... a number of factors, including potency, speed of onset, duration of action of local anesthetics, site and duration of surgery, the degree of muscle relaxation required, and the duration of analgesia desired. Duration of anesthesia ith a given agent varies ith the site of injection and frequently ith the total mass of drug injected. i70s Thus, bupivacaine injected into the epidural space lasts approximately 2 to 3 hours hereas the same dose injected into the brachial plexus may last 10 to 11 hours. Vasoconstrictors, such as epinephrine, are added to increase the duration of action, provide an indication of intravascular injection, and reduce peak serum levels of local anesthetic. The extent to hich epinephrine prolongs the duration of anesthesia depends on the specific local anesthetic used and the site of injection. Vasoconstrictors do not prolong the duration of action of all local anesthetics in all situations Table 2 Table Not Available . Epinephrine prolongs the duration of action of all agents for peripheral nerve blocks except ropivacaine. i33s It also prolongs the duration of action of epidural chloroprocaine, lidocaine, and mepivacaine. The local anesthetic properties of the intrinsically more potent and longer acting agents bupivacaine, etidocaine, tetracaine are influenced less by the addition of epinephrine, particularly hen such agents are used epidurally. Epinephrine does not markedly prolong the duration of motor block by epidural bupivacaine or etidocaine hoever, it does extend the sensory block by these epidural agents. i71s The effects of epinephrine added to agents used for spinal anesthesia are discussed later.The optimal dose of epinephrine is one that ould produce maximal increase in the duration of a local anesthetic agent and minimal hemodynamic effects. Kennedy et al i36s shoed that a supraclavicular brachial plexus block ith 30 mL of 1.6 lidocaine has virtually no hemodynamic effects hereas the same agent ith epinephrine 1200,000 produced a dose-related increase in cardiac rate, cardiac output, and stroke volume that persisted for 90 minutes and decreases in peripheral resistance and concomitant changes in mean arterial pressure that persisted for 120 minutes. Absorbed epinephrine produces predominantly beta-adrenergic effects ith little evidence of alpha-adrenergic effects at TABLE 2 -- DURATION OF ACTION OF LOCAL ANESTHETICS AND EFFECT OF EPINEPHRINE Adapted from Ellis JS Local anesthetics. In Kirby, Gravenstein Clinical Anesthesia Practice. Philadelphia, B Saunders, 1994. Not Available714doses up to 400 mug. Furthermore, epinephrine produced a dose-related increase in mean duration of anesthesia, but only up to a concentration of 1200,000, above hich the cardiocirculatory changes continued to increase ithout any further increase in the duration of anesthesia. The optimal dose of epinephrine in the above study as a 1200,000 solution or 5 mugml concentration. If the use of epinephrine is desirable it should be added to the local anesthetic just before the local anesthetic is used. The reason for this is that commercial solutions of epinephrine containing epinephrine are buffered to a loer pH than the standard solution of that agent in an effort to oxidation of the epinephrine. Such acidification moves the pH farther from the pKa of that solution, reducing the availability of the free base and the rate of diffusion of the local anesthetic. The speed of onset of local anesthetics is primarily related to the agent selected and the site of injection. Thus, agents such as chloroprocaine have a more rapid onset in the epidural space than agents such as lidocaine and bupivacaine, and lidocaine and mepivacaine have a more rapid onset than bupivacaine hen used for peripheral nerve blocks. Efforts to increase the speed of onset of local anesthetics by the addition of bicarbonate have yielded contradictory results and appear to be minimally effective.SEDATIONANALGES ... Download