Why does lidocaine work faster than bupivacaine?
Pharmacology — Local Anaesthetic Pharmacology, pKa and Differential Block
The question
A patient has a wrist block for carpal tunnel decompression. Surgical anaesthesia is established about five minutes after infiltration with lidocaine 2%, whereas an identical block performed on the contralateral hand a week earlier with bupivacaine 0.5% took closer to twenty minutes to become dense. Lidocaine has a pKa of approximately 7.9 and bupivacaine approximately 8.1. Which property best accounts for the difference in speed of onset?
- Bupivacaine is more extensively bound to plasma proteins than lidocaine
- Bupivacaine has the higher pKa, so a smaller fraction of it is un-ionised at tissue pH
- Lidocaine has the higher pKa of the two agents
- Lidocaine is the more lipid-soluble agent, so it penetrates the nerve sheath more readily
- Lidocaine blocks the sodium channel from the extracellular surface without needing to cross the membrane
The answer
Bupivacaine has the higher pKa, so a smaller fraction of it is un-ionised at tissue pH
Bupivacaine is more extensively bound to plasma proteins than lidocaine — Protein binding determines duration, not onset. Bupivacaine is approximately 95% protein-bound compared with 65% for lidocaine, which accounts for its longer duration of action. However, receptor binding affinity has no bearing on how rapidly the drug reaches its site of action.
Bupivacaine has the higher pKa, so a smaller fraction of it is un-ionised at tissue pH — Correct. Only the un-ionised base form is sufficiently lipid-soluble to cross the nerve membrane. At pH 7.4, approximately 25% of lidocaine exists in the un-ionised form compared with roughly 15-17% of bupivacaine. A larger un-ionised fraction translates to more rapid membrane penetration and faster onset of block.
Lidocaine has the higher pKa of the two agents — The stem states that lidocaine has a pKa of 7.9 (the lower value) and bupivacaine a pKa of 8.1 (the higher value). A lower pKa yields a greater proportion of un-ionised drug at physiological pH, which is the basis for the more rapid onset.
Lidocaine is the more lipid-soluble agent, so it penetrates the nerve sheath more readily — This is incorrect. Bupivacaine is considerably more lipid-soluble than lidocaine. Lipid solubility correlates with potency rather than speed of onset. In fact, very high lipid solubility can delay onset by sequestering drug in myelin sheaths and perineural fat.
Lidocaine blocks the sodium channel from the extracellular surface without needing to cross the membrane — Both agents bind to a site on the cytoplasmic face of the sodium channel. They must therefore cross the axonal membrane to reach their receptor, which is precisely why the proportion of un-ionised drug determines onset speed.
Explanation
Local anaesthetics are weak bases. In solution, they exist in an equilibrium between two forms: an un-ionised base and an ionised cation. The Henderson-Hasselbalch equation governs the ratio, and the pKa is the pH at which the two forms are present in equal proportions. When tissue pH falls below the pKa, the equilibrium shifts toward the ionised form.
The distinction matters because the two forms serve different functions. The un-ionised base is sufficiently lipid-soluble to diffuse through the nerve sheath and cross the axonal membrane. Once inside the axoplasm, re-equilibration occurs, and it is the ionised cation that binds the receptor on the intracellular face of the sodium channel. Onset therefore depends on how much un-ionised drug is available to cross the membrane. A pKa closer to tissue pH yields a greater proportion of base form at physiological pH, and thus a faster block.
Lidocaine has a pKa of approximately 7.9. At pH 7.4, roughly 25% is un-ionised. Bupivacaine has a pKa of approximately 8.1, leaving only 15-17% in the base form. A greater number of molecules crossing per unit time produces faster block.
These relationships are best retained as three distinct pairings, since examination questions frequently conflate them. pKa governs onset. Lipid solubility governs potency (more lipid-soluble molecules partition into the membrane more readily, reducing the dose required). Protein binding governs duration (drug bound at the receptor and to plasma proteins dissociates slowly). Each property has its own physicochemical basis, and one cannot be inferred from another.
The notable exceptions merit attention. 2-Chloroprocaine has a pKa of approximately 8.7, which would predict slow onset. However, it is used at 3% concentration, and the sheer number of un-ionised molecules present at that concentration is sufficient to produce one of the fastest onsets encountered in clinical practice. Tissue pH operates in the opposite direction: injection into an infected, acidotic abscess shifts the equilibrium toward the ionised form, resulting in slow and patchy block. Alkalinisation of the solution with sodium bicarbonate reverses this shift and accelerates onset by the same mechanism.
Tips for the candidate
When a stem compares two local anaesthetics, the first step is to determine which of three questions is being asked: how fast (onset), how strong (potency), or how long (duration). Onset points to pKa, potency to lipid solubility, and duration to protein binding. Examiners construct distractors by pairing the correct property with the wrong question. With these three pairings firmly established, two or three options can usually be eliminated without any calculation.
When a stem provides a pKa value, only the direction matters. A pKa above 7.4 indicates that most of the drug is ionised at physiological pH. The further above 7.4, the smaller the un-ionised fraction and the slower the onset. Calculating exact percentages is unnecessary unless a formula is provided, but the candidate must be able to identify which of two agents has a greater proportion of free base available.
The acid-tissue trap deserves particular attention. It appears in several guises: injection into an abscess, blockade in inflamed tissue, or a local anaesthetic crossing the placenta to an acidotic fetus (ion trapping). Each scenario represents the same equilibrium shifted by pH, and the answer invariably concerns the change in the ionised-to-un-ionised ratio rather than any alteration in the drug itself.
One exception is worth retaining. If a stem mentions 2-chloroprocaine's rapid onset despite its high pKa, the explanation lies in concentration, not pKa. Recognising this prevents unnecessary doubt about a principle that has been correctly understood.