Desflurane

Introduction:Desflurane Vaporizer

  • Classification: Halogenated methyl ethyl ether inhalational anesthetic.
  • Brand Name: Suprane®.
  • Core Identity: The fastest-acting potent volatile anesthetic regarding induction (though rarely used for this) and especially emergence. Think of it as the “sports car” of volatiles – incredibly quick off the line and stops on a dime.
  • Primary Use: Maintenance of general anesthesia after IV induction and airway securing.

Chemical Structure & Physical Properties

  • Structure: CF3-CHF-O-CF2H (2-(Difluoromethoxy)-1,1,1,2-tetrafluoroethane). Notice the fluorine atoms replacing hydrogens – this is key to its low solubility and stability.
  • Molecular Weight: 168 g/mol.
  • Boiling Point: 23.5°C (74.3°F). This is CRITICAL! It’s very close to room temperature.
  • Vapor Pressure at 20°C: 681 mmHg. Extremely high! (Compare to Sevoflurane ~157 mmHg, Isoflurane ~238 mmHg).
  • Odor: Pungent, irritating, ether-like. This is a major clinical limitation.
  • Color Coding: Blue. Remember this for identifying the vaporizer and agent bottle!

The Desflurane Vaporizer: Why It’s Special (and Different!)

This is non-negotiable knowledge. You CANNOT use a standard variable-bypass vaporizer (like those for Sevo or Iso) for Desflurane. Here’s why and how it’s different:

Feature
Standard Vaporizer (Sevo/Iso)
Desflurane Vaporizer (e.g., Tec 6, Aladin Cassette)
Principle
Variable Bypass (Splitting Flow)
Pressurized, Electrically Heated, Injection
Heating
None (Relies on ambient temp)
Electrically Heated (to ~39°C / 102°F)
Pressure
Operates at atmospheric pressure
Pressurized (to ~1500 mmHg / 2 atm)
Reason
Agent has lower VP, stable at room temp
High VP & Low BP: Would boil/vaporize uncontrollably at room temp. Pressurization prevents boiling. Heating ensures consistent VP despite cooling during vaporization.
Output
Concentration dial controls bypass ratio
Concentration dial controls injection rate of pressurized vapor into fresh gas flow.
Filling
Pour-fill or keyed filler
Specifically designed filling system (often pressurized bottle or cassette).
Key Takeaway
Simple, mechanical splitting
Complex, thermostatically controlled, pressurized injection system. NEVER use a standard vaporizer for Des!

Pharmacokinetics:

This is where Desflurane shines brightest.

  • Blood:Gas Partition Coefficient (λB:G): 0.42 (Lowest of all potent volatiles).
    • Meaning: Very low solubility in blood. Minimal blood “sink.”
    • Consequence: Rapid equilibration between alveoli (FA) and inspired (FI) concentrations -> Fast rise in FA/FI ratio.
  • Oil:Gas Partition Coefficient (λO:G): 18.7 (Similar to Isoflurane, lower than Halothane).
    • Meaning: Moderate potency. MAC is higher than Halothane but lower than Nitrous Oxide.
  • Tissue Solubility: Low solubility in muscle and fat (low tissue:blood PC).
  • Metabolism: Minimal! Only ~0.02% metabolized by CYP2E1 (to trifluoroacetate and inorganic fluoride).
    • Consequence: Negligible fluoride ion release -> Low risk of nephrotoxicity (unlike Methoxyflurane, much lower than Sevo). No significant Compound A production.
  • Elimination: Almost entirely exhaled unchanged via the lungs.
  • Clinical Impact of Kinetics:
    • Induction: Theoretically fast due to low λB:G, but clinically impractical due to severe airway irritation (see Pharmacodynamics). IV induction is standard.
    • Emergence: Extremely Rapid! Due to low solubility, when the vaporizer is turned off, Desflurane quickly diffuses from the brain back into the blood and is exhaled. Patients wake up significantly faster than with Sevo or Iso, especially after long procedures.

Pharmacodynamics:

  • Minimum Alveolar Concentration (MAC):

    • MAC (100% O2): ~6-7% (Age-dependent, decreases with age).
    • MAC (70% N2O): ~2.8% (Significant MAC-sparing effect of N2O).

  • Central Nervous System (CNS):

    • Dose-dependent CNS depression (anesthesia).
    • Increases Cerebral Blood Flow (CBF) and Intracranial Pressure (ICP): Similar to other volatiles. Use cautiously in patients with head injury or space-occupying lesions.
    • Cerebral Metabolic Rate (CMRO2): Decreases CMRO2.
    • EEG: Produces dose-dependent EEG changes similar to other volatiles. Can cause epileptiform activity at high concentrations (rare).

  • Cardiovascular System (CVS):

    • Myocardial Depression: Dose-dependent direct negative inotropy (similar to Iso).
    • Systemic Vasodilation: Dose-dependent decrease in Systemic Vascular Resistance (SVR) -> Dose-dependent hypotension.
    • Heart Rate (HR): Key Difference: Often causes tachycardia and increased sympathetic nervous system activity, especially with rapid increases in inspired concentration. This is less pronounced with Sevo.
    • Cardiac Output (CO): Generally better preserved than with older agents like Halothane, due to maintained HR and less severe myocardial depression than Halothane.
    • Coronary Steal: Theoretical concern (like Iso), but clinical significance debated.

  • Respiratory System:

    • Respiratory Depression: Dose-dependent depression of minute ventilation and response to hypoxia/hypercapnia (similar to other volatiles).
    • Airway Irritation: MAJOR LIMITATION. Highly irritating to the respiratory tract.
      • Causes coughing, breath-holding, laryngospasm, bronchospasm.
      • Contraindicated for inhalational induction, especially in children and patients with reactive airways (asthma, COPD). Requires IV induction and a secured airway (ETT/LMA).

  • Other Effects:

    • Muscle Relaxation: Provides some skeletal muscle relaxation, potentiates neuromuscular blocking agents (similar to other volatiles).
    • Uterine Tone: May relax uterine smooth muscle (theoretical concern for increased blood loss, but less than Halothane).
    • Renal: Minimal metabolism -> Low nephrotoxic potential.
    • Hepatic: Minimal metabolism -> Low hepatotoxic potential.

Clinical Applications: When to Choose Desflurane

  • Rapid Emergence is Paramount: Outpatient surgery, neurosurgery (for rapid neurological exam), long cases requiring fast OR turnover.
  • Cases Requiring Minimal Metabolism: Patients with significant renal or hepatic impairment (though Sevo is also often used safely here).
  • Low-Flow Anesthesia: Its stability and low metabolism make it suitable, though Sevo is also commonly used effectively with low-flow techniques.

Clinical Considerations & Cautions

  • Airway Reactivity: Avoid in patients with active bronchospasm, severe asthma, or COPD exacerbation. Use with extreme caution in any patient with reactive airways.
  • Pediatrics: Generally avoided for induction (Sevo is gold standard). Can be used for maintenance after IV induction/airway secure, but airway irritation remains a concern.
  • Increased ICP: Use cautiously; ensure adequate depth before noxious stimuli, consider hyperventilation if needed.
  • Hemodynamic Lability: Be prepared for tachycardia and hypotension, especially during rapid concentration changes or in patients with cardiovascular compromise. Titrate slowly.
  • Cost: Higher cost per mL than Sevo/Iso. Cost-effectiveness depends on case length and utilization of low-flow techniques to minimize waste.
  • Environmental Impact: A potent greenhouse gas (though lower global warming potential than Sevo). Minimize waste gas scavenging.

Summary:

  1. Speed King: Unmatched emergence speed due to lowest blood solubility (λB:G = 0.42).
  2. Pungent & Irritating: Contraindicated for inhalational induction. Requires IV induction and secured airway.
  3. Special Vaporizer: MUST use a dedicated pressurized, heated vaporizer (Tec 6/Aladin). Never use a standard vaporizer!
  4. Minimal Metabolism: Very low risk of organ toxicity (renal/hepatic).
  5. Cardiovascular: Causes hypotension (vasodilation) and often tachycardia/sympathetic activation.
  6. Best For: Cases demanding the fastest possible wake-up (outpatient, neuro, long cases).
  7. Avoid In: Reactive airways, inhalational induction, pediatrics (induction), uncontrolled ICP.

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