Local Anaesthesia: Definition, Composition, Classification, Doses, Mechanism of Action and Complications

Local Anaesthesia
Definition, Composition, Classification, Doses, Mechanism of Action and Complications

Local anaesthetics (LA) are drugs that are used to cause a reversible loss of sensation by inhibiting nociception in a defined body area. This loss of sensation is achieved by targeting specific nerves or the local infiltration or topical application to give predictable and targeted anaesthesia by blocking the sodium channels that facilitate nerve impulses in tissue.

Local anaesthesia, as defined by Malamed.

"Local anaesthesia has been defined as loss of sensation in a circumscribed area of the body caused by depression of excitation in nerve endings or inhibition of the conduction process in peripheral nerves."

An important feature of local anaesthesia is producing the loss of sensation without inducing loss of consciousness. In this one major area, local anaesthesia differs dramatically from general anaesthesia. Local anaesthesia aims to minimise pain so that procedures can be conducted as efficiently and comfortably as possible.

Local anesthesia is used in multiple dental procedure like RCT (click here to read about root canal treatment in detail), Extraction, Dental Implants, etc.

Composition of Local Anesthetic agent with their function.

Local Anaesthetic agent: Lignocaine HCL – 2% (20 mg/ml)
Vasoconstrictor: Adrenaline - 1:80,000 (0.012mg) or Epinephrine. It decreases blood flow to the site of injection to decrease the risk of local toxicity, and a higher volume of local anaesthetic agent remains in and around the nerve for a more extended period.
Reducing Agent: Sodium Metabisulphite – 0.5 mg. These act as preservatives for vasoconstrictor agents to prevent the oxidation of vasoconstrictors.
Preservative: Methylparaben – 0.1% (1mg) maintains the stability of local anaesthesia.
Isotonic Solution: Sodium Chloride – 6 mg
Fungicide: Thymol
Vehicle: Ringer's Solution. All the above solutions and local anaesthetic agents are dissolved in a modified ringer solution. This isotonic vehicle minimises discomfort during injection.
Diluting Agent: Distilled water
To adjust pH: Sodium Hydroxide
Nitrogen Bubble: 1-2mm in diameter and is present to prevent Oxygen from being trapped in the cartridge and potentially destroying the Vasopressor or vasoconstrictor.

Types of Local Anaesthetics

Local anaesthetics are categorised into two different types based on their chemical structure:

Para-Aminobenzoic acid (PABA)–based anaesthetics, termed esters
Non-PABA-based anaesthetics are termed amides

Different Classifications of Local Anaesthetics

1. Classification of Local Anesthetic based on functional groups:

Esters
Amides

Use this handy mnemonic tip for remembering: LA's with one i' are esters, and LA's with two' i's are amides.

Easters

Cocaine
Procaine
Chloroprocaine
Tetracaine
Benzocaine

Amides

Lignocaine
Bupivacaine
Prilocaine
Etidocaine
Ropivacaine
Dibucaine

2. Classification of Local Anesthetic Substances based on Biological Site and Mode of Action

Classification	Definition	Chemical Substance
Class A	Agents acting at receptor site on the external surface of nerve membrane	Biotoxins (e.g., tetrodotoxin, saxitoxin)
Class B	Agents acting at receptor site on the internal surface of nerve membrane	Quaternary ammonium analogues of lidocaine Scorpion venom
Class C	Agents acting by a receptor-independent Physico-chemical mechanism	Benzocaine
Class D	Agents acting by a combination of receptor and receptor-independent mechanisms	Most clinically useful local anaesthetic agents (e.g., articaine, lidocaine, mepivacaine, prilocaine)

3. Classification of Local Anesthetic based on potency

Low	Procaine, Chloroprocaine
Intermediate	Lidocaine, Mepivacaine
High	Tetracaine Bupivacaine Dibucaine

Mechanism of action of Local Anaesthetics

Act by inhibition of potential action generation within nerve cells.
Bind reversibly to Na+ channels in the neuron cell membrane.

Pass across lipid membrane in a non-ionised state and become ionised inside the axon (intracellular environment more acidic)
The ionised form is able to bind to the Na+ channel intracellular surface, preventing Na+ ion entry and action potential generation.

Nerve fibres are blocked in a predictable sequence.

Preferentially block small diameter, myelinated and high-frequency nerve fibres.
The sequence of fibres blocked: B fibres (autonomic), C & A∂ fibres (pain and temperature sensation), Aß fibres (Light touch and pressure sensation), A⍺ & Aγ fibres (Motor and proprioception.

Preventing Local Anesthesia Allergy by Skin Test

You can perform an allergy test to diagnose potential allergies to local anaesthesia in a specific patient. First, a tiny amount of anaesthesia is pricked subcutaneously (into the skin) of the patient's arm. In case of sensitivity, a red, raised, itchy hive will appear on your skin within 15-20 minutes.

Some patients are allergic to the preservative or any other components in the LA Agent; hence it is essential to perform a skin test to check for any allergic reaction before using it for the nerve block.

Heart disease patients should not be administered vasoconstrictors like adrenaline or Epinephrine; instead, use a pure LA.

Doses

Esters	Max Dose (mg/Kg)	Duration (in hours)
Chloroprocaine	12	0.5 - 1
Procaine	12	0.5 - 1
Cocaine	3	0.5 - 1
Tetracaine	3	1.5 - 6

Amides	Max Dose (mg/Kg)	Duration (in hours)
Lidocaine	4.5/(7 with epi)	0.75 - 1.5
Mepivacaine	4.5/(7 with epi)	1 - 2
Prilocaine	8	0.5 - 1
Bupivacaine	3	1.5 - 8
Ropivacaine	3	1.5 - 8

Calculating the maximum dose of Local Anaesthetic for a child

Factors Affecting Local Anesthetic Action

Factor	Action Affected	Description
pKa	Onset	Lower Pka = More rapid onset of action, more RN molecules present to diffuse through nerve sheath; thus onset time is decreased
Lipid Solubility	Anesthetic Potency	Increased lipid solubility = Increased potency (e.g. procaine = 1; etidocaine = 140)
		Etidocaine produces conduction blockade at very low concentrations, whereas procaine poorly suppresses nerve conduction, even at higher concentrations
Protein Binding	Duration	Increased protein binding allows anesthetic cations (RNH+) to be more firmly attached to proteins located at receptor sites; thus duration of action is increased
Nonnervous tissue diffusibility	Onset	Increased diffusibility = Decreased time of onset
Vasodilator activity	Anesthetic potency and duration	Greater vasodilator activity = Increased blood flow to region = Rapid removal of anesthetic molecules from injection site; thus anesthetic potency and duration are decreased

Complications

Complications can also be divided based on Local and Systemic Effects

Local Complications

Burning on injection
Pain on injection
Persistent anaesthesia (Paresthesia)
Facial Nerve Paralysis
Infection
Edema
Trismus
Hematoma
Slugging of tissue
Postoperative intraoral lesions – Recurrent Aphthous Stomatitis, Herpes Simplex

Systemic Complications

Allergy
Idiosyncrasy
Drug interaction
Toxicity
Syncope
Serum hepatitis
Hyperventilation
Occupational dermatitis
Respiratory arrest
Cardiac arrest

The important points to remember for LA

Rate of systemic absorption of Local Anesthetics

Mnemonic: In descending order – BICEPS

Blood (Intravenous)
Bronchial (Tracheal)
Intercostal
Caudalpara
Cervical
Epidural
Plexus (Brachial plexus)
Sacral
Subcutaneous

List of long-acting to short-acting local anaesthetics in descending order with the following phrase:

'Delhi To Bombay Love Priyanka Chopra'

D – Dibucaine
T – Tetracaine
B – Bupivacaine
L – Lignocaine
P – Prilocaine
C – Cocaine

References:

1. Handbook of Local Anesthesia, Stanley Malamed

DrDent_Notes