Pathophysiology

Compartment syndromes are due to increased pressure within closed tissue spaces that compromises the flow of blood through nutrient capillaries in muscles and nerves. The complex relationships between time, Starling forces, systemic and venous pressure, and reperfusion injury are not completely understood. The clinical variables of each case make a definitive explanation of how capillary blood flow is compromised a difficult exercise. However, a common factor is elevated tissue pressure. Normal tissue pressure is about zero and usually less than 10 mmHg. Capillary blood flow within the compartment is compromised at pressures greater than about 20 mmHg, and muscle and nerves are at risk for ischemic necrosis at pressures greater than about 30 to 40 mmHg. Of the tissues within the compartments, nerve is most sensitive, followed by muscle tissue. Blood flow through arteries, arterioles, and collaterals is not compromised significantly at these pressures. Nevertheless, tissues within the compartment that are dependent on the nutrient capillaries become ischemic and then necrotic if the compartment pressure is not reduced promptly. By the time that distal pulses are reduced, muscle necrosis has occurred. Ischemic muscles hurt, and this pain is exacerbated by active muscle contraction and by passive stretching of the muscle.

An increase in compartmental pressure can be caused by (1) compression of the compartment, for example, by burn eschar, a circumferential cast, or a pneumatic pressure garment; and (2) by a volume increase within the compartment due to hematoma and edema. Direct trauma with resulting bleeding and edema is probably the most common cause, but overexertion (shin splints) and limb compression during recumbency as a result of alcohol or drug overdose are also common causes. Mubarak and Hargens1 developed a classification of the acute compartment syndromes (Ta.bIe,2Z0,-l) listing the myriad of possible causes. Their text is highly recommended for readers wishing to learn more about this topic.

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