Adrenaline vasoconstrictor and local anaesthetics
Adrenaline has been used for many years as an adjuvant to local anaesthesias aiming to prolong sensory nerve blockade and delaying systemic uptake of the anaesthetic agent,therefore reducing the risk of anesthetic toxicity.
Adrenaline is a potent vasoconstrictor due to its effect on alpha adrenergic receptors. Addition of adrenaline vasoconstrictor to a local anaesthetic creates a relative ischemia in the surgical field by reducing the blood flow to the site of the injection. This causes:
- A reasonably long-lasting local anaesthetic effect,
- Reduced rate of absorption,
- Minimal systemic action and toxicity by confining the drug to its site of action.
Norepinephrine or a vasopressin derivative ie felypressin is sometimes used for the same purpose.
The effects are useful in nerve blocks and infiltration anaesthesia but not in epidural or spinal when used with bupivacaine.
Coadministration of adrenaline should not be used intrathecally.
As blood flow is diminished, diffusion from the endoneural space into the capillary blood decreases because the critical concentration gradient between endoneural space and blood quickly becomes small when inflow of drug-free blood is reduced.
Catecholamine-type vasoconstrictors are associated with side effects such as:
Reactive hyperemia following washout of the constrictor agent and cardiostimulation when epinephrine gains entry to the systemic circulation.
In the absence of epinephrine, felypressin (analogue of vasopressin) can be used as an adjunctive vasoconstrictor with less pronounced reactive hyperemia and no arrhythmogenic action, but danger of coronary constriction is higher.
Coadministration of local anaesthetics with vasoconstrictors should not be used in local anaesthesia involving the extremities (e.g., fingers, toes) because of the risk of necrosis due to vasoconstriction.
To obtains required intense vasoconstrictor effect, a very small concentration of adrenaline is needed. Usually adrenaline used ranges from 1:80 000 to 1:200 000. Adrenaline is expressed as the weight of adrenaline in grams per volume of solution in Ml.
20 mL of 1:80 000 is the maximum safe dose representing 250 micrograms or 50mL of 1:200 000.