vasoconstricting agent like epinephrine, is often added to local anesthetic solutions. This provides the following advantages:
- Decreases the rate of systemic absorption
- Reduces the risk of systemic side effects
- Prolongs the duration of action of the anesthetic
- Improved hemostasis because of its vasoconstrictive effects
Premixed solutions containing epinephrine are acidified even further than plain local anesthetics. This increased acidity delays the onset of action and is more painful on injection. There's no utility in using higher than 1:100,000 epinephrine solutions. No additional vasoconstrictive benefit is offered, whereas the risk of toxicity increases in a dose-dependent manner. Adequate hemostasis relies significantly on allowing adequate time for the vasoconstrictive effects to occur. This usually takes 7-10 minutes.
Contraindications to the use of epinephrine-containing solutions include patients with unstable angina, cardiac dysrhythmias, severe uncontrolled hypertension, or pregnant patients with placental insufficiency. Relative contraindications include hyperthyroidism and concurrent use of MAOI or tricyclic antidepressants. When contraindicated, phenylephrine (1:20,000) may be substituted, however it's not as efficient as epinephrine.
The Addition of Bicarbonate
Sodium bicarbonate may be added to local anesthetics in order to alkalinize the answer. This neutralization of the low pH creates a solution that is less irritating to the tissues and less painful on administration. The limiting factor in the addition of bicarbonate is the tendency for the lipid soluble agents, such as bupivicaine, to precipitate at the more neutral pH values. As a result, bicarbonate may be added to lidocaine but ought to usually not be used with bupivicaine.
Lidocaine is the most widely utilized local anesthetic. It's ready as a 1% (10 mg/ml) or 2% (20 mg/ml) solution with or with out epinephrine. Its duration of action is about 1.5 hours without epinephrine, and this is doubled to 3 hours with the addition of epinephrine to the solution (1:100,000). Lidocaine may also be utilized as a dilute answer (0.2%-0.5%) for certain procedures like a rhytidectomy. This answer is adequately anesthetizing and vasoconstrictive.
A commonly utilized dilute answer, the modified Klein solution, may be ready as follows: 20 ml of 2% lidocaine, 5 ml of sodium bicarbonate, and 1 ml of 1:1,000 epinephrine all mixed in 500 ml of lactated Ringer's answer. The maximum safe dose for plain lidocaine is reported as 3-4 mg/kg. With the addition of epinephrine, this increases to 7 mg/kg. Recent research, nevertheless, refutes this number, providing evidence for a much higher maximal safe dose-up to 35 mg/kg when combined with epinephrine.