PERIOPERATIVE ACUTE KIDNEY INJURY (AKI)

INTRODUCTION

I. DEFINITIONS AND STAGING CRITERIA

RIFLE Criteria (Acute Dialysis Quality Initiative)

KDIGO (Kidney Disease: Improving Global Outcomes) Staging

The KDIGO definition focuses on relative and absolute changes in creatinine from baseline. Even small rises in creatinine (0.3 mg/dL) are associated with increased mortality - hence the shift away from requiring dialysis to define AKI.

II. PATHOPHYSIOLOGY

A. Prerenal AKI

• Caused by decreased renal perfusion pressure
• Reversible if identified early
• Mechanism: reduced cardiac output, hypovolemia, vasodilation, renal artery disease
• Characterized by: oliguria (<0.5 mL/kg/h), high urine osmolality, low urinary sodium, BUN:creatinine ratio >20:1

B. Intrinsic/Intrarenal AKI - Acute Tubular Necrosis (ATN)

• Interruption of blood flow >30-60 minutes leads to irreversible cell damage
• The kidneys receive 1000-1250 mL/min (3-5 mL/min/g of tissue) - far exceeding intrinsic O2 requirement
• The renal cortex (contains most glomeruli, depends on oxidative metabolism) is first injured - particularly the pars recta of proximal tubules
• As ischemia progresses: glycogen consumed, medulla (relies on glycolysis) becomes increasingly affected
• Early: reversible cell organelle swelling (especially mitochondria)
• Late: ATP depletion → Na-K-ATPase failure → cellular swelling → irreversible necrosis

• Aminoglycosides, cisplatin, radiocontrast dye, pigments (hemoglobin, myoglobin)
• No preceding oliguria (sudden insult, unlike ischemic ATN)
• Patients with prerenal azotemia are at markedly increased risk for toxic ATN

• I-R generates reactive oxygen species (ROS) via xanthine oxidase and NADPH oxidase
• Activates complement, recruits neutrophils, triggers inflammatory cascade
• Tubular epithelial cell apoptosis and necrosis
• Tubular obstruction from shed cellular debris and casts

C. Postrenal AKI

• Obstruction of urine flow (urolithiasis, BPH, malignancy, retroperitoneal mass)
• Must be excluded early with bladder catheterization and ultrasound

D. Multifactorial Nature

III. RISK FACTORS

A. Patient-Related (Preoperative) Risk Factors

B. Surgical Risk Factors

• Cardiac surgery with cardiopulmonary bypass (CPB): 5% develop AKI requiring dialysis after CABG
• Major aortic reconstructive surgery: infrarenal cross-clamping affects GFR via neurohormonal mechanisms
• Supraceliac aortic cross-clamping: direct compromise of renal artery flow
• Major abdominal, hepatic, and transplant surgeries
• Prolonged CPB duration, use of multiple inotropes, intra-aortic balloon pump insertion

C. Intraoperative Risk Factors

• Intraoperative hypotension - even transient, is a major modifiable risk factor
• Hypovolemia/volume depletion (especially in fasted patients)
• Anemia/extreme hemodilution (hematocrit <20% during CPB)
• Nephrotoxic drug exposure (aminoglycosides, NSAIDs, contrast agents, ACE inhibitors)
• Aprotinin, hetastarch (perioperative medication exposures)
• Prolonged use of vasopressors, sepsis

IV. MONITORING AND DIAGNOSIS

Conventional Markers (Limitations)

• Gold standard clinically, but lags behind actual injury by 24-48 hours
• Affected by muscle mass, age, sex, diet, dilution from large fluid shifts
• Intraoperative dilution from blood/fluid loss can artificially lower creatinine, masking early AKI

• Target: >0.5 mL/kg/h intraoperatively
• Urine output <0.3 mL/kg/h during major abdominal surgery is associated with creatinine-based AKI
• However, the 0.3-0.5 mL/kg/h range was NOT associated with creatinine-based AKI in major studies
• Intraoperative urine output has not been validated as a reliable measure of postoperative renal dysfunction risk

Novel Biomarkers

• Cystatin C: produced by all nucleated cells at constant rate; theoretically superior to creatinine for mild CKD and early AKI detection; however, sensitivity is inconsistent and conditions such as malignancy and steroid therapy elevate it without renal impairment
• penKid (proenkephalin A 119-159): correlated with GFR; elevated levels in critical illness associated with adverse outcomes; role in perioperative setting still developing

• NGAL (Neutrophil Gelatinase-Associated Lipocalin): urine and plasma; rises within 2 hours of injury
• KIM-1 (Kidney Injury Molecule-1): proximal tubule injury
• L-FABP (Liver-type Fatty Acid Binding Protein): tubular stress
• IGFBP7 x TIMP-2: cell cycle arrest markers; FDA-cleared (NephroCheck); predict development of AKI within 12 hours

The key advantage of novel biomarkers is detecting injury at the time of insult rather than after GFR decline - analogous to troponin in myocardial infarction - though their routine clinical role in perioperative settings is still an area of active investigation.

V. PREVENTION OF PERIOPERATIVE AKI

A. Preoperative Optimization

1. Identify and stratify risk: document baseline creatinine, proteinuria, CKD staging
2. Optimize volume status: ensure adequate hydration preoperatively - hypovolemia + diabetes mellitus increases AKI risk 100-fold
3. Withhold nephrotoxins: NSAIDs, ACE inhibitors/ARBs, contrast agents (if avoidable)
4. Correct anemia: severe anemia worsens renal oxygen delivery
5. Control blood glucose: diabetic nephropathy is a strong risk factor

B. Intraoperative Management

• Avoid sustained hypotension - maintain MAP near patient's individual baseline (within 10%)
• Two large multicenter RCTs comparing MAP >75 vs. >60 mm Hg showed no significant difference in outcomes - optimal targets remain debated
• Continuous invasive arterial pressure monitoring for procedures with anticipated rapid hemodynamic changes

• Use balanced crystalloid solutions (Plasma-Lyte, Ringer's lactate) - avoid 0.9% normal saline which causes hyperchloremic metabolic acidosis, associated with worse renal outcomes
• Avoid hypovolemia (preoperative fasting patients are already depleted)
• Avoid volume overload - excess fluid is an independent risk factor for adverse outcomes in AKI
• Caution with goal-directed therapy (GDT): restrictive fluid protocols in GDT have been associated with increased AKI risk

• When hypotension is refractory to volume, use vasopressors to maintain MAP
• Norepinephrine vs. phenylephrine: a recent pragmatic trial showed no difference in AKI outcomes between the two
• Continuous vasopressor infusion gaining popularity to correct anesthesia-induced vasodilation

• Avoid severe arterial hypoxemia (PaO2 <40 mmHg causes renal vasoconstriction)
• Manage hemodilution on CPB: hematocrit <20% associated with AKI; moderate hemodilution (Hct 20-30%) is acceptable
• Follow STS/SCA guidelines: conservative transfusion triggers except in patients at risk for end-organ ischemia

• Propofol: increases BMP-7 production, suppressing TNF-α-induced inflammatory cascade during sepsis-induced AKI; also decreases injury during ischemia-reperfusion
• Dexmedetomidine: alpha-2 agonist; alters RBF and Na/water handling; may stimulate BMP-7; meta-analysis showed OR 0.43 (95% CI 0.29-0.63) reduction in postoperative AKI with lower NGAL and higher creatinine clearance
• Sevoflurane at low gas flows over prolonged periods: may generate Compound A (potentially nephrotoxic in animals) - use with caution, maintain FGF ≥2 L/min
• All other volatile agents and TIVA are acceptable

VI. SPECIFIC SURGICAL CONTEXTS

A. Cardiac Surgery / CPB

• AKI after CABG: ~5% require dialysis; associated with multiorgan dysfunction and increased mortality (~350,000 CABG/year in USA)
• Risk factors: age >75, diabetes, hypertension, ventricular dysfunction, prolonged CPB, multiple inotropes, IABP insertion
• During CPB: RBF decreases to 12-13% of total pump flow; predicted by flow rate and perfusion pressure
• Off-pump vs. on-pump CABG: KDIGO recommends against selecting off-pump surgery solely to reduce AKI risk (ROOBY trial: no significant benefit, but CKD patients often excluded)

B. Aortic Surgery

• Infrarenal aortic cross-clamping: indirect effects via myocardial dysfunction, sympathetic activation, renin-angiotensin-aldosterone system, changes in SVR
• Supraceliac clamping: direct renal artery compromise
• Bilateral renal artery revascularization may be required in selected cases

C. Obstructive Jaundice (Hepatic Surgery)

• Lactulose (30 mL orally q6h for 3 days preoperatively, last dose within 12 hours) or oral bile salts with IV hydration beginning night before surgery may reduce perioperative AKI risk

VII. MANAGEMENT OF ESTABLISHED PERIOPERATIVE AKI

A. Conservative/Supportive

1. Identify and treat the cause: correct hypovolemia, remove nephrotoxins, relieve obstruction
2. Fluid management: avoid both hypovolemia and fluid overload
3. Electrolyte management: treat hyperkalemia (calcium gluconate, insulin+dextrose, sodium bicarbonate, kayexalate, dialysis if severe)
4. Acid-base correction: bicarbonate supplementation for metabolic acidosis
5. Nutritional support: enteral nutrition preferred; protein restriction controversial

B. Pharmacological (Limited Evidence)

C. Renal Replacement Therapy (RRT)

• Acidosis - refractory metabolic acidosis (pH <7.1)
• Electrolytes - hyperkalemia unresponsive to medical therapy (K+ >6.5 mEq/L)
• Intoxication - dialyzable drug/toxin
• Overload - refractory fluid overload causing respiratory compromise
• Uremia - symptomatic (encephalopathy, pericarditis, bleeding)

Current data do NOT support one modality over another, nor the use of early (prophylactic) dialysis for perioperative AKI. Dialysis can treat acidosis, hyperkalemia, and hypervolemia, but does not clearly decrease perioperative AKI mortality except in aortic surgery where it reduces 30-day mortality in patients who develop loss of renal function.

VIII. OUTCOMES AND PROGNOSIS

• Perioperative AKI is associated with prolonged ICU and hospital stay, increased resource utilization, and high cost
• Even KDIGO stage 1 AKI is associated with increased 30-day and 1-year mortality
• AKI survivors are at significantly increased risk of developing CKD and ESRD
• AKI in the perioperative period represents an independent risk factor for long-term cardiovascular events
• The concept of "Acute Kidney Disease (AKD)" has been proposed for kidney dysfunction lasting 7-90 days (bridging AKI to CKD), highlighting the importance of follow-up nephrology care

SUMMARY TABLE: Anesthesiologist's AKI Prevention Checklist

• Miller's Anesthesia, 10th Edition, Chapter 38 (Renal Function and Anesthesia) - Elsevier
• Morgan and Mikhail's Clinical Anesthesiology, 7th Edition, Chapter 30 - McGraw-Hill
• KDIGO Clinical Practice Guideline for Acute Kidney Injury, 2012
• Prowle JR et al. Nat Rev Nephrol. 2021;17:605-618 (ADQI/PeriOperative Quality Initiative Joint Consensus Report)