[Correlation between the molar potency of atracurium and its onset time and speed of action].
OBJECTIVE
To study the correlation between the subparalyzing dose of atracurium and its onset time and speed of action.


METHODS
Six groups of patients received 0.36, 0.68, 0.98, 1.44, 1.77, or 2.06 times the median effective dose of atracurium. The maximal effect, onset time, molar potency (microM kg(-1)) and speed of action (relationship between time in seconds and percentage of maximum effect and between percentage of maximum effect and time) of each dose were measured. The values were converted to logarithms and square roots to test for correlations. The regression line provided the equation, coefficient of determination (R2), and statistical significance.


RESULTS
The correlation between the logarithms of molar potency and onset time was poor and not statistically significant (R2 = 0.564, P = .138), whereas the correlations between the logarithms of molar potency and speed of action (seg/%) (s/maximum % effect, R2 = 0.952; and maximum % effect/s, R2 = 0.984) were strong and statistically significant (P < .0001) The regression formulas were able to predict the onset time. The results were similar when the values were converted to their square roots.


CONCLUSIONS
The onset time of a neuromuscular blocker is linked to its effect and its permeability is linked to the size of its molecules or the molar potency, which is dose-dependent. In the case of atracurium, this correlation was poor and not significant. This finding coincides with those of other authors, who found that atracurium was distant from the regression line for equipotent quantities of different neuromuscular blockers. Substituting onset time with speed of action, however, made it possible to establish a strong and significant correlation. Our results suggest that the speed of action of atracurium depends on the size of its molecule.
Steinberg
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Departamento de Anestesia, Hospital de Clínicas Caracas, Policlínica Méndez Gimón, Caracas, Venezuela. dsteinberg@cantv.net
D000328:Adult; D001279:Atracurium / Q000008:administration & dosage* / Q000493:pharmacokinetics*; D054796:Drug Dosage Calculations; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D003473:Neuromuscular Nondepolarizing Agents / Q000008:administration & dosage* / Q000493:pharmacokinetics*
D003473:Neuromuscular Nondepolarizing Agents; D001279:Atracurium