Caval Compression

The principal site of venous obstruction in the supine hypotensive syndrome is at the pelvic brim. In the majority of patients, the compensatory mechanisms are sufficient to maintain arterial blood pressure.

When caval compression occurs, blood is diverted into the vertebral and azygos system. This system constitutes a 'protected' conduit for blood returning from the lower part of the body.

As the vertebral veins distend, so the volume of the subarachnoid and epidural space is reduced. This reduction in the volume of the epidural space can enhance the spread of recently-administered epidural solutions.