2. General Anesthesia Techniques
Learning Objectives
- Define general anesthesia and describe its four defining characteristics
- Explain the primary mechanisms by which anesthetic agents act on the CNS
- Identify the three stages of anesthesia and recognize their clinical significance
- Compare inhalational and intravenous anesthesia in terms of onset, offset, and titration
- Describe the balanced anesthesia approach and explain why it reduces side effects
- List the main inhalational and IV agents used in US clinical practice with key properties
- Outline patient monitoring requirements during general anesthesia
Quick Answer
General anesthesia is a pharmacologically induced, reversible state characterized by unconsciousness, amnesia, immobility, and analgesia. Anesthetic drugs produce these effects primarily by depressing the central nervous system through modulation of ion channels, GABA receptors, and NMDA receptors. The anesthetic is administered in three stages: an initial excitement phase (usually brief), surgical anesthesia (the target state), and medullary depression (dangerous — must be avoided). Modern practice uses a balanced technique combining inhaled agents like sevoflurane with intravenous drugs such as propofol, opioids, and muscle relaxants to achieve optimal conditions with lower doses of any single drug.
Introduction
General anesthesia is a crucial aspect of modern medicine, particularly in surgical procedures. It involves inducing unconsciousness and immobility in patients to prevent pain and discomfort during medical interventions.
Principles of General Anesthesia
Definition
General anesthesia is defined as a state of controlled unconsciousness characterized by:
- Loss of consciousness (unresponsiveness)
- Amnesia (memory loss)
- Immobility
- Analgesia (pain relief)
Mechanisms of Action
Anesthetic agents work through various mechanisms:
- Depressant effects on the central nervous system
- Alteration of neurotransmitter function (enhancing GABA-mediated inhibition, blocking NMDA receptors)
- Modulation of ion channels and receptors
Stages of Anesthesia
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Stage 1: Induction / Analgesia
- Patient is conscious but experiences analgesia
- Often fleeting and blends into Stage 2
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Stage 2: Excitement
- Patient may experience confusion, breath-holding, laryngospasm, and irregular respiration
- Short-lived and typically skipped by using rapid IV induction agents
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Stage 3: Surgical Anesthesia
- Patient becomes unconscious, immobile, and has regular controlled respiration
- The target operating state; depth is adjusted throughout the case
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Stage 4: Medullary Depression
- Most dangerous stage; patient loses brainstem reflexes and vital cardiovascular function
- Requires immediate dose reduction and resuscitation
Types of General Anesthesia
Inhalational Anesthesia
- Uses volatile gases (e.g., sevoflurane, desflurane, isoflurane)
- Easily titrated by adjusting the vaporizer concentration
- Rapid onset and recovery
- Potency measured by minimum alveolar concentration (MAC)
Key inhalational agents:
| Agent | Key Properties | Common Use |
|---|---|---|
| Sevoflurane | Pleasant odor, rapid onset, low airway irritation | Inhalational induction, maintenance |
| Desflurane | Fastest offset, pungent odor, not used for induction | Maintenance in cases needing fast wake-up |
| Isoflurane | Economical, slightly pungent | Maintenance; less common now |
| Nitrous oxide | Analgesic, rapid on/off, supports other agents | Adjunct to reduce MAC of volatile agent |
Balanced Anesthesia
- Combination of multiple agents from different classes
- Allows more precise control of individual components (hypnosis, analgesia, relaxation)
- Reduces total dose and side effects of any single drug
A typical balanced anesthetic includes:
- Hypnotic: propofol (induction) or sevoflurane (maintenance)
- Opioid: fentanyl or remifentanil for analgesia
- Muscle relaxant: rocuronium or vecuronium for intubation and immobility
- Reversal: neostigmine/sugammadex at the end of the case
Total Intravenous Anesthesia (TIVA)
- All agents delivered intravenously (typically propofol infusion + remifentanil infusion)
- Avoids volatile agents; preferred when malignant hyperthermia risk exists or PONV is a concern
- Requires target-controlled infusion (TCI) pumps for precise dosing
Induction of Anesthesia
Rapid Sequence Induction (RSI)
Used when aspiration risk is high (full stomach, bowel obstruction, emergency surgery):
- Preoxygenate with 100% oxygen for 3–5 minutes
- Apply cricoid pressure (Sellick maneuver) — controversial but still used
- Administer fast-acting induction agent (propofol or ketamine)
- Administer succinylcholine (or high-dose rocuronium) for rapid intubation
- Secure airway immediately without mask ventilation
Standard Induction
For elective cases in fasted patients:
- IV access and monitoring (ECG, SpO2, NIBP, capnography)
- Propofol 1.5–2.5 mg/kg IV over 30–60 seconds
- Opioid pre-dose (fentanyl) to blunt laryngoscopy response
- Muscle relaxant when intubation is planned
- Laryngoscopy and tracheal intubation or LMA placement
Airway Management During General Anesthesia
| Device | Description | When Used |
|---|---|---|
| Endotracheal tube (ETT) | Cuffed tube placed in trachea | Most general anesthetics; aspiration risk |
| Laryngeal mask airway (LMA) | Sits above larynx; no intubation needed | Short elective procedures; low aspiration risk |
| Nasopharyngeal airway | Soft tube through nostril | Airway maintenance during sedation |
| Video laryngoscope | Camera-assisted blade for difficult airway | Predicted or encountered difficult intubation |
Maintenance of Anesthesia
- Depth titrated using end-tidal agent concentration or bispectral index (BIS) monitoring
- Vital signs (blood pressure, heart rate, SpO2, ETCO2) guide adjustments
- Muscle relaxation monitored with train-of-four (TOF) peripheral nerve stimulator
- Temperature maintenance critical, especially in long or pediatric cases
Emergence and Recovery
- Agents weaned as surgical closure begins
- Muscle relaxants reversed with neostigmine/glycopyrrolate or sugammadex
- Patient extubated when meeting criteria: awake, following commands, sustaining airway, adequate ventilation
- Recovery in PACU with monitoring for pain, nausea, desaturation, and hemodynamic instability
Key Terms
| Term | Definition | Related Concept |
|---|---|---|
| MAC (minimum alveolar concentration) | End-tidal concentration of inhaled agent preventing movement in 50% of patients in response to surgical incision | Potency of inhalational agents |
| Propofol | IV hypnotic agent; rapid onset and offset; used for induction and TIVA | Balanced anesthesia, TIVA |
| Sevoflurane | Non-pungent volatile agent; preferred for inhalational induction and maintenance | MAC, inhalational anesthesia |
| Balanced anesthesia | Combining multiple drug classes to achieve all components of anesthesia at lower doses | Reduced side effects |
| Rapid sequence induction | Fast induction technique to secure airway before gastric contents can be aspirated | Full stomach, emergency surgery |
| Bispectral index (BIS) | EEG-derived number (0–100) estimating depth of anesthesia | Awareness under anesthesia |
| Train-of-four (TOF) | Four nerve stimuli used to assess degree of neuromuscular blockade | Muscle relaxants, reversal |
| TIVA | Total intravenous anesthesia using propofol infusion instead of inhaled agents | Malignant hyperthermia, PONV |
| Sugammadex | Selective reversal agent for rocuronium/vecuronium | Neuromuscular blockade reversal |
| PONV | Postoperative nausea and vomiting; common complication of general anesthesia | Ondansetron, dexamethasone |
| Laryngospasm | Reflexive closure of vocal cords; can occur in Stage 2 | Stage 2 anesthesia, airway emergency |
| Malignant hyperthermia | Rare, life-threatening hypermetabolic reaction to volatile agents and succinylcholine | Dantrolene, TIVA |
Common Mistakes
Misconception: The MAC value gives you the correct dose for every patient. Why it's wrong: MAC is a population median (50th percentile) and varies significantly with age, temperature, concurrent drugs, alcohol use, and acute pain. Elderly patients require roughly 30–40% lower MAC than young adults. Correct understanding: MAC is a starting reference point, not a prescription. The anesthesiologist titrates the agent against clinical signs, BIS monitoring, and hemodynamic responses.
Misconception: Once a patient is intubated, the airway is completely safe. Why it's wrong: Endotracheal tubes can be displaced by patient movement, surgical manipulation, or tape failure. An undetected esophageal intubation is immediately life-threatening. Tube position is confirmed with capnography, chest auscultation, and chest rise — and monitored continuously. Correct understanding: Securing an airway is the beginning, not the end, of airway management. Continuous capnography is the gold standard for confirming ongoing correct placement.
Misconception: General anesthesia and deep sedation are the same thing. Why it's wrong: Deep sedation still allows some protective reflexes and the patient may respond to painful stimulation. General anesthesia involves complete loss of consciousness, loss of airway reflexes, and often apnea requiring ventilatory support. Correct understanding: Sedation and general anesthesia exist on a continuum, but only general anesthesia consistently eliminates protective airway reflexes and requires a provider trained to manage a compromised airway.
Comparison and Connections
| Feature | Inhalational Anesthesia | IV Anesthesia (TIVA) |
|---|---|---|
| Primary agents | Sevoflurane, desflurane | Propofol infusion + remifentanil |
| Titration | End-tidal concentration on monitor | Infusion rate; harder to measure effect-site |
| Emergence speed | Fast (desflurane fastest) | Fast with remifentanil; propofol slightly slower |
| PONV risk | Higher (volatile agents increase PONV) | Lower (propofol has antiemetic properties) |
| Malignant hyperthermia risk | Yes (volatile agents trigger MH) | No (safe in MH-susceptible patients) |
| Cost | Lower drug cost | Higher (propofol, TCI pumps) |
| Best for | Most routine surgeries | MH-susceptible patients, high PONV risk |
Practice Questions
Recall
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What are the four defining characteristics of general anesthesia? Guidance: Unconsciousness (hypnosis), amnesia, immobility, and analgesia — the "anesthetic triad" plus amnesia. Know these in order.
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Name two inhalational agents and one key distinguishing property of each. Guidance: Sevoflurane (pleasant odor, suitable for inhalational induction); desflurane (fastest offset, pungent — not used for induction). Isoflurane is economical but less commonly used now.
Understanding
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Why is Stage 2 (excitement) of anesthesia considered dangerous? Guidance: Irregular respirations, breath-holding, laryngospasm, vomiting, and exaggerated reflexes can all occur. Modern practice uses fast-acting IV agents to transit through Stage 2 rapidly.
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Explain why balanced anesthesia reduces side effects compared to using a single high-dose agent. Guidance: By using lower doses of each drug class and targeting only the needed component (e.g., adding an opioid for analgesia instead of deepening the volatile agent), overall drug burden decreases and side-effect profiles do not compound.
Application
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A patient with a family history of malignant hyperthermia needs cholecystectomy. How would you modify the anesthetic plan? Guidance: Avoid all volatile agents and succinylcholine. Use a TIVA technique with propofol and rocuronium. Prepare dantrolene at the room. Alert the OR team.
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A patient has a rapid desaturation after induction and intubation. What are your first three actions? Guidance: Confirm tube placement with capnography and auscultation, hand-ventilate with 100% oxygen, and check for bilateral breath sounds. If tube is in the esophagus, remove and re-intubate.
Analysis
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Compare the advantages and disadvantages of using sevoflurane versus desflurane for maintenance of anesthesia. Guidance: Desflurane has faster offset (better for long cases requiring prompt emergence) but is pungent (cannot be used for induction), environmentally problematic (high global warming potential), and more expensive. Sevoflurane has pleasant odor and suits induction but slightly slower offset.
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Why is continuous capnography considered the most critical monitor during general anesthesia? Guidance: Capnography confirms correct ETT placement, detects esophageal intubation, circuit disconnection, sudden decrease in cardiac output (pulmonary embolism), and hypo/hyperventilation — all potentially lethal if missed.
FAQ
What stops a patient from remembering the surgery under general anesthesia? The amnesia component of general anesthesia is primarily produced by benzodiazepines (e.g., midazolam given preoperatively) and the hypnotic agents themselves (propofol, volatile agents). These drugs impair the formation of new memories (anterograde amnesia) at subanesthetic concentrations. Intraoperative awareness — waking up and forming memories during surgery — can occur if anesthetic depth becomes insufficient, which is why BIS monitoring and clinical vigilance are maintained throughout.
Why do patients sometimes feel sick after general anesthesia? Postoperative nausea and vomiting (PONV) is one of the most common complaints after general anesthesia, affecting roughly 30% of patients. Volatile inhalational agents and opioids are the primary contributors. Risk factors include female sex, non-smoking status, history of motion sickness, and use of opioids. Anesthesiologists use prophylactic antiemetics such as ondansetron and dexamethasone, and may choose TIVA with propofol in high-risk patients to reduce PONV.
How does the anesthesiologist know the patient is deeply enough anesthetized? Depth of anesthesia is assessed using clinical signs (no movement, stable heart rate and blood pressure, no tearing), end-tidal agent concentration relative to MAC, and processed EEG monitors such as the bispectral index (BIS). A BIS value between 40 and 60 is generally associated with adequate surgical anesthesia. No single monitor is perfect; anesthesiologists integrate multiple data streams to make depth judgments.
What happens if someone is given too much general anesthetic? Overdose of anesthetic drugs can cause cardiovascular depression (low blood pressure, reduced cardiac output), respiratory depression (apnea), and ultimately cardiac arrest. This is Stage 4 — medullary depression. Because all patients under general anesthesia are mechanically ventilated or closely monitored and supported, profound overdose is rare in controlled settings. The treatment is to reduce or stop the anesthetic, provide hemodynamic support, and address reversible causes.
Can general anesthesia be given without IV access? Inhalational induction (mask induction) using sevoflurane is commonly used in pediatric patients who refuse or cannot tolerate IV placement while awake. The child breathes an increasing concentration of sevoflurane until unconscious, after which IV access is established. In adults, IV induction is almost always preferred because it is faster and avoids the unpleasant dissociation of Stage 2. In rare emergency situations without IV access, intramuscular ketamine can be used.
Quick Revision
- General anesthesia: unconsciousness + amnesia + immobility + analgesia
- Mechanisms: GABA enhancement, NMDA blockade, modulation of ion channels
- Four stages: induction, excitement (Stage 2 — dangerous), surgical anesthesia (target), medullary depression (overdose — life-threatening)
- Inhalational agents: sevoflurane (induction/maintenance), desflurane (fast offset), isoflurane (economical)
- MAC = concentration preventing movement in 50% of patients during surgical incision
- Balanced anesthesia: hypnotic + opioid + muscle relaxant at lower doses than any agent alone
- TIVA: propofol infusion + remifentanil; preferred in malignant hyperthermia or high PONV risk
- RSI: preoxygenate, no mask ventilation, fast induction + succinylcholine/high-dose rocuronium
- Capnography is the most critical intraoperative monitor — confirms ETT position continuously
- BIS 40–60 = adequate surgical anesthesia depth
- Sugammadex reverses rocuronium/vecuronium; neostigmine reverses older agents
- PONV risk higher with volatile agents and opioids; propofol reduces PONV
Related Topics
Prerequisites: Introduction to Anesthesiology, basic pharmacology, neurophysiology, respiratory physiology
Related Topics: Regional Anesthesia Techniques, airway management, neuromuscular blockade, perioperative monitoring
Next Topics: Regional Anesthesia Techniques, Pain Management