• Clinical science

Local anesthetic agents

Abstract

Local anesthetics (LAs) are drugs that block the sensation of pain in the region where they are administered. LAs act by reversibly blocking the sodium channels of nerve fibers, thereby inhibiting the conduction of nerve impulses. Nerve fibers which carry pain sensation have the smallest diameter and are the first to be blocked by LAs. Loss of motor function and sensation of touch and pressure follow, depending on the duration of action and dose of the LA used. LAs can be infiltrated into skin/subcutaneous tissues to achieve local anesthesia or into the epidural/subarachnoid space to achieve regional anesthesia (e.g., spinal anesthesia, epidural anesthesia, etc.). Some LAs (lidocaine, prilocaine, tetracaine) are effective on topical application and are used before minor invasive procedures (venipuncture, bladder catheterization, endoscopy/laryngoscopy). LAs are divided into two groups based on their chemical structure. The amide group (lidocaine, prilocaine, mepivacaine, etc.) is safer and, hence, more commonly used in clinical practice. The ester group (procaine, tetracaine) has a higher risk of causing allergic reactions or systemic toxicity and is, therefore, reserved for patients with known allergies to drugs of the amide group. Overdose or inadvertent injection of an LA into a blood vessel can cause systemic toxicity, which mainly affects the CNS (tinnitus, seizures, etc.) and the CVS (bradycardia, arrhythmias, etc.).

Overview

Local anesthetics Duration of action

Ester group

Procaine Short
Tetracaine long

Amide type

Lidocaine Intermediate
Prilocaine Intermediate
Mepivacaine Intermediate
Bupivacaine long
Etidocaine long
Ropivacaine long

Ester LAs do not have an “i” in their names preceding “-caine.” Amide LAs have an "i" in their name preceding “-caine.”

References:[1][2][3]

Effects

  • Pain pathway: Thermal, mechanical, or chemical stimuli → nociceptor stimulation → conversion of stimulus to an electric signal (action potential) → neural conduction of electric signal to the CNS → perception of pain
  • Local anesthetics (LA) bind to voltage-gated sodium channels of the nerve fibers; → reversible blockage of sodium channelsinhibition of nerve excitation and impulse conduction (pain signals) → interruption of pain pathwaylocal anesthesia in the area supplied by the nerve
  • Nerve fibers have different degrees of susceptibility to LA, depending on their size and myelination
    • Small diameter nerves are the first to be anesthetized
    • Myelinated nerves are blocked faster than unmyelinated nerves
  • Factors which affect the efficacy of LA
    • Use of vasoconstrictors; (e.g., adrenaline): reduce the systemic absorption; of LAs and prolong the anesthetic effect
    • Inflamed/infected tissue: Decrease the efficacy of LAs

References:[4][5][6][7][8][9]

Side effects

References:[5][9][10]

We list the most important adverse effects. The selection is not exhaustive.

Indications

References:[9]