Desflurane

 

This new-ish volatile agent is useful because of it’s low blood:gas solubility co-efficient making it an agent that is rapid in offset. This is especially useful in prolonged procedures, and for those for patients with large adipose stores to facilitate more rapid recovery.  Watch for the tachycardia/hypertension if titrating rapidly.  (You can also make the most of the warming chamber when your fingers are chilly on winter mornings)

 

Dose

•           MAC 5.75 – 10.65 (age dependent)

Pharmaceutics

•           fluorinated etyhyl methyl ether, molecular weight 168
•           structure:

     F         H    F

     |         |    |

H - C - O - C - C - F

     |         |    |

     F         F    F

•           boiling point 23.5 ⁰c rendering it extremely volatile (can’t use conventional vapouriser) \ delivered via vapouriser that heats the liquid to 39⁰c at 2 atm and injects into the anaesthetic circuit
•           saturated vapour pressure 89.2 kPa (20⁰c)
•           flammable at 17% concentration
•           blood gas partition co-efficient 0.45 (very low J )
•           oil gas partition co-efficient 29
•           stable in presence of moist soda lime

Pharmacokinetics

•    onset of volatile agents is dependent on:
•    agent solubility (desflurane has a quick onset due to it’s low blood solubility)
•    cardiac output (higher cardiac output spreads distribution further thus slows onset)
•    concentration of agent between alveoli and venous blood (desflurane is able to be delivered in high inspired concentrations which speeds onset)
•    ventilation (faster with high tidal volumes and lower FRC)
•    second gas effect (faster if administered with nitrous oxide)
•    distribution to organs with a high blood flow (brain, heart, kidneys, liver) precedes distribution to less well perfused organs such as muscle then fat then bone
•     0.02% metabolised (thus minimal potential for toxicity) predominately to trifluoroacetic acid which is eliminated in the urine

Pharmacodynamics

Mechanism of action:

The mechanism of action of volatile agents is becoming better understood with the agents known to interrupt synaptic transmission in the CNS, particularly the ventrobasal thalamus.  General anaesthetic agents appear to potentiate GABA and glycine receptors whilst inhibiting signal transmission via NMDA receptors probably via expansion of hydrophobic cell membrane proteins.

 

Desflurane’s systemic effects are similar to other volatiles, in particular:

CVS

•           decreased SVR and MAP with resultant reflex tachycardia
•           minimal decrease in myocardial contractility and cardiac output
•           no myocardial sensitisation to catecholamines
•           no coronary “steal” seen in humans
•           decreased vascular resistance to both cerebral and coronary circulationsRESP
•           respiratory depression – decreased tidal volume, ­ increased respiratory rate, reduced respiratory drive resulting from ­ increased PaCO2
•           pungent odour causing upper airway irritation (coughing, breath holding) especially at concentrations of >6%, making it unsuitable for use in inhalation induction

CNS

•           ­increased cerebral blood flow (due to vasodilation)
•           reduced cerebral oxygen consumption
•           not associated with epileptiform activity
•           centrally mediated reduction in skeletal muscle tone

 

Precautions

•           as with all volatile agents, desflurane is a trigger for MH

 

Monitoring for administration:

•           as desflurane is a general anaesthetic agent full monitoring (as outlined in the college guidelines) is required

 

Specific interactions

•           potentiates co-administered muscle relaxant drugs.