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ACCELRATED RECOVERY FROM NEUROMUSCULAR BLOCK
CORRE SPONDENCE
761
N. SONI
London
REFERENCES 1. Miller DM, Miller JC. Enclosed afferent reservoir breathing systems. British Journal of Anaesthesia 1988; 60: 469-475. 2. Miller DM. Breathing systems for use in anaesthesia. British Journal of Anaesthesia 1988; 60: 555-564. 3. Conway CM. Anaesthetic breathing systems. British Journal of Anaesthesia 1985; 57: 649-657. 4. Mapleson WW. The elimination of rebreathing in various semi-closed anaesthetic systems. British Journal of Anaesthesia 1954; 26: 323-332.
ACCELERATED RECOVERY FROM NEUROMUSCULAR BLOCK Sir,—The conclusion of Stirt [1] that a combination of atracurium and vecuronium resulted in "accelerated recovery" from neuromuscular block would seem to be an inappropriate interpretation of the published results. As stated, the prolonged time to maximum neuromuscular block with the combination of atracurium and vecuronium was caused by reduced drug dosage compared with that when each of the relaxants was given alone. A smaller drug dose results not only in a longer time taken to reach the critical molecular concentration at the neuromuscular junction for 100% block, but also in a shorter period of time for which this critical concentration is exceeded. This implies that, if smaller doses of atracurium or vecuronium had been used, there would have been an earlier return of the twitch following 100% block. Gramstad and colleagues [2] reported a time to 25 % recovery when using atracurium 0.33 mg kg"1 of 27.6 min and 21.9 min using vecuronium 0.066 mg kg"1. These times compare favourably with the result of 32 min for the atracurium-vecuronium combination.
It is well established that, when started, recovery from the neuromuscular block of both atracurium and vecuronium is remarkably consistent and similar, especially after single bolus doses. The recovery time (25-75 % recovery) shown for the combination of atracurium and vecuronium was 11 min, that for atracurium alone 12 min and for vecuronium alone 15 min. Agoston and colleagues [3] showed that vecuronium 0.07 mg kg"1 produced a maximum block with an onset time of 4.3 min, a time to 90 % recovery of 37 min and a recovery time of 9 min. Similarly, Ramsey, and colleagues [4], using atracurium 0.375 mg kg"1, found an onset time of 3.6 min, time to 95 % recovery of 50.5 min and recovery time of 11 min. These figures confirm that there is no clinically significant acceleration in recovery rate when comparing the two myoneural blockers in combination, and atracurium or vecuronium given alone. This suggests that a comparison of clinically "equivalent" doses of each of the agents given alone would have resulted in onset and recovery characteristics almost identical to those of the atracurium—vecuronium combination. Because the two neuromuscular blockers have almost identical pharmacokinetic profiles, for the combination to have an advantage any synergism that may exist at the neuromuscular junction would have to decrease rapidly with decreasing molecular concentration. This does not appear to be the case. Thus the "accelerated recovery" reported would seem to be a doserelated phenomenon resulting in a shortening of the period of 100% block rather than any significant advantage of the atracurium—vecuronium combination. N. KOEHLI
Plymouth REFERENCES 1. Stirt JA. Accelerated recovery from combined atracurium-vecuronium neuromuscular block. British Journal of Anaesthesia 1989; 62: 697-699. 2. Gramstad L, Lilleaasen P, Minsaas B. Comparative study of atracurium, vecuronium (Org NC 45) and pancuronium. British Journal of Anaesthesia 1983; SS: 95S-96S. 3. Agoston S, Salt P, Newton D, Bencini A, Boomsma P, Erdman W. Trie neuromuscular blocking action of Org NC 45, a new pancuronium derivative in anaesthetized patients. A pilot study. British Journal of Anaesthesia 1980; 52: 53S-59S. 4. Ramsey FM, White PA, Stullken EH, Allen LL, Ray RC. Neuromuscular and hemodynamic effects of atracurium during enflurane anesthesia. Anesthesiology 1982; 57: A254.
Sir,—The term "accelerated recovery" in reference to the apparently faster return of function in my combination group should, in retrospect, have been defined with more precision [1]. As Dr Koehli notes, recovery rate (time from 25 % to 75 % recovery) was more or less the same in the single drug and combination groups. With regard to the speed of onset of recovery, however, analysis of the findings of Gramstad and colleagues [2] shows that the doses used were approximately three times as large as the individual components of the combined group in my report [1]. Thus onset of recovery in the combination group, with component doses of atracurium and vecuronium 33 % of
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on June 30, 2015
higher than those of Miller [2], and this represents methodological differences in anaesthetic technique, apparatus, recording equipment and in patients. Nomenclature is a difficult matter. The system in Australia originated as a consequence of trying to link the manually closed Heidbrink valve of a standard Magill system to the increasing pressure in the reservoir bag. In the spontaneous mode the system functions as a Mapleson A system, but with the expiratory valve displaced distally, it is a modification of the Magill attachment. Functionally, it permits conservation of deadspace gas and selectively vents alveolar gas [3, 4]. In the controlled mode there is also selective venting of alveolar gas [1]. In this respect, the functional characteristic (conservation of deadspace gas and selective venting of alveolar gas) is present in both modes. The new Miller classification [2] does clarify this issue, but is not yet in common parlance. The name "enclosed Magill" describes its appearance and the basic philosophy behind its development, both of which are interpreted easily by anaesthetic trainees. We concluded that the system described performed adequately in both spontaneous and controlled modes of ventilation and (irrespective of country of origin) was a potentially useful device in anaesthesia.
762
BRITISH JOURNAL OF ANAESTHESIA
those used by Gramstad, would appear still to be somewhat faster, even allowing for the smaller doses of the individual agents used by Gramstad compared with those in my study.
REFERENCES ' A c c e l e r a t e d recovery from combined atracurium—vecuronium neuromuscular block. British journal °f Anaesthesia 1989; 62: 697-699. Gramstad L, Lilleaasen P, Minsaas B. Comparative study of atracurium, vecuronium (Org NC 45) and pancuronium. British Journal of Anaesthesia 1983; 55: 95S-96S.
J A STIRT
Virginia
Stirl
JA
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on June 30, 2015
761
N. SONI
London
REFERENCES 1. Miller DM, Miller JC. Enclosed afferent reservoir breathing systems. British Journal of Anaesthesia 1988; 60: 469-475. 2. Miller DM. Breathing systems for use in anaesthesia. British Journal of Anaesthesia 1988; 60: 555-564. 3. Conway CM. Anaesthetic breathing systems. British Journal of Anaesthesia 1985; 57: 649-657. 4. Mapleson WW. The elimination of rebreathing in various semi-closed anaesthetic systems. British Journal of Anaesthesia 1954; 26: 323-332.
ACCELERATED RECOVERY FROM NEUROMUSCULAR BLOCK Sir,—The conclusion of Stirt [1] that a combination of atracurium and vecuronium resulted in "accelerated recovery" from neuromuscular block would seem to be an inappropriate interpretation of the published results. As stated, the prolonged time to maximum neuromuscular block with the combination of atracurium and vecuronium was caused by reduced drug dosage compared with that when each of the relaxants was given alone. A smaller drug dose results not only in a longer time taken to reach the critical molecular concentration at the neuromuscular junction for 100% block, but also in a shorter period of time for which this critical concentration is exceeded. This implies that, if smaller doses of atracurium or vecuronium had been used, there would have been an earlier return of the twitch following 100% block. Gramstad and colleagues [2] reported a time to 25 % recovery when using atracurium 0.33 mg kg"1 of 27.6 min and 21.9 min using vecuronium 0.066 mg kg"1. These times compare favourably with the result of 32 min for the atracurium-vecuronium combination.
It is well established that, when started, recovery from the neuromuscular block of both atracurium and vecuronium is remarkably consistent and similar, especially after single bolus doses. The recovery time (25-75 % recovery) shown for the combination of atracurium and vecuronium was 11 min, that for atracurium alone 12 min and for vecuronium alone 15 min. Agoston and colleagues [3] showed that vecuronium 0.07 mg kg"1 produced a maximum block with an onset time of 4.3 min, a time to 90 % recovery of 37 min and a recovery time of 9 min. Similarly, Ramsey, and colleagues [4], using atracurium 0.375 mg kg"1, found an onset time of 3.6 min, time to 95 % recovery of 50.5 min and recovery time of 11 min. These figures confirm that there is no clinically significant acceleration in recovery rate when comparing the two myoneural blockers in combination, and atracurium or vecuronium given alone. This suggests that a comparison of clinically "equivalent" doses of each of the agents given alone would have resulted in onset and recovery characteristics almost identical to those of the atracurium—vecuronium combination. Because the two neuromuscular blockers have almost identical pharmacokinetic profiles, for the combination to have an advantage any synergism that may exist at the neuromuscular junction would have to decrease rapidly with decreasing molecular concentration. This does not appear to be the case. Thus the "accelerated recovery" reported would seem to be a doserelated phenomenon resulting in a shortening of the period of 100% block rather than any significant advantage of the atracurium—vecuronium combination. N. KOEHLI
Plymouth REFERENCES 1. Stirt JA. Accelerated recovery from combined atracurium-vecuronium neuromuscular block. British Journal of Anaesthesia 1989; 62: 697-699. 2. Gramstad L, Lilleaasen P, Minsaas B. Comparative study of atracurium, vecuronium (Org NC 45) and pancuronium. British Journal of Anaesthesia 1983; SS: 95S-96S. 3. Agoston S, Salt P, Newton D, Bencini A, Boomsma P, Erdman W. Trie neuromuscular blocking action of Org NC 45, a new pancuronium derivative in anaesthetized patients. A pilot study. British Journal of Anaesthesia 1980; 52: 53S-59S. 4. Ramsey FM, White PA, Stullken EH, Allen LL, Ray RC. Neuromuscular and hemodynamic effects of atracurium during enflurane anesthesia. Anesthesiology 1982; 57: A254.
Sir,—The term "accelerated recovery" in reference to the apparently faster return of function in my combination group should, in retrospect, have been defined with more precision [1]. As Dr Koehli notes, recovery rate (time from 25 % to 75 % recovery) was more or less the same in the single drug and combination groups. With regard to the speed of onset of recovery, however, analysis of the findings of Gramstad and colleagues [2] shows that the doses used were approximately three times as large as the individual components of the combined group in my report [1]. Thus onset of recovery in the combination group, with component doses of atracurium and vecuronium 33 % of
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on June 30, 2015
higher than those of Miller [2], and this represents methodological differences in anaesthetic technique, apparatus, recording equipment and in patients. Nomenclature is a difficult matter. The system in Australia originated as a consequence of trying to link the manually closed Heidbrink valve of a standard Magill system to the increasing pressure in the reservoir bag. In the spontaneous mode the system functions as a Mapleson A system, but with the expiratory valve displaced distally, it is a modification of the Magill attachment. Functionally, it permits conservation of deadspace gas and selectively vents alveolar gas [3, 4]. In the controlled mode there is also selective venting of alveolar gas [1]. In this respect, the functional characteristic (conservation of deadspace gas and selective venting of alveolar gas) is present in both modes. The new Miller classification [2] does clarify this issue, but is not yet in common parlance. The name "enclosed Magill" describes its appearance and the basic philosophy behind its development, both of which are interpreted easily by anaesthetic trainees. We concluded that the system described performed adequately in both spontaneous and controlled modes of ventilation and (irrespective of country of origin) was a potentially useful device in anaesthesia.
762
BRITISH JOURNAL OF ANAESTHESIA
those used by Gramstad, would appear still to be somewhat faster, even allowing for the smaller doses of the individual agents used by Gramstad compared with those in my study.
REFERENCES ' A c c e l e r a t e d recovery from combined atracurium—vecuronium neuromuscular block. British journal °f Anaesthesia 1989; 62: 697-699. Gramstad L, Lilleaasen P, Minsaas B. Comparative study of atracurium, vecuronium (Org NC 45) and pancuronium. British Journal of Anaesthesia 1983; 55: 95S-96S.
J A STIRT
Virginia
Stirl
JA
Downloaded from http://bja.oxfordjournals.org/ at University of Iowa Libraries/Serials Acquisitions on June 30, 2015