Local anaesthetic with maximum ionized form at physiological pH ?
## **Core Concept**
Local anesthetics are weak bases that exist in both ionized and non-ionized forms at physiological pH. The pKa of a local anesthetic determines the proportion of its ionized form at physiological pH. A lower pKa results in a higher proportion of the non-ionized form, while a higher pKa results in a higher proportion of the ionized form.
## **Why the Correct Answer is Right**
Lidocaine has a pKa of approximately 7.9. At physiological pH (around 7.4), this results in a significant proportion of lidocaine existing in its ionized form. The ionized form is more soluble in water and is the form that interacts with the sodium channel to produce the local anesthetic effect. Among the options provided, lidocaine has a pKa closest to physiological pH, but actually, the correct answer is the one with the highest pKa value among the options.
## **Why Each Wrong Option is Incorrect**
* **Option A:** If the pKa of this option is lower than that of , it would have a lower proportion of ionized form at physiological pH.
* **Option B:** Similarly, if the pKa of this option is lower than that of , it would have a lower proportion of ionized form.
* **Option D:** If the pKa of this option is lower than that of , it would have a lower proportion of ionized form.
## **Clinical Pearl / High-Yield Fact**
The effectiveness of a local anesthetic can be influenced by its pKa. Local anesthetics with a pKa close to physiological pH will have a higher proportion of ionized form, which affects their onset and duration of action. A higher proportion of ionized form can slow the onset of action but may prolong the duration.
## **Correct Answer:** .