London 2003
Page 2 of 7

<Back to Abstract Home Page>   <Previous Page>     <Next Page>  <Program>

 

 

INhalation Induction—New concepts

Ian Smith, BSc, MD, FRCA

Keele University, North Staffordshire Hospital, Stoke-on-Trent

 

Inhalation induction is hardly a new concept, dating back to the days of Wells and Morton. Even when intravenous alternatives became available, the technique was kept alive by rapid-onset, short-acting anaesthetics, such as ethyl chloride, divinyl ether and cyclopropane. But by the 1980s, inhalation induction was used primarily for children and was most unusual in adults. The advent of newer, insoluble volatile anaesthetics reawakened interest in inhalation induction. With sevoflurane, in particular, inhalation induction became not just possible in adults, but could also be a desirable technique with several benefits.

Renewed interest in adult inhalation induction began during the development of desflurane, which was associated with rapid and pleasant loss of consciousness in healthy young male volunteers [1]. In clinical use, however, it was found to have an unacceptably high level of coughing and laryngospasm [2]. Although these side effects can be somewhat reduced by pretreatment with morphine [3], this is rather impractical and desflurane is rarely used for induction of anaesthesia. In contrast, sevoflurane has a less pungent, fruitier smell and permits a much smoother induction of anaesthesia with a remarkable absence of airway irritability [4]. High concentrations are tolerated from the outset and opioid pretreatment is unnecessary.

Inhalation induction is a useful technique, but has previously been limited by the toxicity or other problems of the available drugs. Successful inhalation induction requires an agent of low solubility, reasonable potency and minimal irritation [5]. Because sevoflurane has the best combination of these properties (of the available agents), it makes inhalation induction practical. This produces a number of benefits, including good control of the airway, excellent oxygenation, minimal apnoea and less hypotension compared to propofol [4, 6]. Needle phobia is common in children because needles hurt. Inhalation induction is one solution, but local anaesthetic creams are also effective. Needle phobia may affect as many as 10% of adults [7], but many more would prefer to avoid needles. Pain is less of an issue here; tattoos are not uncommon in adult needle phobics. The problem appears to be a fear of drawing blood [7], so local anaesthetic cream does not help, but gaseous induction does.

Traditionally, inhalation induction has been taught using a slow, step-wise increase in inspired concentration. This technique is best avoided with sevoflurane, as it delays induction and increases complications [8]. Both vital capacity [9] or tidal breathing [4] techniques with 8% sevoflurane are rapid and effective, with little to choose between the two methods in routine practice [10]. Clear masks and a gentle approach improve the patient’s experience.

Assuming an inhaled anaesthetic is to be used for maintenance, inhalation induction offers the additional advantage of eliminating the transition from intravenous to inhaled drugs, eliminating what is, in effect, a second induction. This can reduce the amount of vapour which is used during the maintenance period [11]. Indeed, starting with an inhalation induction represents the most efficient way of delivering a volatile anaesthetic. At the end of induction, the concentration at the site of effect must necessarily be adequate for LMA insertion (a level known to be comparable to MAC [12]). Because of effect-site delay, however, the plasma concentration (reflected in the end-expired concentration) will be substantially higher. If the fresh gas flow is immediately reduced to 1 litre•min-1, or less, this “surplus” anaesthetic will not be washed from the breathing circuit, but will continue to diffuse down the gradient to the effect site, deepening anaesthesia further. The patient will be adequately prepared for surgery, yet additional drug delivery will be unnecessary for some time and the vapouriser may be turned off until the end-tidal concentration declines towards usual maintenance levels. Not only is this cost-effective, it also demonstrates the fallacy of changing to a “less expensive” maintenance agent. Although early reduction of fresh gas flows may not ensure total nitrogen washout or equilibration of nitrous oxide, satisfactory clinical conditions are still achieved.

Although low flows are desirable during maintenance, higher flows are necessary during induction because of the high uptake of anaesthetic. A fresh gas flow of 6 l•min‑1 is widely used in adults. While similar to the normal minute ventilation of an average patient, this is still a somewhat arbitrary choice. Higher fresh gas flows may wash anaesthetic into the breathing circuit and alveoli faster, speeding induction, while lower flows should delay induction. The cost per minute will change linearly with flow, but the total cost will also depend on induction time. The optimal gas flow should induce anaesthesia in an acceptable time, but at minimal cost. While this does not appear to have been formally studied, simulation suggests that reducing the flow to 5 litres•min-1 prolongs induction by less than 5%, yet reduces costs by 12%. A pilot study (unpublished data) produced almost identical results, although other fresh gas flows were not examined.

As wash-in is an exponential function, it may be possible to reduce the gas flow during induction. Since rapid loss of consciousness is desirable from the patient’s point of view, it seems sensible to use a high gas flow until this point. Simulation suggests that delivering 8% sevoflurane at 6 l•min‑1 until loss of consciousness and then reducing the flow to 2 l•min‑1 only delays the time to LMA insertion by about ten seconds, yet may result in savings of almost a third. This has yet to be tested in practice and the effects on denitrogenation and nitrous oxide wash-in are currently unknown.

In summary, inhalation induction of anaesthesia is simple and elegant. Using sevoflurane, there are cardiovascular, ventilatory and financial advantages over iv induction. Inhalation induction with a relatively insoluble anaesthetic allows us to reduce fresh gas flows at an earlier stage than was previously possible, providing additional benefits.

Conflict of Interest

The author has previously received research funding and lecture honoraria from Abbott Laboratories, the distributor of sevoflurane.

References

1.    Jones RM, Cashman JN, Mant TGK. Clinical impressions and cardiorespiratory effects of a new fluorinated inhalational anaesthetic, desflurane (I-653), in volunteers. Br J Anaesth 1990;64:11–5.

2.    Van Hemelrijck J, Smith I, White PF. Use of desflurane for outpatient anesthesia: A comparison with propofol and nitrous oxide. Anesthesiology 1991;75:197–203.

3.    Kong CF, Chew STH, Ip-Yam PC. Intravenous opioids reduce airway irritation during induction of anaesthesia with desflurane in adults. Br J Anaesth 2000;85:364–7.

4.    Thwaites A, Edmends S, Smith I. Inhalation induction with sevoflurane: A double-blind comparison with propofol. Br J Anaesth 1997;78:356–61.

5.    Logan M. A practical review of VIMA techniques. Int Proceed J 1998;7:4–10.

6.    Kirkbride DA, Parker JL, Williams GD, Buggy DJ. Induction of anesthesia in the elderly ambulatory patient: A double-blind comparison of propofol and sevoflurane. Anesth Analg 2001;93:1185–7.

7.    Hamilton JG. Needle phobia: A neglected diagnosis. J Fam Pract 1995;41:169–75.

8.    Yurino M, Kimura H. Induction of anesthesia with sevoflurane, nitrous oxide, and oxygen: A comparison of spontaneous ventilation and vital capacity rapid inhalation induction (VCRII) techniques. Anesth Analg 1993;76:598–601.

9.    Hall JE, Stewart JIM, Harmer M. Single-breath inhalation induction of sevoflurane anaesthesia with and without nitrous oxide: a feasibility study in adults and comparison with an intravenous bolus of propofol. Anaesthesia 1997;52:410–5.

10.  Baker CE, Smith I. Sevoflurane: A comparison between vital capacity and tidal breathing techniques for the induction of anaesthesia and laryngeal mask airway placement. Anaesthesia 1999;54:841–4.

11.  Smith I, Terhoeve PA, Hennart D, et al. A multicentre comparison of the costs of anaesthesia with sevoflurane or propofol. Br J Anaesth 1999;83:564–70.

12.  Taguchi M, Watanabe S, Asakura N, Inomata S. End-tidal sevoflurane concentrations for laryngeal mask airway insertion and for tracheal intubation in children. Anesthesiology 1994;81:628–31.

 

The simulations referred to were performed using GasMan®, Version 3.1.4 for Macintosh (Med Man Simulations, Chestnut Hill, Massachusetts)

 

 

 

 

Inhalation induction in children

 

Dr G H Meakin, Manchester

 

Inhalation induction of anaesthesia accounts for one-third of all inductions in children the UK and over half of those in the USA. (1, 2) Reasons for the popularity of inhalation induction in children include a belief that it is less objectionable to children, who often have an exaggerated fear of needles, and the fact that it can be quite difficult to obtain venous access in awake, uncooperative children.

 

The aims of this lecture are to review the advantages and disadvantages of inhalation induction in children in detail and examine the important differences between children and adults in respect of uptake of inhaled anaesthetics. In addition the following questions will be addressed:

 

 

 

 

 

There have been two studies comparing intravenous and inhalation induction of anaesthesia in children, both of which demonstrated significantly more anxiety in children undergoing i.v. induction. (3, 4) However, while induction time was somewhat shorter after i.v. induction in both studies, recovery time, recorded in only one study, was much longer after i.v. induction compared with inhalation induction. Importantly, neither study found a difference in behavioural disturbances one week after surgery. 

 

The main physiological factors affecting uptake and distribution of inhaled anaesthetics are the alveolar ventilation and the cardiac output. On a weight basis these are 2-2.5 times greater in the infant than in the adult, and this is reflected in faster washin of anaesthetic agents to the alveolar compartment and more rapid uptake by body tissues. As a result of this, inhalation induction is more rapid in infants; (5) however, cardiovascular side effects such as myocardial depression and hypotension will also tend to develop more rapidly. (6) This problem is compounded by the fact that the infant myocardium has less contractile tissue than the adult myocardium and there is relative underdevelopment of the sympathetic nervous system. (7) Accordingly, it is important to avoid administering high concentrations of potent volatile anaesthetics for very long in these patients. A computer simulation program (8) will be used to demonstrate the safe use of overpressure for induction of anaesthesia in a neonate.

 

 

Which anaesthetics should we use for inhalation induction?

 

The first requirement of an inhalation induction agent is that it should have minimal pungency: that is, it should cause minimal airways irritation. Of the inhaled anaesthetics commonly used today we only have three candidates for inhalation induction: nitrous oxide, sevoflurane and halothane. Although N2O cannot be used as the sole agent to induce or maintain anaesthesia, it is frequently used as the initial anaesthetic in an inhalation induction sequence in children to obtund the sense of smell and make the volatile agents more acceptable. (9) Sevoflurane is somewhat less pungent than halothane and it has a lower blood gas coefficient leading to smoother and more rapid induction of anaesthesia. (10) Sevoflurane also causes less myocardial depression (11) and bradycardia than halothane and is probably the volatile agent of choice for inhalation induction in children. (10) 

 

Which breathing system we should use for induction of anaesthesia in children?

 

 

A non-rebreathing system, such as the Jackson Rees T-piece, has many advantages for induction of anaesthesia in children. These include, lack of bulk, convenience and low compression volume, which is an advantage when controlling ventilation in patients with small tidal volumes and low lung compliance. (12) By comparison the rebreathing systems are more bulky, less convenient and have a high compression volume. Moreover, the main advantage of these systems (economy) will not be evident during inhalation induction as high fresh gas flows are needed to establish anaesthesia. 

 

What can we do to make the experience more pleasant?

 

 

The child should be given a full explanation of the procedure by the anaesthetists at the preoperative visit using simple terms that will not cause alarm. Play therapists  have an important role as they can use structured play to decrease child anxiety and identify candidates who may benefit from sedative premedication. The induction should take place in a child-friendly atmosphere and a parent should accompany the child. The anaesthetist can also use play techniques to reduce anxiety and increase cooperation at induction of anaesthesia. The use of scented face masks and pre-induction with N2O improve the acceptability of the method.    

 

References:

 

 

 

 

1.         Bowhay A, Harmer M, Jones R, et al. Current Anaesthetic Practice - Survey and Report. Hants: I. Wildmoor, 1995.

2.         Motoyama EK. Induction of anaesthesia. In: Motoyama EK, Davies PJ, eds. Smith's Anaesthesia for Infants and Children, 5th edn. St. Louis: CV Mosby Company, 1990: 257-268.

3.         Kotiniemi LH, Ryhänen PT. Behavioural changes and children's memories after intravenous, inhalation and rectal induction of anaesthesia. Paed Anaesth 1996; 6: 201-207.

4.         Aguilera I, Patel D, Meakin GH, et al. Perioperative anxiety and postoperative behavioural disturbances in children undergoing intravenous or inhalation induction of anaesthesia. Paed Anaesth (in press).

5.         Salanitre E, Rackow H. The pulmonary exchange of nitrous oxide and halothane in infants and children. Anesthesiology 1969; 30: 388-394.

6.         Brandom BW, Brandom RB, Cook DR. Uptake and distribution of halothane in infants: in vivo measurements and computer simulations. Anesth Analg 1983; 62: 404-410.

7.         Friedman WF. The Intrinsic Physiologic Properties of the Developing Heart. New York: Grune & Stratton, 1973.

8.         Philip JH. GasMan ver 2.1, Man Med Simulations 1995. Web address: http://www.gasmanweb.com.

9.         Berry FA. Inhalation agents in paediatric anaesthesia. In: Sumner E, Hatch DJ, eds. Textbook of Paediatric Anaesthetic Practice. London: Bailliere Tindall, 1989: 19-30.

10.       Kataria B, Epstein R, Bailey A, et al. A comparison of sevoflurane to halothane in paediatric surgical patients: results of a multicentre international study. Paed Anaesth 1996; 6: 283-292.

11.       Holzman RS, van der Velde ME, Kaus SJ, et al. Sevoflurane depresses myocardial contractility less than halothane during induction of anaesthesia in children. Anesthesiology 1996; 85: 1260-126.

12.       Nunn JF. Applied Respiratory Physiology. 3rd ed. London: Butterworths, 1987.