Ch10: Anesthesia II — Local & Regional Anesthesia（局部麻醉与区域麻醉）

Preparatory Mindset

Local/regional anesthesia = reversible loss of sensation in a body region without unconsciousness. The exam tests: classification (esters vs amides), maximum doses, local anesthetic toxicity (中毒), and the techniques (surface, infiltration, field block, nerve block, spinal, epidural) (CM exam tested). The prep PDFs give the exact facts: "酯类：普鲁卡因1g、丁卡因10mg；酰胺类：利多卡因400mg、布比卡因150mg"; "局麻药中毒表现与防治".

Exam mindset: esters vs amides (which letter, which metabolism — pseudocholinesterase vs liver), the dose table, toxicity (CNS excitation → depression, CV collapse; treatment = stop, support, lipid emulsion for bupivacaine), and spinal vs epidural (levels, complications — hypotension, post-dural puncture headache, high block).

Core Concepts

1. Local anesthetics — classification (MUST KNOW)

• Mechanism: block voltage-gated Na+ channels in nerve membranes → prevent depolarization → conduction block (small/demyelinated fibers first — pain, then temp, touch, motor)

2. Maximum doses (MUST KNOW — from prep PDFs)

• Epinephrine added: vasoconstriction → ↓ systemic absorption → longer duration, higher max dose (avoid in end-artery regions: fingers, toes, penis, ear — digital ischemia)
• Bupivacaine: long-acting, cardiotoxic (refractory arrhythmia) — limit dose; ropivacaine = safer cardiac profile

3. Local anesthetic toxicity (局麻药中毒) — MUST KNOW

1. Early (CNS): circumoral numbness, metallic taste, tinnitus, lightheadedness, agitation, confusion, muscle twitching → generalized convulsions 2. Late (CNS depression): unconsciousness, respiratory depression, apnea 3. Cardiovascular: hypotension, bradycardia, arrhythmias → cardiac arrest (esp. bupivacaine)

1. STOP injection; maintain airway & breathing (100% O2, intubate if needed) 2. Treat convulsions: benzodiazepines (midazolam/diazepam), thiopental, propofol; avoid respiratory depression 3. Support circulation: IV fluids, vasopressors (ephedrine/norepinephrine) 4. Cardiac arrest (bupivacaine): IV lipid emulsion (20% intralipid) — binds the drug; prolonged CPR; avoid lidocaine/amiodarone? (use lipid + standard ACLS) 5. Monitor & ICU

• Definition: toxic reaction due to high blood level (overdose, inadvertent IV injection, rapid absorption from vascular site)
• Causes/predisposition: excessive dose, IV injection, vascular area (intercostal, caudal), poor technique, reduced metabolism (liver disease, neonates)
• Clinical features (CNS excitation → depression → CV collapse):
• Prevention: aspirate before injection (no IV), use correct dose, add epinephrine, use test dose, monitor
• Treatment (防治):

4. Techniques of local/regional anesthesia (MUST KNOW)

• Surface (topical) anesthesia (表面麻醉): ointment/spray on mucosa (eye, ENT, bronchoscopy) — lidocaine, tetracaine
• Infiltration anesthesia (局部浸润麻醉): inject into the tissue layers — lidocaine 0.5-1%; field block (surround the area)
• Nerve block (神经阻滞): inject around a nerve/plexus — brachial plexus (axillary/supraclavicular/interscalene), femoral, digital
• Spinal anesthesia (蛛网膜下腔阻滞/腰麻) (lab P-5): inject into the subarachnoid space (CSF) at L3/L4 or L4/L5 (below cord end L2) — rapid dense block; level of anesthesia = dermatome; agents: lidocaine/bupivacaine (± opioids)
• Epidural anesthesia (硬膜外阻滞): inject into the epidural space (outside the dura) — catheter for continuous (labor analgesia, postoperative); slower onset, segmental block; "loss of resistance" technique

5. Spinal vs epidural — comparison (MUST KNOW)

6. Complications of neuraxial (spinal/epidural) anesthesia (MUST KNOW)

• Hypotension (most common): sympathetic block → vasodilation → treat with fluids + vasopressors (ephedrine), position
• High/total spinal block: spread of anesthetic upward → respiratory paralysis, bradycardia, unconsciousness → support airway/ventilation, vasopressors, atropine, may need intubation
• Post-dural puncture headache (PDPH): CSF leak → headache worse upright, relieved supine → bed rest, fluids, caffeine; blood patch if severe (autologous blood injection into the epidural space)
• Neurological injury (rare): direct needle trauma, epidural hematoma (anticoagulated patients!), abscess
• Urinary retention, nausea, backache
• Contraindications to neuraxial: patient refusal, coagulopathy/anticoagulants (epidural hematoma risk), local infection at the site, raised ICP, severe hypovolemia, sepsis/bacteremia

7. Regional techniques in surgery

• Brachial plexus block: axillary (most common), supraclavicular (pneumothorax risk), interscalene (phrenic nerve involvement — avoid in respiratory disease)
• Digital nerve block: NO epinephrine (end artery)
• IV regional (Bier block): tourniquet + IV lidocaine into the exsanguinated limb (upper limb, short procedures)
• Peripheral nerve catheters for postoperative analgesia (femoral, adductor canal, TAP block)

High-Yield Points

• Esters (procaine 1g, tetracaine 10mg) metabolized by plasma esterase; Amides (lidocaine 400mg, bupivacaine 150mg) by liver
• Toxicity: circumoral numbness → tinnitus → twitching → convulsions → CNS depression → CV collapse
• Treatment: stop, O2/airway, benzodiazepines for seizures, fluids/vasopressors; bupivacaine arrest → lipid emulsion
• Prevention: aspirate, correct dose, epinephrine (not in digits), test dose
• Spinal: subarachnoid L3/4-4/5, fast, dense, single-shot; Epidural: epidural space, catheter, slow, segmental
• PDPH: upright headache → blood patch
• High block: respiratory failure — support ventilation
• Contraindications: coagulopathy, infection at site, raised ICP, hypovolemia, sepsis
• Digital blocks: no epinephrine
• Max doses memorized: 普鲁卡因1g, 丁卡因10mg, 利多卡因400mg, 布比卡因150mg

Topic Summary

Local anesthetics: esters (procaine 1g, tetracaine 10 mg — plasma esterase metabolism, more allergy) vs amides (lidocaine 400 mg, bupivacaine 150 mg — liver metabolism). Toxicity follows CNS excitation (circumoral numbness, tinnitus, twitching, seizures) → depression → CV collapse; treat by stopping injection, O2/airway, benzodiazepines for seizures, fluids/vasopressors, and IV lipid emulsion for bupivacaine cardiotoxicity. Techniques: surface, infiltration, nerve block, spinal (subarachnoid, L3/4-4/5, dense/fast) and epidural (catheter, segmental, slower). Complications of neuraxial: hypotension (most common), high block (respiratory support), PDPH (blood patch), neurological injury; contraindications: coagulopathy, site infection, raised ICP, hypovolemia, sepsis. Never use epinephrine in digital blocks.