CHANGES IN HAEMODYNAMIC VARIABLES WITH THIOPENTONE, METHOHEXITONE, PROPOFOL AND ETOMIDATE

CORRESPONDENCE personnel. British Journal of Anaesthesia 1992; 69: 320-321. 2. Barker P, Langton JA, Murphy PJ, Rowbotham DJ. Regurgitation of gastric contents during general anaesthesia using the laryngeal mask airway. British Journal of Anaesthesia 1992; 69: 314-315. 3. Davies PRF, Tighe SQM, Greenslade GL, Evans GH. Laryngeal mask airway and tracheal tube insertion by unskilled personnel. Lancet 1990; 336: 977-979. 4. Maltby JR, Loken RG, Watson NC. The laryngeal mask airway: clinical appraisal in 250 patients. Canadian Journal of Anaesthesia 1990; 37: 509-513. 5. Brodrick PM, Webster NR, Nunn JF. The laryngeal mask airway. A study in 100 patients during spontaneous breathing. Anaesthesia 1989; 44: 238-241. 6. Jesudian MCS, Harrison RR, Keenan RL, Maull KI. Bag and mask ventilation; two rescuers are better than one: preliminary report. Critical Care Medicine 1985; 13: 122-123. 7. Greenslade GL. Single operator cardiopulmonary resuscitation in ambulances. Anaesthesia 1991; 46: 391-394. Sir,—We are grateful for the opportunity to reply to the points made in these letters. The aim of the study was to assess the useof the LMA by inexperienced operators with the intention that it be available on general wards for use by nurses and junior doctors in emergencies. Training these large numbers of staff to use the device would necessarily be brief. The operators in our study had all trained at the same medical school and had received a short period of resuscitation training as undergraduates. This usually involved only a small amount of "hands on" experience of airway management. However, our proposed users in emergencies on wards would all be expected to have had a similar small amount of previous experience with face mask and bag, and perhaps none with patients. The inexperienced operators to whom Dr Tighe refers were naval medical trainees who had a short training programme using mannequins. The fact that the success rates differ reflects only that different populations were studied, and no inaccuracy need be invoked to explain it. With the LMA, his inexperienced group performed as well as anaesthetists, and ours did not. The group we used and the training given seemed to us to be a reasonable representation of the final ward users and the depth of training they can expect. No specific criteria were used to exclude patients who might present with difficult tracheal intubation, but the investigators who recruited patients erred on the side of caution in their choice. None of the patients presented difficulty with intubation subsequently. Operators were shown how to use the mask and airway by demonstration by one of the investigators. Although the means of demonstration, age ranges and degree of neuromuscular block were not standardized, we feel that the crossover design of the study, with each patient being subjected to both airway management techniques in random order, effectively excluded any potential bias caused by these factors. Reasons for failure in the mask group were not recorded, but were mainly as expected: an inability simultaneously to maintain a seal and a clear airway. All the failures in the LMA group were caused by jamming of the mask at the posterior pharyngeal wall. Times and failure rates for the first and fifth attempts for each operator were not statistically different, so no learning effect was demonstrated. It is of interest that in only two patients was the LMA successful where the face mask had failed, whereas there were 11 patients in whom the reverse was true. The source of the 40-s cut-off time was the paper by Davies and colleagues [1]. We agree that 40 s is a very short time; however, for reasons of patient safety the time was accepted to reduce the risk of desaturation. As there were no episodes of desaturation during the trial, a less conservative time might be allowed in future. The ranges of times were 15-40 s for the LMA and 14—40 s for the bag and mask. We felt that, although a difference in mean rimes of only 5 s is probably not clinically significant, the difference in failure rate was. In other words, the added long-term advantage of the LMA is lost if it cannot be inserted at all. It is possible that this difference would disappear if more time was allowed, but are we going to advise users to persevere with attempts to insert the LMA after 40 s without success when the patient is apnoeic, while with the face mask they may have delivered two or three breaths of 100% oxygen in this time? Our conclusion was based on the results and conditions in this trial, which were intended to imitate use by inexperienced ward

115 staff with minimal training. We believe that the results indicate that the LMA is not suitable as afirstline airway management tool to be used by staff who have had only a brief demonstration of its use. This does not preclude its use as a second line of management by staff with more training, as in the paper by Davies and colleagues [1]. We agree that the LMA deserves further consideration for emergency airway management, for the reasons given. Further work might also define if the use of expired tidal volumes, as in our paper, or detection of end-tidal carbon dioxide as in the paper of Davies and colleagues, is a more accurate measure of adequate ventilation. P. TOLLEY A. WATTS J. HICKMAN

Wrexham Maelor Hospital Wrexham 1. Davies PRF, Tighe SQM, Greenslade GL, Evans GH. Laryngeal mask airway insertion by unskilled personnel. Lancet 1990; 336: 977-979.

CHANGES IN HAEMODYNAMIC VARIABLES WITH THIOPENTONE, METHOHEXITONE, PROPOFOL AND ETOMIDATE Sir,—I read with interest the paper by Price and colleagues [1], and I agree that their results may indicate that etomidate produced some degree of myocardial depression, as has been shown by other investigators [2,3]. However, I believe that their study design precludes comparison with other induction agents because they did not account for the effects of either changes in blood-gas tensions or the accompanying medications (nitrous oxide and alfentanil). Both hypoxaemia and hypercapnia may increase cardiac output and decrease systemic vascular resistance. Because thiopentone, methohexitone and propofol depress ventilation more than etomidate [4, 5], the different results in these groups possibly could be explained by hyopoventilation and hypoxaemia. This could have occurred in these patients, as Fi o was 0.3, ventilation was assisted only after 30 s of apnoea and, because there was no documentation that either ventilatory frequency or tidal volume were measured, it seems possible that both could have been less than optimal for the maintenance of normal blood-gas tensions. Also, the combination of alfentanil and nitrous oxide may have depressant effects on cardiac output [6] and should have been used as a control to establish if the effects in the etomidate group were attributable to the administration of these drugs. Until the above issues are addressed, I believe that the conclusions drawn from their results are probably invalid. E. L. WILLIAMS

University School of Medicine St Louis, U.S.A. 1. Price ML, Millar B, Grounds M, Cashman J. Changes in cardiac index and estimated systemic vascular resistance during induction of anaesthesia with thiopentone, methohexitone, propofol and etomidate. British Journal of Anaesthesia 1992; 69: 172-176. 2. Firestone S, Kleinman CS, Jaffe CC, Taunt RT, Schneiderman S, Talner NS, Barash PD. Human research and noninvasive measurement of ventricular performance: an echocardiographic evaluation of etomidate and thiopental. Anesthesiology 1978; 51: S22. 3. Criado A, Maseda J, Navarro E, Escarpa A, Avello F. Induction of anaesthesia with etomidate: a study of 36 patients. British Journal of Anaesthesia 1980; 52: 803-805. 4. Choi SD, Spaulding BC, Gross JB, Apfelbaum JL. Comparison of the ventilatory effects of etomidate and methohexital. Anesthesiology 1985; 62: 442-447. 5. Streisand JB, Nelson P, Bubbers S, Stocking-Korzen R, Posthuma W, East KA, Gillmor ST, Stanley TH. The respiratory effects of propofol with and without fentanyl. Anesthesia and Analgesia 1987; 66: S171. 6. Stanley TH, Liu W-S. Cardiovascular effects of meperidine-nitrous oxide anaesthesia before and after pancuronium. Anesthesia and Analgesia 1977; 56: 669-673.