing blood hemoglobin concentration.