Local Anesthetics Basics

Systemic Effects of Local Anaesthetics (LA)

• Local anaesthetics (LA) are not easily absorbed through unbroken skin.
• When applied to mucous membranes or injected into tissues, they are absorbed into the bloodstream.
• Local anaesthetics are distributed throughout the body.
• They undergo metabolism and elimination.

Systemic Actions of Local Anaesthetic Agents

• Local anaesthetics cause a reversible blockage of conduction in nerve endings and nerve trunks.
• They cause direct relaxation of smooth muscle.
• They can cause myocardial depression (decreasing conduction rate, force of contraction, and electrical excitability).
• Initially, there may be excitatory effects on the central nervous system.

Absorption of Local Anaesthetic Agents into Circulation

• Absorption of local anaesthetic agents into the circulation causes the action of the LA to wear off.
• Systemic absorption can lead to toxicity.
• Factors affecting systemic absorption rate include:
  • The type and concentration of the anaesthetic agent (dose).
  • The vascularity of the tissues.
  • The presence or absence of vasoconstrictors.
  • Accidental intravascular injection.

Metabolism and Elimination of LA

• Metabolism and elimination of local anaesthetics depend on the patient's physiology.
• Most local anaesthetics are metabolized in the liver to produce inactive metabolites; a notable exception is articaine which is also metabolized in the plasma.
• The metabolites are then excreted in the urine.
• Reduced liver and kidney function may necessitate using lower doses of LA for safety.

Signs of Systemic Toxicity

• Signs of systemic toxicity are extremely rare in dentistry.
• Perioral signs include tingling and numbness of the lips.
• Visual and hearing disturbances, such as tinnitus, can occur.
• Mood changes, including irritability, agitation, anxiety, and restlessness, are possible.
• Light-headedness or dizziness may be experienced.
• Breathing may become deeper and faster.
• Patients may experience drowsiness, coma, and/or convulsions.
• Severe cases can lead to respiratory arrest and cardiac arrest.

Reducing Systemic Toxicity

• Avoiding intravascular injection, through aspiration prior to injection, is crucial.
• Injecting LA slowly helps reduce the risk of systemic toxicity.
• Using safe doses of LA.
• Including vasoconstrictors (e.g., 3% prilocaine with felypressin) can help reduce systemic absorption.
• Selecting less toxic drugs (e.g., prilocaine 3%) can also help regulate systemic toxicity.

Maximum Safe Dose

• A table is provided listing maximum safe doses for various local anaesthetics, expressed in mg/kg, along with the maximum volume of cartridges per dose for adults (70kg) and children (20kg).
• Lidocaine 2%: 4.4 mg/kg, 6.8 cartridges (adult); 2.0 cartridges (child)
• Prilocaine 3%: 5.0 mg/kg, 6.0 cartridges (adult); 1.8 cartridges (child)
• Mepivacaine 3%: 4.4 mg/kg, 4.5 cartridges (adult); 1.3 cartridges (child)
• Articaine 4%: 7.0 mg/kg, 5.0 cartridges (adult); 1.5 cartridges (child)

Contraindications and Cautions of LA Agents

• Hypersensitivity is a very rare contraindication to most local anesthetics.
• Other cautions and contraindications specific to each local anesthetic are as follows:
• Lidocaine: Beta-blockers (caution); Hypersensitivity (very rare)
• Prilocaine: Hypersensitivity (very rare); Sulfonamides; Beta-blockers (caution)
• Mepivacaine: Hypersensitivity (very rare); Children under 4 years; Pregnancy; Breastfeeding; Beta-blockers (caution)
• Articaine: Hypersensitivity; Under 4 years; Plasma cholinesterase deficiency; Pregnancy (use only if benefit outweighs risk); Avoid breastfeeding for 48 hours; Beta-blockers; ID block (risk of post-op paraesthesia)
• Epinephrine/Adrenaline: Cocaine use within the past 24 hours; Tricyclic antidepressants; Monoamine oxidase inhibitors (MAOIs); Beta-blockers; Calcium channel blockers; Diuretics; Cardiac conditions; Hypertension; Circulatory problems (e.g., prior strokes).
• Felypressin: Late stages of pregnancy (theoretical risk of inducing labour); similar to natural hormone oxytocin
• Note: Specific contraindications/cautions should be considered alongside the general contraindication and caution of Hypersensitivity.

LA Equipment

• Equipment includes cartridges, plungers, and needles.
• Specific examples of types of equipment and supplies are given.

Summary of Today's Session

• Different local anaesthetics (LA) used in dentistry and their modes of action were examined.
• Systemic effects of LA were outlined, including factors modulating absorption and elimination.
• Contraindications and cautions applicable to several LA medications were explored.
• The safe use of LA equipment was demonstrated.