A 35-year-old woman is admitted 2 hours after a severe crush injury to her left forearm sustained in a machinery accident. She reports severe pain in the forearm that is out of proportion to the visible swelling. On examination, pain is elicited with passive extension of the fingers. Forearm compartments are tense. Sensation over the dorsal first web space is normal. Radial and ulnar pulses are palpable. Compartment pressure in the deep flexor compartment is measured at 58 mmHg; her blood pressure is 130/80 mmHg. Which of the following statements about the pathophysiology of compartment syndrome in this patient is most accurate?

Explanation

## Pathophysiology of Compartment Syndrome: Pressure Dynamics ### Critical Pressure Threshold Mechanism **Key Point:** Compartment syndrome develops when the pressure within a closed fascial space rises to a level that compromises microvascular perfusion. The critical threshold is NOT absolute systolic pressure, but rather the **pressure gradient** between the capillary bed and the compartment. ### Perfusion Pressure Concept **High-Yield:** The perfusion pressure (ΔP) is defined as: $$\Delta P = \text{Diastolic BP} - \text{Compartment Pressure}$$ When ΔP falls below 30 mmHg, microvascular flow becomes critically compromised: - In this patient: ΔP = 80 − 58 = **22 mmHg** (critically low) - Tissue oxygenation fails despite palpable pulses (which reflect large-vessel flow, not capillary perfusion) ### Why Pulses Are Misleading **Clinical Pearl:** Palpable radial and ulnar pulses do NOT exclude compartment syndrome. Large arteries remain patent at pressures well below those that obliterate capillary flow. Compartment syndrome is a **microvascular emergency**, not a macrovascular one. ### Timeline of Ischemic Injury **Mnemonic: "ISCHEMIA SEQUENCE"** 1. **Capillary collapse** (ΔP <30 mmHg) — first event 2. **Venous stasis** → edema → further pressure rise 3. **Muscle ischemia** (6–8 hours) → rhabdomyolysis 4. **Nerve ischemia** (later, after muscle) — NOT the first event 5. **Irreversible necrosis** (>8 hours) ### Comparison Table: Pressure Thresholds | Pressure Parameter | Threshold | Clinical Significance | |---|---|---| | Absolute compartment pressure | >30 mmHg | Concerning in normotensive patients | | Delta P (Diastolic − Compartment) | <30 mmHg | **CRITICAL** — microvascular compromise | | Compartment pressure vs. systolic BP | Rarely relevant | Systolic is too high; diastolic is the driver | | Time to irreversible damage | 6–8 hours | Window for fasciotomy | **Warning:** Do not confuse "palpable pulses" with "adequate tissue perfusion." Pulses reflect large-vessel flow; compartment syndrome is a capillary perfusion crisis. ### In This Patient - Compartment pressure: 58 mmHg - Diastolic BP: 80 mmHg - ΔP = 22 mmHg (critically low) - **Diagnosis: Compartment syndrome confirmed; fasciotomy urgently needed** [cite:Rockwood & Green's Fractures in Adults Ch 1; Orthopedic Surgery Essentials]