Anesthesia Study Guide

📖 Core Concepts Anesthesia – a controlled, temporary loss of sensation, awareness, or both for medical procedures. Endpoints – hypnosis (unconsciousness), analgesia (pain relief), and muscle relaxation (paralysis). General vs. Regional vs. Sedation General: suppresses CNS → unconsciousness + all three endpoints. Regional/Local: blocks nerve impulse transmission in a specific area; patient stays conscious. Sedation: mild CNS depression; reduces anxiety and creates amnesia but does not cause unconsciousness. Minimum Alveolar Concentration (MAC) – the inhaled‑agent concentration that prevents a painful response in 50 % of patients; higher MAC = lower potency. ASA Physical Status – 6‑tier scale (Ⅰ–Ⅵ) predicting peri‑operative risk; higher class → higher morbidity/mortality. Pre‑emptive Analgesia – treating pain pathways before surgical stimulus to lessen acute and chronic pain. 📌 Must Remember Primary endpoints: hypnosis, analgesia, muscle relaxation. MAC relationship: ↑MAC → ↓ potency. Anesthesia awareness: 1–2/1,000 cases. Mortality rate: 1 death per 185 056 anesthetics; patient health dominates risk. Risk multipliers: Age 60–79: ×2.3; >80: ×3.3. ASA 3‑5: ×10.7 vs. ASA 1‑2. Emergency surgery: ×4.4. Neuraxial block physiology: arterial + venous vasodilation → ↓ BP; venous capacitance holds 75 % of blood volume. Post‑op nausea/vomiting (PONV): 9.8 % overall; varies by anesthetic type. POCD risk: highest after cardiac surgery; older age is a major factor. 🔄 Key Processes Pre‑operative Assessment History → Physical → Labs → ASA classification → Identify patient‑/procedure‑related risk factors. Induction of General Anesthesia Pre‑oxygenate → Administer IV induction agent → Secure airway → Begin inhalational agent (adjust to target MAC). Neuraxial Block Placement Position → Identify interspace → Aseptic prep → Needle insertion → Test dose → Inject local anesthetic → Observe hemodynamic response. Ultrasound‑Guided Peripheral Nerve Block Scan anatomy → Identify nerve sheath → Needle in‑plane → Aspirate → Inject local anesthetic (stay ≤ max safe dose). Emergence & Recovery Stop anesthetic agents → Monitor airway, breathing, circulation → Assess for nausea, hypothermia, confusion → Discharge criteria (stable vitals, adequate pain control, orientation). 🔍 Key Comparisons General Anesthesia vs. Sedation Consciousness: none vs. retained. Airway: secured (ET tube) vs. usually not needed. Cardiovascular impact: greater with GA; sedation often lighter. Spinal vs. Epidural Location: subarachnoid space (spinal) vs. epidural space. Onset: rapid (spinal) vs. slower (epidural). Duration: single dose (spinal) vs. catheter‑titrated (epidural). Peripheral Nerve Block vs. Central Neuraxial Block Complication risk: lower neurologic injury for peripheral blocks. Coverage: isolated limb vs. trunk/lower extremities. ⚠️ Common Misunderstandings “Sedation = General Anesthesia” – Sedation never produces unconsciousness; airway support is usually unnecessary. “Higher MAC means deeper anesthesia” – MAC is a potency index; a higher MAC value actually means the agent is less potent. “All regional techniques give analgesia only” – Some peripheral blocks also produce muscle relaxation; spinal/epidural can cause profound sympathetic block and hypotension. “Awareness only occurs with equipment failure” – It can also result from light anesthesia, patient factors, or inadequate dosing. 🧠 Mental Models / Intuition “Three‑leg stool” – Hypnosis, analgesia, and muscle relaxation are three legs; if any leg is missing, the patient may experience intra‑operative awareness or movement. “Blood‑volume reservoir” – Think of the venous capacitance system as a 75 % “spongy” reservoir; neuraxial vasodilation empties it → BP drop. “MAC as a 50 % lottery ticket” – At MAC, half the patients won’t move to pain; increasing concentration wins more “tickets” (more patients immobile). 🚩 Exceptions & Edge Cases Malignant hyperthermia – Rare, triggered by certain inhalational agents & succinylcholine; requires immediate dantrolene. Local anesthetic systemic toxicity (LAST) – Exceeds maximum safe dose → CNS & cardiac toxicity; use aspiration, incremental dosing, and lipid rescue. Pediatric/Third‑trimester exposure – Repeated or prolonged anesthetic use may affect brain development (FDA warning). High thoracic neuraxial block – Produces more pronounced hemodynamic effects than lumbar/caudal blocks. 📍 When to Use Which Choose General Anesthesia when: airway control needed, surgery is lengthy/complex, or patient cannot tolerate regional techniques. Select Regional (e.g., spinal/epidural) for: lower‑extremity, pelvic, or abdominal surgeries where analgesia plus sympathetic block is advantageous; also when aiming to reduce postoperative pain. Prefer Sedation for: short, minimally invasive procedures, diagnostic endoscopies, or patients at high risk for GA hemodynamic depression. Use Peripheral Nerve Block for: isolated limb surgery, postoperative analgesia, or when minimizing opioid use. 👀 Patterns to Recognize “Drop in BP + high block level” → Likely neuraxial sympathetic block → treat with fluids/vasopressors. “Patient reports pain despite adequate MAC” → Consider inadequate analgesia (add opioid or regional technique). “Delayed emergence + inhalational agent” → Check for low MAC, drug accumulation, or hypothermia. “Nausea + opioid use → Antiemetic prophylaxis indicated. 🗂️ Exam Traps Confusing MAC with concentration needed for unconsciousness – Remember MAC is for 50 % immobility, not loss of consciousness. Assuming all regional blocks avoid hypotension – Spinal/epidural can cause significant vasodilation; expect BP drop. Choosing sedation for a painful procedure – Sedatives alone provide limited analgesia; combine with narcotics or regional block. Over‑relying on ASA class alone – Procedural complexity and emergency status independently raise risk. “Local anesthetic toxicity only with accidental intravascular injection” – Large volume systemic absorption can also cause toxicity; respect maximum dose limits.