Makindo Medical Notes

⚠️ Sometimes there can be more than one cause at the same time. 💡 Always check that any catheter is not blocked or that the tip is not sitting in the urethra.

📖 Definition

• Oliguria: < 500 ml urine output (UO) per day.
• Also defined as UO < 0.5 ml/kg/h for ≥ 6 consecutive hours (≈ 35 ml/hour for a 70 kg adult).
• Anuria: < 50 ml UO per day — usually suggests bilateral renal injury or obstruction of both kidneys.

📊 Incidence

• In ICU patients, nearly 50% experience at least one episode of oliguria during their stay.

🧬 Pathophysiology

• Average solute excretion = ~600 mOsm/day.
• Maximal urinary concentrating ability = 1200 mOsm/L → at least 0.5 L urine output is physiologically required.
• Urine output depends on:
  • Glomerular filtration rate (GFR).
  • Tubular secretion and reabsorption.
• Glomerular filtration is directly dependent on renal perfusion.
• Oliguria = either reduced GFR or mechanical obstruction to urine flow.

🩺 Causes (same classification as AKI)

• Pre-renal:
  • Dehydration / hypovolaemia.
  • Cardiac failure.
  • Liver failure (hepatorenal syndrome).
• Renal (intrinsic):
  • Nephrotoxic drugs (penicillin, NSAIDs, cephalosporins, gentamicin).
  • Radiocontrast-induced nephropathy.
  • Ischaemic acute tubular necrosis (ATN).
• Post-renal (obstructive):
  • Renal pelvis: Papillary necrosis, tumour (often unilateral → oliguria not anuria).
  • Ureter: Stone, compression, tumour (unilateral usually → oliguria).
  • Bladder outlet: Transitional cell carcinoma, prostate cancer, BPH, blocked catheter, clot retention.
  • Urethra: Stricture.
  • Other: High intra-abdominal pressure.

🔎 Investigations

• Bloods: FBC, U&E, calcium, phosphate, LFTs, bone profile, glucose.
• Inflammatory markers: CRP (infection/inflammation, vasculitis).
• Imaging: CXR (oedema), AXR (abdominal pathology).
• Special tests: Serum protein electrophoresis.
• Urinalysis: Haematuria, leucocytes, protein, glucose.
  • Haematuria + proteinuria → GN.
  • Broad brown casts → ATN (ischaemic/toxic).
  • RBC casts → acute glomerulonephritis.
  • WCC casts → infection or interstitial nephritis.
• Urine sodium: Low (<10 mmol/L) in pre-renal.
• Urine osmolality: High (>500 mOsm/kg) in pre-renal.
• Autoimmune screen: ANA, dsDNA, ANCA, Anti-GBM if vasculitis suspected.
• Renal USS: To exclude obstruction/hydronephrosis (within 24h).

💊 Management

• 🔍 Check simple causes first: Blocked catheter or dehydration.
• 💧 Fluid challenge if volume depleted (unless pulmonary oedema present). Review response.
• 📈 Strict input/output chart; measure daily weight.
• 🩺 Assess BP and rule out shock — treat underlying cause, consider inotropes if needed.
• 💉 Central line may be required to guide fluid replacement and monitor response.
• 🖼️ Renal ultrasound within 24 hrs to rule out obstruction.
• ⛔ Stop nephrotoxic agents (NSAIDs, contrast, aminoglycosides).
• 📞 Early renal referral if oliguria/anuria persists despite optimisation, or if AKI worsens.
• 🔧 Obstructive causes → urology referral (may need stenting/nephrostomy).

📚 References

• Oliguria in Critically Ill Patients – Narrative Review

Clinical Pearl: 🚨 Always consider reversible causes first (blocked catheter, dehydration). In exams, the key triad = Check catheter ➝ Assess volume ➝ Exclude obstruction.