Why is adrenaline added to local anesthetics?
Most local anesthetics share an undesirable side effect: they dilate blood vessels (cause vasodilation). As a result, the administered drug is absorbed too rapidly into the systemic circulation, leaving the tissues of the maxillofacial region. Consequently, the depth of anesthesia decreases, and the duration of action is drastically shortened.
To compensate for this drawback, dental practice employs vasoconstrictors, the primary one being adrenaline (epinephrine). Adding it to the anesthetic solution solves several clinical problems at once:
- Delayed absorption: the drug remains in the tissues longer, reducing its overall systemic toxicity.
- Enhanced anesthesia: the depth of nerve impulse blockade increases.
- Prolonged action: the duration of effective pain relief increases several fold.
- Improved visibility: vasoconstriction reduces capillary bleeding in the surgical field, which is critical in oral and maxillofacial surgery.
Interaction of adrenaline with procaine (novocaine)
Procaine (Procainum) has moderate local anesthetic activity and a wide therapeutic index. However, among all anesthetics, it exhibits the most pronounced vasodilatory effect.
Due to such strong vasodilation, procaine requires the addition of a high concentration of a vasoconstrictor. Without adrenaline, infiltration or conduction anesthesia of soft tissues lasts only 15 to 30 minutes. With the addition of adrenaline, this figure increases up to 60 minutes.
It is important to remember that the use of procaine with high doses of adrenaline is undesirable in at-risk patients (e.g., those with severe heart, liver, and kidney diseases, as well as hypotension). The maximum injectable dose of procaine without a vasoconstrictor is 500 mg (1–2% solutions are used). At the same total dose, the toxicity of the drug increases in proportion to the increase in its solution concentration.
Use of adrenaline with trimecaine and amide anesthetics
Trimecaine (Trimecainum) belongs to the amide group. In terms of its anesthetic activity, it surpasses procaine by 2–3 times, acts faster and longer, without irritating tissues. Nevertheless, it also causes vasodilation and is somewhat more toxic (especially at high concentrations). The maximum total injectable dose is 300 mg.
In dental practice, trimecaine is used in combination with vasoconstrictors, which enhance and prolong its effect. To slow down absorption, adrenaline is added to a 2% trimecaine solution in a strict proportion: 0.1% solution, 1–2 drops per 5–10 ml of anesthetic solution.
Modern local anesthetics of the amide group (such as lidocaine, mepivacaine, articaine, bupivacaine, and others) diffuse better into tissues, act faster, and provide a larger zone of anesthesia compared to esters. However, the principles of their combination with vasoconstrictors remain similar: adrenaline is necessary to achieve the most effective and safe anesthesia during surgical interventions on the upper and lower jaws.
Risks of systemic effects upon rapid absorption
If the anesthetic is administered without adrenaline or if its concentration is insufficient, the drug quickly enters the systemic circulation, exerting a resorptive effect. Using procaine as an example, this manifests in the following effects:
- Central nervous system: depressive effect, decreased excitability of the motor areas of the cerebral cortex.
- Cardiovascular system: decreased myocardial excitability (antiarrhythmic effect).
- Autonomic nervous system: ganglion-blocking effect, inhibition of acetylcholine formation.
In case of overdose (which adrenaline helps prevent by retaining the drug locally), neuromuscular transmission is disrupted. Symptoms progress with increasing concentration: from dizziness and paresthesia to motor excitation, convulsions, dyspnea, and collapse.
The rule of three "P"s for adrenaline in dentistry: Prolongs anesthesia, Potentiates the effect, Prevents (reduces) bleeding.
Class and exam questions
How long does adrenaline prolong the action of procaine?
Plain procaine anesthetizes soft tissues for only 15–30 minutes. With the addition of a vasoconstrictor (adrenaline), this period increases to 60 minutes.
In what proportion is adrenaline mixed with trimecaine?
A 0.1% adrenaline solution is used to slow absorption. It is added at a rate of 1–2 drops per every 5–10 ml of trimecaine solution.
Why does procaine require high doses of a vasoconstrictor?
This drug has the strongest vasodilatory effect among all local anesthetics. Without a high concentration of adrenaline, it is washed out of tissues into the systemic circulation too quickly.
Can anesthetics with adrenaline be used in at-risk patients?
The use of combinations with a high concentration of a vasoconstrictor in at-risk patients (e.g., those with severe heart disease) is highly undesirable and requires extreme caution.
What else to study on this topic
- Comparative characteristics of ester and amide local anesthetics.
- Pharmacokinetics and resorptive effects of procaine on body systems.
- Indications and contraindications for the use of trimecaine in dentistry and cardiology.
- Mechanism of action of local anesthetics during therapeutic nerve blocks.
- Pathogenesis and symptoms of local anesthetic overdose.
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