With the approval of our institutional ethics committee and after having obtained written informed consent, patients were recruited for this prospective, randomized clinical study (German Clinical Trial Register number DRKS00000760). Inclusion criteria were: age >18
years and scheduled elective surgical intervention in the supine position with predicted anaesthesia duration between 60 and 180
minutes. Exclusion criteria were: BMI >35
, ASA status III or higher, known risk of aspiration, known or predicted difficult airway. The patients were assigned to their groups with a computer-generated randomisation list (http://www.randomizer.org
). The sealed envelope method was used for blinding.
Leak pressure (LP) was the primary endpoint. Secondary endpoints were speed of insertion, insertion success rates, fibreoptically assessed in situ position, dynamic airway compliance and signs of airway morbidity.
The patients were premedicated with midazolam (7.5
mg p.o.) thirty minutes prior to anaesthesia induction. Anaesthesia was induced with sufentanil (0.3-0.5
μg/kg) and propofol (1.5-2.5
mg/kg) and maintained during the measurement period with a propofol infusion (5-8
mg/kg/h) and repeated injections of sufentanil when necessary. No muscle relaxants or opioids other than sufentanil were given.
Airway management and ventilation
Two senior anaesthesia registrars (SC and TG) skilled in placing SGA performed all cases. Although the recommended insertion techniques of the three SGA are quite similar the investigators were required to perform a minimum of 15 insertions with all three devices before starting patient recruitment, particularly because they had previous experience with reusable laryngeal mask type devices (classical laryngeal mask airway and PLMA) but not i-gel™ and LMA-S. Depth of anaesthesia was assessed by performing a jaw thrust manoeuvre [9
]. The size of the device was determined according to the manufacturers’ weight-based recommendations.
If two attempts to insert the initially randomized SGA failed, the study protocol prescribed a change to one of the other two devices selected randomly (coin toss). If two attempts with the second SGA device were also unsuccessful the trachea was intubated.
Failed insertion of the SGA was defined as the inability to position the device within 60
seconds, an air leak through the drainage channel during positive pressure ventilation despite corrective manoeuvres (e.g. deeper insertion or up-and-down-manoeuvre) [11
], inability to introduce a suction catheter (12Ch for the i-gel™, 16Ch for the LMA-S and LTS-D) beyond the tip of the device, inability to establish successful ventilation with a stable end-expiratory CO2
signal with a targeted expiratory tidal volume of 7
ml/kg because of leakage or airway obstruction.
The time required for successful insertion was defined as the time from placing the SGA in the front of the patient’s mouth to its placement in the correct position. A non-blinded observer who was not involved in the study recorded the time needed for insertion.
After successful insertion, the cuff of the LMA-S was inflated to a pressure of 60
O. The cuff of the LTS-D was initially inflated with the volume indicated on the syringe provided by the manufacturer for emergency use. The cuff pressure was measured at this inflation volume and air was then withdrawn until cuff pressure was also 60
Ventilation was pressure-controlled (PCV) with a positive end-expiratory pressure (PEEP) of 3
O, a respiratory rate between 14 and 16 and an inspiratory to expiratory ratio of 1:1.5. It was adapted to give an end-tidal CO2
Leak pressure was determined as a function of cuff pressure for the SGA with inflatable cuffs (LMA-S and the LTS-D). Cuff pressure was measured with a manometer (VBM Medical GmbH, Sulz, Germany; range from 0 to 110
O) that was connected to the SGA through a three-way stopcock with an attached syringe. Air was injected until the pre-determined cuff pressure was obtained and the required inflation volume was recorded. Cuff pressures started at 0
0 (completely deflated and equilibrated to ambient pressure) and were increased in 10
0 increments to 60
0. It was further increased in the LTD-S in two 20
0 increments to a maximum of 100
O. The pressure limit of the anaesthesia circuit was set to 35
O and airway pressure was increased steadily with a continuous flow of oxygen (3
l/min). Leakage was defined as air escape audible with a stethoscope placed on the larynx, and leak pressure was defined as the airway pressure at which leakage was first detected [12
]. After these measurements the cuff was inflated to 60
O and kept at that pressure for the remainder of the study duration.
Airway pressures (PAW) and tidal volumes (Vt
) were recorded and averaged over one minute after insertion, as well as after equilibration of ventilator settings. Dynamic airway compliance (Cdyn
) was calculated using the formula
. The LP defined the maximum inspiratory airway pressure setting.
Fibreoptic (FO) evaluation of the SGA’s position was performed after successful insertion and determination of the airway pressures and tidal volumes. The position was assessed using a previously described four points score (1
only vocal cords seen; 2
cords and/or arytenoids seen; 3
only epiglottis seen; 4
other (e.g. cuff, pharynx, etc)) [13
If an unexpected reduction in tidal volume occurred during stable anaesthesia and unchanged ventilator settings, the airway was assessed with the fibrescope for dislocation of the device or obstruction due to glottic narrowing or laryngospasm. Glottic narrowing was differentiated from laryngospasm by a partial closure of the vocal cords that was not reversed by deepening anaesthesia or by the administration of a neuromuscular blocker.
All devices were evaluated for traces of blood on the mask bowl (LMA-S, i-gel™) or the airway apertures and cuff (LTS-D). The patients were questioned about sore throat, discomfort during swallowing and hoarseness at one hour and at 24
hours after anaesthesia. These complaints were classified as none, mild, or severe.
Published data on leak pressures were used to estimate the necessary sample size. Assuming a mean LP of 24
O for the i-gel™ [3
O for the LMA-S [6
], and 28
O for the LTS-D [8
] and an assumed standard deviation of 5
O for all devices, a total sample size of 83 was calculated to detect differences with 90% power and a significance level of 0.05 [17
]. To allow for potential dropouts, a sample size of 40 patients per group was chosen.
The data were documented in an Excel™ spreadsheet and analyzed using SPSS Statistics™ software (IBM SPSS Inc., Chicago, IL, USA). Depending on the level of measurement of the dependent variables, ANOVA, rank variance analysis (Kruskall-Wallis), or multinomial logistic regressions and chi-square tests were used.