A primer on tissue pH and local anesthetic potency

The relationship between pH, pK_a, 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 - pK_a, which requires conversion to pH, based on the pK_a for each local anesthetic, a complex process. We present a graphical solution that clarifies the interrelationships between pH, pK_a, and degree of ionization by plotting pK_a 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 pK_a.