During treatment of a seizure induced by lidocaine during an interscalene block, which rationale for hyperventilating with 100% O2 is incorrect?

During treatment of a seizure induced by lidocaine during an interscalene block, which rationale for hyperventilating with 100% O2 is incorrect?

• A. Helps to prevent and treat hypoxia
• B. Hyperventilation decreases blood flow and delivery of lidocaine to the brain
• C. Hyperventilation induces hypokalemia which elevates the seizure threshold
• D. Hyperventilation induces alkalosis which converts lidocaine to the protonated (ionized) form

Answer: (D)

Hyperventilating with 100% oxygen in this situation is aimed at reducing brain exposure to lidocaine while keeping the patient well-oxygenated. The main physiological effects are respiratory alkalosis from blowing off CO2, cerebral vasoconstriction that lowers cerebral blood flow and thus lidocaine delivery to the brain, and a shift of potassium into cells that tends to raise the seizure threshold. The chemistry of lidocaine supports these effects: lidocaine is a weak base with a pKa around 7.9, so increasing pH (alkalosis) shifts the equilibrium toward the non-ionized form, which actually crosses membranes more readily and can increase CNS penetration and toxicity. Therefore, the idea that alkalosis converts lidocaine to the protonated (ionized) form is incorrect; alkalosis decreases protonation and increases the non-ionized fraction, potentially increasing brain exposure rather than reducing it.

Hyperventilating with 100% oxygen in this situation is aimed at reducing brain exposure to lidocaine while keeping the patient well-oxygenated. The main physiological effects are respiratory alkalosis from blowing off CO2, cerebral vasoconstriction that lowers cerebral blood flow and thus lidocaine delivery to the brain, and a shift of potassium into cells that tends to raise the seizure threshold. The chemistry of lidocaine supports these effects: lidocaine is a weak base with a pKa around 7.9, so increasing pH (alkalosis) shifts the equilibrium toward the non-ionized form, which actually crosses membranes more readily and can increase CNS penetration and toxicity. Therefore, the idea that alkalosis converts lidocaine to the protonated (ionized) form is incorrect; alkalosis decreases protonation and increases the non-ionized fraction, potentially increasing brain exposure rather than reducing it.