Blood flow in the lower limbs in the knee-chest position. Ultrasonographic study in unanaesthetised volunteers.

The knee-chest position for lumbar spine surgery is favoured because decreased filling of the epidural veins is associated with reduced peroperative bleeding. However, the position may be unfavourable from a circulatory point of view. In the present study, non-invasive assessment of circulation in the lower limbs was performed in 21 unanaesthetised, healthy volunteers who were placed in the surgical knee-chest position. Measurements included blood flow velocity (colour Doppler ultrasonography), oscillotonometric arterial blood pressure from upper and lower limbs and pulse oximetry from a toe. There was a statistically significant reduction in the posterior tibial artery flow velocity, maximally 31.6%, when the subject was moved from the prone to the knee-chest position. An enlargement of the trunk-femoral angle at the hip did not improve arterial flow. In 10 of the 21 volunteers, no flow in the posterior tibial vein was detected in the knee-chest position. In spite of the deteriorated blood flow, pulse oximetry indicated sufficient capillary flow in the very periphery of the lower limb. The change from prone to knee-chest position resulted in an increase in arterial blood pressure of the upper limb; the increase in diastolic arterial pressure was statistically significant (p < 0.001). It is concluded that the surgical knee-chest position involves deterioration of both the arterial and venous flow of the lower limbs. This should be considered in patients undergoing surgery in this position and, in particular, in those at risk of developing cardiovascular complications.