Tetracaine

Chemical classification and structure

To understand the properties of local anesthetics, they are traditionally divided into two main groups depending on the type of chemical bond between the aromatic ring and the amino group.

Tetracaine belongs to the ester group, featuring an ester bond. This same group includes such well-known drugs as procaine hydrochloride (novocaine) and benzocaine (anesthesizin). By comparison, the second large group—substituted acid amides (with an amide bond)—includes lidocaine, bupivacaine, and articaine (the latter also containing a thiophene ring).

In its chemical structure, tetracaine is the 2-dimethylaminoethyl ester of p-aminobenzoic acid. In medical practice, it is used as the hydrochloride, which possesses an important physicochemical property—it is readily soluble in water and alcohol, making the preparation of anesthetic solutions convenient.

Pharmacological profile and toxicity

When evaluating its pharmacological action, tetracaine is frequently compared to cocaine. The drug exhibits exceptionally strong local anesthetic activity: its potency exceeds that of cocaine by approximately 10 times. However, this efficacy is associated with serious risks.

The toxicity of tetracaine is 2–5 times higher than that of cocaine. The drug is officially classified under List A (poisons). The main danger lies in its pharmacokinetic features: tetracaine has an extremely high capacity for absorption through mucous membranes into the systemic circulation. This creates a high risk of developing resorptive toxic effects.

The clinical presentation of severe tetracaine poisoning develops in several stages:

• Phase 1: Marked excitation of the central nervous system (CNS).
• Phase 2: Profound CNS depression.
• Severe intoxication: Can be fatal due to paralysis of the respiratory center.

Use in ophthalmology

Despite its limitations, tetracaine is actively used in ophthalmology due to the rapid onset of its effect (pronounced anesthesia develops within 1–2 minutes). The drug has a number of significant advantages over cocaine: it has absolutely no effect on intraocular pressure and does not cause mydriasis (i.e., does not dilate the pupils).

The choice of solution concentration strictly depends on the purpose of the procedure:

• 0.1% solution: used for routine measurement of intraocular pressure (tonometry).
• 0.25–1% or 2% solutions: used for the removal of foreign bodies and various eye surgeries.
• 0.5% solution: considered the optimal standard for most surgical interventions.

Important limitations: The use of solutions with a concentration above 2% is strictly not recommended, as they can cause damage to the corneal epithelium. Additionally, the drug is capable of causing significant dilation of conjunctival vessels. If prolonged eye anesthesia is required, special ophthalmic drug films containing a strictly dosed amount of tetracaine (0.75 mg) are used instead of drops.

Use in ENT practice and the role of vasoconstrictors

In addition to ophthalmology, the drug is used for terminal anesthesia of the nasal and nasopharyngeal mucous membranes. For these purposes, solutions at concentrations ranging from 1% to 2% are generally used.

To minimize the colossal risk of resorptive toxicity associated with the rapid absorption of the drug, a specific pharmacological strategy is employed in clinical practice. Vasoconstrictors—specifically epinephrine (adrenaline)—are invariably added to tetracaine solutions.

This combination pursues several goals at once:

1. A sharp reduction in the systemic absorption of anesthetic molecules.
2. A significant decrease in the overall toxic load on the patient's body.
3. Prolongation of the duration of action and enhancement of the local analgesic effect by retaining the drug in the tissues.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Chemical classification of anesthetics: esters versus substituted amides
• Mechanisms of resorptive toxic effects of local anesthetics on the central nervous system
• Tetracaine-impregnated ocular films: pharmacokinetics of sustained-release formulations
• Comparative profile of procaine, benzocaine, and tetracaine

Other topics in the "Analgesics" section

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