Local Anesthetic

Local anesthetics produce anesthesia by inhibiting excitation of nerve endings or by blocking conduction in peripheral nerves. Cocaine, a compound indigenous to the Andes Mountains, West Indies, and Java, was the first anesthetic to be discovered and is the only naturally occurring local anesthetic; all others are synthetically derived. Cocaine was introduced into Europe in the 1800s following its isolation from coca beans. Sigmund Freud, the noted Austrian psychoanalyst, used cocaine on his patients and became addicted through self-experimentation.

Contents:

  • Brief overview of structure & mode of action of local anesthetic
  • Different types of local anesthetic
  • Safe dosage calculations
  • Complications of peripheral nerve blocks
  • Treatment of toxicity
  • Contraindications

How do local anesthetic work?

Modern local anesthetics are amides supplied in a water soluble weak acid solution

The unionised base diffuses into nerve cells and causes reversible blockade of Na+ channels in excitable neural tissue, preventing action potential propagation

They are less effective in infected tissue due to the relative acidity of the tissue

Local anesthetics types
Local anesthetics types

Maximum safe dosages of local anesthetics

  • Depends on vascularity of injection site
  • Depends on health of patient
  • Cautiously increased in presence of adrenaline (vasoconstrictor)
Anesthetic

route of administration

Maximum single dose without vasoconstrictor (mg/kg)Maximum single dose with vasoconstrictor (mg/kg)Onset of action

(min)

Duration of action in isolation (min)

[w/ vasoconstrictor, if available]

Esters

Procaine

infiltration, subcutaneous

7-10

– not to exceed 1000 mg total

1020 – 30

[30-45 w/ epinephrine]

Chloroprocaine

infiltration, subcutaneous

10-12

– not to exceed 800 mg per dose

14

– not to exceed 1000 mg per dose

6-1230-60

[60-90 w/ epinephrine]

Tetracaine

topical, skin & mucous membranes

infiltration, subcutaneous

1-3

– topical skin, adults: 7g/24h

– topical skin, children: 2g/24h

– topical mucous membranes: 20 mg/dose

– infiltration, subcutaneous: 3 mg/kg per dose

1.53-8120-180

Amides

Lidocaine

topical, skin & mucous membranes

infiltration, subcutaneous

3-4.5

– topical skin: 4.5 mg/kg per dose, not to exceed 300 mg

– topical mucous membranes: 4.5 mg/kg per dose, not to exceed 300 mg per dose, maximum 2400 mg/24h

– infiltration, subcutaneous: 4.5 mg/kg per dose

6-7 (infiltration)

– not to exceed 500 mg per dose

– infiltration 1-3

– topical skin 3-5

30-120

[120-240 w/ epinephrine]

Mepivacaine

infiltration, subcutaneous

4.5-5

– not to exceed 400 mg per dose

– maximum 1000 mg/24h

6.6

– not to exceed 400 mg per dose if with levonordefrin

– not to exceed 500 mg per dose if with epinephrine

3-2045-90

[60-330 w/ levonordefrin, 120 w/ epinephrine]

Bupivacaine

infiltration, subcutaneous

2-2.5

– not to exceed 175 mg per dose

– maximum 400 mg/24h

2.5-3

– not to exceed 225 mg per dose

– maximum dose 400 mg/24h

2-10120-175

[180-480 w/ epinephrine]

Levobupivacaine

infiltration, subcutaneous

2

– not to exceed 150 mg per dose

3180-360
Ropivacaine

infiltration, subcutaneous

2-3

– not to exceed 225 mg per dose

3-4

– not to exceed 225 mg per dose

3-15120-240

[180-480 w/ epinephrine]

Articaine

infiltration, subcutaneous

n/a71-9[60-230 w/ epinephrine]

ECG Interpretation and How to Read ECG

Which local anesthetic is better to use?

  • Lignocaine (lidocaine)
    • Fast onset (< 10 minutes)
    • Duration of action 2 hours Max.
    • Safe dose: 3 mg/kg
    • Cautious dose increase to 5 mg/kg with vasoconstrictor

Local anesthetic dosing for a 70 kg man

  • ALWAYS consult the anaesthetist before administration as they may have already performed a regional technique such as a femoral nerve block

The following is a guide for appropriate maximum dosing for lignocaine and bupivacaine Maximum safe dosing

  • Lignocaine –
    • Max. safe dose = 3 mg/kg – 5 mg/kg if combined with adrenaline
    • Available in 1% or 2% solution
  •  Bupivacaine –
    • Max. safe dose = 2mg/kg
    • Available in 0.25% or 0.5% solution

Remember: a 1% solution = 10mg/ml and sums are easy!

Calculation of Maximum Recommended Dosage of local anesthetics:

  1. The concentration of the local anesthetic should be known.
    • Percent concentration must be converted to concentration of drug (mg/mL) as follows (table modeled from AAP, AAPD, CJ Cote, S Wilson, Work Group on Sedation)
    • Concentration %mg/mL
      440
      330
      2.525
      220
      110
      0.55
      0.252.5
      0.1251.25
  2. The weight of the patient should be known.

How to Read Chest X-Ray

Local Anesthetic Examples

1% Lignocaine Example

How much 1% solution can I give to a 70kg man?

  • Max. safe dose = 3mg/kg = 210mg
  • 1% solution = 10mg/ml so 210/10 = 21ml
  • With adrenaline = 5mg/kg – 350mg/10 = 35ml

2% Lignocaine Example

How much 2% solution can I give to a 70kg man?

  • Max. safe dose = 3mg/kg = 210mg
  • 2% solution = 20mg/ml so 210/20 = 10.5ml
  • With adrenaline = 5 mg/kg – 350mg/20 = 17.5 ml

Bupivacaine

How much 0.5 % solution can I give to a 70kg man?

  • Max. safe dose is 2mg/kg = 140mg
  • 0.5% = 5mg/ml – 140mg/5 = 28ml

Bupivacaine

How much 0.25 % solution can I give to a 70kg man?

  • Max. safe dose is 2mg/kg = 140mg
  • 0.25% = 2.5mg/ml – 140mg/2.5 = 56ml

More examples about dosage of local anesthetic (Lidocaine):

  • Total dose of local anesthetic that can be used
    • Maximum dose of lidocaine (plain, without vasoconstrictor)  is 4.5 mg/kg (not to exceed 300 mg)
    • Example patient weight – 10 kg
    • Total dose that can be used for this patient = 4.5 mg/kg x 10 kg = 45 mg
  • Maximum volume of lidocaine administered
    • Depends on concentration (see conversion table above)
    • E.g. for 1% lidocaine: contains 10 mg of lidocaine per 1 mL
    • Max volume of 1% lidocaine that can be administered to a 10 kg patient  = 45 mg / 10mg/mL = 4.5 mL

ABG interpretation and arterial blood gases analysis

Complications of peripheral nerve block

Technique

  • Neural damage
  • Bleeding/hematoma
  • Intravascular injection of LA
  • Infection
  • Undesired effects e.g. temporary nerve palsy

Drug

  • Toxicity
  • Anaphylactoid response (v.rare)
  • Methaemaglobinaemia (prilocaine)

Toxicity of local anesthetics

Local anesthetics toxicity
Local anesthetics toxicity
  • Immediate: due to intravascular injection
  • Delayed: due to absorption from vascular site, accumulation, repeated injection, relative overdose

Systemic toxicity may occur from administration of local anesthetics and is related to the serum concentration of the drug as it is absorbed into the circulation. This serum concentration is influenced by the dose, site, and method of drug administration (Berde & Strichartz, Butterworth et eds, Catterall & Mackie, Curtis et al, Rosenberg et al, Schwartz & Kaufman).

What are the signs of Local Anesthetic toxicity?

  1. CNS:
    1. Agitation
    2. Sleepiness
    3. Perioral numbness/tingling
    4. Slurring of speech
    5. Fitting
  2. CVS:
    1. – Hypotension
    2. – Arrhythmias, often VT/VF
    3. – Bradycardias ( May be masked by addition of adrenaline to LA)

Management of LA Toxicity

  • Call for help
  • STOP INJECTING
  • Remember A B C
  • Administer oxygen, support airway
  • Ventilate with bag & mask if necessary
  • Ensure iv access, give fluids
  • Begin CPR if necessary

The only books you need to read about local anesthesia: