Perioperative Acute Kidney Injury (10 Marks - DNB)

Definition and Incidence

• Miller's Anesthesia, 10e

Diagnostic Criteria / Staging (KDIGO 2012)

• Brenner and Rector's The Kidney, p. 1244-1245

Risk Factors

• Pre-existing kidney dysfunction / CKD
• Diabetes mellitus, age >50 years
• Cardiac dysfunction, hepatic failure
• Sepsis, volume depletion
• Crush injury, prior nephrotoxin exposure

• Hypovolemia (bleeding, insensible losses)
• Kidney ischemia, intraoperative hypotension
• Inflammation (surgical stress)
• Increased intra-abdominal pressure
• Decreased cardiac output (anesthetic effect)
• Vasodilatation (anesthetic effect)
• Embolism (atheroemboli, air embolism)
• Nephrotoxin exposure

• Continued hypovolemia and ischemia
• Urinary obstruction
• Acute lung injury / mechanical ventilation
• Ongoing inflammation
• Miller's Anesthesia, 10e, Fig. 38.1

Pathophysiology

1. Ischemia-Reperfusion Injury

• The kidney's outer medulla is susceptible to ischemia due to its high metabolic demand and marginal oxygen supply (medullary pO2 ~10-20 mmHg at baseline).
• Anesthetic agents cause vasodilation and negative inotropy, reducing renal blood flow (RBF).
• Intraoperative hypotension with mean arterial pressure (MAP) below the autoregulatory threshold leads to a fall in GFR. The renal cortex is primarily supplied by the peritubular capillary network, and ischemia-reperfusion injury damages the S3 segment of the proximal tubule.
• Reperfusion generates reactive oxygen species (ROS) causing further cellular damage.

2. Inflammation

• Surgery activates the systemic inflammatory response (SIRS) via complement, cytokines (TNF-α, IL-1, IL-6), and neutrophil-endothelial adhesion.
• Inflammatory mediators cause afferent arteriolar vasoconstriction and tubular epithelial injury.

3. Nephrotoxin-Mediated Injury

• Nephrotoxins disturb either renal O2 delivery or utilization and promote ischemia.
• Key culprits: aminoglycoside antibiotics, amphotericin B, radiocontrast agents, NSAIDs, cyclosporine/tacrolimus, ACE inhibitors/ARBs, myoglobin (rhabdomyolysis), free hemoglobin.
• Barash Clinical Anesthesia, 9e

4. Hemodynamic Prerenal Mechanism

• Prerenal azotemia is the commonest precursor. The kidney maximizes solute/water retention, but if hypoperfusion persists, ATN supervenes.
• ACE inhibitors and ARBs impair glomerular efferent arteriolar tone, reducing GFR during low-flow states.

5. Atheroembolism

• Especially relevant in vascular surgery (aortic cross-clamping) - cholesterol crystal emboli occlude small renal vessels.

Causes - Classification

• Bailey & Love's Short Practice of Surgery, 28e, Table 24.4

Effects of Anesthetic Techniques

Regional Anesthesia

• Neuraxial blocks reduce sympathetic tone (T4-L1) innervating the renal vasculature. If hypotension results, RBF falls. Meta-analysis data suggest a 30% reduction in odds of postoperative mortality with neuraxial blockade (Rodgers et al.), with some reduction in renal failure, though confidence intervals were wide.

Inhaled Anesthetics

• Methoxyflurane and enflurane (historical) caused fluoride-induced polyuric renal insufficiency. Sevoflurane generates inorganic fluoride and compound A but is NOT associated with clinically significant AKI - likely due to shorter duration of fluoride elevation and lower intrarenal metabolism compared to methoxyflurane.

Intravenous Anesthetics

• Propofol increases bone morphogenetic protein-7 (BMP-7), which suppresses TNF-α-induced inflammatory cascade in sepsis-induced AKI and reduces ischemia-reperfusion injury.
• Dexmedetomidine (α2-adrenoceptor agonist) - meta-analysis data suggest a reduction in postoperative AKI; may stimulate BMP-7 in sepsis/ischemia-reperfusion models.
• Miller's Anesthesia, 10e

Biomarkers

Nephrotoxic Drug Considerations (Key Perioperative Points)

1. ACE inhibitors/ARBs: Should be withheld 48 hours before elective cardiac surgery - meta-analysis of >50,000 procedures links continuation to higher mortality (20%) and AKI rates (17%).
2. NSAIDs: Impair prostaglandin-mediated renal autoregulation; risk magnified in volume depletion.
3. Loop diuretics: Chronic therapy and high-dose furosemide independently associated with postoperative AKI.
4. Radiocontrast: AKI risk 24-48 h after exposure, peaks at 3-5 days. Mitigation: pre-hydration, minimize contrast dose, withhold NSAIDs/nephrotoxins.
5. Statins (protective): Pre-CABG statin use associated with lower dialysis and mortality rates (meta-analysis of 17 studies, >47,000 patients).
6. Aminoglycosides/Amphotericin B: Avoid concomitant hypovolemia, fever, other nephrotoxins; monitor electrolytes (hypomagnesemia, hypokalemia worsen toxicity).

• Barash Clinical Anesthesia, 9e

Prevention / Management

General Principles

• Hemodynamic optimization: Maintain MAP above autoregulatory threshold; avoid intraoperative hypotension. Goal-directed therapy (GDT) is beneficial, but overly restrictive fluid protocols have recently been associated with increased AKI risk - balance is key.
• Fluid choice: Balanced salt solutions preferred over normal saline - avoids hyperchloremic metabolic acidosis; may modestly reduce AKI rates.
• Avoid volume overload: Volume overload is an independent risk factor for adverse AKI outcomes.
• Minimize nephrotoxin exposure: Hold ACE inhibitors/ARBs perioperatively; avoid NSAIDs; use ISO-osmolar/low-osmolarity contrast; monitor aminoglycoside levels.
• Urine output monitoring: Carefully monitor intraoperative urine output, though it is not validated as a sole predictor of postoperative renal dysfunction.

Pharmacological Interventions with Evidence

• Miller's Anesthesia, 10e

Renal Replacement Therapy (RRT)

• Indicated for severe AKI with refractory hyperkalemia, acidosis, uremia, or volume overload.
• Does not prevent AKI, but treats its metabolic consequences.
• Current evidence does not support early initiation of dialysis or one modality (CRRT vs. IHD) over another.
• Up to 75% of survivors of AKI after major vascular surgery regain kidney function and become dialysis-independent.
• Miller's Anesthesia, 10e

Outcomes and Long-term Consequences

• AKI increases risk of acute kidney disease (AKD) and progression to CKD.
• Even minor elevation in serum creatinine postoperatively correlates with increased short-term mortality.
• After cardiac/major vascular surgery: AKI requiring dialysis is associated with 30-day mortality significantly higher than those without AKI.
• KDIGO defines AKD as AKI or reduction in GFR <60 mL/min/1.73m² or a 35% GFR decrease or >50% SCr increase, lasting <3 months - a bridging category between AKI and CKD.

Summary Points for Exam

1. KDIGO defines AKI as SCr rise ≥0.3 mg/dL in 48h OR ≥50% in 7 days OR UO <0.5 mL/kg/h for >6h.
2. Primary mechanism of perioperative AKI is ATN, via ischemia-reperfusion, inflammation, and nephrotoxins.
3. Perioperative AKI is a mosaic of pure nephropathies; no single treatment reverses it.
4. Sevoflurane is safe; avoid methoxyflurane/enflurane (historical).
5. Withhold ACE inhibitors/ARBs 48h before cardiac surgery.
6. Balanced crystalloids preferred over saline; avoid HES.
7. No proven pharmacological renoprotection exists - hemodynamic optimization is the mainstay.
8. Dopamine and ANP are NOT renoprotective.