Geneva is reviewing for her upcoming quiz in Pharmacology. She should be aware that local and regional anesthesia act by:
A. Inhibiting depolarization.
B. Increasing depolarization.
C. Producing a semiconscious state.
D. Inhibiting motor movement.
Correct Answer: A. Inhibiting depolarization.
When local anesthesia is used, sensation is removed and the area anesthetized by inhibition of depolarization. Local anesthetics work to anesthetize skin, subcutaneous tissue, and peripheral nerves for invasive or surgical procedures. Local anesthetics block nerve impulse transmission in the peripheral and central nervous system without causing central nervous system depression or altered mental status. Choices C and D do not occur with local anesthesia.
Option B: Local anesthetics block voltage-gated sodium channels, which prevents sodium influx into the cell and blocks impulse transmission. Local anesthetics are also class I antiarrhythmic drugs due to the blockade of cardiac sodium channels, with lidocaine being the class IB prototype. They selectively block channels that are frequently depolarizing (tachyarrhythmias) and slow transmission.
Option C: Local anesthetics work in the nonionic form. In the presence of a low pH, the ionized form is dominant, and this can delay the onset of action; this also explains why local anesthetics are not effective in sites of inflammation, where an acidic environment is common. Thus, many clinicians add sodium bicarbonate to overcome the acidity and increase the efficacy of the local anesthetic.
Option D: 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.