Local Anesthetic Duration of Action depends on a number of factors. The important factors effecting the duration of action of Local anesthetics are given below:
1. Dose: Increased dose of the local anesthetic increases the duration of action of the nerve block.
2. Pharmacokinetic profile of drug: It includes:
i. Plasma protein binding (Alpha1 acid glycoprotein) Agents with high protein binding like bupivacaine have prolonged action.
ii. Metabolism: Esters are metabolized by pseudo cholinesterase and amides are metabolized in liver b Microsomal enzymes. Esters have shorter duration of action than amides.
3. Addition of vasoconstrictors: Vasoconstrictors decrease the systemic absorption of local anesthetics in blood, so increases the concentration thereby increasing the duration of action. Most commonly used constrictors are:
a. Adrenaline: It is commonly used for increasing the Local Anesthetic Duration. The Adrenaline mechanism for this has already been explained in another article.
b. Phenylephrine: In concentration of I in 20,000. Other vasoconstrictors which can be used to increase Local Anesthetic Duration are noradrenaline and felypressin (octopressin), a synthetic derivative of vasopressin.
4. Sodium bicarbonate: Addition of sodium bicarbonate (1 ml of 8.4% to 10 ml of lignocaine) increases both Local Anesthetic Duration of action and its onset. Sodium bicarbonate increases the onset by making the pH more alkaline so more drug is available in unionized form. Later carbon dioxide released from sodium bicarbonate metabolism enters intracellularly making the pH more acidic making more drug to be available in ionized form, thereby increasing the duration of action also.
So addition of sodium bicarbonate:
a. Enhances the onset of action.
b. Increases the Local Anesthetic Duration of action.
c. Improves the quality of block.
d. Decreases the pain of injection.