A primer on tissue pH and local anesthetic potency

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