A primer on tissue pH and local anesthetic potency

Abstract

Pope RL, Brown AM. A primer on tissue pH and local anesthetic potency. Adv Physiol Educ 44: 305-308, 2020; doi:10.1152/advan. 00018.2020.-The relationship between pH, pKa, and degree of local anesthetic ionization is quantified by the Henderson-Hasselbalch equation. As presented in standard textbooks, the effect of pH on the degree of ionization of any particular local anesthetic is not immediately clear due to the x-axis displaying pH, pKa, which requires conversion to pH, based on the pKa for each local anesthetic, a complex process. We present a graphical solution that clarifies the interrelationships between pH, pKa, and degree of ionization by plotting pKa on the x-axis versus the percentage of unionized local anesthetic on the y-axis. The vertical intercept from the x-axis to the pH curves allows rapid and accurate estimation of the degree of ionization of any local anesthetic of known pKa.