Transmural pressure of epidural veins in the thoracic and lumbar spine of pigs

The aim of this study was to determine the transmural pressure gradient of epidural veins located within the spinal column and to investigate the effects of increased intra-abdominal and intrathoracic pressure on this gradient. Fourteen pigs were sedated with ketamine and anesthetized with halothane. Ventilation was controlled after tracheotomy. Needles were inserted into the epidural and subarachnoid space, and a catheter was threaded into an epidural vein. Pressures in these structures were measured during controlled and spontaneous ventilation, increased abdominal pressure, and thoracic compression. Measurements were made in both the thoracic and lumbar regions. The pressure gradient between the epidural vein and the surrounding epidural space was low-11 or 2 mmHg--in both the lumbar and thoracic areas. This gradient was not affected by the mode of ventilation or increased abdominal or thoracic pressure (or both), even though the absolute intramural pressure increased. The pressure gradient between the epidural vein and the surrounding space is low and does not change when abdominal or thoracic pressures are increased.