Patients and protocol
This prospective, single-center, randomized, single-blinded, parallel group comparison study was approved by the Institutional Review Board of the National Medical Center (authorization number H-1305/029-001). Written informed consents were obtained from all patients. This randomized controlled trial was registered at ClinicalTrials.gov (NCT02058030). Eighty adult patients (age range 18–90 years; American Society of Anesthesiologists physical status 1–2) scheduled for minor surgery in the supine position were enrolled. The patients were recruited from November 2013 to November 2014. Patients were excluded if they had a known or predicted difficult airway, recent sore throat, mouth opening less than 2.5 cm, or risk of aspiration (non-fasted or gastroesophageal reflux disease). Anesthesiologists who did not perform the anesthesia enrolled the participants and assigned them to one of the two groups (3 cm group, 3 cm head elevation; 6 cm group, 6 cm head elevation), using a computer-generated randomization table (generated by Mi-Jung Yun at www.randomizer.org). The allocation ratio was 1:1. The assignment was concealed in an envelope until the start of anesthesia. Both patients and evaluators were blinded to the study.
The standard anesthesia protocol was as follows: monitoring devices were connected before anesthetic induction; these included an electrocardiograph, pulse oximeter, gas analyzer and non-invasive blood pressure monitor. Anesthesia was induced with intravenous (IV) propofol (1–2 mg/kg) and inhalation of 6–8 vol% sevoflurane. Neuromuscular blockade was achieved with IV rocuronium (0.6 mg/kg). The patient’s head was elevated using a firm 3 cm pillow (3 cm group) or 6 cm pillow (6 cm group) and PLMA was inserted in the sniffing position. Anesthesia was maintained with 1.5–2.5 vol% sevoflurane in 50 % O2 and air. Water-based gel without a local anesthetic was applied to the posterior and lateral surface of the PLMA for lubrication, and the cuff was fully deflated before insertion.
PLMA size was determined based on age and weight. Heart rate (HR) and mean blood pressure (MBP) were recorded 1 min before and 1 min after PLMA insertion. All insertions were performed by a single experienced PLMA user who was not blinded to the pillow height. The standard insertion technique was applied in both groups using an index finger, according to the manufacturer’s instructions [3]. The patient’s head was elevated with a pillow and the head was extended using the anesthesiologist’s non-dominant hand. The index finger of the dominant hand was placed in the retaining strap of the PLMA. The PLMA was pressed against the hard palate and advanced into the hypopharynx until resistance was felt.
The laryngeal and oral axes were measured to assess their alignment. It was postulated that insertion would be more difficult with a greater difference in angle between the two axes and the PLMA would be more likely to buckle against the posterior pharyngeal wall. The laryngeal and oral axes were assessed using images obtained on the right side of the patient during PLMA insertion. A physician, who was blinded to the study, acquired the images while the pillow was covered using a barrier. The airway axes were defined as follows: angle of the ventral neck (an imaginary line along the long axis of the trachea) and the oral axis (an imaginary midline perpendicular to the line between the upper and lower lip). The angles were measured in images of all patients. Using a protractor, each angle was assessed three times by three individual anesthesiology residents, who were blinded to the study and could not determine pillow height on the images. The difference in angle of adjacent two axes was considered the alignment between the two axes. A lesser angle difference (oral axis - ventral neck angle) indicated greater alignment of the two airway axes (Fig. 1).
The PLMA was connected to a breathing circuit after insertion, and the cuff was inflated with air until an effective airway was secured. An effective airway was defined as normal thoracoabdominal movement and a square-wave capnograph tracing. Airway pressure and end-tidal CO2 concentration were monitored. If insertion failed after two attempts using a pillow of the same height (head elevation), a third insertion trial was performed using a pillow height of the opposite group. If the third attempt was unsuccessful, it was regarded as a failure, and tracheal intubation was performed. The number of insertion attempts was recorded. Airway seal pressure was measured by setting the adjustable pressure limiting valve to 30 cmH2O and manually ventilating the patient while listening with a stethoscope over the mouth [6] and epigastrium [7] to detect oropharyngeal and gastric air leaks, respectively. The PLMA was repositioned if air leaked up the drainage tube or if ventilation was ineffective (expired tidal volume <8 ml/kg). The PLMA was removed after surgery when patients were able to breathe spontaneously and open their eyes. The following predefined complications were documented: airway reflex activation (coughing, gagging, retching, laryngospasm, bronchospasm); airway obstruction (poor air entry in the absence of upper airway reflex activation); aspiration, regurgitation, vomiting; blood staining on the PLMA surface; displacement of the PLMA from the pharynx; gastric distension (visible increase in abdominal girth with air entry into the stomach detected by stethoscopy); persistent leaks (persistent peak airway pressure < 12 cmH2O); hypoxia (SpO2 < 90 %). Sore throat and hoarseness were assessed before discharge to the ward by an anesthesia nurse, who was blinded to the study.
Statistical analysis
Sample size was calculated based on a pilot study of 10 patients in each group. The success rate of the first attempt at PLMA insertion was 100 % in the 3 cm group and 70 % in the 6 cm group. With a 30 % difference in first attempt success being considered significant, 35 patients were required in each group, accepting a type 1 error (two-tailed) of 0.05 and a power of 90 %. An additional five patients per group were enrolled to compensate for possible loss.
Within each group, the effect on MBP and HR due to LMA insertion interaction was compared by paired t-test. Gender ratio, success rate, incidence of repositioning and the occurrence of complications were compared using a Fisher’s exact or chi-squared test. A value of P < 0.05 was considered to indicate statistical significance.