Cirrosis Hepática

REVIEW ARTICLE
David C. Warltier, M.D. Ph.D., Editor

Anesthesiology 2001; 94:888 –906

© 2001 American Society of Anesthesiologists, Inc. Lippincott Williams & Wilkins, Inc.

Current Issues in Spinal Anesthesia
Spencer S. Liu, M.D.,* Susan B. McDonald, M.D.†

SPINAL anesthesia has enjoyed a long history of success and recently celebrated a centennial anniversary.1 Anesthesiologistsmaster spinal anesthesia early during training with achievement of competence ( 90% technical success rate) after only 40 –70 supervised attempts.2,3 The ease and long history of spinal anesthesia may give the impression that it is a simple technique with little sophistication. However, much has been learned recently regarding the anatomy, physiology, pharmacology, and applications of spinalanesthesia. This review article focuses on what is new, interesting, and clinically relevant for this simple and popular technique.

the area of the neural root cuffs.8 Here, unidirectional transport of materials from the CSF into the epidural space occurs and may contribute to clearance of spinal anesthesia agents. Another potential clinical consideration of the lamellar structure of the arachnoid iseasy separation of the arachnoid membrane from the dura during spinal puncture. This mechanical arrangement allows easy subdural deposition of spinal agents despite the free return of CSF during spinal injection, which may help to explain individual effects of spinal anesthesia.9 Spinal Cerbrospinal Fluid Volume After injection of spinal anesthetics, dilution with the CSF occurs before arrival ateffector sites in the central nervous system. Thus, individual variation in lumbosacral volumes of CSF and distribution within this volume will affect spinal anesthesia. Recent use of magnetic resonance imaging (MRI) demonstrates great variability between individuals in volume of lumbosacral CSF, with a range of 28 – 81 ml.10 Interestingly, obese individuals have substantially less CSF (~10 mlless), which is partly caused by compression of the neural foramina. Clinical correlation between volume of lumbosacral CSF and spinal anesthesia with hyperbaric lidocaine and isobaric bupivacaine is excellent, with CSF accounting for 80% of the variability for peak block height and regression of sensory and motor block (fig. 1).11 Unfortunately, volume of lumbosacral CSF does not correlate withexternal physical measurements other than weight (r 0.4, P 0.05); therefore, volume cannot be easily estimated from physical examination.11 Other important considerations include the observation on MRI that the CSF is not a “still lake” of fluid but vigorously oscillates with arterial pulsations.9 These wavelike movements may be another important factor in distribution and clearance of spinal agents andmay influence neurotoxicity from exposure to concentrated agents (see Transient Neurologic Symptoms–Neurotoxicity). Spinal Nerve Roots The target sites of spinal anesthetics are the spinal nerve roots and spinal cord. In a similar fashion to volume of CSF, individual variability in anatomy of spinal nerve roots may also explain variability in spinal anesthesia.12,13 Recent autopsy and microscopicstudies have 888

Anatomy
Meninges Many anatomic structures important for spinal anesthesia have only recently been investigated. The arachnoid membrane is a structure of obvious interest, as spinal agents must be delivered within its confines. The arachnoid membrane is composed of overlapping layers of epithelial cells connected by tight junctions.4 This anatomic arrangement allows the arachnoidmembrane, not the dura, to function as the principal meningeal barrier (90% of resistance) to materials crossing in and out of the cerebrospinal fluid (CSF).4 A functional proof of the arachnoid’s importance as gatekeeper to the CSF is that spinal CSF resides in the subarachnoid and not subdural space. The arachnoid membrane serves not only as a passive container of CSF but also actively...