Objective: To compare popliteal vein (PV) diameter and peak flow velocity generated by a calf compressor, when the knee is either flexed or hyperextended.
Method: Subjects lay supine on a couch with a calf compressor on each leg. Peak flow velocities in the PV were measured by duplex ultrasound during activation of the compressor, when the knee was either flexed or hyperextended. Hyperextension of the knee is seen when the heel is elevated to prevent heel pressure ulcers.
Results: The results were analysed using the Wilcoxon signed rank test. This showed a significant increase in peak flow measurements related to narrowing of the PV when the knee was hyperextended.
Conclusion: There is a functional narrowing of the PV during hyperextension of the knee. We developed a model, which postulates an increase in the risk of deep vein thrombosis in patients whose knee is hyperextended. Based on this model, we suggest that the knee should be flexed slightly when patients are supine and anaesthetized.