Renal function and intrarenal hemodynamics in acutely hypoxic and hypercapnic rats

Pathophysiology of Red Blood Cell Trapping in Ischemic Acute Kidney Injury

Red blood cell (RBC) trapping describes the accumulation of RBCs in the microvasculature of the kidney outer medulla that occurs following ischemic acute kidney injury (AKI). Despite its prominence in human kidneys following AKI, as well as evidence from experimental models demonstrating that the severity of RBC trapping is directly correlated with renal recovery, to date, RBC trapping has not been a primary focus in understanding the pathogenesis of ischemic kidney injury. New evidence from rodent models suggests that RBC trapping is responsible for much of the tubular injury occurring in the initial hours after kidney reperfusion from ischemia. This early injury appears to result from RBC cytotoxicity and closely reflects the injury profile observed in human kidneys, including sloughing of the medullary tubules and the formation of heme casts in the distal tubules. In this review, we discuss what is currently known about RBC trapping. We conclude that RBC trapping is likely avoidable. The primary causes of RBC trapping are thought to include rheologic alterations, blood coagulation, tubular cell swelling, and increased vascular permeability; however, new data indicate that a mismatch in blood flow between the cortex and medulla where medullary perfusion is maintained during cortical ischemia is also likely critical. The mechanism(s) by which RBC trapping contributes to renal functional decline require more investigation. We propose a renewed focus on the mechanisms mediating RBC trapping, and RBC trapping-associated injury is likely to provide important knowledge for improving AKI outcomes. © 2024 American Physiological Society. Compr Physiol 14:5325-5343, 2024.

Influence of arterial blood gases on the renal arterial resistive index in intensive care unit

BACKGROUND

Renal artery Doppler sonography with resistive index (RI) determination is a noninvasive, fast, and reliable diagnostic tool increasingly used in the intensive care unit (ICU) to predict and assess the reversibility of acute kidney injury (AKI). However, interpreting the RI can be challenging due to numerous influencing factors. While some studies have explored various confounding factors, arterial blood gases have received limited attention. Therefore, our study aims to evaluate the impact of arterial blood gases on the RI in the ICU setting.

METHODS

This prospective observational study enrolled ICU patients who required blood gas analysis and had not experienced significant hemodynamic changes recently. The RI was measured using standardized Doppler ultrasound within an hour of the arterial blood gases sampling and analysis.

RESULTS

A total of sixty-four patients were included in the analysis. Univariate analysis revealed a correlation between the RI and several variables, including PaCO2 (R = 0.270, p = 0.03), age (R = 0.574, p < 0.0001), diastolic arterial pressure (DAP) (R = - 0.368, p = 0.0028), and SaO2 (R = - 0.284, p = 0.0231). Multivariate analysis confirmed that age > 58 years and PaCO2 were significant factors influencing the RI, with respective odds ratios of 18.67 (p = 0.0003) and 1.132 (p = 0.0267).

CONCLUSION

The interpretation of renal arterial RI should take into account thresholds for PaCO2, age, and diastolic arterial pressure. Further studies are needed to develop a comprehensive scoring system that incorporates all these cofactors for a reliable analysis of RI levels. Trial registration This observational study, registered under number 70-0914, received approval from local Ethical Committee of Toulouse University Hospital.

Organ Crosstalk in Acute Kidney Injury: Evidence and Mechanisms

Acute kidney injury (AKI) is becoming a public health problem worldwide. AKI is usually considered a complication of lung, heart, liver, gut, and brain disease, but recent findings have supported that injured kidney can also cause dysfunction of other organs, suggesting organ crosstalk existence in AKI. However, the organ crosstalk in AKI and the underlying mechanisms have not been broadly reviewed or fully investigated. In this review, we summarize recent clinical and laboratory findings of organ crosstalk in AKI and highlight the related molecular mechanisms. Moreover, their crosstalk involves inflammatory and immune responses, hemodynamic change, fluid homeostasis, hormone secretion, nerve reflex regulation, uremic toxin, and oxidative stress. Our review provides important clues for the intervention for AKI and investigates important therapeutic potential from a new perspective.

Ischemic Renal Injury: Can Renal Anatomy and Associated Vascular Congestion Explain Why the Medulla and Not the Cortex Is Where the Trouble Starts?

The kidneys receive approximately 20% of cardiac output and have a low fractional oxygen extraction. Quite paradoxically, however, the kidneys are highly susceptible to ischemic injury (injury associated with inadequate blood supply), which is most evident in the renal medulla. The predominant proposal to explain this susceptibility has been a mismatch between oxygen supply and metabolic demand. It has been proposed that unlike the well-perfused renal cortex, the renal medulla normally operates just above the threshold for hypoxia and that further reductions in renal perfusion cause hypoxic injury in this metabolically active region. An alternative proposal is that the true cause of ischemic injury is not a simple mismatch between medullary metabolic demand and oxygen supply, but rather the susceptibility of the outer medulla to vascular congestion. The capillary plexus of the renal outer medullary region is especially prone to vascular congestion during periods of ischemia. It is the failure to restore the circulation to the outer medulla that mediates complete and prolonged ischemia to much of this region, leading to injury and tubular cell death. We suggest that greater emphasis on developing clinically useful methods to help prevent or reverse the congestion of the renal medullary vasculature may provide a means to reduce the incidence and cost of acute kidney injury.

Renal and segmental artery hemodynamic response to acute, mild hypercapnia

Profound increases (>15 mmHg) in arterial carbon dioxide (i.e., hypercapnia) reduce renal blood flow. However, a relatively brief and mild hypercapnia can occur in patients with sleep apnea or in those receiving supplemental oxygen therapy during an acute exacerbation of chronic obstructive pulmonary disease. We tested the hypothesis that a brief, mild hypercapnic exposure increases vascular resistance in the renal and segmental arteries. Blood velocity in 14 healthy adults (26 ± 4 yr; 7 women, 7 men) was measured in the renal and segmental arteries with Doppler ultrasound while subjects breathed room air (Air) and while they breathed a 3% CO₂, 21% O₂, 76% N₂ gas mixture for 5 min (CO₂). The end-tidal partial pressure of CO₂ ([Formula: see text]) was measured via capnography. Mean arterial pressure (MAP) was measured beat to beat via the Penaz method. Vascular resistance in the renal and segmental arteries was calculated as MAP divided by blood velocity. [Formula: see text] increased with CO₂ (Air: 45 ± 3, CO₂: 48 ± 3 mmHg, P < 0.01), but there were no changes in MAP (P = 0.77). CO₂ decreased blood velocity in the renal (Air: 35.2 ± 8.1, CO₂: 32.2 ± 7.3 cm/s, P < 0.01) and segmental (Air: 24.2 ± 5.1, CO₂: 21.8 ± 4.2 cm/s, P < 0.01) arteries and increased vascular resistance in the renal (Air: 2.7 ± 0.9, CO₂: 3.0 ± 0.9 mmHg·cm^-1·s, P < 0.01) and segmental (Air: 3.9 ± 1.0, CO₂: 4.4 ± 1.0 mmHg·cm^-1·s, P < 0.01) arteries. These data provide evidence that the kidneys are hemodynamically responsive to a mild and acute hypercapnic stimulus in healthy humans.

Acute Kidney Injury in Mechanically Ventilated Patients: The Risk Factor Profile Depends on the Timing of Aki Onset

BACKGROUND

Acute kidney injury (AKI) is a frequent complication in patients under mechanical ventilation (MV). We aimed to assess the risk factors for AKI with particular emphasis on those potentially preventable.

STUDY DESIGN, SETTING, AND PARTICIPANTS

Retrospective analysis of a large, multinational database of MV patients with >24 h of MV and normal renal function at admission. AKI was defined according to creatinine-based KDIGO criteria. Risk factors were analyzed according to the time point at which AKI occurred: early (≤48 h after ICU admission, AKIE) and late (day 3 to day 7 of ICU stay, AKIL). A conditional logistic regression model was used to identify variables independently associated with AKI.

RESULTS

Three thousand two hundred six patients were included. Seven hundred patients had AKI (22%), the majority of them AKIE (547/704). The risk factor profile was highly dependent upon the timing of AKI onset. In AKIE risk factors were older age; SAPS II score; postoperative and cardiac arrest as the reasons for MV; worse cardiovascular SOFA, pH, serum creatinine, and platelet count; higher level of peak pressure and Vt/kg; and fluid overload at admission. In contrast, AKIL was linked mostly to events that occurred after admission (lower platelet count and pH; ICU-acquired sepsis; and fluid overload). None ventilation-associated parameters were identify as risk factors for AKIL.

CONCLUSIONS

In the first 48 h, risk factors are associated with the primary disease and the patient's condition at admission. Subsequently, emergent events like sepsis and organ dysfunction appear to be predictive factors making prevention a challenge.

[Pulmonary-renal crosstalk in the critically ill patient]

Despite advances in the development of renal replacement therapy, mortality of acute renal failure remains high, especially when occurring simultaneously with distant organic failure as it is in the case of the acute respiratory distress syndrome. In this update, birideccional deleterious relationship between lung and kidney on the setting of organ dysfunction is reviewed, which presents important clinical aspects of knowing. Specifically, the renal effects of acute respiratory distress syndrome and the use of positive-pressure mechanical ventilation are discussed, being ventilator induced lung injury one of the most common models for studying the lung-kidney crosstalk. The role of renal failure induced by mechanical ventilation (ventilator-induced kidney injury) in the pathogenesis of acute renal failure is emphasized. We also analyze the impact of the acute renal failure in the lung, recognizing an increase in pulmonary vascular permeability, inflammation, and alteration of sodium and water channels in the alveolar epithelial. This conceptual model can be the basis for the development of new therapeutic strategies to use in patients with multiple organ dysfunction syndrome.

Kidney-lung cross-talk and acute kidney injury

There is a growing appreciation for the role that acute kidney injury (AKI) plays in the propagation of critical illness. In children, AKI is not only an independent predictor of morbidity and mortality, but is also associated with especially negative outcomes when concurrent with acute lung injury (ALI). Experimental data provide evidence that kidney-lung crosstalk occurs and can be bidirectionally deleterious, although details of the precise molecular mechanisms involved in the AKI-ALI interaction remain incomplete. Clinically, ALI, and the subsequent clinical interventions used to stabilize gas exchange, carry consequences for the homeostasis of kidney function. Meanwhile, AKI negatively affects lung physiology significantly by altering the homeostasis of fluid balance, acid-base balance, and vascular tone. Experimental AKI research supports an "endocrine" role for the kidney, triggering a cascade of extra-renal inflammatory responses affecting lung homeostasis. In this review, we will discuss the pathophysiology of kidney-lung crosstalk, the multiple pathways by which AKI affects kidney-lung homeostasis, and discuss how these phenomena may be unique in critically ill children. Understanding how AKI may affect a "balance of communication" that exists between the kidneys and the lungs is requisite when managing critically ill children, in whom imbalance is the norm.

Impact of mild hypoxemia on renal function and renal resistive index during mechanical ventilation

RATIONALE

Short-term hypoxemia affects diuresis and natriuresis in healthy individuals. No data are available on the impact of the mild hypoxemia levels usually tolerated in critically ill patients receiving mechanical ventilation.

OBJECTIVES

To assess the renal effects of mild hypoxemia during mechanical ventilation for acute lung injury (ALI).

METHODS

Prospective, physiological study in 12 mechanically ventilated patients with ALI. Patients were studied at baseline with an arterial saturation (SaO(2)) of 96% [94-98] then a comparison was performed between SaO(2) values of 88-90% (mild hypoxemia) and 98-99% (high oxygenation).

MAIN RESULTS

FiO(2) was set at 0.25 [0.23-0.32] and 0.7 [0.63-0.8], respectively, to obtain SaO(2) of 89 [89-90] and 99% [98-99]. Hemodynamic or respiratory parameters were not significantly affected by FiO(2) levels. Compared with high oxygenation level, mild hypoxemia using low FiO(2) was associated with increase in diuresis (median [interquartile range], 67 [55-105] vs. 55 [45-60] ml/h; P = 0.003) and in doppler-based renal resistive index (RI) (0.78 [0.66-0.85] vs. 0.72 [0.60-0.78]; P = 0.003). The 2-h calculated creatinine clearance also increased (63 [46-103] vs. 35 [30-85] ml/min; P = 0.005) without change in urinary creatinine (P = 0.13). No significant change in natriuresis was observed. Half of the patients were under norepinephrine infusion and the renal response did not differ according to the presence of vasopressors.

CONCLUSION

In patients with ALI, mild hypoxemia related to short-term low FiO(2) induce increases in diuresis and in renal RI. This latter point suggests intra-renal mechanisms that need to be further investigated.

Mechanical ventilation and acute renal failure

OBJECTIVE

To review the current literature on possible mechanisms by which mechanical ventilation may initiate or aggravate acute renal failure.

DATA SOURCE

A Medline database and references from identified articles were used to perform a literature search relating to mechanical ventilation and acute renal failure.

DATA SYNTHESIS

Acute renal failure may be initiated or aggravated by mechanical ventilation through three different mechanisms. First, strategies such as permissive hypercapnia or permissive hypoxemia may compromise renal blood flow. Second, through effects on cardiac output, mechanical ventilation affects systemic and renal hemodynamics. Third, mechanical ventilation may cause biotrauma-a pulmonary inflammatory reaction that may generate systemic release of inflammatory mediators. The harmful effects of mechanical ventilation may become more significant when a comorbidity is present. In these situations, it is more difficult to maintain normal gas exchange, and moderate arterial hypoxemia and hypercapnia are often accepted. Renal blood flow is compromised due to a decreased cardiac output as a consequence of high intrathoracic pressures. Furthermore, the effects of biotrauma are not limited to the lungs but may lead to a systemic inflammatory reaction.

CONCLUSIONS

The development of acute renal failure during mechanical ventilation likely represents a multifactorial process that may become more important in the presence of comorbidities. Development of optimal interventional strategies requires an understanding of physiologic principles and greater insight into the precise molecular and cellular mechanisms that may also play a role.

Effect of laparoscopic antireflux surgery upon renal blood flow

BACKGROUND

Hypercapnia and local pressure effects unique to CO(2) base minimally invasive surgery alter renal blood flow. We have demonstrated laparoscopic antireflux surgery to have an additional impact upon hemodynamics (decreased cardiac output), potentially extending known effects upon renal blood flow.

METHODS

We measured renal blood flow with radioactive microspheres during laparoscopic antireflux surgery in a porcine model. Six pigs were anesthetized, monitoring lines were placed, and microspheres injected five time points associated with a laparoscopic antireflux operation. After euthanasia kidneys were retrieved and fixed, and representative samples counted for radioactivity specific for each of the five time points.

RESULTS

The greatest reduction in renal blood flow was 36% below baseline (p<0.05). Concurrently, cardiac output had a maximum reduction of 39%.

CONCLUSIONS

Laparoscopic Nissen fundoplication in this pig model is associated with a significant reduction in renal blood flow, probably related to reduction in cardiac output. Caution is warranted when considering laparoscopic antireflux surgery in patients with a compromised renal blood flow.

Differential control of intrarenal blood flow during reflex increases in sympathetic nerve activity

The role of renal sympathetic nerve activity (RSNA) in the physiological regulation of medullary blood flow (MBF) remains ill defined, yet regulation of MBF may be crucial to long-term arterial pressure regulation. To investigate the effects of reflex increases in RSNA on intrarenal blood flow distribution, we exposed pentobarbital sodium-anesthetized, artificially ventilated rabbits (n = 7) to progressive hypoxia while recording RSNA, cortical blood flow (CBF), and MBF using laser-Doppler flowmetry. Another group of animals with denervated kidneys (n = 6) underwent the same protocol. Progressive hypoxia (from room air to 16, 14, 12, and 10% inspired O(2)) significantly reduced arterial oxygen partial pressure (from 99 +/- 3 to 65 +/- 2, 51 +/- 2, 41 +/- 1, and 39 +/- 2 mmHg, respectively) and significantly increased RSNA (by 8 +/- 3, 44 +/- 25, 62 +/- 21, and 76 +/- 37%, respectively, compared with room air) without affecting mean arterial pressure. There were significant reductions in CBF (by 2 +/- 1, 5 +/- 2, 11 +/- 3, and 14 +/- 2%, respectively) in intact but not denervated rabbits. MBF was unaffected by hypoxia in either group. Thus moderate reflex increases in RSNA cause renal cortical vasoconstriction, but not at vascular sites regulating MBF.

Methods of renal blood flow measurement

Variations in regional renal blood flow have been implicated in a variety of disease states. Many techniques have been developed in an attempt to accurately assess these changes. The microsphere technique is the most widely used method at the present time. This technique allows focal measurements to be performed, but there is a conflict between the resolution of the method and the number of microspheres necessary in each sample. New imaging techniques such as tomography and autoradiography enable visual assessment of renal blood flow. Though there is no ideal method, these techniques have opened up new possibilities in the quantification of regional renal blood flow.

The effects of systemic hypoxia on renal function in the anaesthetized rat

1. In rats anaesthetized with Saffan, renal function was monitored from the left kidney from the 5th minute of spontaneous breathing of 12% O2 for two 20 min periods and during air breathing before, between and after the hypoxic periods. Two groups of animals (I and II) were used, each group comprising two subgroups in which the left kidney was innervated or denervated, respectively; in Group II, renal perfusion pressure (RPP) was maintained during the 2nd hypoxic period by occl97uding the distal aorta. 2. In both subgroups of Group I, both hypoxic periods produced hyperventilation, arterial PO2 falling to approximately 50 mmHg. Concomitantly, mean arterial pressure (MABP) fell by similar extents (approximately 23%, from a baseline level of 140 mmHg during the 2nd hypoxic period). In the innervated subgroup, renal vascular conductance (RVC) increased, but glomerular filtration rate (GFR) fell (by 48 and 6%, respectively, during the 2nd hypoxic period), while urine flow, absolute sodium excretion (UNaV) and fractional sodium excretion (FENa) fell (by 52, 63 and 61%, respectively). Baseline urine flow, UNaV and FENa were higher in the denervated subgroup, but hypoxia produced similar percentage changes from baseline in all variables. 3. In Group II, both subgroups showed similar changes during the 1st hypoxic period as the corresponding subgroups of Group I. However, during the 2nd hypoxic period when the fall in MABP was reduced to approximately 7%, the increase in RVC persisted only in the denervated subgroup; there was no significant change in GFR, urine flow, UNaV or FENa in either subgroup. 4. These results indicate that, in the rat, moderate hypoxia produces antidiuresis and antinatriuresis that are not dependent on the renal nerves, but are dependent on the hypoxia-induced fall in MABP. The fall in renal perfusion pressure (RPP) may directly determine renal function, but reflex influences upon the kidney initiated by, for example, arterial baroreceptor unloading, may play a role. The fall in GFR and increase in RVC, which persisted after denervation or when renal perfusion was controlled, implies a local dilatatory influence acting preferentially on the efferent arterioles.

Direct renal effects of endothelin in chronic hypoxic spontaneously hypertensive rats

1. The direct renal effects of endothelin (ET) were studied in eight chronic hypoxic rats (HA) and eight sea level (SL) spontaneously hypertensive rats (SHR). 2. After 4 weeks of exposure to simulated 5486 m (18,000 ft) hypoxia, all HA rats were in apparently good health, and baseline renal function, except effective renal blood flow, was not significantly different from SL rats. 3. Intrarenal arterial administration of ET (600 ng/kg per h) reduced ipsilateral renal excretion of water, sodium and potassium, glomerular filtration rate and effective renal plasma flow in both SL and HA rats to almost the same extent. 4. Administration of ET antiserum, however, increased the renal excretion of water in HA rats. 5. It is concluded that ET may play a role in the renal regulation of chronic hypoxic SHR.