Alfentanil Dosage When Inserting the Classic Laryngeal Mask Airway
Andrea L.Y. Yu, Lester A.H. Critchley, Anna Lee, and Tony Gin
(Anesthesiology, 105:684-688, 2006)
Department of Anaesthesia and Intensive Care, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, New Territories, Hong Kong, People's Republic of China
Reliable conditions for inserting a laryngeal mask airway (LMA) have been investigated, with several induction agents and adjuncts proposed to assist the clinician with this task. Propofol seems to be the most appropriate induction agent, but the choice of an adjunct is not certain. In a comparison of propofol (2.5 mg/kg) administered with either fentanyl (1 μg/kg) or alfentanil (10 μg/kg), alfentanil created better conditions for LMA insertion but also induced prolonged apnea. The present study determined an optimum dose of alfentanil, coadministered with 2.5 mg/kg propofol, when inserting a classic LMA.
The 75 adult Chinese patients were randomly assigned to 5 dosage groups: placebo or 5, 10, 15, or 20 μg/kg alfentanil. Anesthesia was induced by first injecting the study drug over 10 seconds, followed by propofol, 2.5 mg/kg, also over 10 seconds. The LMA was inserted 90 seconds after the injection of the alfentanil. The LMA was checked for patency, and if obstructed, it was removed after another dose of propofol (1 mg/kg) was given, followed by another insertion attempt 60 seconds later. After 3 failed attempts, the trachea was intubated. Insertion conditions were graded with a 3-point 6-category scale including resistance to mouth opening, resistance to insertion, swallowing, coughing and gagging, head or body movement, and laryngospasm. ED50 and ED95 were estimated.
The 25 men and 50 women were 18 to 59 years old; the dosage groups were similar in age, sex, American Society of Anesthesiologists physical status, and weight. Six patients in the propofol-only group required more than 1 attempt at LMA placement. In 1 patient in the 5-μg/kg group, insertion was impossible, and intubation was performed. One patient in the 10-μg/kg group and 2 in the 15-μg/kg group required more than 1 attempt at insertion. The incidence of swallowing, gagging and coughing, movement, and laryngospasm decreased with increasing doses of alfentanil. As the dose of opiate increased, the duration of apnea became longer, and the number of patients with prolonged apnea (>5 minutes) increased. Baseline mean arterial pressure and heart rate were comparable in all dosage groups and decreased after injection of propofol and alfentanil in all groups; the decreases ranged from 20% to 27% across the 5 groups and did not differ substantially. After LMA insertion, the mean arterial pressure in the propofol-only group increased, but it continued to decrease in the alfentanil groups. Heart rate did not change after propofol or study drug injection. After LMA insertion, the groups differed notably; patients in the 15-μg/kg group had significantly lower heart rates than patients in the propofol-only group. The ED50 and ED95 could not be predicted with any confidence for mouth opening and ease of insertion. However, for swallowing, gagging, movement, and laryngospasm, the ED50 and ED95 could be predicted; an alfentanil dose of 10 μg/kg, at which insertion conditions would be optimal in 95% of cases, was estimated.
Alfentanil coadministered with propofol can provide ideal insertion conditions in most patients. Mouth opening and resistance to insertion were poor predictors of dose because these 2 assessments are influenced mainly by anatomical features of the upper airway and are not especially useful in assessing insertion conditions. However, swallowing, gagging, movement, and laryngospasm can be used to determine an optimal dosage. The authors recommend administering 10 μg/kg alfentanil when using 2.5 mg/kg propofol in young healthy adult patients.