Snapshot A 36-year-old man presents to the emergency department with a wound laceration of the hand. He is a construction worker that accidently cut himself with his tools approximately 2 hours prior to presentation. His most recent tetanus vaccination was given 2 years ago. There does not appear to be any possible contamination or foreign body at the site. The wound extends beyond the dermis. Wound irrigation is commenced and a local anesthetic is administered prior to suturing. Introduction Local anesthetics prevent sensory nerve impulses from reaching the central nervous system (CNS) this is accomplished by blocking the inner portion of the sodium channel which in turn prevents the propagation of an action potential most effective in rapidly firing neurons structure a lipophilic group is joined to a hydropilic group via an amide or ester linkage biotransformation of amides mainly occur in the liver tertiary amine local anesthetics cross membrane in uncharged form and undergo ionic change in order to bind to sodium channel in charged form biotransformation of esters are accomplished by plasma cholinesterases (pseudocholinesterase) onset and duration of action influenced by tissue pH infection can decrease pH (more acidic) in the affected tissue alkaline anesthetics will therefore become charged, which impairs its ability to penetrate the membrane to block sodium channels more anesthetic would be needed lipid solubility of the drug drug concentration nerve morphology effects local anesthetics can result in vasodilation, which causes the drug to diffuse away from the site of action vasoconstrictors (e.g., epinephrine) can correct this thus enhancing the local action of the drug order of nerve blockade small-diameter fibers > large diameter fibers and myelinated fibers > unmyelinated fibers size predominates over myelination order of loss (first) pain → temperature → touch → pressure (last) Medications Medication esters benzocaine, cocaine, procaine, and tetracaine amides bupivacaine, lidocaine, and mepivacaine Clinical use minor surgical procedures spinal anesthesia Adverse effects CNS symptoms excitation or depression cardiovascular toxicity (bupivacaine) arrhythmia methemoglobinemia (benzocaine)