General Anesthesia: Routes, Effects, Indications, Contraindications, and Drug Regimens
Routes of Administration
General anesthesia is primarily administered via two routes: intravenous (IV) and inhalational, with IV being the preferred route for induction and inhalational for maintenance. 1
Intravenous Route
- Primary method for induction - provides rapid, smooth onset
- Requires establishment and maintenance of vascular access throughout the procedure and until cardiorespiratory depression risk resolves 1
- Drugs must be administered in small, incremental doses or by infusion, titrating to desired endpoints
- Sufficient time must elapse between doses to assess peak effect before subsequent administration 1
Inhalational Route
- Preferred for maintenance of anesthesia - allows more precise control of anesthetic depth at lower cost 2
- Avoids claustrophobia associated with mask induction 2
Alternative Routes (Less Common)
- Oral, rectal, intramuscular, transmucosal 1
- When using non-IV routes, allow sufficient time for absorption and peak effect before supplementation 1
- One study showed IV ketamine had shorter onset and recovery times versus intramuscular administration 1
Physiological Effects
General anesthesia produces reversible loss of consciousness through hypnotic drugs, requiring simultaneous management of multiple physiological systems 3:
Core Effects
- Loss of consciousness - induced and maintained by hypnotics
- Analgesia/antinociception - inhibits pain transmission and sympathetic/endocrine responses to nociceptive stimuli 3, 4
- Muscle relaxation - facilitates intubation and surgery 3
Cardiorespiratory Effects
- Respiratory depression - risk of hypoxemia, hypoventilation, and apnea 1
- Cardiovascular effects - variable blood pressure and heart rate changes depending on agent 1
- Propofol combined with ketamine causes more respiratory depression and hypoxemia than propofol alone 1
Agent-Specific Risks
- Volatile anesthetics in Duchenne Muscular Dystrophy: Risk of extreme hyperthermic events, rhabdomyolysis, hyperkalemia, and sudden cardiac arrest 5
- Succinylcholine contraindicated in DMD due to acute rhabdomyolysis risk 5
Indications
General anesthesia is indicated when:
- Surgical procedures require complete unconsciousness, immobility, and analgesia
- Regional or local anesthesia is inadequate or contraindicated
- Patient cooperation cannot be achieved
- Airway protection is necessary
- Procedures require muscle relaxation 6
Special Considerations
- For high-risk patients (morbid obesity, sleep apnea, difficult airway, significant cardiac/pulmonary disease), preprocedure anesthesiology consultation decreases adverse outcomes 7
- In emergency situations, benefits of awaiting consultation must be weighed against delay risks 7
Contraindications
Absolute Contraindications
- Patient refusal (when patient has decision-making capacity)
- Known malignant hyperthermia susceptibility - volatile agents and succinylcholine contraindicated
- Duchenne Muscular Dystrophy - volatile anesthetics (halothane, isoflurane, sevoflurane) and succinylcholine are contraindicated 5
Relative Contraindications (Require Special Precautions)
- Severe cardiac disease - requires specialized monitoring and regimens 7
- Severe pulmonary disease - especially restrictive disease with FVC <50% predicted 5
- Hepatic or renal dysfunction - affects drug metabolism and clearance 7
- Morbid obesity and sleep apnea - increased risk of airway complications 7
- Pregnancy - requires careful agent selection 7
- Unprepared/emergency patients - increased aspiration risk 7
Drugs for General Anesthesia
Induction Agents (IV)
Propofol is the preferred IV induction agent due to rapid onset, smooth induction, and rapid clearance 1, 2:
- Propofol: Fastest recovery times versus midazolam; shorter sedation time and less recall versus diazepam 1
- Ketamine: Equivocal outcomes versus propofol for sedation scores, pain, and recovery 1
- Etomidate: Shorter sedation times versus midazolam and pentobarbital, but higher myoclonus frequency 1
Maintenance Agents
Volatile Anesthetics (Inhalational)
Sevoflurane is preferred over desflurane and isoflurane when using volatile agents 8:
- Sevoflurane - adequate potency, appropriate solubility, minimal hepatotoxicity risk 2
- Desflurane - acceptable alternative but higher environmental impact 8, 2
- Isoflurane - acceptable but less preferred 2
- Halothane - avoid due to excessive solubility and severe hepatotoxicity risk 2
- Nitrous oxide - should NOT be used due to environmental impact; inadequate potency alone 8, 2
Total Intravenous Anesthesia (TIVA)
- Propofol infusion - allows precise control, faster recovery 1
- Monitor depth of anesthesia to reduce consumption 8
Analgesics (Opioids)
Combined with hypnotics to inhibit pain transmission 3, 4:
- Fentanyl - most commonly used
- Remifentanil - ultra-short acting
- Morphine - longer duration
- Propofol combined with opioids provides lower pain scores versus propofol alone 1
Adjunctive Agents
Benzodiazepines
- Midazolam - most common
- Diazepam - alternative
- Avoid routine preoperative midazolam for enhanced recovery 9
Muscle Relaxants
- Minimize use and ensure appropriate reversal of residual paralysis 9
- Succinylcholine absolutely contraindicated in DMD 5
Other Adjuncts
- Dexmedetomidine - alternative to benzodiazepines on case-by-case basis 1
- Magnesium - multimodal approach 4
Reversal Agents
Naloxone and flumazenil must be immediately available 7:
- Naloxone - reverses opioid effects, increases alertness and respiratory rate 1
- Flumazenil - antagonizes benzodiazepine sedation within 15 minutes 1
Key Practice Principles
Drug Administration Strategy
- Use minimal drug combinations at lowest effective doses 9
- Titrate IV drugs to effect - allow peak effect assessment before additional dosing 1
- Avoid deep anesthesia - use depth monitoring 9
- Opioid-sparing approach recommended 9
Monitoring Requirements
- Continuous monitoring of vital signs, oxygen saturation, and when possible, end-tidal CO₂ 5
- Depth of anesthesia monitoring reduces drug consumption 8
- Neuromuscular monitoring mandatory when using muscle relaxants 3
Safety Measures
- Maintain IV access throughout procedure and recovery period 1
- Supplemental oxygen improves efficacy and reduces adverse outcomes 7
- Immediate availability of personnel trained in airway management 7
- For deep sedation or general anesthesia drugs (propofol, ketamine, etomidate), provide care consistent with general anesthesia requirements 1
Environmental Considerations
With equal clinical benefit, choose between TIVA and volatile agents recognizing different environmental impacts: volatile agents emit greenhouse gases while propofol has ecotoxicity concerns for water and soil 8. When using volatile agents, employ low fresh gas flows 8.