Postoperative Acute Kidney Injury

Postoperative acute kidney injury is acute kidney injury occurring within 7 days of surgery. It complicates 5% to 15% of major noncardiac surgery, 20% to 30% of cardiac surgery and over 30% of emergency vascular and abdominal surgery, and is independently associated with prolonged admission, chronic kidney disease, cardiovascular events and mortality that rises stepwise with severity.

Perioperative injury is multifactorial:

• Haemodynamic: intraoperative hypotension (mean arterial pressure below 65 mmHg for more than 10 to 20 minutes, below 55 mmHg for any duration), hypovolaemia from blood loss, third space loss and bowel preparation, low cardiac output, vasodilation from anaesthesia and neuraxial block
• Ischaemia reperfusion: aortic cross-clamping, cardiopulmonary bypass, major vascular surgery, prolonged tourniquet use
• Inflammation and sepsis: peritonitis, surgical site infection, anastomotic leak
• Nephrotoxins: NSAIDs, aminoglycosides, vancomycin, iodinated contrast, hydroxyethyl starch, colistin, calcineurin inhibitors, intravenous immunoglobulin with sucrose
• Pigment: rhabdomyolysis (prolonged positioning, tourniquet, obese surgery), haemolysis (cardiopulmonary bypass, transfusion)
• Abdominal compartment syndrome and intra-abdominal hypertension (pressure above 12 mmHg)
• Postrenal: ureteric ligation or injury (gynaecological and colorectal surgery), bladder outlet obstruction from opioids and catheter blockage, retention, haematoma
• Renal: embolism, renal vein thrombosis, atheroembolism
• Metabolic: hyperchloraemia from large volumes of 0.9% sodium chloride

Risk factors: preoperative eGFR below 60 mL/min/1.73 m2, age above 65 years, diabetes, heart failure, hypertension, peripheral vascular disease, liver disease, emergency surgery, high risk surgery (cardiac, aortic, major abdominal), anaemia, preoperative renin angiotensin blockade, and exposure to nephrotoxins.

• Oliguria (below 0.5 mL/kg/h for 6 hours or more) is often the first sign; creatinine rises 24 to 72 hours after the insult
• Hypotension, tachycardia, signs of hypovolaemia or overload (positive balance, weight gain, pulmonary oedema)
• Surgical context: abdominal distension, tense abdomen, wound or drain output, signs of sepsis, limb swelling, dark urine
• Palpable bladder, flank pain or leaking urine in ureteric injury
• Hyperkalaemia, metabolic acidosis, rising lactate, hypocalcaemia
• Fever and rash in drug induced interstitial nephritis

Red flags: anuria after pelvic or colorectal surgery (ureteric ligation), intra-abdominal pressure above 20 mmHg, potassium above 6.0 mmol/L, pH below 7.15, lactate above 4 mmol/L, hypotension needing vasopressors, dark urine with creatine kinase above 5000 U/L.

• Baseline preoperative creatinine; postoperative creatinine at 24, 48 and 72 hours after major surgery; hourly urine output for high risk patients.
• Electrolytes, bicarbonate, lactate, haemoglobin, creatine kinase, C-reactive protein.
• Bladder scan and catheter check (blocked catheter is the commonest reversible cause).
• Renal tract ultrasound with Doppler if obstruction or vascular occlusion is suspected; CT with delayed phase for ureteric injury (urinoma, extravasation); creatinine in drain fluid at least 3 times serum creatinine confirms urine leak.
• Bladder pressure measurement for abdominal compartment syndrome.
• Urinalysis and microscopy for tubular casts, red cell casts, white cells.
• Fractional excretion of sodium below 1% suggests prerenal hypoperfusion, above 2% tubular injury; interpret with caution after diuretics.
• Biomarkers (urinary TIMP-2 multiplied by IGFBP7) above 0.3 (ng/mL)2/1000 identify risk after surgery where available.

Differentials: prerenal azotaemia, sepsis, acute tubular injury, obstruction, rhabdomyolysis, contrast injury, interstitial nephritis, cardiorenal syndrome.

Prevention (preoperative and intraoperative)

• Identify high risk patients and optimise: control heart failure, correct anaemia, avoid fasting related volume depletion (allow clear fluids until 2 hours before surgery), minimise bowel preparation.
• Stop NSAIDs and aminoglycosides; hold ACE inhibitors and angiotensin receptor blockers for 24 hours before major surgery; hold diuretics and metformin on the day.
• Maintain mean arterial pressure at least 65 mmHg (75 mmHg in chronic hypertension); treat hypotension within minutes with vasopressors (phenylephrine 50 to 100 microg or noradrenaline 0.02 to 0.1 microg/kg/min).
• Balanced crystalloids in preference to 0.9% sodium chloride; avoid hydroxyethyl starch and gelatin; goal directed fluid therapy using stroke volume or pulse pressure variation.
• Limit contrast exposure and aortic clamp time; avoid hyperglycaemia (glucose 6 to 10 mmol/L).
• Do not use dopamine, fenoldopam, mannitol or furosemide for renal protection; they do not prevent injury.

Established postoperative acute kidney injury

• Relieve obstruction first: flush or replace catheter, bladder scan, image for ureteric injury.
• Restore volume with balanced crystalloid 250 to 500 mL aliquots with reassessment; treat sepsis early (antibiotics within 1 hour, source control).
• Support perfusion pressure and cardiac output; avoid positive balance above 10% of body weight.
• Stop nephrotoxins and renally adjust drugs; avoid NSAIDs; continue analgesia with paracetamol 1 g four times daily and opioids at reduced dose (fentanyl or hydromorphone rather than morphine).
• Treat hyperkalaemia: calcium gluconate 10% 10 mL for ECG changes, insulin 10 units with glucose 25 g, salbutamol 10 to 20 mg nebulised, sodium zirconium cyclosilicate 10 g three times daily for up to 48 hours.
• Abdominal compartment syndrome: decompression laparotomy when pressure is above 20 mmHg with new organ failure.
• Rhabdomyolysis: crystalloid 200 to 300 mL/h targeting urine output 200 to 300 mL/h.
• Ureteric injury: nephrostomy or stent immediately; surgical repair or reimplantation depends on timing and level.

Renal replacement therapy for refractory hyperkalaemia, pH below 7.15, overload unresponsive to diuretics, uraemic complications. Early initiation without indication is not beneficial; continuous therapy for haemodynamic instability.

Follow up. Creatinine and albumin:creatinine ratio at 3 months for all patients; those who required dialysis or had stage 2 or 3 injury need nephrology review; avoid nephrotoxins during recovery.

Special groups. Older adults: lower thresholds for monitoring, smaller fluid aliquots and avoidance of benzodiazepines. Chronic kidney disease: restart renin angiotensin blockade once creatinine is stable and the patient is eating and drinking.