Correspondence Anaesthesia, 2008, 63, pages 433–445 . ....................................................................................................................................................................................................................
prolonged attempts. Again, we reply that if he reads what we wrote he will see that the truth is exactly the opposite. He is confused by discrepancies in the reported incidence of grade 3 laryngoscopies, an issue resolved many years ago, as he will see from our Editorial and reference 20 therein. Informed consent is a complex issue which cannot be covered in a short space – his comments are extremely simplistic. He states that our drill is unethical, but does not mention the far more serious ethical issue of making trainees do emergency work having had no instruction in difficult intubation; the dire effects that can follow were spelt out many years ago. Litigation following future disasters will call to account those responsible for training. Surprisingly, Cook’s lengthy discussion of side-issues says nothing about that most important question, namely how to improve intubation training. In recent years a motley array of airway devices have appeared, some prompted more by commercial speculation than coherent logic. There is scant evidence of benefit and clear proof of harm – for example, the original gum elastic bougie has given trouble-free service over half a century, but a new ‘improved’ rigid version has already caused two deaths [13]. Should research focus on new devices, or on better training methods? A full-scale study of a new device would be a colossal waste of time and money if existing methods, properly taught, can solve the problem. Lastly, may we take this opportunity to thank many former trainees and others who have contacted us with their keen support.
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associated with difficult intubation. Anesthesiology 1992; 77: 67–73. West MRJ, Jonas MM, Adams AP, Carli F. A new tracheal tube for difficult intubation. British Journal of Anaesthesia 1996; 76: 673–9. Miller CG. Management of difficult intubation in closed malpractice claims. ASA Newsletter 2000; 64: 13–9. Heidegger T, Gerig HJ, Ulrich B, et al. Structure and process quality illustrated by fibreoptic intubation: 1612 cases. Anaesthesia 2003; 58: 734– 9. Cormack RS, Smith JE. Training requirements for intubation. British Journal of Anaesthesia 2006; 97: 433P. Chambers WA. Difficult airways – difficult decisions. Anaesthesia 2004; 59: 631–2. Mendelson CL. Aspiration of stomach contents into the lungs during obstetric anesthesia. American Journal of Obstetric Anesthesia 1946; 52: 191–205. Cook TM, Lee G, Nolan JP. The ProSealTM laryngeal mask airway: a review of the literature. Canadian Journal of Anesthesia 2005; 52: 739–60. Cook TM, Gibbison B. Analysis of 1000 consecutive uses of the Proseal laryngeal mask airwayTM. British Journal of Anaesthesia 2007; 99: 436–9. Beveridge WIB. The art of scientific investigation. Melbourne: Heinemann, 1950.
13 Hodzovic I, Latto IP. Single-use tracheal introducers. Anaesthesia 2007; 62: 1189. Calculating the volume of the oxygen reservoir bag for the Triservice anaesthetic apparatus
It is good to see that research continues into improving draw-over anaesthetic equipment [1]. The authors have correctly shown that the use of a bag, which incorporates valves to release any over-pressure or allows air entry if the bag collapses, gives a slightly higher inspired-oxygen concentration than an open-ended reservoir tube in a drawover breathing system similar to that of the Triservice apparatus. This may be attributable to the diffusion of oxygen out of an open-ended reservoir tube [2]. The size of the reservoir bag required to utilise all the supplementary oxygen may be calculated from the formula below, assuming that the supplementary flow-rate of oxygen is equal to or less than the patient’s inspired minute volume: Vr ¼ V o2 te 60 where VO2 ¼ flow rate of supplementary oxygen (ml.min)1), te = length of expiration (s) and Vr ¼ minimum
R. S. Cormack Northwick Park Hospital, Harrow HA1 3UJ, UK E-mail:
[email protected]
References 1 Lyons G. Failed intubation: 6 years experience. Anaesthesia 1985; 40: 759– 62. 2 Samsoon GLT, Young JRB. Difficult tracheal intubation: a retrospective study. Anaesthesia 1987; 42: 487–90. 3 Rocke DA, Murray WB, Rout CC, et al. Relative risk analysis of factors 438
Figure 1 Laerdal reservoir valve. (Reproduced with kind permission of the Heritage
Section of the Association of Anaesthetists of Great Britain and Ireland.)
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Anaesthesia, 2008, 63, pages 433–445 Correspondence . ....................................................................................................................................................................................................................
volume of reservoir bag to utilise all the supplementary oxygen in millilitres [2]. Laerdal used to make a simple and neat valve arrangement which combined an over-pressure and air-intake valve (Part no. 531902) as illustrated in Fig. 1. This mated readily with a bag oxygen reservoir (Part no. 531901). I. T. Houghton Westferry Circus London E14 8RP, UK E-mail: ivan.houghton@ btinternet.com
References 1 Eales M, Rowe P, Tully R. Improving the efficiency of the drawover anaesthetic breathing system. Anaesthesia 2007; 62: 1171–4. 2 Yoganathan S, Johnston IG, Houghton IT. Reservoir volume in a drawover system. Anaesthesia 1987; 42: 1137.
Use of propofol in patients with food allergies
We read with interest the recent correspondence highlighting the inclusion of peanut allergy in the list of contraindications in the product literature for propofol [1]. The potential for peanutallergic patients to be sensitive to propofol is due to the fact that soya oil is one of the excipients of propofol, and that cross-reactivity occurs in legume allergy, for example between soya and peanut. However, it is the protein component that is responsible for allergic reactions in these patients, and as the soya oil present in propofol is refined, it is unlikely to contain significant quantities of allergenic particles. In the food industry soya-containing mixed vegetable oil may be sold for consumption without listing soya as an ingredient [2]. The reason for this is that the dose of protein contained in refined soya oil is too small to provoke a reaction when ingested. Although the minimum dose of protein required to trigger a reaction after oral ingestion has been identified, the same is not true of parenteral administration [3]. There is no mention of egg allergy in the product literature for propofol. The
main triggers for egg anaphylaxis are three proteins found in egg white: ovoalbumin, ovomucoid and conalbumin. Lecithin, a purified egg phosphatide present in propofol, is not thought to be problematic for patients who are allergic to eggs. Allergic reactions to propofol have been shown to be triggered by the iso-propyl or phenol groups rather than the lipid vehicle [4, 5]. One case report describes an anaphylactic reaction following administration of propofol to a child with multiple food allergies including egg and peanut. However, other drugs including rocuronium had also been administered. Skin prick testing was not performed and the specific causative agent remains unclear [6]. We have spoken to the Medical Information Department at AstraZeneca who informed us that the final decision to use Diprivan in patients with egg allergy remains at the discretion of the individual anaesthetist concerned (personal communication, D. Gupta, AstraZeneca Ltd). In summary, the negligible protein content of refined soya oil in propofol suggests the drug is unlikely to trigger a reaction in patients with peanut allergy. The fact that the product literature cites this as a contra-indication must, however, be given careful consideration prior to administration. If the clinician decides propofol is unsuitable, inhalational induction of anaesthesia may provide a useful alternative. A. E. D. Bradley K. E. S. Tober R. E. Brown Musgrove Park Hospital Taunton TA1 5DA, UK E-mail:
[email protected]
References 1 Gangineni K, Scase AE, Fearn J. Propofol and peanut allergy. Anaesthesia 2007; 62: 1191. 2 Eat well, be well – Soya allergy. http:// www.food.gov.uk (accessed 30 October 2007). 3 European Medicines Agency. Public Statement on the Allergenic Potency of Herbal Medicinal Products Containings Soya or Peanut Protein. London: European Medicines Agency, 2006. http://
2008 The Association of Anaesthetists of Great Britain and Ireland
www.emea.europa.eu/pdfs/human/ hmpc/13813905en.pdf (accessed 30 October 2007). 4 De Leon-Casasola OA, Weiss A, Lema MJ. Anaphylaxis due to propofol. Anesthesiology 1992; 77: 384–6. 5 Laxenaire MC, Mata-Bermejo E, Moneret-Vautrin DA, et al. Life threatening anaphylactoid reactions to propofol (Diprivan). Anesthesiology 1992; 77: 275–80. 6 Hofer KN, McCarthy MW, Buck ML, et al. Possible anaphylaxis after propofol in a child with food allergy. Annals of Pharmacotherapy 2003; 37: 398–401. Hypertension and general anaesthesia: guidance for general practitioners and results of a questionnaire
Hypertension in the peri-operative period is associated with peri-operative ischaemic events, arrhythmias and increased cardiovascular lability. A metaanalysis of 30 observational studies demonstrated an odds ratio for the association between hypertensive disease and perioperative cardiac outcomes of 1.35 (1.17–1.56) [1]. Consequently, hypertensive patients may be cancelled on the day of surgery, causing distress and compromising efficiency. There is potential to reduce this problem by careful determination of a hypertensive patient’s suitability for general anaesthesia. Patients with dangerously elevated arterial blood pressure should ideally not be referred from Primary Care, or detected at pre-assessment clinics and referred back to their general practitioner. Conversely, general practitioners or pre-assessment staff may feel that patients with moderate hypertension are yet to be adequately controlled and require further optimisation prior to anaesthesia and surgery, when evidence is lacking to support this practice. We attempt to provide a template, based on current anaesthetic teaching, which could reduce this problem. We envisage that this information would be useful to general practitioners and pre-assessment clinics. Evidence for adverse outcomes associated with hypertension is sometimes 439
