How pH and pKa Impact the Effectiveness of Anesthetic Drugs?

Abstract

By and large, drugs exhibit weak acid or base properties that result in an alkaline or acidic solution when dissolved. The degree of ionization in an aqueous solution can be determined by the pKa, which is unique to each drug. The ionization results in two forms: ionized (IF) and non-ionized (NIF). The IF is water-soluble, while the NIF is fat-soluble. The process is reversible and modifiable based on pH and temperature. When pH equals pKa, the IF and NIF are present in equal proportions, as determined by the Henderson-Hasselbalch equation. For weak acids, the proportion of IF is directly related to pH and the proportion of NIF is inversely related. For weak bases, the opposite is true. Many drugs used in anesthesia are weak bases, such as local anesthetics and opioids. Acidosis reduces the proportion of NIF, slowing down the drug’s efficacy as only the NIF can pass through the cell membrane. In clinical practice, local anesthetic can be injected around an abscess for incision and drainage. However, its efficacy may decrease if the surrounding tissues are acidic. To solve this problem, the physician may increase the alkalinity of the local anesthetic solution with sodium carbonate or administer antibiotics prior to the surgical procedure to enhance the drug’s effect.