Muscle Relaxation Produced By Atracurium Given By Different Methods Of Administration Under Inhalational Anesthesia With Isoflurane

Objective To compare the muscle relaxation produced by atracurium intermittent .iv. bolus injection (IBI), continuous infusion (CI) or target controlled infusion (TCI) under inhalational anesthesia with isoflurane during abdominal surgery of gynecology. Methods Forty-five patients aged (20-60)yr weighing (45-65) Kg undergoing elective abdominal surgery of gynecology were randomly diveded into three groups (n=15 each): group IBI, group CI ang group TCI. Patients with hepatic, renal or neuromuscular diseases and those on medications known to affect neuromuscular blocking drugs were excluded. All patients were fasted at least 8 hr before surgery. All patients were premedicated with injection atropine 0.5mg and phenobarbital 0.1g.Anaesthesia was induced with midazolam 0.05mg/Kg, fentanyl 4ug/Kg and propofol 4ug/ml TCI. Neuromuscular transmission was monitored by recording the force of contraction of adductor pollicis of the right hand to supramaximal TOF stimulation of the ulnar nerve using accelerography (TOF-WATCH SX). After induction, supramaximal stimulation need to be secured and the responses to train-of -four(TOUR) nerve stimulation were stable at least 5 minutes. In group IBI a blous of 0.6mg/Kg atracurium was given to faciliate intubation followed by intermittent iv boluses of 0.2mg/Kg to maintain T₁ at 10% of the control height. In group CI, after a bolus dose of 0.6mg/Kg atracurium for intubation ,the initial infusion rate was set at 12ug/Kg/min by increament or decrement of 10-20% every 5 minutes to maintain T₁ at 10% of the control height. While in group TCI ,atracurium was given by a TCI system of which the effect site concentration was set at 2ug/ml for intubation followed by increament or decrement of 0.lug/ml to maintain T₁ at 10% of the control height. Mechnical ventilation was adjusted to maintain normocapnia. Anaesthesia was maintained with isoflurane and remifentanil 2-5ng/ml TCI. During the operation ,concentration of isoflurane at the end of expiration was maintained at 1 MAC. Infusion of atracurium was discontinued 20 minutes before terminal of the operation in group CI and group TCI, and in group IBI atracurium was no longer added. At the same time ,isoflurane was halted. Spontaneous recovery of muscle function was then allowed to proceed. The criteria of full recovery from muscular blockade was TOFR greater than 75%, tidal volume greater than 400ml and ability to sustain a head lift for no less than 5 seconds. The onset time and duration of action of initial dose was recorded as well as interval time of group IBI. Condition for intubation was evaluated. Dosage of induction , average dosage during operation ,recovery time and recovery index were also recorded. Results During maintenance of anesthesia T₁ could maintained at 10% of the control height in group CI and group TCI, but not in group IBI. The amount of atracurium used for induction of anesthesia is much smaller in group TCI than in group IBI and in group CI, the onset time is significantly longer in group TCI than in group IBI and in group CI. T₁ suppression in group IBI and in group CI were above 95%, while in group TCI, the maximum T₁ suppression was 86%±13%.The score of intubation condition in group TCI was significantly lower than in group IBI and group CI. Duration of action of initial dose of group TCI was much shorter than that of group IBI and group CI. Three groups were comparable with regard to average dosage of atracurium during operation. Stable infusion rate of group TCI and group CI were achieved after 30 mins constant infusion, which were 1.0±0.18ug/ml and 5.2±1.01ug/Kg/min respectively. The interval time of group IBI was (24-45)min with an average of 32±5min. Recovery time was similar in group TCI and group CI which was much shorter than that in group IBI. Recovery index were similar among three groups. Conclusion 1 Atracurium given by TCI of which effect concentration was set at 2ug/ml is not suitable for induction of anesthesia because of longer onset time and poorer intubation condition.2 Stable muscle relaxation can be achieved in both group TCI and group CI but not in group IBI with shorter recovery from termination.3 Atracurium given by TCI and more suitable than CI for maintenance of anesthesia.