A 17-year-old male presents with altered mental status. He was recently admitted to the hospital due to a tibial fracture suffered while playing soccer. His nurse states that he is difficult to arouse. His temperature is 98.6 deg F (37 deg C), blood pressure is 130/80 mm Hg, pulse is 60/min, and respirations are 6/min. Exam is notable for pinpoint pupils and significant lethargy. Which of the following describes the mechanism of action of the drug likely causing this patient's altered mental status?

Neuronal hyperpolarization due to potassium efflux

Neuronal hyperpolarization due to sodium influx

Neuronal depolarization due to sodium efflux

Neuronal depolarization due to potassium influx

Neuronal hyperpolarization due to chloride influx

A 36-year-old male with fluctuating levels of consciousness is brought to the emergency department by ambulance due to a fire in his home. He currently opens his eyes to voice, localizes painful stimuli, responds when asked questions, but is disoriented and cannot obey commands. The patient’s temperature is 99°F (37.2°C), blood pressure is 86/52 mmHg, pulse is 88/min, and respirations are 14/min with an oxygen saturation of 97% O2 on room air. Physical exam shows evidence of soot around the patient’s nose and mouth, but no burns, airway obstruction, nor accessory muscle use. A blood lactate is 14 mmol/L. The patient is started on intravenous fluids. What is the next best step in management?

100% oxygen, hydroxycobalamin, and sodium thiosulfate

Sodium thiosulfate and sodium nitrite

Two hours after undergoing elective cholecystectomy with general anesthesia, a 41-year-old woman is evaluated for decreased mental status. BMI is 36.6 kg/m2. Respirations are 18/min and blood pressure is 126/73 mm Hg. Physical examination shows the endotracheal tube in normal position. She does not respond to sternal rub and gag reflex is absent. Arterial blood gas analysis on room air shows normal PO2 and PCO2 levels. Which of the following anesthetic properties is the most likely cause of these findings?

Low brain-blood partition coefficient

High minimal alveolar concentration

A 28-year-old man comes to the emergency department for an injury sustained while doing construction. Physical examination shows a long, deep, irregular laceration on the lateral aspect of the left forearm with exposed fascia. Prior to surgical repair of the injury, a brachial plexus block is performed using a local anesthetic. Shortly after the nerve block is performed, he complains of dizziness and then loses consciousness. His radial pulse is faint and a continuous cardiac monitor shows a heart rate of 24/min. Which of the following is the most likely mechanism of action of the anesthetic that was administered?

Inactivation of sodium channels

Activation of acetylcholine receptors

Inactivation of ryanodine receptors

Inactivation of potassium channels

A 60-year-old man is brought to the emergency department 25 minutes after falling and hitting his left flank on a concrete block. He has severe left-sided chest pain and mild shortness of breath. He underwent a right knee replacement surgery 2 years ago. He has type 2 diabetes mellitus. He has smoked one pack of cigarettes daily for 42 years. Current medications include metformin, sitagliptin, and a multivitamin. He appears uncomfortable. His temperature is 37.5°C (99.5°F), pulse is 102/min, respirations are 17/min, and blood pressure is 132/90 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 96%. Examination shows multiple abrasions on his left flank and trunk. The upper left chest wall is tender to palpation and bony crepitus is present. There are decreased breath sounds over both lung bases. Cardiac examination shows no murmurs, rubs, or gallops. The abdomen is soft and nontender. Focused assessment with sonography for trauma is negative. An x-ray of the chest shows nondisplaced fractures of the left 4th and 5th ribs, with clear lung fields bilaterally. Which of the following is the most appropriate next step in management?

Adequate analgesia and conservative management

Admission and surveillance in the intensive care unit

Prophylactic antibiotic therapy

Continuous positive airway pressure

Local anesthesia and nerve blocks for USMLE Step 2 CK: High-Yield Surgery Lessons

⚡ Drug class/MOA - Block That Shock

MOA: Reversibly block voltage-gated sodium ($Na^+$) channels from inside the neuron. This increases the threshold for electrical excitation, slowing impulse propagation and ultimately blocking the action potential.
Two Classes:
- Amides: (two "i"s) Lidocaine, Bupivacaine, Ropivacaine.
  - Metabolized by the liver (CYP450).
- Esters: (one "i") Procaine, Cocaine, Benzocaine.
  - Metabolized by plasma pseudocholinesterases.
  - ⚠️ Higher allergy risk due to PABA metabolite.

⭐ Differential Blockade: Smaller, myelinated fibers are blocked first. Sensation is lost sequentially:

Pain & Temperature (Aδ, C fibers)

Touch & Pressure

Motor (Aα fibers)

💉 PK/PD - How It Gets In & Out

Absorption & Distribution: Systemic absorption is ↑ by vascularity. Epinephrine (vasoconstrictor) is co-administered to ↓ absorption, which ↑ duration of action and ↓ systemic toxicity.
Metabolism:
- Esters (one 'i', e.g., Procaine): Metabolized by plasma pseudocholinesterases.
- Amides (two 'i's, e.g., Lidocaine): Metabolized by the liver (CYP450). 📌 Amides have two i's, like liver.
Excretion: Metabolites are renally excreted.

⭐ Ion Trapping: In physiologic pH, the non-ionized (lipid-soluble) form of the LA crosses the nerve membrane. Inside the more acidic axon, it becomes ionized and "trapped," allowing it to block the Na+ channel.

💉 Clinical uses - Numb's The Word

Topical & Infiltration: Minor procedures (e.g., suturing, skin biopsies, dental work).
Peripheral Nerve Blocks (PNB): Anesthesia/analgesia for a specific region.
- Upper Extremity: Brachial plexus block (shoulder, arm, hand surgery).
- Lower Extremity: Femoral, sciatic, ankle blocks (leg, foot surgery).
- Truncal: Intercostal (rib fracture pain), TAP block (abdominal wall analgesia).
Neuraxial Anesthesia:
- Spinal/Epidural: Labor analgesia, C-sections, major lower abdominal/limb surgery.
IV Regional (Bier Block): Short procedures on extremities.

⭐ Peripheral nerve blocks are a cornerstone of multimodal analgesia, significantly ↓ postoperative opioid requirements and side effects.

Lower extremity nerve anatomy and cutaneous distribution

⚠️ Adverse Effects - When Numb Goes Wrong

Local Anesthetic Systemic Toxicity (LAST): Dose-dependent, often from accidental intravascular injection.
- 📌 B-LAST: Bupivacaine is the most cardiotoxic.
- Progression:

Allergic Reactions: More common with esters (PABA metabolite) than amides (often preservative-related).
Methemoglobinemia: Associated with benzocaine, prilocaine. Presents with cyanosis, "chocolate-colored" blood.
Local: Nerve damage, hematoma, infection.

⭐ Treat LAST emergently with 20% lipid emulsion and supportive care. Avoid vasopressin, calcium channel blockers, and beta-blockers if possible.

⚠️ Interactions & C/I - Danger Zone Drugs

⭐ Epinephrine + Non-selective β-blocker (e.g., Propranolol) → Unopposed α1 agonism → Severe hypertension & reflex bradycardia.

Key Drug Interactions:
- TCAs, MAOIs, Cocaine: Block norepinephrine reuptake, potentiating sympathomimetic effects of epinephrine → hypertensive crisis, arrhythmias.
- Prilocaine, Benzocaine: Can induce methemoglobinemia (Fe²⁺ → Fe³⁺ in heme). Risk ↑ with oxidizing agents (dapsone, nitrates). Treat with methylene blue.
- Cholinesterase Inhibitors: Prolong duration of ester-type anesthetics (e.g., procaine) by ↓ metabolism.
Contraindications:
- Absolute: True allergy (rare; esters > amides), active infection at injection site.
- Relative: Severe coagulopathy, severe liver disease (amides), severe heart block (bupivacaine).

⚠️ Avoid epinephrine in end-arterial fields (fingers, toes, nose, penis) to prevent ischemic necrosis.

⚡ Biggest Takeaways

Local anesthetics block voltage-gated Na+ channels, preventing depolarization. They are most effective on small, myelinated fibers and in alkaline tissue.

Amides (2 'i's, e.g., lidocaine) are liver-metabolized; Esters (1 'i', e.g., procaine) use plasma pseudocholinesterase.

Systemic toxicity causes CNS excitation (seizures) then depression, and cardiotoxicity. Bupivacaine is the most cardiotoxic.

Treat local anesthetic systemic toxicity (LAST) with intravenous lipid emulsion.

Epinephrine adds vasoconstriction, prolonging duration and reducing systemic absorption. Avoid in end-arterial fields.

Interscalene blocks risk phrenic nerve palsy (hemidiaphragmatic paralysis).

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