Objective: In patients with extreme stature or build, estimation of individual dosage requirements of muscle relaxants by body weight is unreliable. To define a more precise guideline for dosage of atracurium in clinical practice we compared in this prospective study in patients with a wide range of body weights the cumulative effective dose for a 95% twitch depression (ED95), the dosage necessary to maintain a 95% twitch depression (DD95) and the recovery from a 95% neuromuscular block with simple demographic data such as body weight, body size, body surface area and lean body mass (LBM).
Methods: 30 patients were divided into three groups according to the individual body weight: underweight, normal, overweight. The electromyographic response was monitored using train-of-four stimuli applied to the ulnar nerve. Neuromuscular block was induced by constant infusion of atracurium; the dose required for a 95% twitch depression was registered as ED95. The infusion rate was then adjusted to maintain a constant electromyographic response of the first twitch of 5 +/- 1% for at least 30 minutes and the required dose of atracurium was recorded as DD95. The neuromuscular recovery was studied regarding T1 and T4-ratio. Data are given as medians (25%-/75%-quartiles).
Results and discussion: The cumulative ED95 in underweight patients (0.34 (0.31/0.48) mg/kg) exceeded (P < 0.05) those in normal (0.29 (0.26/0.30) mg/kg) and in overweight patients (0.22 (0.18/0.26) mg/kg). In contrast to calculations according to the normal weight (Mn) no difference in ED95 was seen between groups (normal: 0.29 (0.26/0.30) mg/kg Mn; underweight: 0.28 (0.26/0.32) mg/kg Mn; overweight: 0.31 (0.25/0.32) mg/kg Mn). The individual DD95 was best correlated with LBM (r = 0.465, p < 0.01). In view of the difficulty of estimating LBM in routine clinical practice it is emphasized that DD95 correlates only slightly less with normal weight (r = 0.404, p < 0.05). In spite of the variability of DD95 with regard to body weight, the recovery of neuromuscular transmission in the patients of the three groups is comparable. As a constant neuromuscular block cannot be maintained without monitoring muscular evoked responses, even if body build is taken into account, neuromuscular monitoring is advocated for longer infusion of atracurium.