title: Molar potency is not predictive of the speed of onset of atracurium.
abstract: In an effort to determine the extent to which atracurium may represent an exception to the rule that molar potency predicts onset time, we studied the onset profile of atracurium after a dose selected to produce approximately 95% twitch depression. We compared these results with data obtained in a previous study after the administration of vecuronium, rocuronium, and cisatracurium. Eighteen ASA physical status I and II patients were studied. After the induction of anesthesia, tracheal intubation was accomplished without relaxants. The evoked electromyographic response to 0.10-Hz single stimuli was continuously recorded. After baseline stabilization, a single bolus of atracurium, averaging 0.21 mg/kg, was administered. If peak twitch depression did not fall within the range of 90%-98%, the patient was excluded. The time to 50% and 90% of peak effect was recorded. The time to 90% of maximal effect (192 +/- 23 s) was not different from that previously observed for vecuronium (201 +/- 20 s). The time to 50% of peak effect (110 +/- 15 s) was shorter (P < 0.05) after atracurium administration than after vecuronium (125 +/- 9 s). The onset times recorded for atracurium were slower than previously observed after rocuronium and more rapid than that which was seen after cisatracurium (P < 0.001). The observed onset profile of atracurium was considerably slower than anticipated, based on the drug's molar potency. The 95% effective dose (microM/kg) may not be a reliable predictor of a muscle relaxant's onset time, when the drug administered is a mixture isomers of varying potency.
  
  
  CONCLUSIONS
  The speed of onset of atracurium is slower than predicted, based on its molar potency. Potency of a relaxant may not be a reliable predictor of its time to peak effect, when the drug administered is a mixture of isomers with widely different neuromuscular activities.
authors.affiliation: Department of Anesthesiology, New York Medical College, Valhalla, USA. AKopman@interport.net
authors.forename: A F
authors.initials: AF
authors.lastname: Kopman
authors.forename: M M
authors.initials: MM
authors.lastname: Klewicka
authors.forename: G G
authors.initials: GG
authors.lastname: Neuman
mesh_terms: D000328:Adult; D000732:Androstanols / Q000008:administration & dosage / Q000737:chemistry; D001279:Atracurium / Q000008:administration & dosage* / Q000031:analogs & derivatives / Q000737:chemistry; D004576:Electromyography / Q000187:drug effects; D019054:Evoked Potentials, Motor / Q000187:drug effects; D005260:Female; D005544:Forecasting; D006801:Humans; D007442:Intubation, Intratracheal; D007536:Isomerism; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction / Q000187:drug effects; D009466:Neuromuscular Blocking Agents / Q000008:administration & dosage / Q000737:chemistry; D003473:Neuromuscular Nondepolarizing Agents / Q000008:administration & dosage* / Q000737:chemistry; D009994:Osmolar Concentration; D015203:Reproducibility of Results; D000077123:Rocuronium; D013997:Time Factors; D014459:Ulnar Nerve / Q000187:drug effects; D014673:Vecuronium Bromide / Q000008:administration & dosage / Q000737:chemistry
chemical_list: D000732:Androstanols; D009466:Neuromuscular Blocking Agents; D003473:Neuromuscular Nondepolarizing Agents; D001279:Atracurium; D014673:Vecuronium Bromide; C101584:cisatracurium; D000077123:Rocuronium