Charles W Buffington1, Elisabet U M Blix. 1. Department of Anesthesiology, University of Pittsburgh, Pittsburgh, PA, USA. Buffingtoncw@anes.upmc.edu
Abstract
BACKGROUND AND OBJECTIVES: Air injected into the epidural space of the spine reaches the heart within 15 secs, suggesting easy access to the bloodstream. We wished to quantify the washout of a macromolecular tracer (albumin labeled with Evans blue) from the thoracic epidural space. METHODS: Eleven juvenile pigs were anesthetized with isoflurane and positioned on their sides. We injected a bolus of the tracer into the thoracic epidural space followed by serial saline infusions (total, 50-70 mL). We measured plasma concentrations of the tracer during washout. Finally, we injected Microfil (a liquid rubber compound that hardens on standing) into the epidural space and later inspected with a dissecting microscope the nerve roots and veins draining the spine. RESULTS: More than half (60% ± 12%; range, 31%-79%) of the tracer injected in the epidural space was detected in the bloodstream. Microfil was detected in the veins draining the cervical or high thoracic spine, but never in veins draining the lumbar spine or sacrum. CONCLUSIONS: Because the macromolecular tracer cannot enter the bloodstream by absorption, these results indicate that pathways capable of handling bulk flow connect the spinal epidural space to the venous circulation. These pathways are located in the cervical and high thoracic spine of the pig.
BACKGROUND AND OBJECTIVES: Air injected into the epidural space of the spine reaches the heart within 15 secs, suggesting easy access to the bloodstream. We wished to quantify the washout of a macromolecular tracer (albumin labeled with Evans blue) from the thoracic epidural space. METHODS: Eleven juvenile pigs were anesthetized with isoflurane and positioned on their sides. We injected a bolus of the tracer into the thoracic epidural space followed by serial saline infusions (total, 50-70 mL). We measured plasma concentrations of the tracer during washout. Finally, we injected Microfil (a liquid rubber compound that hardens on standing) into the epidural space and later inspected with a dissecting microscope the nerve roots and veins draining the spine. RESULTS: More than half (60% ± 12%; range, 31%-79%) of the tracer injected in the epidural space was detected in the bloodstream. Microfil was detected in the veins draining the cervical or high thoracic spine, but never in veins draining the lumbar spine or sacrum. CONCLUSIONS: Because the macromolecular tracer cannot enter the bloodstream by absorption, these results indicate that pathways capable of handling bulk flow connect the spinal epidural space to the venous circulation. These pathways are located in the cervical and high thoracic spine of the pig.