Which local anesthetic has an onset of 2 minutes?

Which local anesthetic has an onset of 2 minutes?

• A. Bupivacaine
• B. Lidocaine
• C. Ropivacaine
• D. Procaine

Answer: (B)

Onset time for a local anesthetic is driven by how quickly it can cross the nerve membrane to block sodium channels. The key factor is the fraction that is non-ionized at tissue pH, which depends on the drug’s pKa. The more the drug exists in the non-ionized form at physiological pH, the faster it diffuses into the nerve and produces anesthesia. Lidocaine has a pKa value that makes a larger portion of it non-ionized at body pH compared with the other agents listed, so it penetrates the nerve membrane quickly and produces a rapid onset—around 2 minutes. The other agents—bupivacaine and ropivacaine—tend to have slower onset due to their higher pKa and greater lipid solubility affecting diffusion, and procaine also has a slower onset profile. That’s why lidocaine is the best choice for a rapid onset.

Onset time for a local anesthetic is driven by how quickly it can cross the nerve membrane to block sodium channels. The key factor is the fraction that is non-ionized at tissue pH, which depends on the drug’s pKa. The more the drug exists in the non-ionized form at physiological pH, the faster it diffuses into the nerve and produces anesthesia.

Lidocaine has a pKa value that makes a larger portion of it non-ionized at body pH compared with the other agents listed, so it penetrates the nerve membrane quickly and produces a rapid onset—around 2 minutes. The other agents—bupivacaine and ropivacaine—tend to have slower onset due to their higher pKa and greater lipid solubility affecting diffusion, and procaine also has a slower onset profile. That’s why lidocaine is the best choice for a rapid onset.