Local Anesthetics in Pain Medicine

By Chris Faubel, MD —

Local anesthetics (anaesthetics) are the most frequently used medication in pain medicine.  From numbing up the skin prior to inserting larger needles, to anesthetizing joints and deeper nerves, these drugs are essential for both therapeutic and diagnostic purposes in our daily practice.

MOST COMMONLY USED LOCAL ANESTHETICS

• Lidocaine – Xylocaine (1%, 2%)
• Bupivacaine – Marcaine (0.25%, 0.5%, 0.75%)
• Ropivacaine – Naropin (0.2%, 0.5%, 0.75%, 1%)

MECHANISM OF ACTION

• Block voltage-gated sodium channels, thus blocking signal conduction —  “Membrane Stabilizers”
  • Remember: A large influx of sodium ions through sodium channels is needed to depolarize a nerve cell membrane and allow for saltatory conduction of the action potential up/down the neuron

WORKS BEST ON SMALL-DIAMETER NEURONS

• Aα (alpha) – proprioception (muscle sense)
• Aβ (beta) – touch, pressure, vibration
• Aδ (delta) – fast pain, cold
• C – slow pain, warmth and itch

ESTERS vs AMIDES

• Esters (cocaine, procaine / novocaine)
  • Risk of severe allergic reactions secondary to the breakdown product paraaminobenzoic acid (PABA)
• Amides (lidocaine, bupivacaine, ropivacaine)  [Hint:  the ones with two I‘s are amIdes]
  • Much less common allergic reactions

ADDING EPINEPHRINE

• Epinephrine is added to local anaesthetics (LAs) to decrease vascular absorption, allowing for longer anesthetic effect
• About 50% prolonged duration of action

BUFFERING WITH SODIUM BICARBONATE

• The burning sensation of lidocaine being injected into the patient’s skin may quite possibly be the most painful aspect of the injection process.

• Lidocaine is more acidic than human tissue, and this is believed to be the reason for the burning sensation.

• Bicarbonate neutralizes the acidic lidocaine –> Less pain, Faster onset
  

• Directions
  • Add 1-ml of 8.4% sodium bicarbonate to a 10ml bottle of lidocaine (with or without epinephrine) – 10:1 ratio
  • Note: the buffered lidocaine decreases the effectiveness of the epinephrine. But since we rarely use epinephrine with our injections in pain medicine, this is less relevant to us.
• The buffered lidocaine has also been shown to have even more antibacterial activity

OTHER WAYS TO TAKE THE STING OUT OF INJECTING LOCAL ANESTHETICS

• Warming the lidocaine and injecting it slowly
  • Body temperature lidocaine produces less pain than room temperature
  • One study found that the rate of administration of lidocaine had a greater impact on the perceived pain of lidocaine infiltration than did buffering.

COMPARISON OF LOCAL ANESTHETICS (click the chart to the right to enlarge)

• Quickest onset = Lidocaine
  • Least ionized, therefore quickest to pass through tissue and anesthetize the neurons
• Longest duration = Bupivacaine(3-6 hours)
  • Because the most protein-bound and fat soluble
• Ropivacaine is in-between lidocaine and bupivacaine with regards to onset and duration — though it costs MUCH more
  

CONTRAINDICATIONS

• True anaphylactic allergy with AMIDES is extremely rare
• Local sepsis and coagulopathy are relative contraindications
  • Intraarticular hemorrhage risk is very low

MAXIMUM SAFE DOSES

• The maximum doses are NOT evidence-based
• The levels are meant to prevent a toxic dose
• Lidocaine
  • 30ml of 1% or 15ml of 2% (based on 3mg per kg of body weight)
  • Can increase this if using epinephrine
• Bupivacaine
  • 25ml of 0.5% or 50ml of 0.25%
  • As this all depends on vascular absorption, these numbers will be higher if injected into areas with less vascular uptake (subcutaneously, brachial plexus, epidural)

CENTRAL NERVOUS SYSTEM (CNS) and CARDIAC TOXICITY

• CNS is more sensitive to the effects of LAs, so it is the first to demonstrate an intravascular injection
• CNS toxicity signs:
  • Tingling tongue/lips, lightheadedness, shivering, tinnitus, slurred speech, muscle twitching, loss of consciousness, coma, respiratory arrest
  • MUCH more sensitive to bupivacaine
• Cardiac toxicity signs:
  • Myocardial depression, arrhythmias, ventricular arrest
  • Bupivacaine is much more cardiotoxic
• Treating CNS and cardiac toxicities
  • IV administered lipid emulsion (LE) has been shown effective in local anesthetic toxicity

SKELETAL MUSCLE TOXICITY

• Seen in experimental settings
• Bupivacaine seems to induce myocyte apoptosis
• Only a few case reports exist, and this is with large amounts such as with continuous peripheral blockade or infiltration of a wound.
  • Very rare in the clinical setting

ARTICULAR CARTILAGE / CHONDROCYTE TOXICITY

• At high doses (30-60ml), bupivacaine and all epinephrine-containing LA formulationsdemonstrate increased chondrocyte apoptosis
  • Note: with doses in pain medicine being typically 3-10 ml, these chondrocytic doses aren’t relevant to our practices
• Dilutional effects of joint fluid and active absorption
  • Intraarticular injections of LAs are unlikely to expose the chondrocytes to the concentrations used for in vitro experiments, because of the dilutional effect of the joint fluid and the active systemic absorption

CLOSING THOUGHTS

• Bupivacaine is the Triamcinolone of local anesthetics — see the “Corticosteroids in Pain Medicine” post for an explanation of this