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The Effects of Induced Hypertension and Acute Graded Compression on Impulse Propagation in the Spinal Nerve Roots of the Pig

Lind, Bengt, MD, PhD*; Massie, Jennifer B., BS*; Lincoln, Todd, MD*; Myers, Robert R., PhD; Swenson, Michael R., MD; Garfin, Steven R., MD*

The Effects of Induced Hypertension and Acute Graded Compression on Impulse Propagation in the Spinal Nerve Roots of the Pig: PDF Only
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Injuries caused by compression of spinal nerve roots are frequently encountered clinically. Experimental studies show that several different factors affect the pathophysiologic changes that occur after these injuries. However, the effect of hypertension in conjunction with graded compression of spinal nerve roots is yet unclear. A previously established porcine model was employed, in which the spinal nerve roots were exposed and compressed by an inflatable balloon. Impulse propagation across the compressed nerve segment was studied by the recording of efferent and afferent nerve action potentials, and nerve conduction velocity. The systemic blood pressure was increased by administration of Neo-Synephrine hydrochloride (phenylephrine HCI) (Winthrop Pharmaceuticals, New York, NY) and elevated 40 ± 5 mm Hg above the normal (100 ± 5 mm Hg), and electrophysiologic baseline values were recorded. The spinal nerve roots were then compressed for 2 hours with either 0 (control), 50, 100, or 200 mm Hg. The balloon was deflated and the nerve roots were allowed to recover for 1.5 hours. Impulse propagation was studied every 15 minutes and hypertension was maintained throughout the experiment. The results showed no significant changes of the efferent and afferent nerve action potentials at 0.50 or 100 mm Hg. At 200 mm Hg, efferent and afferent nerve action potentials decreased rapidly and were almost abolished within 20 minutes of compression. Some but not significant recovery was seen of the nerve impulse. Compared to data from normotensive pigs in an earlier experiment, the current study showed that hypertension significantly decreases the susceptibility of the spinal nerve roots to compression at and below 100 mm Hg. Histologic sections of the compressed nerve segments revealed more structural changes with an increase of compression pressure.

*From the Departments of Orthopaedics and Neuroscience, University of California, San Diego, California.

Supported by VA-grant # B/535R and by grants from the Swedish Medical Research Council and the Göteborg Medical Society.

Accepted for publication March 29, 1993.

© Lippincott-Raven Publishers.