Copyright 0 Actu Annesthesiol Scnnd 1995

Acta Anaesthesiologica Scand i navi ca

ISSN 0.515-2720

The new fluranes in paediatric day surgery: des and sevo JERROLD LERMAN

The Hospital for Sick Children and University of Toronto, Toronto, Canada

Desflurane and sevoflurane are two new inhalational anaesthetics soon to be or already available for use in paediatric anaesthesia around the world. Although both of these anaesthetics have favourable kinetic profiles that are reflected in rapid emergence from anaesthesia, desflurane is not suitable for administra- tion by mask. For ambulatory surgery, the versatility of sevoflurane (ie., it may be used for both induction and maintenance of anaesthesia together with its ease of use by mask insufflation) provides it with an edge over desflurane in the busy ambulatory unit. However, for those children who are intubated, recovery after both agents is rapid. In this lecture, I shall compare and contrast the pharmacology of both of these inhalational anaesthetics viewed from an ambulatory surgery perspective.

DESFLURANE. Desflurane (difluoromethyl 1 -fluoro-2,2,2-tri- flouro-ethyl-ether), a methyl ethyl ether compound, differs from isoflurane in only one substitution--a fluoride for chloride ion at the alpha ethyl carbon. This substitution confers several differences in the physical properties of desflurane compared to con- ventional inhalational anaesthetics. First, it has a boiling point of 23.5”C, just above room tempera- ture. This requires a special delivery device which is commercially available for anaesthetic machines. Second, it has low blood (0.42) and tissue solubili- ties.’.* This property leads to a very rapid wash-in and wash-out of the anae~thetic.~ Third, its odor irri- tates the upper airway reflexes when the inspired concentration exceeds 5%. The airway irritability results in breathholding, laryngospasm and hemoglo- bin oxygen desaturation in 50% or more of children precluding this anaesthetic from use as an induction agent in children4s5 Finally, its chemical structure confers stability in that it is metabolized (= 0.02%) far less than isoflurane and is stable in the presence of carbon dioxide absorbents.

The anaesthetic requirements (MAC) desflurane in 100% oxygen in adults ranges between 6 and 7.25%. The MAC of desflurane in children increases as age decreases reaching a zenith in infants 6-12 months of age and then decreases with age to neonates.6 The peak MAC in infants 6-12 months was 9.92 f 0.44% while that in neonates was 9.16 & 0.02%. The MAC- sparing effect of N20 is decreased in the presence of desflurane.’

Cardiovascular effects of desflurane are similar to those of isoflurane: heart rate and blood pressure are maintained.6 Although sympathetic hyperactivity has been reported in adults,8 it has not occurred in chil- dren. Little is known of the respiratory effects of des- flurane in children because of the absence of studies with spontaneous ventilation.

Recovery from desflurane anaesthesia is extremely rapid, as predicted by the low blood and tissue solu- bility of the drug.9 Accordingly, a pain management strategy must be in place (parenteral opioids or regional block) before withdrawal of the anaesthetic. The incidence of excitement during emergence may in part be explained by inadequate pain control but may also be a property of the anaesthetic itself. Some clinicians administer midazolam in such an instance.

Desflurane is relatively resistant to degradation in vivo (0.2%) and in vitro. Accordingly it poses little threat to organ function.

SEVOFLURANE. Sevoflurane (fluoromethyl 2,2,2-trifluoro-l-[trifluo- romethyl] ethyl ether) is a polyfluorinated methyl isopropyl ether anaesthetic with similar physico- chemical properties as other inhalational anaesthet- ics. However, it has low blood (0.63)’O and tissue2 solubilities which results in a rapid wash-in and wash-out. It also has a pleasant non-irritating odor.]’J Thus, it should provide for a rapid smooth inhalational induction and rapid recovery from anaes- thesia in ~h i ld ren .~ . ” , ’~

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Induction of general anaesthesia in infants and chil- dren with sevoflurane is smooth and uneventful. It does not trigger reflex responses of the upper airway. However, transient involuntary movement (agitation) of the extremities may occur.

The MAC of sevoflurane in adults is 1.7 to 2.O%.I3 The MAC in children between 6 months and 12 years is 2.5% and 3.2% in neonates and infants < 6 months of age.11 Surprisingly, 70% N,O only decreases the MAC =25%.

Cardiorespiratory effects of sevoflurane are similar to those of isoflurane. Arrhythmias are infrequent. In some studies, transient tachycardia of 15-20% above baseline occurs early in the induction period. Sevoflurane is a respiratory depressant and at con- centrations > 1.4 MAC, depresses respiration greater than halothane in adults.

Sevoflurane is metabolized in vivo (-4%) to inor- ganic fluoride. Plasma levels of inorganic fluoride correlate with the duration of exposure to sevoflurane although the levels tend to be less than in adults.ll1: Despite isolated instances of high plasma levels of fluoride, sevoflurane has not caused renal dysfunc- tion in any patients. This may be explained by the relatively small fraction of sevoflurane that under- goes renal metabolism.15 Sevoflurane is also degraded in the presence of carbon dioxide absorbent.16 Among the degradation products, compound A is produced in the greatest quantity. The concentration of compound A in the inspired limb of the circuit in children corre- lates with the temperature of the absorbent and the body surface area of the child.17 Since most clinicians use Mapleson D or F circuits in ambulatory surgery, degradation of sevoflurane in soda lime is unlikely to pose a significant obstacle.

Recovery from sevoflurane anaesthesia is rapid.11 ~ l *

Recovery after brief anaesthesia is rapid and complete, although slightly slower than after desflurane.

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