Local Anesthetics: Lignocaine, Articaine, Vasoconstrictors

Local anaesthetics block pain in a specific area without unconsciousness, and they are dentistry's most-used drugs. The essentials are the agent (lignocaine, articaine), the vasoconstrictor added to it (adrenaline), and the maximum safe dose, calculated for every patient.

This matters because overdose is preventable but real: too much drug, or accidental injection into a vessel, can cause seizures and cardiac collapse. Dose mathematics before the injection keeps routine dentistry routine.

Key Concepts

• Mechanism: blockade of voltage-gated sodium channels in nerves stops the pain impulse; sensation returns as the drug wears off.
• Lignocaine (lidocaine) 2%: the global standard; onset 2-3 minutes, pulpal anaesthesia ~60 minutes plain, 90-120 minutes with adrenaline.
• Articaine 4%: thiophene ring gives superior bone diffusion; excellent for infiltrations, safe within its dose limit.
• Vasoconstrictors: adrenaline 1:100,000 or 1:80,000 keeps the anaesthetic local longer, reduces bleeding, and slows systemic absorption. Used cautiously in severe cardiac disease.
• Concentration facts: 2% = 20 mg per mL, so a 1.8 mL cartridge of 2% lignocaine holds 36 mg. Adrenaline 1:100,000 = 10 micrograms per mL.
• Maximum doses: lignocaine 4.4 mg/kg plain (max 300 mg), 7 mg/kg with adrenaline (max 500 mg); articaine 7 mg/kg (max 500 mg).

Formulas

• Maximum dose (mg) = weight (kg) x mg/kg limit. Example: 60 kg adult, lignocaine + adrenaline: 60 x 7 = 420 mg.
• Cartridges allowed = maximum dose / mg per cartridge. 420 / 36 = 11.6, so at most 11 cartridges, far above routine 1-2.
• mg per cartridge = mg/mL x 1.8 mL. For 4% articaine: 40 x 1.8 = 72 mg.
• Safety habit: always aspirate before injecting, inject slowly (~1 mL per minute), use the minimum effective volume.

Worked Example

A healthy 60 kg adult needs a molar extraction under 2% lignocaine with 1:100,000 adrenaline.

1. Maximum dose: 60 x 7 = 420 mg (under the 500 mg ceiling).
2. Per cartridge: 20 x 1.8 = 36 mg.
3. Maximum cartridges: 420 / 36 = 11.67, rounded down to 11.
4. The extraction needs only 1-2 cartridges; the maths simply proves the planned dose is a small fraction of the toxic threshold.

Common Misconceptions

• "I am allergic to the injection." True lignocaine allergy is rare; most reactions are fainting from anxiety or palpitations from adrenaline.
• "More cartridges mean better numbness." Failed numbness is about technique and anatomy, not volume; extra cartridges risk toxicity without benefit.
• "Articaine is unsafe." It is safe within 7 mg/kg and widely used; concerns relate to specific blocks, not general toxicity.

Real-World Applications

• Dentists calculate the maximum dose from weight before every injection, especially in children.
• History screening flags cardiac disease (minimize adrenaline) and true allergy.
• Every clinic's emergency kit carries oxygen and seizure-management drugs for the rare toxic event.

Local anaesthesia is safe because it is calculated: sodium-channel blockade silences the nerve, adrenaline keeps the drug local, and mg/kg limits turn body weight into a maximum cartridge count. Aspirate, inject slowly, use the least that works, respect the ceiling: painless dentistry with a built-in safety margin.