Comprehensive Guide to Nerve Blocks, Anaesthetic Agents, and Toxicity Management

Anaesthetic Dosage and Calculations

• Fundamentals of Solution Concentrations:

  • $1\,ml$ of water weighs exactly $1\,g$.
  • A $100\%$ solution is defined as $1\,g/ml$ (which is equivalent to $1000\,mg/ml$).
  • A $10\%$ solution contains $100\,mg/ml$.
  • A $1\%$ solution contains $10\,mg/ml$.
  • A $0.5\%$ solution contains $5\,mg/ml$.

• Formula for Calculating Maximum Volume:

  • $\text{Max volume allowed (ml)} = \frac{\text{Maximum dose (mg)}}{\text{Concentration (\%)} \times 10\,mg/ml}$

Clinical Pearls and Additives

• General Injection Tips:

  • Use small-gauge needles (specifically $25G$ or $27G$) to minimize trauma.
  • Warm the anaesthetic solution to body temperature prior to the procedure to reduce the burning sensation/pain associated with injection.

• Sodium Bicarbonate ($NaHCO_3$):

  • Preparation: Add $1\,ml$ of $8.4\%$ sodium bicarbonate to $9\,ml$ of $1\%$ Lignocaine.
  • Clinical Benefits:
    • Decreases pain perceived during injection.
    • Results in a shorter onset of action.
    • Increases the effectiveness of the local anaesthetic in acidic (e.g., infected) environments.

• Adrenaline (Epinephrine):

  • Function: Increases the total duration of action and allows for a higher maximum dose by reducing systemic absorption through local vasoconstriction.
  • Caution: Use care when administering in extremities (fingers, toes, nose, penis), although current evidence suggests the risks of ischemia may be overstated.

Local Anaesthetic Agent Comparison

• Lignocaine ($0.5-1\%$\text{)}:

  • Onset: Rapid ($2-5\,min$).
  • Duration: $1-2\,hrs$ (extends to $4\,hrs$ with adrenaline).
  • Maximum Dose (Plain): $4\,mg/kg$ up to a total of $300\,mg$.
  • Maximum Dose (with Adrenaline): $7\,mg/kg$ up to a total of $500\,mg$.

• Bupivacaine ($0.25\%$\text{)}:

  • Onset: Slow ($2-5\,min$).
  • Duration: $4\,hrs$ (extends to $8\,hrs$ with adrenaline).
  • Maximum Dose (Plain): $2\,mg/kg$ up to a total of $175\,mg$.
  • Maximum Dose (with Adrenaline): $3\,mg/kg$ up to a total of $225\,mg$.

• Ropivacaine ($0.75\%$\text{)}:

  • Onset: Medium ($1-15\,min$).
  • Duration: $2-3\,hrs$ (extends to $6\,hrs$ with adrenaline).
  • Maximum Dose: $3\,mg/kg$.

Nerve Block Procedures

• Digital Nerve Block (Transthecal Approach):

  • Technique: Insert the needle at the center of the palmar digital crease until bone is contacted. Withdraw slightly until the loss of resistance to injection is felt.
  • Agents: Use $1\%$ Lignocaine or $0.25-0.5\%$ Bupivacaine.
  • Restriction: Specifically avoid the use of adrenaline for finger blocks.

• Median Nerve Block:

  • Positioning: Place the wrist in slight extension.
  • Technique: Insert the needle at the radial border of the palmaris longus muscle, just proximal to the proximal wrist crease. Advance until a "slight pop" is felt or until the needle is approximately $1\,cm$ deep.
  • Volume: Use $3-5\,ml$ of $1\%$ Lignocaine or $0.25-0.5\%$ Bupivacaine.
  • Note: Apply a subcutaneous wheal while withdrawing the needle to ensure coverage of the palmar branch.

• Radial Nerve Block:

  • Main Injection: Inject $2-5\,ml$ of $1\%$ Lignocaine just lateral to the radial artery (always aspirate first to ensure no intravascular injection).
  • Supplemental: Provide a $5-6\,ml$ subcutaneous field block extending from the initial insertion site to the dorsal midline of the wrist.

• Ulnar Nerve Block:

  • Approach: Target the nerve under the Flexor Carpi Ulnaris (FCU) at the proximal palmar crease.
  • Technique: Inject a $3-5\,ml$ wheal of $1\%$ Lignocaine as the needle is withdrawn.
  • Branch Coverage: Block cutaneous branches with a $5-6\,ml$ subcutaneous band extending from the lateral border of the FCU to the dorsal midline.

• Tibial Nerve Block:

  • Technique: Insert needle $1\,cm$ proximal to the medial malleolus, perpendicular to the skin. Aim just posterior to the Posterior Tibial artery at a depth of $0.5-1\,cm$.
  • If Paresthesia occurs: If a "needle wiggle" triggers paresthesia, aspirate and inject $3-5\,ml$ of $1\%$ Lignocaine.
  • If no Paresthesia: Continue advancing to the tibia, withdraw $1\,mm$, aspirate, and inject $5-7\,ml$ as the needle is withdrawn over a distance of $1\,cm$.
  • Alternative Landmark: If the Tibial Artery (TA) cannot be palpated, enter just anterior to the Achilles tendon.

Local Anaesthetic Systemic Toxicity (LAST)

• Central Nervous System (CNS) Manifestations:

  • Typically occurs before Cardiovascular (CVS) effects, except in massive intravenous overdoses where cardiac arrest may be the presenting symptom.
  • Symptoms: Tongue paresthesia, metallic taste, lightheadedness, tinnitus (ringing in ears), visual disturbances, muscle twitching, anxiety, confusion, seizures, coma, and apnea.

• Cardiovascular System (CVS) Manifestations:

  • Bupivacaine is noted as being particularly cardiotoxic due to its prolonged binding to myocardial tissue.
  • Effects: Myocardial depression, hypotension, bradycardia, supraventricular and ventricular arrhythmias.

• Methemoglobinemia:

  • Associated with Lignocaine or Prilocaine.
  • Note: This is not dose-related.
  • Symptoms: Blue mucous membranes (cyanosis), progressing to CNS and CVS manifestations of hypoxia.

• Differential Diagnosis (DDx):

  • Allergy/Anaphylaxis: Presenting with rash, angioedema, tachycardia, hypotension, wheeze, or stridor.
  • Vasovagal Response: Presenting with lightheadedness, visual/hearing disturbances, bradycardia, and hypotension.
  • Excess Catecholamines: Presenting with tachycardia, hypertension, anxiety, and tremors.

Management of Toxicity

• Immediate Resuscitative Steps:

  1. Call for help.
  2. Move the patient immediately to the Resuscitation (Resus) area.
  3. Establish IV access, apply $O_2$, and place on monitors. Have an advanced airway ready; intubate if there are signs of cardiac toxicity.
  4. Encourage mild hyperventilation to decrease the seizure threshold; hyperventilate if the patient is intubated.

• Specific Pharmacological Management:

  • Seizures: Usually self-limiting; treat with Midazolam.
  • Hypotension and Bradycardia: Administer fluids, Atropine, or an Adrenaline infusion.
  • ECG Changes: Administer Sodium Bicarbonate ($NaHCO_3$) boluses of $2\,mmol/kg$ until stabilization.
  • Cardiac Arrest (Lipid Rescue):
    • Use $20\%$ Lipid Emulsion (Intralipid).
    • Stat Dose: $100\,ml$ (or $1-1.5\,ml/kg$) IV bolus; repeat $1-2\times$.
    • Infusion: Follow with $0.25\,ml/kg/min$ until hemodynamic stability is restored.
  • Methemoglobinemia: Administer Methylene Blue to all symptomatic patients at a dose of $1-2\,mg/kg$ IV over $5\,min$.

• Investigations:

  • Blood Gas: To confirm the presence of methemoglobinemia.
  • ECGs: Look for signs of $Na^+$ (Sodium) channel blockage, including:
    • Right axis deviation.
    • Prolonged PR interval.
    • Prolonged QRS duration.
    • Large terminal R wave in lead aVR.