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INTUBATING CONDITIONS WITH VECURONIUM
BRITISH JOURNAL OF ANAESTHESIA
932 to establish an extradural block is widely used and avoids some of the risks of systemic toxicity. However, this technique is unsuitable for the type of study in which agents are to be compared if it entails the use of a wide range of volumes. In the case of 0.5 % bupivacaine, the repeated bathing of the spinal nerve roots may be necessary to achieve satisfactory blockade for surgery (which is unnecessary with the more concentrated solutions) and leads to the considerably slow onset time (mean 42 min) found in this study. J. H. MCCLUKB D. B. SCOTT
Edinburgh
agents, particularly regarding the onset of autonomic blockade, differed markedly. As we pointed out in the discussion section of the paper, we had to modify the procedure of fluid pre-loading in the women who received etidocaine and, for this reason, it was useful for the anaesthetist to be aware of which agent was being administered. Drs McClure and Scott are correct in their assertion that the technique we have described is widely used in the practice of obstetric anaesthesia. For this reason, and also for the reason that the procedure of Caesarean section is one of the commonest surgical procedures carried out under extradural anaesthesia alone, we cannot agree that such a technique is inappropriate for the study of different local anaesthetic drugs. D. A. DUTTON
D. D. Mom H. B. HOWIE J. THORBURN
Glasgow
INTUBATING CONDITIONS WITH VECURONIUM
Sir,—The paper by Bencini and Newton (1984) appears to be planned essentially to test the old suggestion of differential paralysis of the respiratory and peripheral muscles by neuromuscular blocking agents. However, 9ince most previous studies have not made direct comparisons, the authors may be Sir,—Thank you for allowing us the opportunity to reply to the attributing to previous workers views which they did not hold. letter from Drs McClure and Scott. Mirakhur and colleagues (1983) simply stated "that intubation We apologize for the misquote in the first paragraph of our could be carried out before the onset of complete neuromuscular paper. This unfortunately occurred as a result of an oversight block". However, in another paper on a similar topic, Clarke during our scrutiny of the edited manuscript immediately and Mirakhur (1983) stated that "intuba ting conditions when before publication. Our original manuscript read " .. .with a studied in a clinical sequence after thiopentone cannot be greater frequency of adequate analgesia (Scott et al., related closely to onset of neuromuscular block". This 1980)" — and made no reference to the use of higher emphasizes what we believe to be the view of most concentrations of either drug during labour. anaesthetists, that thiopentone (with or without opiate) In response to their criticism of our study, we can only normally contributes greatly to the quality of intubating reiterate that we based our comparison of the efficacy of sensory conditions. Therefore, we agree with Bencini and Newton in analgesia upon observations of patients in whom we established their statement that "One possible reason for these disparities extradural blockade which we considered adequate for the [from the findings of ourselves and other workers] is the commencement of surgery, that is sensory blockade (loss of technique of anaesthesia associated with the intubation", since pinprick sensation) from S5 to T6 spinal segments bilaterally, the anaesthesia used by Bencini and Newton (1984) was with demonstrable motor blockade in the lower limbs. These deliberately light. appear to be the most common criteria recognized in this Criticism is also levelled at other workers who use different clinical situation and, as such, provide a logical end-point in criteria for assessing intubating conditions. Certainly, we agree determining the duration of onset of blockade. We believe that that the patients who do not respond at all to intubation should we have presented a valid clinical comparison of these local be noted and that is what we have indicated (Mirakhur et al., anaesthetic drugs despite the variation in volumes admin1983) by excellent conditions. The alternative term "ideal istered. We would point out that, in addition to the intubating conditions" (with a score of 1 or less) also used by considerable variation in the volumes between the etidocaine Bencini and Newton corresponds to our "acceptable" and bupivacaine groups, there was also a wide variation in conditions. It is not possible in any one patient to define a volumes administered to subjects within each group. One may "Time to Optimal Intubation", since one can only assess conclude from this observation that, in the pregnant woman at, conditions at the particular time at which intubation is or near, term, the spread of a given volume of local anaesthetic attempted. One can only assess on a statistical basis the solution within the extradural space is not easy to predict. We percentage of patients with a predetermined grading at a remain unconvinced that, in this particular situation and in the specified time. Certainly, intubating conditions recorded in face of other variable factors (for example the patient's height, patients having received a minimum amount of anaesthetic advancement in pregnancy, and the degree of distension of the drugs would seem to be of little clinical value. extraduraJ venous plexus), the injection of standard volumes Surely expressing an intubation score (mean±SD) has no of each local anaesthetic solution will produce a fairer or more statistical validity? valid comparison. R. S. J. CLARKE We accept that double-blinding may have been advantageous R. K. MIRAKHUR in this situation. It was, however, obvious to us before and Belfast during the formal study that the characteristics of the two
Downloaded from http://bja.oxfordjournals.org/ at Université Laval on July 8, 2015
REFERENCES Dutton, D. A., Moir, D. D., Howie, H. B., Thorbum, J., and Watson, R. (1984). Choice of local anaesthetic drug for extradural Caesarean section. Comparison of 0.5% and 0.75% bupivacaine and 1.5% etidocaine. Br. J. Anaesth., 56, 1361. Scott, D. B., McClure, J. H., Giasi, R. M., Seo, J., and Covino, B. G. (1980). Effects of concentration of local anaesthetic drugs in extradural block. Br. J. Anaesth., 52, 1033.
CORRESPONDENCE REFERENCES Bencini, A., and Newton, D. E. F.( 1984). Rate of onset of good intubating conditions, respiratory depression and hand muscle paralysis after vecuronium. Br. J. Anaesth., 56, 959. Clarke, R. S. J., and Mirakhur, R. K. (1983). Intubating conditions after vecuronium: a study with three doses and a comparison with suxamethonium and pancuronium; in Clinical Experiences teith Norcuron (eds S. Agoston, W. C. Bowman, R. D. Miller and J. Viby-Mogensen), p. 145. Amsterdam: Excerpta Medics. Mirakhur, R. S., Ferres, C. J., Clarke, R. S. J., Bali, I. M.,and Dundee, J. W. (1983). Clinical evaluation of Org NC 45. Br. J. Anaesth., 55, 119.
These problems can be overcome by studies under light anaesthesia, simplifying the intubation score (taking as an end-point no reaction at all to intubation), and by noticing the doses as well as time after injection which produce no reaction in all patients. It is evident from most studies, once a certain dose is exceeded, that excellent conditions will be attained if the time to intubation is long enough. This is what we called the Time to Optimal Intubation (TOI). It is certainly not possible to predict such a time interval in any one patient; this value can be achieved in large enough groups of patients by Increasing the dose and the time to intubation in steps of, say, 30 s. The correspondents would have achieved this if, in their investigations, they had kept on increasing the dose of vecuronium and intubated the patients at ever longer intervals after injection of each dose of vecuronium. This method of assessment, besides being more simple and applicable to the most exigent clinical situation, will also do away with the cumbersome expression of an intubation score in terms of mean± SD. A. F. BENCIKI D. E. F. NEWTON
Gromngen
REFERENCES Agoston, S., Salt, P., Newton, D., Bencini, A., Boomsma, P., and Erdmann, W. (1980). The neuromuscular blocking action of Org N C 45, a new pancuronium derivative in anaesthetized patients. Br. J. Anaesth., 52, 53S. Clarke, R. S. J., and Mirakhur, R. K. (1983). Intubating conditions after vecuronium: a study with three doses and a comparison with suxamethonium and pancuronium; in Clinical Experiences with Norcuron (eds S. Agoston, W. C. Bowman, R. D. Miller, and J. Viby-Mogensen), p. 145. Amsterdam: Excerpta Medics. Krieg, N., Mazur, L., Booij, L. H. D. J., and Crul, J. F. (1980). Intubating conditions and reversibility of a nondepolarising neuromuscular blocking agent, Org NC 45. Acta Anaesthesiol Scand., 24, 423. Mirakhur, R. K., Ferres, C. J., Clarke, R. S. J., Bali, I. M., and Dundee, J. W. (1983). Clinical evaluation of Org NC 45. Br. J. Anaesth., 55, 119.
SHORT ACTING NEUROMUSCULAR BLOCKERS AND ECT Sir,—Few anaesthetists now have experience with the administration of electroconvulsive therapy (ECT) modified by the use of a competitive neuromuscular drug instead of suxamethonium. Recently, a patient was encountered who appeared to develop bronchospasm after the administration of suxamethonium for ECT. Although skin sensitivity tests were negative, it was decided to omit suxamethonium and use a competitive neuromuscular blocking agent. Initially, alcuronium 2.5 mg was used, but this did not provide satisfactory modification of the ECT. Vecuronium was then used in view of its short duration of action and the possibility that it is less likely to release histamine. This proved satisfactory using the following technique: After the dose of the induction agent, vecuronium 2.0 mg was given i.v. The patient was ventilated with oxygen-halothane for 2 min and then ECT administered. The
Downloaded from http://bja.oxfordjournals.org/ at Université Laval on July 8, 2015
Sir,—Drs Clarke and Mirakhur's letter addresses the two messages which we hoped our paper would convey to your readers. First, as the correspondents rightly stated, this study was conceived and designed in order to test the hypothesis that vecuronium relaxes the respiratory (and, by inference, the laryngeal) muscle before the peripheral muscles. Intubation should consequently be possible when the peripheral muscles are still partially paralysed and the time to intubation would then be relatively short. This suggestion was put forward by Agoston and colleagues (1980) and later echoed by Krieg and co-workers (1980). Mirakhur and associates (1983) stated " that intubation could be carried out before the onset of complete neuromuscular block". This was, however, said in the context of the above-mentioned hypothesis and seemed to us to be providing supportive evidence for it. Clarke and Mirakhur (1983) later stated that the "intubating conditions when studied in a clinical sequence after thiopentone cannot be related closely to the onset of neuromuscular block". This statement points to the lack of correlation between peripheral neuromuscular block and intubating conditions. Which comes first is the important consideration. Indeed, in the same paper, Clarke and Mirakhur had noted earlier: "it was notable that in most patients tracheal intubating conditions were excellent well before the last twitch in the train-of-four was lost". Second, we felt that the popular mode of assessing the intubating conditions provided by a neuromuscular blocker was inadequate in that it does not provide the practising anaesthetist with some of the most crucial information on the ability of a neuromuscular blocker to provide good clinical intubating conditions. The information that most intubation studies provide is that, under an "average " type of anaesthesia - usually a deeper form of anaesthesia than minimal anaesthesia - a myoneural blocker will produce "excellent" intubating conditions, in a certain percentage of patients, with a given dose, at some minutes after its injection. This information is inadequate to the anaesthetist for several reasons. The "average" type of anaesthesia is of no value with a very ill patient in whom minimal anaesthesia must be applied. Relaxation for intubation will then have to be provided by the myoneural blocker, not by deep anaesthesia. The excellence of the intubating conditions is based on a complex score that is subjective and seems to mean different things to different people. The anaesthetist is thus not sure of the excellence of the intubating conditions that he should expect. Knowing only that a percentage of the patients will not react to tracheal intubation is of no use when faced with a perforating eye injury. The patient must not react to intubation.
933
932 to establish an extradural block is widely used and avoids some of the risks of systemic toxicity. However, this technique is unsuitable for the type of study in which agents are to be compared if it entails the use of a wide range of volumes. In the case of 0.5 % bupivacaine, the repeated bathing of the spinal nerve roots may be necessary to achieve satisfactory blockade for surgery (which is unnecessary with the more concentrated solutions) and leads to the considerably slow onset time (mean 42 min) found in this study. J. H. MCCLUKB D. B. SCOTT
Edinburgh
agents, particularly regarding the onset of autonomic blockade, differed markedly. As we pointed out in the discussion section of the paper, we had to modify the procedure of fluid pre-loading in the women who received etidocaine and, for this reason, it was useful for the anaesthetist to be aware of which agent was being administered. Drs McClure and Scott are correct in their assertion that the technique we have described is widely used in the practice of obstetric anaesthesia. For this reason, and also for the reason that the procedure of Caesarean section is one of the commonest surgical procedures carried out under extradural anaesthesia alone, we cannot agree that such a technique is inappropriate for the study of different local anaesthetic drugs. D. A. DUTTON
D. D. Mom H. B. HOWIE J. THORBURN
Glasgow
INTUBATING CONDITIONS WITH VECURONIUM
Sir,—The paper by Bencini and Newton (1984) appears to be planned essentially to test the old suggestion of differential paralysis of the respiratory and peripheral muscles by neuromuscular blocking agents. However, 9ince most previous studies have not made direct comparisons, the authors may be Sir,—Thank you for allowing us the opportunity to reply to the attributing to previous workers views which they did not hold. letter from Drs McClure and Scott. Mirakhur and colleagues (1983) simply stated "that intubation We apologize for the misquote in the first paragraph of our could be carried out before the onset of complete neuromuscular paper. This unfortunately occurred as a result of an oversight block". However, in another paper on a similar topic, Clarke during our scrutiny of the edited manuscript immediately and Mirakhur (1983) stated that "intuba ting conditions when before publication. Our original manuscript read " .. .with a studied in a clinical sequence after thiopentone cannot be greater frequency of adequate analgesia (Scott et al., related closely to onset of neuromuscular block". This 1980)" — and made no reference to the use of higher emphasizes what we believe to be the view of most concentrations of either drug during labour. anaesthetists, that thiopentone (with or without opiate) In response to their criticism of our study, we can only normally contributes greatly to the quality of intubating reiterate that we based our comparison of the efficacy of sensory conditions. Therefore, we agree with Bencini and Newton in analgesia upon observations of patients in whom we established their statement that "One possible reason for these disparities extradural blockade which we considered adequate for the [from the findings of ourselves and other workers] is the commencement of surgery, that is sensory blockade (loss of technique of anaesthesia associated with the intubation", since pinprick sensation) from S5 to T6 spinal segments bilaterally, the anaesthesia used by Bencini and Newton (1984) was with demonstrable motor blockade in the lower limbs. These deliberately light. appear to be the most common criteria recognized in this Criticism is also levelled at other workers who use different clinical situation and, as such, provide a logical end-point in criteria for assessing intubating conditions. Certainly, we agree determining the duration of onset of blockade. We believe that that the patients who do not respond at all to intubation should we have presented a valid clinical comparison of these local be noted and that is what we have indicated (Mirakhur et al., anaesthetic drugs despite the variation in volumes admin1983) by excellent conditions. The alternative term "ideal istered. We would point out that, in addition to the intubating conditions" (with a score of 1 or less) also used by considerable variation in the volumes between the etidocaine Bencini and Newton corresponds to our "acceptable" and bupivacaine groups, there was also a wide variation in conditions. It is not possible in any one patient to define a volumes administered to subjects within each group. One may "Time to Optimal Intubation", since one can only assess conclude from this observation that, in the pregnant woman at, conditions at the particular time at which intubation is or near, term, the spread of a given volume of local anaesthetic attempted. One can only assess on a statistical basis the solution within the extradural space is not easy to predict. We percentage of patients with a predetermined grading at a remain unconvinced that, in this particular situation and in the specified time. Certainly, intubating conditions recorded in face of other variable factors (for example the patient's height, patients having received a minimum amount of anaesthetic advancement in pregnancy, and the degree of distension of the drugs would seem to be of little clinical value. extraduraJ venous plexus), the injection of standard volumes Surely expressing an intubation score (mean±SD) has no of each local anaesthetic solution will produce a fairer or more statistical validity? valid comparison. R. S. J. CLARKE We accept that double-blinding may have been advantageous R. K. MIRAKHUR in this situation. It was, however, obvious to us before and Belfast during the formal study that the characteristics of the two
Downloaded from http://bja.oxfordjournals.org/ at Université Laval on July 8, 2015
REFERENCES Dutton, D. A., Moir, D. D., Howie, H. B., Thorbum, J., and Watson, R. (1984). Choice of local anaesthetic drug for extradural Caesarean section. Comparison of 0.5% and 0.75% bupivacaine and 1.5% etidocaine. Br. J. Anaesth., 56, 1361. Scott, D. B., McClure, J. H., Giasi, R. M., Seo, J., and Covino, B. G. (1980). Effects of concentration of local anaesthetic drugs in extradural block. Br. J. Anaesth., 52, 1033.
CORRESPONDENCE REFERENCES Bencini, A., and Newton, D. E. F.( 1984). Rate of onset of good intubating conditions, respiratory depression and hand muscle paralysis after vecuronium. Br. J. Anaesth., 56, 959. Clarke, R. S. J., and Mirakhur, R. K. (1983). Intubating conditions after vecuronium: a study with three doses and a comparison with suxamethonium and pancuronium; in Clinical Experiences teith Norcuron (eds S. Agoston, W. C. Bowman, R. D. Miller and J. Viby-Mogensen), p. 145. Amsterdam: Excerpta Medics. Mirakhur, R. S., Ferres, C. J., Clarke, R. S. J., Bali, I. M.,and Dundee, J. W. (1983). Clinical evaluation of Org NC 45. Br. J. Anaesth., 55, 119.
These problems can be overcome by studies under light anaesthesia, simplifying the intubation score (taking as an end-point no reaction at all to intubation), and by noticing the doses as well as time after injection which produce no reaction in all patients. It is evident from most studies, once a certain dose is exceeded, that excellent conditions will be attained if the time to intubation is long enough. This is what we called the Time to Optimal Intubation (TOI). It is certainly not possible to predict such a time interval in any one patient; this value can be achieved in large enough groups of patients by Increasing the dose and the time to intubation in steps of, say, 30 s. The correspondents would have achieved this if, in their investigations, they had kept on increasing the dose of vecuronium and intubated the patients at ever longer intervals after injection of each dose of vecuronium. This method of assessment, besides being more simple and applicable to the most exigent clinical situation, will also do away with the cumbersome expression of an intubation score in terms of mean± SD. A. F. BENCIKI D. E. F. NEWTON
Gromngen
REFERENCES Agoston, S., Salt, P., Newton, D., Bencini, A., Boomsma, P., and Erdmann, W. (1980). The neuromuscular blocking action of Org N C 45, a new pancuronium derivative in anaesthetized patients. Br. J. Anaesth., 52, 53S. Clarke, R. S. J., and Mirakhur, R. K. (1983). Intubating conditions after vecuronium: a study with three doses and a comparison with suxamethonium and pancuronium; in Clinical Experiences with Norcuron (eds S. Agoston, W. C. Bowman, R. D. Miller, and J. Viby-Mogensen), p. 145. Amsterdam: Excerpta Medics. Krieg, N., Mazur, L., Booij, L. H. D. J., and Crul, J. F. (1980). Intubating conditions and reversibility of a nondepolarising neuromuscular blocking agent, Org NC 45. Acta Anaesthesiol Scand., 24, 423. Mirakhur, R. K., Ferres, C. J., Clarke, R. S. J., Bali, I. M., and Dundee, J. W. (1983). Clinical evaluation of Org NC 45. Br. J. Anaesth., 55, 119.
SHORT ACTING NEUROMUSCULAR BLOCKERS AND ECT Sir,—Few anaesthetists now have experience with the administration of electroconvulsive therapy (ECT) modified by the use of a competitive neuromuscular drug instead of suxamethonium. Recently, a patient was encountered who appeared to develop bronchospasm after the administration of suxamethonium for ECT. Although skin sensitivity tests were negative, it was decided to omit suxamethonium and use a competitive neuromuscular blocking agent. Initially, alcuronium 2.5 mg was used, but this did not provide satisfactory modification of the ECT. Vecuronium was then used in view of its short duration of action and the possibility that it is less likely to release histamine. This proved satisfactory using the following technique: After the dose of the induction agent, vecuronium 2.0 mg was given i.v. The patient was ventilated with oxygen-halothane for 2 min and then ECT administered. The
Downloaded from http://bja.oxfordjournals.org/ at Université Laval on July 8, 2015
Sir,—Drs Clarke and Mirakhur's letter addresses the two messages which we hoped our paper would convey to your readers. First, as the correspondents rightly stated, this study was conceived and designed in order to test the hypothesis that vecuronium relaxes the respiratory (and, by inference, the laryngeal) muscle before the peripheral muscles. Intubation should consequently be possible when the peripheral muscles are still partially paralysed and the time to intubation would then be relatively short. This suggestion was put forward by Agoston and colleagues (1980) and later echoed by Krieg and co-workers (1980). Mirakhur and associates (1983) stated " that intubation could be carried out before the onset of complete neuromuscular block". This was, however, said in the context of the above-mentioned hypothesis and seemed to us to be providing supportive evidence for it. Clarke and Mirakhur (1983) later stated that the "intubating conditions when studied in a clinical sequence after thiopentone cannot be related closely to the onset of neuromuscular block". This statement points to the lack of correlation between peripheral neuromuscular block and intubating conditions. Which comes first is the important consideration. Indeed, in the same paper, Clarke and Mirakhur had noted earlier: "it was notable that in most patients tracheal intubating conditions were excellent well before the last twitch in the train-of-four was lost". Second, we felt that the popular mode of assessing the intubating conditions provided by a neuromuscular blocker was inadequate in that it does not provide the practising anaesthetist with some of the most crucial information on the ability of a neuromuscular blocker to provide good clinical intubating conditions. The information that most intubation studies provide is that, under an "average " type of anaesthesia - usually a deeper form of anaesthesia than minimal anaesthesia - a myoneural blocker will produce "excellent" intubating conditions, in a certain percentage of patients, with a given dose, at some minutes after its injection. This information is inadequate to the anaesthetist for several reasons. The "average" type of anaesthesia is of no value with a very ill patient in whom minimal anaesthesia must be applied. Relaxation for intubation will then have to be provided by the myoneural blocker, not by deep anaesthesia. The excellence of the intubating conditions is based on a complex score that is subjective and seems to mean different things to different people. The anaesthetist is thus not sure of the excellence of the intubating conditions that he should expect. Knowing only that a percentage of the patients will not react to tracheal intubation is of no use when faced with a perforating eye injury. The patient must not react to intubation.
933