Hi all,
I’ve often been confused by the differences between the myriad of choices we have for local anesthetics floating around our emergency department, so I’m dedicating this POTD to lining out some of the key differences.
Local anesthetics vary in their potency, allowing for concentrations that range typically from 0.5 to 4%. This is largely the result of differences in lipid solubility, which enhances diffusion through nerve sheaths and neural membranes. They will interrupt neural conduction by inhibiting the influx of sodium ions through channels or ionophores within neuronal membranes.
Local anesthetics have greater affinity for receptors within sodium channels during their activated and inactivated states than when they are in their resting states. Therefore, neural fibers having more rapid firing rates are most susceptible to local anesthetic action. Also, smaller fibers are generally more susceptible, because a given volume of local anesthetic solution can more readily block the requisite number of sodium channels for impulse transmission to be entirely interrupted.
Epinephrine is often added to a local anesthetic solution, which allows the clinician to use a lower dose of the anesthetic and improve safety. Further, epinephrine acts as a vasoconstrictor and delays absorption of the anesthetic into the peripheral arteriole, thus increasing the duration of action. The addition of epinephrine can also improve hemostasis by inducing vasoconstriction in the surgical field.
To best compare between some common choices, I figured a graph would be the best visual. I’ve highlighted the most common anesthetics we use in our emergency department.
Interestingly, bupivacaine exists in two enantiomers (yeah I know, sorry for the PTSD from organic chemistry), which are mirror images of each other. Although structurally identical, enantiomers can exhibit clinical differences including potency and adverse effects. The discovery of a selective blockade of cardiac Na+ channels by the dextro-enantiomer of bupivacaine led to the creation and widespread use of two levo-enantiomers: levobupivacaine and ropivacaine. These exhibit lower potency at myocardial Na+ and K+ channels and have less effect on myocardial electrical conduction and contractility compared to bupivacaine. Hence our move away from bupivacaine.
Also, don’t be fooled by all the fancy brand names – these will not tell you whether there is epinephrine present or a specific concentration. For example, Xylocaine may be on the bottle but this isn’t some specific formulation with/without epinephrine or any specific concentration – it just means lidocaine. Read your bottles carefully and calculate your maximum dose before injecting, especially on large wounds. Here for your reference:
