Although reliable and proven existing anaesthetic techniques are available both within and outside hospitals, this does not diminish the need for continued improvement and technical development. Anaesthesia is necessary in prehospital emergency medicine [PHEM] for strictly biological/clinical reasons as well as for compassion and alleviation of suffering.
Introduction of new methods for prehospital anaesthesia should be based on actual field observations and experience but will ultimately have to be reproduced and studied in greater detail in a more controlled setting of a proper trial. The symbiosis between experience driven change and experimental verification does to some degree exist in all fields of medicine, but the balance between the two elements varies from speciality to speciality. In PHEM, the critical constraints of the field work stress the need for a more aggressive and proactive development of new experience based methods on behalf of a perhaps greater reliance on methodology derived from classical trials. In other fields of clinical medicine, where the input can be standardized to a greater degree and the immediate time-horison for any therapy initiated is longer, this balance may more properly be skewed towards controlled trials.
Some simple and practical prerequisites should be considered before putting forward new suggestions for PHEM anaesthesia. In hospitals, general anaesthesia is rarely induced without muscular relaxants [MR] and, accordingly, observations on the impact on the airway reflexes by anaesthetics are rarely mentioned in the literature. Moreover, it is hardly possible to standardize both the stimulus (summarized as “laryngoscopy” or “intubation”) and the outcome – its reflex (pharyngeal and laryngeal) muscular response, – as ideally required in a randomized controlled trial. One must therefore rely on clinical observations for data on airway reflexes until appropriate experimental methodology becomes available.
Prehospital patients are rarely fasting in anticipation of anaesthesia and, in consequence, most physicians utilize the in-hospital technique of rapid sequence induction (RSI) with thiopental and the short-acting MR succinylcholine for orotracheal intubation [OI] in order to minimise the risk of pulmonary aspiration. This works generally when used by anaesthetists, but there has been and increase in reports of serious adverse effects in connection with this procedure. These involve unrecognized oesophageal intubation and inability to carry out endotracheal intubation [EI] in the apnoeic patient, both of which are potentially lethal. To this must be added the potential pharmacological side-effects of the drugs used, such as bronchospasm and cardiac depression. In consequence, Deller [1] and Sefrin et al.[2] recommended EI carried out without the use of MR, but although this makes the procedures even more difficult, no advise was given as to how such EI could then be carried out.
The frequent use of MR and the difficulty in quantifying upper airway reflexes makes the impact of anaesthetics on upper airway reflexes a neglected topic in anaesthesia. Circulatory shock is also known to potentiate anaesthesia but the general condition of the patient has not been given attention in the more recent anaesthesia literature. Since considerable reflex differences are present between animals and humans, non-human experimental studies are less suitable for gaining knowledge in this field. This makes a comprehensive review of the scattered information upon the subject in humans desirable.
In the 1950s, British researchers [3-5] developed a method for visualization of aspiration during dental surgery through installation of radiopacque material in the pharynx and subsequent X-ray of the chest. This technique was revived in the 1970s: Taylor et al.[6] found aspiration after ketamine anaesthesia in all 7 volunteers and in 12 of 17 patients [7]. Utilizing a similar technique for 50% N2O/O2 for 14 volunter adults, Cleaton-Jones [8] found no aspiration cases, and neither did Roberts and Wignall[9] among 50 children for dental treatment, whereas Rubin et al.[10] found two aspiration cases among 10 volunteers. The same working group found aspiration of radiopacque material in all 8 patients, receiving neuroleptic drugs (droperidol, diazepam and fentanyl in combination) for carotid angiography [11]. Wise et al.[12] described aspiration resulting in visualization of the bronchial tree in 13 of 30 patients given methohexitat for conservative dentistry.
Without employing anaesthesia, and assessing upper airway reflex sensivity by means of a stimulus with ammonia vapour, Pontoppidan and Beecher [13] found a progressive loss of laryngeal closure reflexes with age, raising sixfold in latency to stimulation from the 2nd to the 8th of 9th decade. This protective, physiological measure is described as the Kretschmer-reflex after its description in 1870 [14] and rediscovered by Scandinavian authors in 1950 [15]. At the other end of the age spectre, an EI is known to be more easy to carry out without any drugs, the shorter the gestation age of a neonate [16], suggesting that not only the integrity but also the maturity of the nervous system is of importance. Though this phenomenon was discussed in relation to silent aspiration pneumonis, it may also be important in PHEM in making EI without MR possible.
Utilizing a similar technique, Groves et al.[17] found that two benzodiazepines with the same degree of sedation impaired the reflex differently: diazepam needed higher concentrations of ammoniumchloride than lormetazepam and was thus found to be reflex-depressing. With an improved technique, using better adjusted ammonia concentrations [18], Murphy et al.[19] found glottic closure considerably depressed 30-150 min after oral administration of 20 mg diazepam to 10 volunteers.
In ten enflurane-anesthetized but spontaneously breathing humans, airway defensive reflexes were induced by instillation into the trachea of 0.5 ml of distilled water [20]. This caused a variety of reflex responses including apnea, spasmodic panting, expiration reflex, cough reflex without any particular difference for CPAP (PEEP-level 10 cm H2O) or normal breathing. In studies of awake humans, Nishino et al.[21] showed that stimulation of the laryngeal mucosa with a small amount of distilled water causes elicitation of the expiration reflex, cough reflex, and swallowing reflex, but the duration of these responses is remarkably short. In contrast, the same stimulation causes more varied, prolonged, and exaggerated responses during a light depth of enflurane-anesthesia. An increase in depth of anesthesia abolishes expiratory efforts such as coughing and the expiration reflex whereas the apnoeic reflex and laryngeal closure reflex are resistant to the depressant effect of this type of anaesthesia. Both laryngeal and tracheal stimulation cause vigorous respiratory responses whereas bronchial stimulation causes little or no responses from the respiratory airways.
The same authors [22] used spraying of the vocal chords through a larynx mask airway [LMA] to provoke laryngeal stimulation in 22 patients anaesthetized with propofol and with different fentanyl dosages and CO2-levels. Before administration of fentanyl, laryngeal stimulation caused vigorous reflex responses, such as expiration reflex spasmodic panting, cough reflex, and apnea with laryngospasm. Increasing doses of fentanyl reduced the incidences of all these responses, except for apnea with laryngospasm.
Rimaniol et al.[23] measured the swallowing reflex every 3 min after the end of propofol infusion. To initiate swallowing, 0.3 ml of distilled water was injected into the pharynx. The latency periods were significantly increased for the first 12 min after the end of the propofol infusion and returned to control (preanesthetic values) at 24 min. Utilizing a similar technique, Nishino et al. [24] showed that 50% N2O/O2 in adults also resulted in delayed swallowing reflexes, but this was much dependant of the volume used for provoking the response.
These studies demonstrate differences both in patients and in the impact of anaesthetics on the upper airway reflexes; they also illustrate how difficult it is to approach these differences in a suitable methodology and how few patients are actually found in each study.
Comparing the conditions for insertion of an LMA, Molloy et al.[25] found coughing and head movement persistant after both propofol (2.5 mg/kg) and after sevofluran (8% in 50% N2O) without additional drugs. Quite in contrast to these findings, Muzi et al.[26] found no untoward airway responses during mask induction of anesthesia with a three-breath technique of sevofluran. In response to intubation, no adverse airway responses, e.g. jaw tightness, laryngospasm or excessive coughing occurred.
Scanlon et al.[27] compared thiopental with propofol for insertion of the LMA. Thiopental was associated with an adverse response in 76% of patients, compared with propofol in 26%. Gagging, laryngospasm and head movement were more common using thiopentone and in 11% of the thiopental group insertion of the LMA was impossible due to inadequate relaxation.
Djaiani et al.[28] found that after alfentanil, midazolam added to propofol increased the incidense of apnoe compared to propofol alone. Avoiding opioids and sedatives prior to induction, Stoneham et al.[29] found that pretreatment with lignocaine facilitated insertion of the LMA, also reducing both coughing and airway obstruction in response while increasing the success rate of the insertion without further deepening of anaesthesia. Arandia and Path [30] found by fibrelaryngoscopy that high doses of fentanyl were able to cause total occlusion of the vocal chords and assumed that this was caused by the fentanyl-rigidity.
Thus we approach studies using the same technique as employed in this study, an attempted EI on the basis of a hypnotic without the use of MR. In a study on vecuronium pharmacodynamics, deGrood et al.[31] found that propofol relaxed the vocal chords better than thiopental but in both cases, coughing was often precipitated upon succesfull intubation. Similarly, In 158 unpremedicated patients, McKeating et al.[32] found that after an unsupplemented induction dose of thiopentone or propofol, the latter made visualisation of the vocal cords by standard laryngoscopy possible more often after propofol. Of 103 patients anaesthetized with propofol 2.5 mg/kg, EI was succesfull in 99, in contrast to only 9 of 25 receiving thiopental 4 mg/kg and 20 of 30 after thiopental 5 mg/kg. Scheller et al.[33] intubated 60 patients after alfentanil 30-60 mg/kg plus propofol 2 mg/kg and found similar intubation conditions as seen after MR; however, coughing and movements after EI was frequent in the group receiving the lowest amount of alfentanil. Anaesthetic drugs thus appear to have a range of effects on the upper airway reflexes of great importance in clinical practice.
Little support for the suggested effect of etomidate was found in the literature. In early studies of this drug, Carlos and Innerarity [34] observed that pharyngeal and laryngeal reflexes were maintained in unpremedicated patients but delayed in combination with benzodiazepines. After bronchoscopy, McIntosh et al.[35] found laryngospasm in 16% of the patients who had received methohexital but only 2% after etomidate. In 47 adult patients undergoing either microlaryngoscopy or bronchoscopy in etomidate anaesthesia, Helmers et al.[36] noted specifically that mobility of the vocal cord was retained. Clement et al.[37] utilized a combination of diazepam and etomidate for laryngoscopic laser-surgery with maintained breathing in 115 patients, among whom 4 patients needed additional pentazocine while another technique was required in one case only. Hoffmann et al.[38] found that the combination of etomidate with pentazocine enabled spontaneous breathing while myocloni (frequent and very disturbing if etomidate is used alone) were effectively removed.
A study of our own group [39] is given particular attendence here. It was small and not very elegant, therefore it demonstrates the problems in getting deeper insights in this topic. Our local experience has confirmed EI to be possible in a breathing patient without MR, i.e. for toleration of laryngoscope and absence of laryngospasm. In a foreshortened study, 17 patients where scheduled for OI. In four of the six patients given thiopental 5 mg/kg, the intubation attempt precipitated laryngospasm, all six needed ventilation in advance and OI succeeded in 5 cases only after MR. This is in sharp contrast to the patients receiving etomidate 0.3 mg/kg, who all breathed spontaneously and no laryngospasm was noted except for a transient and reversible laryngeal defense in two cases. However, even here, OI succeded only in 6 of 11 cases. Retrospectively, old age and general risk factors (anyhow generally leading to etomidate for induction) was related to successful intubation. In a similar study of intubation without the use of MR, Erhan et al.[40] found clinically acceptable intubating conditions in 93% after propofol 2 mg/kg, 67%, after thiopental 6 mg/kg and 40% of patients after etomidate 0.3 mg/kg, in all cases preceeded by remifentanil 3 mg/kg.
It is noted here that etomidate produces a differential depression of reflexes, affecting pharyngeal more than laryngeal and least of all tracheal reflexes. In order to be of any value, this effect must be obtained in combination with drugs which abolish myocloni but without compromitizing the ventilatory or reflex properties of etomidate. This observation has not been made subject of any studies. The problem in the reported studies is that there is something to observe but little to measure. However important airway reflexes may be to the anaesthetist, they are unlikely to fall into the interest sphere of modern scientists.
In conclusion, there are substantial differences in the reflex depressing properties of various anaesthetics, and these are of importance for prehospital anaesthesia technics. Unfortunately, the topic has been widely ignored thus far in the anaesthetic literature.