6.1 Local and Regional Anesthetics

Introduction to Local and Regional Anesthesia

Local and regional anesthesia provide a reversible loss of sensation in a specific area of the body without a loss of consciousness. This is achieved by temporarily blocking the transmission of pain signals to the brain. For the Certified Surgical First Assistant (CSFA), understanding these agents, their safe administration, and their potential complications is paramount, as the CSFA often assists in drawing up, labeling, and verifying these medications on the sterile field.

Mechanism of Action

Local anesthetics (LAs) work by temporarily binding to and inhibiting voltage-gated sodium channels on the nerve cell membrane. By blocking the influx of sodium ions, they prevent the depolarization of the nerve cell, thereby halting the generation and propagation of action potentials. Without action potentials, sensory impulses—such as pain—cannot reach the central nervous system.

Common Local Anesthetics

Local anesthetics are classified chemically into two distinct groups: amides and esters. Most modern LAs used in surgery are amides. A simple trick to remember: amide generic names contain an "i" before the "caine" (e.g., lidocaine, bupivacaine, ropivacaine), whereas esters do not (e.g., procaine, cocaine).

Clinical Trap

Due to its profound cardiotoxicity, bupivacaine must never be used for intravenous regional anesthesia (Bier block). Accidental intravascular injection of bupivacaine can lead to intractable ventricular arrhythmias that are highly resistant to standard resuscitation protocols.

The Role of Epinephrine

Epinephrine (Adrenalin) is frequently mixed with local anesthetics in concentrations such as 1:100,000 or 1:200,000. It acts as a potent vasoconstrictor and serves two critical purposes on the surgical field:

1. Prolongs Anesthesia: By constricting local blood vessels, it slows the absorption of the anesthetic into the systemic circulation, keeping the drug concentrated at the operative site for a longer period.
2. Reduces Bleeding: Vasoconstriction significantly reduces capillary bleeding, providing a much clearer surgical field for the surgeon and CSFA.

CRITICAL WARNING: Local anesthetics containing epinephrine must never be injected into terminal appendages (digits, nose, penis, or ears). The profound vasoconstriction can severely compromise the limited blood supply in these areas, causing irreversible ischemia, tissue necrosis, and gangrene.

Regional Anesthesia Techniques

Regional anesthesia involves blocking nerves at specific locations to numb larger, targeted regions of the body.

Spinal Anesthesia (Subarachnoid Block)

• Location: The anesthetic is injected directly into the subarachnoid space (cerebrospinal fluid) in the lumbar spine, usually below the L2 vertebra to avoid trauma to the spinal cord.
• Effect: Provides profound, rapid-onset anesthesia and muscle relaxation for the lower abdomen, pelvis, and lower extremities (e.g., total knee arthroplasty, cesarean section).
• Complications: The sympathetic blockade causes widespread vasodilation, frequently resulting in a sudden drop in blood pressure (hypotension). Another common complication is a post-dural puncture headache (spinal headache).

Epidural Anesthesia

• Location: Injected into the epidural space, just outside the dura mater.
• Effect: Slower onset than a spinal block. A catheter can be left in place for continuous infusion or postoperative pain control. Common in labor and delivery, as well as major thoracic and abdominal surgeries.

Peripheral Nerve Blocks

• Brachial Plexus Block: Numbs the entire arm and hand. Frequently used for upper extremity orthopedic surgeries.
• Bier Block (Intravenous Regional Anesthesia): A double-cuff tourniquet is applied to an extremity, and local anesthetic (strictly lidocaine, never bupivacaine) is injected into a vein distal to the tourniquet. The tourniquet prevents the drug from entering systemic circulation. Upon completion of the short procedure, the tourniquet is slowly deflated to prevent a bolus of anesthetic from entering the heart.

Local Anesthetic Systemic Toxicity (LAST)

LAST is a life-threatening emergency caused by high systemic levels of local anesthetics, typically resulting from an accidental intravascular injection or exceeding the maximum weight-based safe dose.

• Signs and Symptoms: The earliest signs are neurological, including circumoral numbness (tingling around the mouth), a metallic taste, tinnitus (ringing in the ears), and dizziness. If unrecognized, this rapidly progresses to generalized seizures, respiratory arrest, and catastrophic cardiovascular collapse.
• Treatment: Immediate airway management, seizure control (using benzodiazepines), and the rapid administration of 20% Intravenous Lipid Emulsion (Lipid Rescue). The lipid emulsion acts as a "sink" in the blood, binding the circulating anesthetic molecules and pulling them away from the heart and brain.

Maximum Recommended Doses

As a CSFA, maintaining situational awareness of the amount of local anesthetic drawn up and administered on the field is a critical safety check.

• Lidocaine without epinephrine: ~4.5 mg/kg (maximum ~300 mg)
• Lidocaine with epinephrine: ~7.0 mg/kg (maximum ~500 mg)
• Bupivacaine without epinephrine: ~2.5 mg/kg (maximum ~175 mg)
• Bupivacaine with epinephrine: ~3.0 mg/kg (maximum ~225 mg)

By adding epinephrine, the maximum allowable dose increases because systemic absorption is delayed. Always verify these limits, particularly in pediatric patients or when large volumes are infiltrated during plastic surgery procedures.