Background and Objectives: We have investigated how the vascular components of the spine determine the resistance and capacitance of the spinal epidural space and determined the magnitude of the longitudinal pressure gradient in the space during fluid infusion.
Methods: Pigs were studied during isoflurane anesthesia. Tuohy needles were inserted into midthoracic spine at adjacent interspaces, one to measure pressure in the epidural space and one for fluid infusion. A third Tuohy needle was inserted in the lumbar epidural space. Fluid was infused at a constant flow (0.9-3.1 mL/min) until epidural space pressure reached a steady plateau, then the pump was shut off, and pressure returned to baseline over 5 to 6 mins. Resistance was calculated as the ratio of plateau pressure and flow. Initial (first 20-30 secs) and late (1-5 mins) capacitance was calculated. Measurements were made with the animal alive, 15 mins after its life was terminated by an intravenous injection of KCl, and then after exsanguination.
Results: During fluid infusion, the pressure gradient between the lumbar and thoracic epidural space was small, on the order of 1 to 2 mm Hg. Death reduced resistance but not capacitance, whereas exsanguination reduced late capacitance but not resistance. Neither maneuver affected initial capacitance.
Conclusions: There is a small longitudinal pressure gradient within the epidural space during fluid infusion. Hence, the major source of resistance occurs where fluid leaves the epidural space. Death reduced resistance, perhaps by depressurizing spinal arteries in the intervertebral foramina, but did not affect capacitance. Blood in epidural veins is a major determinant of late epidural capacitance.
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This study was supported by a grant from Department of Anesthesiology, University of Pittsburgh, Pittsburgh, PA.
Presented at the ASA Annual Meeting, San Francisco, CA, October, 2007.