Pka affects which characteristic of anesthetic?

Pka affects which characteristic of anesthetic?

• A. Diffusion
• B. Onset
• C. Potency
• D. Duration

Answer: (B)

pKa is the pH at which half the molecules are ionized. Local anesthetics are weak bases, so their ability to produce anesthesia depends on how much is in the non-ionized form to cross nerve membranes. At physiological tissue pH (~7.4), the proportion of unionized drug is determined by the difference between pH and pKa. When the pKa is close to 7.4, a larger fraction is unionized, allowing quicker diffusion through the nerve sheath and into the axoplasm, which leads to a faster onset of action. If the pKa is much higher than the tissue pH, most molecules are ionized and diffusion into the nerve is slower, delaying onset. Therefore, pKa most directly affects onset of anesthesia.

pKa is the pH at which half the molecules are ionized. Local anesthetics are weak bases, so their ability to produce anesthesia depends on how much is in the non-ionized form to cross nerve membranes. At physiological tissue pH (~7.4), the proportion of unionized drug is determined by the difference between pH and pKa. When the pKa is close to 7.4, a larger fraction is unionized, allowing quicker diffusion through the nerve sheath and into the axoplasm, which leads to a faster onset of action. If the pKa is much higher than the tissue pH, most molecules are ionized and diffusion into the nerve is slower, delaying onset. Therefore, pKa most directly affects onset of anesthesia.