Abstracts of Posters Presented at the International Anesthesia Research Society; 72nd Clinical and Scientific Congress; Orlando, FL; March 7-11, 1998: Ambulatory Anesthesia
The laryngeal mask airway (LMA) is a popular alternative to tracheal intubation during general anesthesia. Insertion of the LMA requires suppression of upper airway reflexes and jaw relaxation. Propofol has been recognized as the superior induction drug for the insertion of the LMA as compared to thiopental. [1,2] However, the lack of pungency and the speed of induction makes sevoflurane an attractive alternative to intravenous induction drugs.  The purpose of this study was to compare LMA insertion conditions using three different induction drugs and secondly, to determine the role of intravenous lidocaine, used to decrease injection site pain during propofol administration, in the suppression of upper airway reflexes.
METHODS: With IRB approval 100 consenting PS I & II adult patients, scheduled for elective surgery under general anesthesia using the LMA were studied. After routine monitoring devices were applied and fentanyl/midazolam premedication, the patients were randomly assigned to one of four induction protocols: 1) thiopental 4.0 mg/kg; 2) thiopental 4.0 mg/kg + lidocaine 1.0 mg/kg; 3) propofol 2.0 mg/kg + lidocaine 1.0 mg/kg; and 4) sevoflurane 6% in N2 O/O2. After loss of consciousness, the LMA was inserted by an anesthesiologist having no knowledge of the induction protocol used. Degree of jaw opening, ease of LMA insertion, and degree of coughing and gagging were graded in severity on a 0 to 2 point scale (least severe to most severe).  The number of attempts at LMA insertion, and incidences of upper airway obstruction and postoperative sore throat were also recorded.
RESULTS: The data is displayed in Table 1. All results are reported as a mean +/- SD. No statistically significant differences between groups were found. All patients in the sevoflurane group maintained spontaneous breathing prior to LMA insertion; whereas, patients receiving intravenous induction drugs experienced a period of transient apnea (30-90 secs) requiring assisted positive pressure ventilation in 62 of 73 patients.
CONCLUSIONS: The present study demonstrates that all three drugs are equally acceptable for LMA insertion. However, in patients where inhalation induction of general anesthesia is desired, sevoflurane provides the added benefit of maintaining spontaneous breathing throughout the induction period. Our findings do not support previous reports that propofol is superior to thiopental. Since the addition of lidocaine to thiopental induction did not influence LMA insertion conditions, we conclude that lidocaine in the dose described does not decemably contribute to improving LMA insertion conditions. It is also possible that the effect of concurrent administration of fentanyl and midazolam may have contributed to narrowing the differences between the induction drugs.
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2. Can J Anaesth 1993; 40:816.
3. Anesthesiology 1996; 85:536.