Correspondence  |   May 2003
The Influence of the Duration of Anesthesia on Neuromuscular Potency: In Reply:—
Author Affiliations & Notes
  • Francois Donati, Ph.D., M.D., F.R.C.P.C.
  • *Hopital Maisonneuve Rosemont, Montreal, Quebec, Canada.
Article Information
Correspondence   |   May 2003
The Influence of the Duration of Anesthesia on Neuromuscular Potency: In Reply:—
Anesthesiology 5 2003, Vol.98, 1300-1301. doi:
Anesthesiology 5 2003, Vol.98, 1300-1301. doi:
In Reply:—
We thank Dr. Kopman for his remarks on our article and would like to take this opportunity to interpret our findings in a different way. We measured the onset characteristics of mivacurium-induced neuromuscular blockade at two muscles, the adductor pollicis and the corrugator supercilii muscles, to rule out the possibility of a local effect (temperature or blood flow) at one muscle. 1 The duration of anesthesia before mivacurium injection had a major influence on the level of paralysis at both muscles, suggesting that local effects at the adductor pollicis were not a major factor. We proposed that these results might apply to laryngeal muscles because we have previously demonstrated that after rocuronium injection the pharmacodynamic profile of the corrugator supercilii is the same than at the adductor laryngeal muscles. 2 
It should be noted that muscle blood flow was not measured in our study and that its role in enhancing neuromuscular blockade is only a hypothesis. Other factors, such as the broader plasma concentration profile that occurs after a bolus injection when cardiac output is decreased (a front end kinetics phenomenon 3) could explain the greater neuromuscular blockade seen after 15 min of anesthesia. Likewise, our results are validated by the fact that we observed the same trend in maximum neuromuscular blockade at two different sites of measurement (i.e.  , the adductor pollicis and the corrugator supercilii).
We agree with Dr. Kopman that the adductor pollicis is not a valuable guide for evaluating the time of tracheal intubation. This statement has been demonstrated repeatedly since the original study performed by Bencini and Newton. 4 
Finally, Dr. Kopman emphasizes that the exact intubating dose of muscle relaxant is controversial: some experts recommend twice the ED95measured at the adductor pollicis, 5 others suggest a lesser dose, 6 and others report good results with no muscle relaxant. 7 The debate continues because to date, no data are available regarding the exact minimal dose of the three major components of anesthesia (i.e.  , hypnotic, the opioid, and the muscle relaxant) that will provide good or excellent conditions in > 95% of cases, without major side effects (i.e.  , hypotension with a large dose of hypnotic, pharyngolaryngeal injury when no muscle relaxant is injected, major hypertension, or awareness with underdosage of opioid or hypnotic). All our study provides is an explanation for the occasional failure of muscle relaxants to pro-vide excellent conditions in all cases, even at relatively large doses (2 × ED95).
Plaud B, Debaene B, Donati F: Duration of anesthesia before muscle relaxant injection influences level of paralysis. A nesthesiology 2002; 97: 616–21Plaud, B Debaene, B Donati, F
Plaud B, Debaene B, Donati F: The corrugator supercilii, not the orbicularis oculi, reflects rocuronium neuromuscular blockade at the laryngeal adductor muscles. A nesthesiology 2001; 95: 96–101Plaud, B Debaene, B Donati, F
Krejcie TR, Avram MJ: What determines anesthetic induction dose? It's the front-end kinetics, doctor! Anesth Analg 1999; 89: 541–4Krejcie, TR Avram, MJ
Bencini A, Newton DE: Rate of onset of good intubating conditions, respiratory depression and hand muscle paralysis after vecuronium. Br J Anaesth 1984; 56: 959–65Bencini, A Newton, DE
Savarese JJ, Caldwell JE, Lien CA, Miller RD: Pharmacology of muscle relaxants and their antagonists, Anesthesia, 5th edition. Edited by Miller RD. Philadelphia, Churchill Livingstone, 2000, pp 412–90
Kopman AF, Klewicka MM, Neuman GG: The recommended “intubating dose” for nondepolarizing blockers of rapid-onset. Anesth Analg 2001; 93: 954–9Kopman, AF Klewicka, MM Neuman, GG
Scheller MS, Zornow MH, Saidman LJ: Tracheal intubation without the use of muscle relaxants: a technique using propofol and varying doses of alfentanil. Anesth Analg 1992; 75: 788–93Scheller, MS Zornow, MH Saidman, LJ