Internal Medicine · Year 3 · from Internal Medicine

Case 1: Prerenal AKI

Patient Presentation

A 78-year-old woman is brought from a nursing home with altered mental status and decreased oral intake for 3 days. Staff reports she has been febrile and had multiple episodes of diarrhea. Her medical history includes hypertension, heart failure (EF 35%), and type 2 diabetes. Medications include lisinopril, furosemide, and metformin.

Vital Signs

  • Blood Pressure: 88/52 mmHg
  • Heart Rate: 108 bpm
  • Respiratory Rate: 20/min
  • Oxygen Saturation: 96% on room air
  • Temperature: 38.4°C

Physical Examination

  • General: Lethargic, dry mucous membranes
  • Cardiovascular: Tachycardic, flat JVP
  • Pulmonary: Clear
  • Abdomen: Soft, hyperactive bowel sounds
  • Skin: Decreased turgor, dry axillae
  • Neurologic: Oriented to person only

Laboratory Results

  • Creatinine: 3.2 mg/dL (baseline 1.1)
  • BUN: 68 mg/dL
  • BUN/Cr ratio: 21:1
  • Sodium: 148 mEq/L
  • Potassium: 5.4 mEq/L
  • Urinalysis: SG 1.030, bland sediment, no protein
  • Urine sodium: 8 mEq/L
  • FENa: 0.4%
  • Lactate: 3.8 mmol/L

Clinical Image

Figure 1: Diagram illustrating the pathophysiology of prerenal AKI: decreased renal perfusion leads to reduced GFR while tubular function remains intact, resulting in low urine sodium and FENa.

Image Source: Educational illustration for teaching purposes.

Questions

  1. What are the KDIGO criteria for staging AKI?
  • A) Based on BUN levels alone
  • B) Based on creatinine rise and/or urine output
  • C) Based on need for dialysis
  • D) Based on symptoms
  1. What laboratory findings support prerenal AKI in this patient?
  • A) High urine sodium and high FENa
  • B) BUN/Cr ratio >20:1, low urine sodium (<20), low FENa (<1%), high urine specific gravity
  • C) Muddy brown casts
  • D) Hematuria and proteinuria
  1. What is this patient's AKI stage according to KDIGO?
  • A) Stage 1
  • B) Stage 2
  • C) Stage 3
  • D) Not AKI
  1. What medications should be held in this patient?
  1. What is the initial management priority?

Answers

  1. B) Based on creatinine rise and/or urine output
  • Stage 1: Cr 1.5-1.9x baseline OR ≥0.3 mg/dL rise within 48h OR UOP <0.5 mL/kg/h for 6-12h
  • Stage 2: Cr 2.0-2.9x baseline OR UOP <0.5 mL/kg/h for ≥12h
  • Stage 3: Cr ≥3.0x baseline OR rise to ≥4.0 mg/dL OR UOP <0.3 mL/kg/h for ≥24h OR anuria ≥12h OR RRT initiation
  1. B) BUN/Cr ratio >20:1, low urine sodium (<20), low FENa (<1%), high urine specific gravity - These indicate intact tubular function with appropriate sodium and water reabsorption in response to decreased perfusion. Concentrated urine (high SG) reflects ADH-mediated water retention.
  1. C) Stage 3 - Creatinine rose from 1.1 to 3.2 mg/dL, which is approximately 2.9x baseline, meeting Stage 3 criteria (≥3.0x baseline or ≥4.0 mg/dL).
  1. Medications to hold:
  • Lisinopril (ACE inhibitor): Reduces efferent arteriolar tone, decreasing GFR in hypoperfused state
  • Furosemide: Worsens volume depletion
  • Metformin: Risk of lactic acidosis in AKI, contraindicated with eGFR <30
  • NSAIDs (if taking): Reduce prostaglandin-mediated afferent arteriolar dilation
  • Resume after renal function recovers
  1. Initial management:
  • Aggressive IV fluid resuscitation (crystalloids)
  • Treat underlying cause (infection with diarrhea - consider antibiotics if bacterial)
  • Monitor urine output closely (Foley catheter)
  • Hold nephrotoxic medications
  • Repeat creatinine in 24-48 hours
  • Address hyperkalemia if it worsens
  • Expect creatinine to improve with volume repletion if prerenal

All cases for this lecture as Markdown