7. POSTOPERATIVE EMERGENCIES AFTER UROLOGICAL SURGERY 149
a frequency on the order of 1 in 3500 (Halliwell et al. 1998). Thus, it is rare, but because the consequences of missing the diagnosis of LLCS are devastating, it is important to appreciate its predisposing factors, presentation, and subsequent management.
The leg has four osteofascial compartments, which are bordered by nonelastic fascia and bone. Normal resting tissue pressure in the anterior compartment of the leg ranges between 3 and 22 mmHg.
Any factor that induces ischaemia in the leg can lead to a compartment syndrome. Ischaemia disrupts the integrity of the vascular endothelium, leading to fluid shifts into the extracellular tissue space with a consequent rise in tissue pressure. The lithotomy position causes ischaemia in the leg by the following mechanisms:
1. Reduction in hydrostatic perfusion pressure. Every 1-cm elevation of the limb above the heart reduces mean arteriolar pressure by 1 mm Hg and causes a measurable reduction in ankle-brachial pressure index. This reduction in perfusion pressure is compounded by the head-down position.
2. Calf compression. This can occlude both venous drainage and arterial flow.
3. Knee and hip flexion can compress blood vessels.
4. Dorsiflexion of the foot causes an increase in pressure within the calf.
As compartment pressure rises, the lumen of arterioles is eventually occluded. A vicious cycle of ischaemia sets in. When the limb is returned to the supine position, a reperfusion injury can cause a further rise in compartment pressure.
The major factor determining the likelihood of development of a LLCS is time spent in the lithotomy position. The exaggerated lithotomy position is more likely to lead to a LLCS than is a lower lithotomy position. Hypotension, hypovolaemia, and peripheral vascular disease all predispose to development of the compartment syndrome. Young, large men with an increased muscle bulk may be at greater risk of a compartment syndrome because of tighter, less compliant compartments in the leg.