Anaesthetics Quiz

See Local and Regional Anaesthesia. Option B is correct. A local anaesthetic is a weak base that exists in two forms in the tissue: an uncharged form and a charged (ionised) form. Only the uncharged form can diffuse through the fatty nerve membrane, and once inside the nerve it reverts partly to the charged form, which then blocks the voltage-gated sodium channel from inside. The pKa is the pH at which half of the drug is charged and half uncharged, so it determines the fraction in the uncharged form at tissue pH. A lower pKa means more of the drug is uncharged at tissue pH, so more can diffuse across the membrane and the onset is faster. The three properties to link are: lipid solubility with potency, pKa with speed of onset, and protein binding with duration. Local anaesthetics block sodium channels in nerve fibres, so pain fibres are blocked first, then temperature, touch, pressure and finally motor function.

• Meaning of pKa: The pH at which half of the drug is charged and half uncharged.
• Lower pKa: More drug is uncharged at tissue pH, so it crosses the membrane faster and onset is quicker.
• Uncharged form: The only form that can diffuse through the fatty nerve membrane.
• Properties to link: Lipid solubility with potency, pKa with speed of onset, protein binding with duration.