Explain the advantages of spinal anesthesia over epidural anesthesia.
Although major similarities exist between spinal anes-thesia and epidural anesthesia, significant differences are noted (Table 56.3). Spinal anesthesia is usually accom-plished with a 26- or 22-gauge needle, which is technically easier to manipulate between bony prominences than the typical 17-gauge epidural needle. CSF flowing through the needle is a more reliable indicator of proper needle place-ment than subtle movement of a liquid drop or even loss of resistance as used in epidural techniques. Spinal anesthesia requires minute amounts of local anesthetic which, if injected intravascularly, often might not manifest central nervous system alterations. Epidural techniques require such large amounts of local anesthetic that the risk of toxicity from absorption is markedly increased. Direct intravascular injection is almost certain to result in pro-found central nervous system and cardiovascular collapse. Although spinal techniques are infamous for their poten-tial to produce headaches, the intensity of the discomfort is often less severe than that associated with unintentional dural puncture with the standard 17-gauge epidural needle. Epidural anesthesia offers a lower risk of headache but frequently produces a more severe, incapacitating headache. The incidence of significant neurologic compli-cations other than headache is extremely low for both techniques. Spinal anesthesia tends to provide far more profound analgesia than do epidural techniques. The inci-dence of patchy block is very low with spinal anesthesia but considerably more frequent with epidural anesthesia. Spread of anesthesia is more predictable with spinal tech-niques using varying baricities and positions. Spread of anesthesia associated with epidural techniques is less predictable and cannot be adjusted by position or baricity of the injected anesthetic. The onset of spinal anesthesia is rapid and that of epidural anesthesia tends to be slower. The duration of both techniques is virtually limitless when using catheters. It is debatable whether hemodynamic alterations and hypotension are more pronounced with spinal anesthesia than with epidural anesthesia.