A 48-year-old male with type 2 diabetes and chronic kidney disease (eGFR 35 mL/min/1.73m²) undergoes elective open prostatectomy under general anesthesia. Induction is achieved with propofol 1.5 mg/kg IV, and anesthesia is maintained with isoflurane and nitrous oxide. Postoperatively, the patient develops acute kidney injury (AKI) with serum creatinine rising from 1.8 mg/dL preoperatively to 3.2 mg/dL on postoperative day 2. Urine sodium is 15 mEq/L, and fractional excretion of sodium (FENa) is 0.4%. Which of the following is the MOST likely mechanism of anesthesia-related renal injury in this patient?

Explanation

## Analysis This patient developed **postoperative acute kidney injury** with a **low FENa (0.4%)**, which is diagnostic of **prerenal azotemia** (FENa <1% indicates prerenal cause; >2% indicates intrinsic renal disease). The clinical context—elective surgery, no mention of contrast agents or succinylcholine, and the low urine sodium (15 mEq/L)—points to **renal hypoperfusion**. ### Why Option B is Correct: **Acute tubular necrosis (ATN) secondary to intraoperative hypotension** is the most common anesthesia-related renal complication in high-risk patients. Although the FENa is initially low (suggesting prerenal), **prolonged or severe intraoperative hypotension** can progress to **ATN**. The patient's risk factors (diabetes, pre-existing CKD) make him exquisitely sensitive to hemodynamic changes. Isoflurane causes dose-dependent vasodilation and can contribute to intraoperative hypotension, especially in elderly/comorbid patients. **Key Point:** The combination of: - Pre-existing renal disease (eGFR 35) - Diabetes (impaired renal autoregulation) - General anesthesia with volatile agent (isoflurane) - Likely intraoperative hypotension ...creates a **perfect storm** for **ischemic AKI**. ### High-Yield Mnemonic: **AKIN** (Acute Kidney Injury Network) - **A**cute tubular necrosis from hypotension - **K**idney hypoperfusion (prerenal → ATN) - **I**schaemia (most common anesthesia-related mechanism) - **N**ephrotoxins (secondary) ## Comparison Table | Mechanism | FENa | Urine Na | Urine Osmolality | Anesthesia Link | |-----------|------|----------|------------------|----------| | **Prerenal/Hypotension** | <1% | <20 | >500 | Volatile agents, blood loss | | **ATN** | >2% | >40 | <350 | Severe/prolonged ischemia | | **Fluoride toxicity** | Variable | Variable | Low | High-dose volatile (rare) | | **Contrast nephropathy** | >2% | >40 | Low | Iodinated contrast | **Clinical Pearl:** In this patient, the **low FENa + low urine Na** initially suggest **prerenal** etiology (renal hypoperfusion). However, if hypotension was **severe or prolonged**, it progresses to **ATN**, which is the most likely diagnosis given the magnitude of creatinine rise (1.8 → 3.2) and the patient's vulnerability.