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Pharmacokinetics of Local Anesthetics During Plexus Blocks

ALON P. WINNIE, MD^*, CHENG-HIN TAY, MD, KANCHAN P. PATEL, MD, SOMAYAJI RAMAMURTHY, MD, and ZIA DURRANI, MD

*Professor and Head. Instructor. Assistant Professor. Associate Professor. Department of Anesthesiology, Abraham Lincoln School of Medicine, University of Illinois Hospitals, Chicago, Illinois 60612.

Abstract

A new model for the study of pharmacokinetics of local anesthetics is described which allows the separate determination of onset and recovery of sensory and motor blockade in peripheral (mantle) and central (core) bundles within nerve trunks. The data obtained with this model indicate that in man the sequence and pattern of both onset of and recovery from blockade differ from those predicted on the basis of available laboratory data. Results of this study indicate that in man, following the subclavian perivascular injection of 1 percent lidocaine with 1:200,000 epinephrine the onset of motor blockade precedes, or at least occurs simultaneously with, sensory blockade, 1st in the mantle bundles and then in the core bundles. Furthermore, significant motor blockade persists as long as, if not longer than, sensory blockade in both the mantle and the core fibers, but with recovery occurring 1st in the core bundles and last in the mantle bundles. The reason that motor onset precedes sensory onset is related to the fact that because of the somatotopic arrangement of fibers in a nerve bundle at the level of the trunks, motor fibers are located more peripherally than sensory fibers, and hence a local anesthetic injected perineurally will arrive at (and begin to block) motor fibers before it arrives at the more centrally located sensory fibers. The reason that significant motor blockade outlasts sensory blockade and that the block recedes in the hand before it recedes in the upper arm is presumably that, because of the intraneural vascular architecture, as the intraneural vessels move from periphery to core, branching and rebranching, the interface between blood vessels and nerve fibrils increases progressively. Thus after the establishment of complete blockade, there will be a greater uptake of local anesthetic in the central portions of the nerve than in the periphery. Therefore, the more central fibers, ie, the sensory fibers, recover first, followed by the more peripherally placed motor fibers.

Key Words: PHARMACOKINETICS, local anesthetics • ANESTHETICS, local, pharmacokinetics • ANESTHETIC TECHNICS, regional, pharmacokinetics

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999