Anesthesiology APPLIED Exam › Critical care, resuscitation and trauma

Postoperative oliguria

T1Critical care, resuscitation and trauma·845 words · ~5 min read·v0.1✓ Sourced · 3DRAFT

Why examiners ask this

"The nurse calls from PACU: the urine output has been 15 mL/h for two hours." This is a standard post-op question because the reflex answer, a fluid bolus, is often right and sometimes harmful. Examiners want a structured pre-renal, renal, post-renal approach, a bedside assessment before treatment, and recognition of abdominal compartment syndrome. It tests organization and judgment.

The 60-second answer

Oliguria is less than 0.5 mL/kg/h for 6 hours by the KDIGO definition,¹ but in PACU I act on two hours. I go to the bedside. First I exclude the post-renal cause, because it is the easiest to fix: flush or replace the Foley, and do a bladder scan or ultrasound. Then I assess volume status and perfusion: blood pressure against the patient's baseline, heart rate, cap refill, lactate, hemoglobin, drains, and a dynamic test, a passive leg raise or IVC ultrasound. If the patient is fluid-responsive, I give a 250–500 mL balanced crystalloid bolus and reassess. If not, I raise the MAP with a vasopressor rather than drowning the patient. I review nephrotoxins and send electrolytes, creatinine and a urinalysis. I do not give furosemide to make urine.

Decision points and the "why"

Post-renal first — a kinked or clotted catheter is the most common and most embarrassing cause; a 5-minute check before anything else.
Fluid only if fluid-responsive — a positive passive leg raise (a rise of about 10% in stroke volume or cardiac output, or more than 15% in pulse pressure when I cannot measure output)² or pulse-pressure variation over 13% says volume will help; otherwise fluid causes edema, including renal interstitial edema that worsens kidney function.
Perfusion pressure — renal autoregulation shifts to the right in chronic hypertension, so a MAP of 65 may be too low for that patient; I target a MAP within 10–20% of baseline.
Surgical context — after aortic cross-clamp I think about renal ischemia or embolism; after a major abdominal case, intra-abdominal hypertension; after a long steep Trendelenburg case, rhabdomyolysis; after contrast, contrast nephropathy; after a big transfusion, hemolysis.
Bladder pressure — measured with no more than 25 mL of saline, a sustained or repeated pressure of 12 mmHg or more is intra-abdominal hypertension, and a sustained pressure above 20 mmHg with new organ dysfunction is abdominal compartment syndrome;³ the treatment is decompression, not fluid.
No diuretics to treat a number — furosemide converts oliguric to non-oliguric renal failure without improving outcome; KDIGO suggests diuretics only for volume overload, recommends against low-dose dopamine and suggests against fenoldopam.¹
Stop nephrotoxins — NSAIDs, aminoglycosides, ACE inhibitors and ARBs on the day of surgery, and IV contrast where avoidable.

The complication they will inject

Six hours after a ruptured AAA repair, the patient is intubated, oliguric for three hours despite 3 L of crystalloid, with rising peak airway pressures, a tense abdomen and a MAP of 60 on norepinephrine. I measure bladder pressure through the Foley: it is 26 mmHg. This is abdominal compartment syndrome. I tell the surgeon directly because the treatment is decompressive laparotomy.³ In the meantime I deepen sedation and give a trial of neuromuscular block to improve abdominal wall compliance, decompress the stomach with a nasogastric tube,³ stop further fluid, maintain perfusion with norepinephrine, and ventilate with lower tidal volumes accepting higher plateau pressures. After decompression I expect a reperfusion surge of potassium and acid and prepare calcium and bicarbonate.

Pitfalls that cost points

Giving a fluid bolus over the phone without examining the patient.
Skipping the Foley.
Repeating boluses in a fluid-unresponsive patient.
Giving furosemide to "see if the kidneys work."
Missing abdominal compartment syndrome in a ventilated patient with rising airway pressures.

Rapid-why

Q: Why does the urine output fall in every surgical patient? A: Surgical stress raises ADH, aldosterone and catecholamines; some oliguria is normal physiology and does not mean kidney injury.

Q: Why is the fractional excretion of sodium less useful after surgery? A: Diuretics and saline load distort it; I use fractional excretion of urea below 35% if the patient received diuretics.

Q: What protects the kidney in high-risk surgery? A: Maintaining MAP and cardiac output, avoiding nephrotoxins and hyperchloremia, and limiting contrast; there is no proven pharmacologic protection.

Q: When do you call nephrology for dialysis? A: Refractory hyperkalemia, acidosis, volume overload, uremic complications, or certain toxins.

Numbers to know

Item	Value
Oliguria (KDIGO)	< 0.5 mL/kg/h for 6 h¹
AKI by creatinine	Rise ≥ 0.3 mg/dL in 48 h, or ≥ 1.5× baseline within 7 days¹
Fluid challenge	250–500 mL balanced crystalloid
Intra-abdominal hypertension	Bladder pressure ≥ 12 mmHg, sustained or repeated³
Abdominal compartment syndrome	> 20 mmHg with new organ dysfunction³
Diuretics in AKI	Only for volume overload¹

Sources

1.KDIGO Clinical Practice Guideline for Acute Kidney Injury · KDIGO, 2012 · kdigo.org
2.Surviving sepsis campaign: international guidelines for management of sepsis and septic shock 2021 · SCCM / ESICM, 2021 · pmc.ncbi.nlm.nih.govDynamic measures; passive leg raise with >15% pulse-pressure rise where output cannot be measured.
3.Intra-abdominal hypertension and the abdominal compartment syndrome: updated consensus definitions and clinical practice guidelines from the World Society of the Abdominal Compartment Syndrome · WSACS (Intensive Care Med), 2013 · ncbi.nlm.nih.gov

Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.