Inhalational induction of anaesthesia in adults: time for a breath of fresh air?

For the first century of anaesthesia, beginning with the public demonstration of diethyl ether in 1846, inhalational induction was the standard practice. The rise in the popularity of intravenous induction since the introduction of thiopental in 1934 has been attributable to the rapid, smooth transition from wakefulness to surgical anaesthesia that it provides. During an ether induction, the patient was observed to pass gradually through each of the classic stages of anaesthesia. Uninhibited responses to stimuli during stage two were a major cause of anaesthetic morbidity. Although the alternative agent chloroform provided faster, smoother induction, this was at the expense of life-threatening toxicity. Intravenous anaesthetics brought on sleep swiftly and relatively safely compared with a prolonged, usually unpleasant and often dangerous ether induction. With the advent of the modern volatile agents (starting with halothane in 1956) a new era of rapid and safe inhalational induction began. However, it remained largely confined to paediatrics and the management of difficult airways. Newer, faster agents appeared, but these were relatively pungent and were unpopular as induction agents. Since the arrival in the 1990s of sevoflurane, with its pleasant smell and minimal irritation of the respiratory tract, a flurry of interest in novel methods and uses for inhalational induction has occurred. Concurrently, debate about the technique's usefulness has resurfaced. Outside the field of paediatric anaesthesia, where inhalational induction avoids the need for venous cannulation before unconsciousness, the textbook situation for the use of inhalational induction has been in the management of the difficult airway. Here, obstruction of the airway (and loss of pulmonary ventilation) causes cessation of administration of anaesthetic agent and gradual re-awakening. After initial debate about its suitability, sevoflurane has almost universally replaced halothane in this role. While halothane remains useful when one is struggling to achieve or maintain a deep enough level of anaesthesia for laryngoscopy, there may be increasing difficulty accessing halothane as it all but disappears from routine UK practice. In patients with a known difficult airway who present for elective surgery, there are now a plethora of techniques and equipment for airway management, such that inhalational induction is only one of many options available. In this situation, awake fibreoptic intubation is now commonly employed, the awake patient maintaining his or her own airway until it is secured. It is for patients with a partially obstructed airway that authors have continued to advocate inhalational induction. In their 1999 editorial, Mason and Fielder recommended that the only safe options for patients with perilaryngeal tumours and stridor were awake tracheostomy or inhalational induction [1]. They strongly opposed awake fibreoptic intubation in such patients. Their powerful and clearly presented arguments against fibreoptic intubation also apply to many other patients with perilaryngeal obstruction (e.g. epiglottitis or Ludwig's angina). This traditional use of inhalational induction to manage the partially obstructed airway now faces assault; increasing experience with propofol and remifentanil, particularly with target controlled infusion (TCI) technology, has enhanced awake fibreoptic intubation and even allowed intravenous induction to be revisited. The airway-friendly properties and feasibility of titration of propofol commend it as an excellent candidate for use in difficult airways. An infusion can be gradually increased to allow deepening of anaesthesia, whilst maintaining spontaneous ventilation. If the patient suffers central or obstructive apnoea, the infusion can be stopped and rapid redistribution will allow lightening of anaesthesia or full wakening as appropriate. These properties – maintenance of spontaneous ventilation and redistribution during apnoea – have previously been the reasons for employing inhalational induction in difficult airways. The perceived advantage of inhaled agents over propofol is that, on apnoea, delivery ceases immediately and automatically. Balanced against this are the advantages of propofol: suppression of airway reflexes, achievement of adequate anaesthesia unhampered by poor ventilation, and lack of lightening of anaesthesia during airway intervention. Propofol infusion has been used successfully instead of inhalational induction before both conventional laryngoscopy [2,3] and intubation via a ‘dedicated airway’[4,5], and in patients with stridor [3,6]. For awake fibreoptic intubation, the patient experience and ease of the technique can be improved by careful sedation with propofol or remifentanil; these may be used singly or in combination. The ultra-short acting opioid remifentanil allows easy titration and rapid reversibility of deep levels of analgesia sedation. Randomised controlled trials have suggested that remifentanil sedation is superior to propofol [7] or combined midazolam and fentanyl [8], although at the expense of increased postoperative recall of the intubation. Recently, authorities on airway management have stated that, in partially obstructed airways, where awake tracheostomy is thought unnecessary or not possible, awake fibreoptic intubation can be successfully employed by an experienced operator [9,10]. A recently suggested development of the fibreoptic technique has been described as ‘awake fibrecapnic intubation’, whereby carbon dioxide monitoring is used to aid glottic identification [11]. What seems clear is that thorough assessment of the individual patient, discussion with surgical colleagues, and careful use of familiar and well-practiced techniques by experienced and skilled staff is more important than the specific techniques used. It is possible that inhalational induction for the difficult airway will become consigned to history. Time will tell if the practices of airway enthusiasts can be safely translated into widespread use by the general anaesthetist who rarely encounters such patients. Volatile induction and maintenance of anaesthesia (VIMA) is appealing because of the theoretical advantages of enhanced safety and recovery as a result of mono-pharmacy, and it has been proposed as a rival to total intravenous anaesthesia (TIVA) for day-case surgery. However, a meta-analysis comparing sevoflurane with propofol for induction concluded that, although they are comparable in terms of induction time and induction complications, sevoflurane is associated with significantly more postoperative nausea and vomiting [12]. Subsequently, a randomised controlled trial of over 1000 adult day surgery patients confirmed that sevoflurane VIMA had the highest postoperative nausea and vomiting rate with no apparent benefits, compared with TIVA or propofol induction with volatile maintenance [13]. It also found that, although less costly than TIVA, the total cost per episode for VIMA was greater than propofol induction with volatile maintenance. Inhalational induction for adult day surgery is acceptable, but is not cost effective and cannot be recommended. Six cases have been published of rapid sevoflurane induction for Caesarean section, where intravenous rapid sequence induction was not undertaken owing to difficulty in venous cannulation [14,15], needle phobia [16,17] or status asthmaticus [18]. The reported techniques vary. Some can be criticised, but in none were there any adverse incidents related to the induction. An editorial by Levy on this subject recommends appropriate conduct for obstetric inhalational induction [19]. He suggests employing a 20–30° head-up tilt, in order to decrease the likelihood of passive regurgitation. Although induction may be more rapid after a single vital capacity breath technique [20], this has been questioned [21] and the success of the technique depends on adequate understanding by the patient. None of the above case reports specifically describes employing such a technique and adequate instruction may not be possible in the pressured setting of emergency obstetrics. A compromise would be to ensure full priming of the anaesthetic circuit with sevoflurane 8% in nitrous oxide 66% and encouraging the patient to take slow, deep breaths. Immediately on loss of consciousness, intravenous access should be secured if not already in place. Application of full 30-N cricoid pressure might be safely delayed until after neuromuscular blockade to reduce the risk of inducing vomiting. Finally, the airway should almost certainly be secured before the start of surgery. Levy concludes that there is no evidence that sevoflurane induction is less safe than intravenous induction. Although absence of evidence is not evidence of absence, rapid inhalational induction would appear to be an acceptable alternative to intravenous rapid sequence induction in selected obstetric situations. Inhalational induction is accelerated in pregnancy by a reduced minimum alveolar concentration (MAC) value for volatile anaesthetic agents and increased ventilation of a relatively small functional residual capacity. In addition, pregnancy almost certainly presents a different aspiration risk to other situations such as gastrointestinal obstruction. Thus, it is currently difficult to justify rapid sequence inhalational induction in non-obstetric situations, but there is scope for future investigation. Inhalational induction with sevoflurane can provide relatively stable conditions in patients at risk from hypotension, hypertension and tachycardia. Sevoflurane induction has been shown to be comparable to TCI propofol in terms of cardiovascular stability around the time of tracheal intubation in hypertensive patients [22]. There has been particular interest in anaesthesia for the elderly population, in whom sevoflurane has been demonstrated to provide greater cardiovascular stability than propofol [23,24] and less reduction in left ventricular mechanical performance [25]. The anaesthetic agent employed is probably less important than the manner of its use, but inhalational induction may be the technique of choice when trying to provide a stable induction in patients with little cardiovascular reserve, especially in view of concerns about etomidate [26]. One hundred and sixty years after the birth of anaesthesia, inhalational induction is still widely practiced. We can observe that the momentum of use in adults is away from difficult airways, via an unfruitful sojourn into day-case anaesthesia, towards novel indications such as the fragile patient and rapid sequence inhalational induction. Although intravenous induction has become the preferred method for adults in most circumstances, inhalational induction will remain a core anaesthetic skill for the foreseeable future.

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