Peripheral Neuroanatomy, Human Anatomy, USMLE Step 1 - Full Vignette with Extended Explanations

A 53-year-old man is brought to the ER following his lower leg being pinned under heavy machinery. He presents with severe localized pain, swelling, tense shiny skin, neurologic deficits in the foot, and vital sign abnormalities. What clinical features in this scenario should alert you to potential complications, and how should these findings influence the selection of safe and appropriate pain management strategies in the emergency setting?

VIDEO INFO
Category: Peripheral Neuroanatomy, Human Anatomy, USMLE Step 1
Difficulty: Easy - Basic level - Suitable for medical students
Question Type: Contraindications
Case Type: Emergency - Emergency scenario requiring urgent decision-making

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QUESTION
A 53-year-old man is brought to a United States emergency department after his right lower leg was pinned under a forklift at work 40 minutes ago. He is alert but anxious and in severe pain. Vital signs on arrival are pulse 102/min, respiratory rate 38/min, blood pressure 90/palp (diastolic not obtainable), temperature 36.9 degreesC, and oxygen saturation 99% on a nonrebreather mask placed by EMS....

OPTIONS
A. Initiate a continuous popliteal sciatic catheter in the injured limb using a dense local anesthetic regimen (e.g., 0.5% bupivacaine 20-30 mL bolus, then 0.2% infusion at 6-8 mL/hour in the ED) because a prolonged, dense block can mask evolving compartment pain and delay diagnosis.
B. Perform a single-shot, low-concentration ultrasound-guided popliteal sciatic block (e.g., 0.2% ropivacaine 10 mL) with hourly neurovascular checks and documented compartment-pressure surveillance in a monitored setting.
C. Provide intravenous fentanyl in small titrated doses (e.g., 25-50 micrograms every 5 minutes to effect with continuous pulse oximetry) while repeating serial compartment and neurovascular examinations.
D. Start low-dose intravenous ketamine for analgesia (e.g., 0.1-0.3 mg/kg slow IV push, then 0.05-0.1 mg/kg/hour infusion) with frequent reassessment of pain and motor/sensory function in the affected limb.

CORRECT ANSWER
A. Initiate a continuous popliteal sciatic catheter in the injured limb using a dense local anesthetic regimen (e.g., 0.5% bupivacaine 20-30 mL bolus, then 0.2% infusion at 6-8 mL/hour in the ED) because a prolonged, dense block can mask evolving compartment pain and delay diagnosis.

EXPLANATION
"Initiate a continuous popliteal sciatic catheter in the injured limb using a dense local anesthetic regimen (e.g., 0.5% bupivacaine 20-30 mL bolus, then 0.2% infusion at 6-8 mL/hour in the ED) because a prolonged, dense block can mask evolving compartment pain and delay diagnosis." This is contraindicated because dense, long-duration regional anesthesia can blunt the cardinal symptom of acute compartment syndrome-worsening pain, especially with passive stretch-thereby delaying fasciotomy. The vignette flags evolving compartment syndrome of the leg: severe pain out of proportion, pain with passive toe extension and ankle dorsiflexion, tense shiny compartments below the knee, emerging deep peroneal sensory deficit (first web space), and evolving motor weakness of great toe dorsiflexion....


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