Use Of Low-Dose Suxamethonium To Facilitate Laryngeal Mask Insertion Under Etomidate Anaesthesia

摘要

Despite the popularity of laryngeal masks (LMA) for airway maintenance during general anaesthesia, there is still no optimal induction technique that guarantees good insertion conditions whilst maintaining cardiovascular stability and rapid onset of respiration. The most popular induction agent for LMA insertion continues to be propofol as this agent best obtunds oropharyngeal reflexes1,2. Studies show an incidence of poor insertion conditions ranging from 38 to 60%3,4,5 with standard induction doses (2-3mg kg-1) of propofol. However, its use in doses which allow adequate jaw relaxation and prevent patient reaction to LMA insertion1,5 i.e., movement & laryngospasm commonly results in hypotension 6,7,8,9,10,11 and prolonged apnoea12. Although probably inconsequential in a fit patient, these side effects are undesirable in the elderly or those with cardiovascular disease. Introduction There have been numerous papers that looked into co-induction techniques combining a lower dose of propofol or thiopentone with other agents, including benzodiazepines13,14,15,16, rapidly acting opiates3,16 neuromuscular blocking agents4,17,18,19 and topical20 or intravenous5 local anaesthetic agents. Etomidate is known to have greater cardiovascular stability than the other intravenous induction agents, even in patients with cardiovascular risk factors 11,12,21,23. In this study, we propose to use the combination of etomidate 0.3mg kg -1 with various low doses of suxamethonium so as to obtain good LMA insertion conditions whilst maintaining cardiovascular stability. Methods With Institutional Ethics Committee approval, 60 ASA I & II patients were randomly allocated into 3 groups of 20 patients. We excluded all patients suffering from cardiovascular disorders or taking anti-hypertensive or cardiac drugs that could interfere with normal cardiovascular physiology. The patients age range was 17 to 59 years. All subjects presented to the Singapore General Hospital for surgery under general anaesthesia with spontaneous ventilation through a laryngeal mask airway. Informed consent was obtained from all patients pre-operatively.All patients were unpremedicated. Non-invasive blood pressure (NIBP), electrocardiogram (ECG) and pulse oximetry were monitored using a Hewlett Packard GMX modular system and intravenous access secured prior to pre-oxygenation. Induction of anaesthesia was performed with a rapid bolus of etomidate 0.3mg kg-1. This was followed immediately by the administration of normal saline in group A, suxamethonium 0.25mg kg-1 in group B and suxamethonium 0.5mg kg-1 in group C. The study drug was made up to 2ml with normal saline to ensure that the anaesthetist inserting the laryngeal mask was blinded to the treatment given. The laryngeal mask was then inserted 60 seconds after the administration of the drugs in the manner described in the Intravent manual. If jaw relaxation was found to be inadequate to permit LMA insertion, boluses of propofol 50mg were given until adequate relaxation occurred. The position of the LMA was verified by capnography, chest movement and the absence of gas leak around the cuff.Heart rate (HR) and mean arterial blood pressure (MAP) readings were taken pre-induction, 30 seconds post-induction, and 30 seconds post-LMA insertion. The apnoea time was taken from time of insertion of LMA to resumption of respiration. Inhalational anaesthetic agents were not delivered to the patients until after the resumption of respiration. Ventilation was not assisted unless the patient's oxygen saturation fell below 95%. In that instance, the apnoea time was abandoned. The number of attempts taken to insert the laryngeal mask was noted together with degree of jaw relaxation and overall insertion conditions. The reaction from the patient was also graded.Jaw relaxation was graded as 1=good, 2=incomplete or 3=poor according to the classification by Young, Clark and Dundee21. Overall insertion conditions were graded in a system modified fr