MEDizzy - It is common to include small amounts of epinephrine (EPI)

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Pharmacology Of The Peripheral Nervous Systems 1

It is common to include small amounts of epinephrine (EPI) in solutions of local anesthetics that will be administered by infiltration (injection around sensory nerve endings), as when a skin laceration needs suturing. Which of the following is the most likely reason for, or outcome of, including the EPI?

ExplanationThere are several reasons for and outcomes of adding a vasoconstrictor (usually epinephrine) to some local anesthetics, including those given by infiltration. The vasoconstrictor confines the local anesthetic to the desired site of action (site of administration) by reducing local blood flow and the rate of anesthetic entry into the bloodstream. It is the bloodstream (or, more precisely, it is the presence of anesthetic in the systemic distribution system) that delivers the drug to sites where signs and symptoms of toxicity occur. Slow down anesthetic absorption rates and the drug will enter the circulation slowly enough that it can be metabolized (inactivated) fast enough to prevent accumulation to toxic levels. That is, we reduce the risk of systemic toxicity with added vasoconstrictor. Another outcome of including a vasoconstrictor is to prolong (and certainly not shorten) the duration of local anesthetic effect. That occurs also because reduced local blood flow keeps the anesthetic in the vicinity of sensory nerves longer (since the anesthetic is not being removed as quickly by blood flow). More on this issue will be presented in the CNS questions. But for now: Local anesthetics (except cocaine) can cause vasodilation (not vaso- constriction; a), but unless the local anesthetic dosages are quite high (toxic), the vasodilation is not intense; hypotension is uncommon with usual dosages. Likewise, cardiac depression can occur, but again that is a manifestation of overdose. We don’t routinely include epinephrine in a local anesthetic to combat or prevent these cardiac-depressant problems (b), and the amount of epinephrine found in these preparations is far too low to do anything meaningful to remedy these adverse responses should they occur. Likewise, the amounts of vasoconstrictor are far too low to prevent (or treat) anesthetic-induced anaphylaxis (c)—a reaction that requires only a few molecules of antigen to occur. So what the vasoconstrictor does is essential cause a pharmacologic tourniquet, reducing regional blood flow that otherwise would quickly “wash away” the anesthetic. This essentially confines the anesthetic to the desired site longer (not shorter; e) than otherwise and decreases the potential systemic reactions (d). Some local anesthetics cause vasodilation, which allows more compounds to escape the tissue and enter the blood. Procaine is an ester-type local anethetics with a short duration of action due to rather rapid biotransformation in the plasma by cholinesterases. The duration of action of the drug during infiltration anesthesia is greatly increased by the addition of epinephrine, which reduces the vasodilation caused by procaine. Finally, in plastic surgery and many other types of surgery the direct vasoconstricor effects of EPI not only antagonize the local vasodilator effects of a local anesthetic, but also cause direct vasoconstriction