[Dose-response relationship of atracurium in underweight, normal and overweight patients]

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).

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).

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.