04RC1 Pathophysiology, diagnosis and treatment of peri-operative acute renal failure Fabio Guarracino, Rubia Baldassarri Cardiothoracic Intensive Care Medicine and Anaesthesia, Azienda Ospedaliera Universitaria Pisana, Pisa, Italy

Saturday, June 12, 2010 13:00-13:45

Room: 208

Introduction Acute kidney injury (AKI), previously referred to as acute renal failure (ARF), can be defined as the sudden and sustained fall in glomerular filtration rate with consequent accumulation of catabolites and body water [1-3]. AKI is one of the most frequent peri-operative complications in patients undergoing major surgery (10-23%) and it is associated with high morbidity and mortality that can exceed 60% in patients requiring dialysis. Predisposing factors include pre-existing renal damage, associated co- morbidities, hypovolaemia and sepsis. An increased risk in developing AKI has been demonstrated in patients exposed to radiocontrast dye, in patients undergoing cardiac surgery, especially those that requre cardiopulmonary by-pass, in mechanically ventilated patients and in critically ill patients [4-7]. Despite significant improvement in diagnostic and therapeutic approaches, that allows early evaluation of renal dysfunction and prompt treatment, AKI remains a serious clinical challenge. One of the major limitations in the early diagnosis is the lack of predictive markers, similar to the use of serum troponin in myocardial ischaemia, that can help to identify progressive renal impairment. Commonly AKI is associated with an increase in serum creatinine and blood urea levels.

Definition A uniform definition of AKI is not available because of a lack of consensus [7, 8]. Several definitions exist. Definitions according to a grading system of evaluation of renal function have been recently proposed by the AKIN (Acute Kidney Injury Network). Among them the Second International Consensus Conference of the Acute Dialysis Quality Initiative has suggested a new classification for AKI called RIFLE (Figure 1) [9]. The acronym stands for Risk of renal dysfunction, Injury to the kidney, Failure of kidney function, Loss of kidney function and End-stage renal disease. It is a multi-level classification that incorporates many different factors connected with renal dysfunction. It matches the decreasing rate of glomerular filtration and associated increase in serum creatinine levels with the fall in urine output [8], in an attempt to standardize the definition of AKI across the scientific community by classifying renal dysfunction on the base of serum creatinine changes and urine output.

Pathophysiology of acute renal failure On the basis of aetiology and pathophysiological mechanisms, AKI has been divided into three groups: pre-renal (30-60%), renal or intrinsic (20-40%) and post-renal or obstructive (1-10%) [10]. Under normal conditions the kidney blood flow is about 20% of the total cardiac output (~ 1 l/min) with an oxygen delivery of about 80 ml/min for every 100 gm of tissue. The blood flow distribution in the kidney is not uniform; it is most prominent in the cortex (> 90% of the total blood flow). At the same time, the oxygen consumption is ~ 10% of the total body oxygen extraction. Thus, renal oxygen extraction is very low and suggests that the kidney has a wide oxygen reserve. Despite high perfusion and a low fraction of oxygen extraction, the kidney is extremely sensitive to hypoperfusion and hypotension. This is due to the different distribution of blood flow and oxygen extraction in the two portions of the kidney. The cortex is better perfused but less oxygenated than the medulla. The medulla requires about 80% of the total oxygen supply to support tubular reabsorption of sodium and chloride. A decrease in blood perfusion and oxygen delivery is poorly tolerated by the medulla with consequent ischaemia and acute tubular necrosis even if perfusion decreases by 40-50%. The main determinant of medullary oxygen requirement is the rate of tubular reabsorptive function of sodium and water. Several mediators (vasodilators, vasoconstrictors, diuretics, tubulo-glomerular feedback) can affect renal perfusion and increase sensitivity to ischaemia.

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Figure 1

In the critically ill patients and in the peri-operative setting, acute renal dysfunction is most often initially due to a pre-renal injury caused by renal hypoperfusion secondary to systemic hypotension. Absolute or relative hypovolemia, severe sepsis or septic shock, cardiogenic shock, and cardiac tamponade are some of the major predisposing factors of medullary ischaemia and acute tubular necrosis. Irrespective of the mechanism, once AKI develops renal impairment itself and other organ dysfunction follows.

Diagnosis In current clinical practice AKI continues to be defined and diagnosed only in terms of changes in serum creatinine levels and urine output measurement. However, it must be remembered that serum creatinine rises slower and later than glomerular filtration rate decreases because of a large intrinsic renal reserve. Serum creatinine levels change significantly from the baseline only when the glomerular filtration rate decreases by least of 50%, and the raise in serum creatinine is evident 48-72 h after the initial renal injury [11]. In addition, it is well known that serum creatinine can be influenced by many extra-renal factors such as age, gender, race, metabolism, muscle mass and that it can be altered in various clinical settings. Serum creatinine can increase in trauma, fever and immobilization or decrease when liver dysfunction or reduction in muscle mass occur [11, 12]. Urine output measurement can be influenced by extra-renal factors such as fluid balance or post-operative endocrine alterations. Early diagnosis of AKI is mandatory to permit prompt and adequate treatment. Recently, in an effort to evaluate biomarkers that correlate with renal function and that could be easily and reliably detected after renal injury, several biochemical markers of renal impairment have been identified. An adequate biomarker for clinical application should be easily and routinely detectable at the bedside or in the clinical laboratory by sampling blood or urine; it should be non-invasive and reliable; it should be highly sensitive to allow an early

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diagnosis and it should have a wide cut-off value to perform risk stratification [12]. The evaluation of clinical biomarkers is also helpful in determining: 1. the primary location of the kidney injury (proximal or distal tubuli, interstitium or vasculature); 2. the type and modality of development of the renal dysfunction (AKI, acute-on-chronic failure); 3. the pathogenesis of the injury (hypoxaemia, toxins, sepsis); 4. risk stratification and prognosis (duration of disease, length of stay in ICU, need for dialysis, mortality); and 5. response to treatment. In the last few years several new biomarkers have been identified, while previously known markers for detecting renal dysfunction have been further investigated. Although there have been more than 20 unique biomarkers of AKI identified or under investigation, most of the current interest has focused on Cystatin C and neutrophil gelatinase–associated lipocalin (NGAL) [8, 11, 12]. Cystatin C Cystatin C is a cysteine protease inhibitor that is synthesized by all nucleated cells. Its blood levels are relatively constant because it is completely filtered by the glomerulus, reabsorbed at the proximal tubule level and not secreted. Cystatin C blood levels are not influenced by extra-renal factors such as age, gender or others physical or metabolic affections. It is considered a significant index of altered glomerular filtration, better than serum creatinine, in patients affected by chronic renal disease and a strong predictor of AKI. Several investigations have demonstrated that serum Cystatin C increases earlier than serum creatinine levels but later than NGAL [13, 14]. However, in a published study Cystatin C did not outperform serum creatinine in the early diagnosis of AKI [15]. In summary, Cystatin C can be considered a significant marker of reduction in glomerular filtration but not an index of kidney injury. It is very sensitive marker of injury when the glomerular filtration is affected, but it cannot distinguish among different kinds of injury. Cystatin C levels are available in a few minutes and easy to obtain; in addition Cystatin C blood levels are not influenced by routine clinical storage conditions, freeze/thaw cycles, the presence of interfering substances, and the pathogenesis of the AKI. Neutrophil gelatinase–associated lipocalin Human NGAL is an immunological protein covalently bound to gelatinase from neutrophils. In healthy people it is expressed at very low levels in several human tissues (kidney, lungs, stomach, colon) and its concentration rises after tissue injury as expression of epithelium damage. NGAL concentrations are elevated in patients with acute bacterial infections, the sputum of subjects with asthma or chronic obstructive pulmonary disease, and the bronchial fluid from the lungs of emphysematous patients. Several investigations in different populations of critically ill patients affected by established AKI documented an increase of 50% of the baseline in serum creatinine levels, and demonstrated an increase in serum and urine NGAL levels compared with normal values. In a cross-sectional study of adults in ICU with established AKI (defined as a doubling of the serum creatinine in less than 5 days) secondary to sepsis, ischaemia, or nephrotoxins, there was a greater than ten-fold increase in plasma NGAL and a greater than 100-fold increase in urine NGAL by Western blotting when compared with normal controls [16, 17]. Both plasma and urine NGAL correlated well with serum creatinine levels. These results identified NGAL as a widespread and sensitive response to established AKI in humans. In a prospective study of children undergoing cardiopulmonary bypass, AKI (defined as a 50% increase in serum creatinine) occurred in 28% of subjects, but the diagnosis using serum creatinine was only possible 1–3 days after surgery [18]. In marked contrast, NGAL measurements by Western blotting and by enzyme-linked immunosorbent assay (ELISA) revealed a robust ten-fold or greater increase in the urine and plasma within 2–6 h of surgery in patients who subsequently developed AKI. It should be emphasized that paediatric patients generally do not have associated pathologies such as diabetes, hypertension and atherosclerosis that could influence NGAL expression. Nevertheless these results have been confirmed in a prospective study of adult patients undergoing cardiac surgery who developed postoperative AKI, identified as increase in serum creatinine levels of more than 50% of the baseline, after the third postoperative day. Urinary NGAL levels rose and were measured in the first three postoperative hours. However, patients undergoing cardiac surgery that did not develop postoperative AKI have displayed elevated NGAL concentration in the urine in the early postoperative period, although NGAL values have been less elevated than those evaluated in patients who subsequently developed AKI. NGAL was recently identified as one of the earliest and most robustly induced genes and proteins in the kidney after ischaemic or nephrotoxic injury in animal models, and NGAL protein was easily detected in the blood and urine soon after AKI.

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In summary, the importance of NGAL as biomarker in AKI has been increasing. It seems to be a highly sensitive predictor of AKI with a good potential for early diagnosis. NGAL serum and urinary concentrations can be evaluated bedside in few minutes. However, the role of NGAL should be further investigated in view of the fact that NGAL measurements can be influenced by several factors such as pre-existing renal disease and systemic or urinary tract infections. Nevertheless, NGAL has a significant role in early detection of AKI and as predictive biomarker of AKI.

Treatment of peri-operative acute renal failure Early treatment of AKI is crucial to reduce severity, prevent other organ secondary dysfunction, and reverse pathophysiological mechanisms. Based on pathophysiology, early management of AKI should aim at maintaining adequate volaemia and systemic flow in order to restore kidney perfusion pressure. Therefore, a proper haemodynamic diagnostic and monitoring approach is required to investigate the adequateness of circulating volume and cardiac output, and to decide on cardio- and vaso-active drug administration. Weak data are available to support the use of any drug with renal protective effect. Diuretics are useful to control volume conditions in patients with reduced urine output. Loop diuretics are more commonly used for their rapid onset. When a patient suffering from AKI experiences a marked reduction in urine output (oliguria or anuria), fluid overload, negative metabolic changes such as increased creatinine and uraemia, or acidaemia and hyperkalaemia, renal replacement therapy (RRT) is needed. In severe sepsis and septic shock high flow RRT is also indicated to treat hyperthermia and to remove inflammatory mediators and endotoxins. Continuous RRT is usually preferred in ICU patients due to less aggressive haemodynamic impact and prolonged removal of impurities (cytokines, inflammatory mediators, urea, etc.) from the blood.

Conclusions Peri-operative acute renal failure carries high mortality in surgical patients. It is a challenging syndrome to diagnose and treat. Proper knowledge of predisposing factors and pathophysiology is of great help in understanding the mechanism, assisting early diagnosis and guiding management.

Key learning points • • •

Renal hypoperfusion plays a major role in the pathophysiology of AKI in the surgical patient Early detection based on clinical evaluation and biomarkers is crucial to prevent further worsening of kidney function and other organs involvement Aggressive management, based on circulating volume optimisation, systemic flow adjustment, diuretics and RRT is advocated

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