Factors Affecting Level of Block
Factors affecting the level of epidural anesthesia may not be as predictable as with spinal anesthesia. In adults, 1–2 mL of local anesthetic per segment to be blocked is a generally accepted guideline. For example, to achieve a T4 sensory level from an L4– L5 injection would require about 12–24 mL. For segmental or analgesic blocks, less volume is needed.
The dose required to achieve the same level of anesthesia decreases with age. This is probably a result of age-related decreases in the size or compli-ance of the epidural space. Although there is little correlation between body weight and epidural dos-age requirements, patient height affects the extent of cephalad spread. Thus, shorter patients may require only 1 mL of local anesthetic per segment to be blocked, whereas taller patients generally require 2 mL per segment. Although less dramatic than with spinal anesthesia, spread of epidural local anesthetics tends to be partially affected by gravity. The lateral decubitus, Trendelenburg, and reverse Trendelenburg positions can be used to help achieve blockade in the desired dermatomes.
Additives to the local anesthetic, particularly opioids, tend to have a greater effect on the quality of epidural anesthesia than on the duration of the block. Epinephrine in concentrations of 5 mcg/mL prolongs the effect of epidural lidocaine, mepiva-caine, and chloroprocaine more than that of bupi-vacaine, levobupivacaine, etidocaine, or ropivacaine. In addition to prolonging the duration and improv-ing the quality of block, epinephrine delays vascular absorption and reduces peak systemic blood levels of all epidurally administered local anesthetics.