Popliteal Vein Compression Under General Anaesthesia

ObjectivesTo determine the incidence of popliteal vein compression in supine patients, who are under general anaesthesia and using heel elevators.DesignA prospective cohort study.MethodsThe popliteal veins of 50 patients, lying supine under general anaesthesia, were insonated using duplex ultrasonography to determine the incidence of popliteal vein compression when the knees were flexed and extended.ResultThere was a statistically significant reduction in popliteal vein diameter in extension compared with the diameter in flexion (median diameter: flexed 7.6 mm, extended 2.1 mm, p < 0.001, Wilcoxon test). In extension, 43% of veins occluded, and a further 21% was compressed by ≥50%. There was a significant relationship to body mass index (BMI) but not to height, gender or age; all patients with BMI ≥30 displayed narrowing at least one vein by ≥50%. The relative risk of vein narrowing ≥50% or occlusion was 1.55 for BMI ≥25 versus <25, and 1.67 for BMI ≥30 versus <25.ConclusionKnee hyperextension in supine patients during general anaesthesia leads to popliteal vein compression or occlusion. The likelihood of compression increases with higher BMIs.