Systemic Effects of Local Anaesthetics

Systemic Effects of Local Anaesthetics (LA)

• Local anaesthetics (LA) are not easily absorbed through intact skin.
• LA are absorbed into the bloodstream when applied to mucous membranes or injected into tissues.
• LA are distributed throughout the body.
• LA are metabolized and eliminated.
• The rate of systemic absorption depends on the type of anaesthetic agent, its dose, the vascularity of the tissues, the presence/absence of vasoconstrictors, and accidental intravascular injections.

Systemic Actions of Local Anaesthetics

• Local anaesthetics cause a reversible blockage of conduction within nerve endings and nerve trunks.
• Local anaesthetics directly relax smooth muscle.
• Local anaesthetics cause myocardial depression (decreased conduction rate, decreased force of contraction, decreased electrical excitability).
• Local anaesthetics cause central nervous system depression (initial signs are excitatory).

Absorption of Local Anaesthetic Agents into the Circulation

• Systemic absorption of LA causes the action of the LA to cease.
• Systemic absorption can lead to toxicity.
• Factors affecting Systemic Absorption:
  • Anaesthetic agent and concentration
  • Vessel density of the affected tissue
  • Presence or absence of vasoconstrictor
  • Accidental intravascular injection

Metabolism and Elimination of Local Anaesthetics

• Elimination of LA depends on patient physiology.
• Most commonly used LA agents are metabolised in the liver to inactive metabolites.
  • Articaine is also metabolised in plasma.
• LA metabolites are then excreted in the urine.
• Liver and kidney function impact LA elimination. If impaired, the patient may require a lower dose for safety.

Signs of Systemic Toxicity

• Perioral signs (e.g. tingling, numbness of lips)
• Visual/hearing problems (e.g. tinnitus)
• Mood changes (irritability, agitation, anxiety, restlessness)
• Light-headed/dizziness
• Increased rate and depth of respiration
• Drowsiness
• Coma and convulsions
• Respiratory arrest
• Cardiac arrest

Reducing Systemic Toxicity

• Avoid intravascular injections. Always aspirate first.
• Inject slowly.
• Use safe doses.
• Choose less toxic drugs (e.g., 3% prilocaine with felypressin).
• Use vasoconstrictors (to reduce systemic toxicity).

Maximum Safe Dose

• Information on maximum safe doses(mg/kg) for different types of LA agents, including lidocaine, prilocaine, mepivacaine, and articaine.
• Maximum cartridge volumes (mL) for different patient groups are also included.

Contraindications and Cautions of LA Agents

• Lidocaine: Contraindicated in hypersensitivity. Cautions with beta-blockers.
• Prilocaine: Contraindicated in hypersensitivity. Cautions with sulphonamides and beta-blockers.
• Mepivacaine: Contraindicated in hypersensitivity, children under 4 years old. Cautions in pregnancy, breastfeeding, and with beta-blockers
• Articaine: Contraindicated in hypersensitivity, children under 4 years old, and those with plasma cholinesterase deficiency.. Cautions in pregnancy (only if benefit > risk), avoid breast feeding for 48 hours after administering, cautious use with beta-blockers , and ID block (risk of post-op paraesthesia)
• Epinephrine/Adrenaline: Contraindicated in cocaine use within last 24 hours, and have cautions with tricyclic antidepressants, Mono-amine oxidase inhibitors, Beta-blockers, Calcium-channel blockers, Diuretics, Cardiac conditions, Hypertension, and Circulatory problems (including previous strokes)
• Felypressin: Caution in late stages of pregnancy (theoretical risk of inducing labour)

LA Equipment

• Cartridges: Different types of cartridges are available (e.g., Lignospan Special, Septanest).
• Plunger (handle): Used for delivering LA.
• Safety plus needles: Different lengths and gauges are available.
• Bevel indicator: Used for proper needle insertion.
• Traditional LA system: Includes syringe, cartridge, and cone.

General notes

• Patients with renal or liver failure may require reduced dosages.
• Limit LA usage to 2 x 2.2ml cartridges per day, or 1 cartridge per day, respectively, for patients with renal/liver failure according to the cautions specified.
• Consult with the patient's physician regarding safe dosages when necessary.