Desflurane 5 mark answer anesthesia exam

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DESFLURANE

Introduction

Desflurane is a halogenated volatile inhalation anesthetic that is structurally identical to isoflurane except for the substitution of a fluorine atom in place of isoflurane's chlorine atom. This seemingly minor change produces profound differences in its physical and pharmacological profile.

Physical Properties

Property	Value
Vapor pressure (20°C)	669-681 mmHg (near boiling at room temperature)
Blood:gas partition coefficient	0.42 (lowest of all halogenated agents)
MAC (in O₂)	6-7% (approximately 4x higher than isoflurane)
Oil:gas coefficient	18.7
Boiling point	23.5°C
Molecular formula	C₃H₂F₆O
Flammability	Non-flammable, non-explosive

Because its vapor pressure is so high that it nearly boils at room temperature, desflurane cannot be used in standard variable bypass vaporizers. It requires a specially heated, pressurized, electronically controlled vaporizer (e.g., the Tec 6 or Aladin cassette vaporizer) that delivers pure vapor subsequently diluted with carrier gas.

Pharmacokinetics (ADME)

Uptake and Distribution: The ultralow blood:gas partition coefficient (0.42 - lower than even nitrous oxide at 0.47) means the alveolar concentration (FA) equilibrates with the inspired concentration (FI) faster than any other volatile agent. This gives:

Rapid induction of anesthesia
Rapid and precise changes in anesthetic depth
Very fast emergence and recovery

Metabolism: Desflurane is minimally metabolized - more than 99% of absorbed drug is eliminated unchanged via the lungs. Serum and urine inorganic fluoride levels are essentially unchanged after desflurane anesthesia. This minimal metabolism accounts for its low organ toxicity profile.

Elimination: Primarily pulmonary, rapid due to low tissue solubility (especially low fat solubility). Wake-up times are approximately 50% shorter than with isoflurane.

Effects on Organ Systems

1. Cardiovascular System

Produces dose-dependent hypotension primarily via decreased systemic vascular resistance (SVR)
Cardiac output is relatively preserved (well-maintained at 1-2 MAC)
Moderate rise in heart rate and central venous pressure
Key distinguishing feature: Rapid increases in desflurane concentration cause transient but pronounced surges in heart rate, blood pressure, and catecholamine levels due to sympathetic nervous system stimulation - more pronounced than with isoflurane
These sympathetic responses can be attenuated by fentanyl, esmolol, or clonidine
Does not sensitize the myocardium to epinephrine-induced arrhythmias; safe up to 4.5 mcg/kg of epinephrine
Hypotensive effects do not diminish with prolonged administration

2. Respiratory System

Causes dose-dependent decrease in tidal volume and increase in respiratory rate
Net effect at >1 MAC: decreased alveolar ventilation and elevated PaCO₂
Depresses the ventilatory response to rising PaCO₂
Strongly pungent and irritating to the airway - causes coughing, breath-holding, salivation, laryngospasm, and bronchospasm
Acts as a bronchodilator at deeper planes
Not suitable for inhalation induction because of these irritant properties
Children with reactive airway disease are especially at risk

3. Central Nervous System

Directly vasodilates cerebral vasculature, increasing cerebral blood flow (CBF), cerebral blood volume, and intracranial pressure (ICP)
Simultaneously decreases cerebral metabolic rate of oxygen (CMRO₂)
Cerebrovascular reactivity to CO₂ is preserved - ICP can be controlled with hyperventilation
EEG: initial frequency increase, then EEG slowing, then burst suppression at ~2 MAC (CMRO₂ reduced ~50%)
Contraindicated in raised ICP unless controlled by hyperventilation

4. Neuromuscular

Dose-dependent potentiation of nondepolarizing neuromuscular blocking agents (to the same extent as isoflurane)
Provides direct skeletal muscle relaxation

5. Renal

No significant nephrotoxicity (consistent with minimal metabolism and negligible fluoride release)
Decreases in urine output with falling cardiac output are expected as with any anesthetic

6. Hepatic

Hepatic function tests generally unaffected if perfusion is maintained
Minimal risk of anesthetic-induced hepatitis due to minimal metabolism

Biotransformation and Toxicity

Carbon Monoxide (CO) Production: Desflurane contains a difluoromethoxy group (-OCHF₂) and is degraded by desiccated CO₂ absorbents (particularly those containing strong bases: barium hydroxide lime, sodium hydroxide, potassium hydroxide) to produce potentially dangerous levels of carbon monoxide. The magnitude of CO production among volatile agents follows: desflurane ≥ enflurane > isoflurane >> sevoflurane/halothane. Risk factors include dried-out absorbents and Monday-morning first cases (after no gas flow overnight). Prevention: use calcium hydroxide-based absorbents (e.g., Amsorb), keep absorbents moist.

Environmental: Desflurane is the most potent greenhouse gas among volatile anesthetics - its lifecycle global warming potential is approximately 15x that of isoflurane and 20x that of sevoflurane. This has led to significant scrutiny and withdrawal from some health systems.

Contraindications

Malignant hyperthermia (susceptibility)
Raised intracranial pressure (without hyperventilation control)
Severe hypovolemia
Inhalation induction (especially in pediatric patients)

Drug Interactions

Potentiates nondepolarizing neuromuscular blockers (dose reduction required)
Opioids and nitrous oxide reduce MAC (allowing lower desflurane concentrations)
Fentanyl, esmolol, and clonidine blunt the sympathetic surges with rapid concentration increases

Clinical Uses

Maintenance of general anesthesia (never for induction)
Induction always by intravenous agent, then desflurane for maintenance at 6-8% in O₂ (~1 MAC)
Historically favored for ambulatory surgery and prolonged procedures due to rapid emergence, though meta-analysis shows only ~1-1.2 minutes faster wake-up versus sevoflurane - a clinically modest advantage
Use in morbidly obese patients: theoretical advantage from low fat solubility, but clinical data is mixed
Largely being phased out in several countries due to environmental impact

Summary Comparison with Isoflurane and Sevoflurane

Feature	Desflurane	Sevoflurane	Isoflurane
Blood:gas coefficient	0.42	0.65	1.4
MAC	6-7%	2%	1.2%
Induction	Not suitable	Suitable	Not suitable
Airway irritation	High	Low	Moderate
Sympathetic surge	Yes (rapid increase)	No	Minimal
CO₂ absorbent degradation	High (CO)	Compound A (nephrotoxicity risk)	Moderate CO
Emergence speed	Fastest	Very fast	Slower
Environmental impact	Worst	Moderate	Moderate

Sources: Morgan & Mikhail's Clinical Anesthesiology, 7e - Goodman & Gilman's Pharmacological Basis of Therapeutics - Miller's Anesthesia, 10e - Barash, Cullen & Stoelting's Clinical Anesthesia, 9e

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