Local Anesthetics: Part 4

Tumescent Anesthesia

  • Provides comfort during liposuction.

  • Contains:

    • Sodium chloride

    • Lidocaine

      • Helps decrease pain

    • Epinephrine

      • Decreases vascular uptake of lidocaine

    • Bicarb

  • All components are diluted and injected into the adipose tissue.

  • Effects:

    • Firms the adipose tissue, facilitating removal.

  • Lidocaine Dosage:

    • Should not exceed 55 mg/kg according to the Academy of Dermatology.

    • Serum lidocaine levels:

    • 1.5 mcg/ml is below CNS and cardiovascular toxic threshold.

    • Plasma concentration peaks at 12 hours and is eliminated at 36 hours.

    • Metabolism occurs through CYP3A4 and CYP2D6 enzymes.

  • Anesthesia Types:

    • MAC (Monitored Anesthesia Care):

    • Used for small doses.

    • General Anesthesia:

    • Requires more than 2-3 liters of Tumescent injected.

    • Risk factors include fluid overload and pulmonary edema due to IV volume expansion.

      • Importance of fluid management is emphasized.

Methemoglobinemia

  • Definition:

    • Heme component of hemoglobin contains an iron molecule (ferrous, Fe²⁺) at the oxygen binding site.

    • In methemoglobin, iron is oxidized to the ferric form (Fe³⁺), preventing oxygen binding.

  • Impact on oxygenation:

    • Causes a leftward shift in the oxyhemoglobin dissociation curve due to decreased oxygen carrying capacity (CaO₂).

    • Diagnosis requires co-oximeter because methemoglobin absorbs 660 nm and 940 nm light equally, leading to inaccurate pulse oximetry.

  • Symptoms of Methemoglobinemia:

    • Cyanosis occurs with normal PaO₂ (arterial oxygen partial pressure).

    • Associated signs:

    • Hypoxia

    • Tachycardia

    • Tachypnea

    • Changes in mental status

    • Coma and potential death.

  • Common Drug Risks:

    • Local anesthetics:

      • Benzocaine

      • Cetacaine (contains benzocaine)

      • Prilocaine

      • EMLA cream (contains prilocaine and Lidocaine)

    • Other drugs:

      • Nitroprusside

      • Nitroglycerin

      • Phenytoin

      • Sulfonamides

  • Treatment:

    • Methylene blue:

    • Dosing:

      • 1-2 mg/kg IV over five minutes, maximum dose is 7-8 mg/kg.

    • Mechanism:

      • Acts as an electron donor, converting methemoglobin back to hemoglobin via methemoglobin reductase.

    • Patients with glucose-6-phosphate dehydrogenase deficiency:

      • Might not have methemoglobin reductase, requiring exchange transfusion.

    • Fetal hemoglobin has a deficiency in reductase, increasing oxidation risk leading to toxicity.

EMLA Cream

  • Composition:

    • 5% EMLA cream is a 50/50 combination of:

      • 2.5% Lidocaine

      • 2.5% Prilocaine

  • Pharmacological properties:

    • Lower melting point increases absorption; requires occlusive dressing post-application.

  • Common indications:

    • Arterial or venous cannulation

    • Lumbar puncture

    • Myringotomy.

  • Analgesia onset:

    • Begins in one hour, maximum effect after 2-3 hours.

  • Enhancements:

    • Nitroglycerin can improve absorption.

  • Risk Management:

    • Eczema, psoriasis, or skin wound increases toxicity risk.

      • Must apply to intact skin.

  • Prilocaine Metabolism:

    • Metabolized to o-toluidine, which oxidizes hemoglobin to methemoglobinemia, particularly in pediatrics and infants.

  • Dose Recommendations:

    • Zero to three months or < 5 kg: max dose 1 g, max application area 10 cm².

    • Three to twelve months or > 5 kg: max dose 2 g, max area 20 cm².

    • One to six years or > 10 kg: max dose 10 g, max area 100 cm².

    • Seven to twelve years or > 20 kg: max dose 20 g, max area 200 cm².

Additives to Local Anesthetics

Drugs Prolonging Duration of Action:

  • Epinephrine:

  • Decreases systemic uptake, enhancing block duration and quality, extends duration more for lidocaine than bupivacaine.

  • Decadron:

  • Prolongs action through glucocorticoid activity modifying steroid receptors, enhancing duration of brachial plexus blocks by 50%.

  • Dextran:

  • Low molecular weight; increases action duration by decreasing systemic uptake.

  • Drugs Causing Analgesia with Local Anesthetics:

    • Clonidine:

    • Alpha-2 agonist, dosage is 100 mcg mixed with local anesthetic.

    • Epinephrine:

    • Also provides analgesia due to alpha-2 agonism effects.

    • Opioids:

    • Effective for spinal and epidural anesthesia; mixed results for peripheral blocks.

    • Caution: Chloroprocaine may reduce opioid efficacy in epidural applications.

  • Drugs Affecting Onset of Action:

    • Sodium Bicarbonate:

    • Alkalinization increases lipid-soluble molecules, expediting onset.

    • Mix 1 ml of 8.4% sodium bicarb with 10 ml of lidocaine.

    • Important: Sodium bicarb should not precipitate with local anesthetics.

  • Drugs Enhancing Tissue Diffusion:

    • Hyaluronidase:

    • Hydrolyzes hyaluronic acid present in the interstitial matrix and basement membrane, facilitates anesthetic spread.

    • Used in ophthalmic blocks to enhance onset speed, does not elevate intraocular pressure.

    • Decreases hematomas and lowers postoperative strabismus risks.

    • Notable: Allergy risk associated with hyaluronidase.

  • Conclusion:

    • Summary emphasizes the importance of tumescent anesthesia, methemoglobinemia, EMLA cream applications, and the various additives affecting anesthetic action, all critical for safe and effective anesthesia practice.