Ethics Statement
Ethical approval was obtained from the First Affiliated Hospital of Guangzhou University of Chinese Medicine, Guangzhou, China (NO.ZYYECK[2018]041). Our study was registered with the Chinese Clinical Trial Registry (ChiCTR1900021123). All patients volunteered to take part in this study and provided written informed consent before being selected.
Study Population
We recruited patients between May 1, 2019 and August 31, 2019 who presented with American Society of Anesthesiologists (ASA) physical status I or II and were aged between 18-70 years. Inclusion and exclusion criteria are reported in Table 1.
Table 1. Trial inclusion and exclusion criteria
Inclusion Criteria
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Exclusion Criteria
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Age: 18–70 years
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Patients weighing <30 kg or >75 kg, pregnant women, and parturients
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ASA grade I or II
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Patients with a high risk of regurgitation or aspiration (i.e., large hiatal hernia, history of gastroesophageal reflux disease (GERD), or uncontrolled diabetes mellitus)
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Patients undergoing elective breast cancer surgery under general anaesthesia
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Patients with abnormal pharynx or anatomy, severe cardiovascular or cardiopulmonary diseases, hepatic or renal dysfunction, or coagulation disorders
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Patients who agreed to the use of LMA-classic during surgery
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Patients who requested tracheal intubation during surgery
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Consent to participate
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Refusal to participate
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Study Design
Before study, we did a pilot experiment, measured the upper airway parameters of 10 subjects who using LMA during operation by ultrasound, we found that when the hyomental distance was greater than 4.41 cm, 80% of subjects were suit for class 4 LMA. So we hypothesized that if a patient’s hyomental distance was at least 4.41 cm or their weight was greater than 50 kg, the class 4 LMA would be selected for insertion, all other instances resulted in a class 3 selection[9,10].
In this single-blind study, 70 patients were randomly assigned into two groups via selection of an envelope from a shuffled stack. Each patient selected an envelope immediately before surgery to assign them to one study group. Patients were not made aware of which study group they were assigned. The ultrasound parameters for hyomental distance were recorded for the hyomental distance group.
An anaesthesiologist proficient in ultrasound administration was responsible for measurement and data recording. Two other anaesthesiologists were responsible for LMA-classic selection and insertion.
Preoperative evaluation was performed the day before surgery. After routine fasting (no solids, ≥8 hr; no water, ≥4 hr), patients were sent to operating room (OR) and positioned supine on the operating table with their head in sniffing position [11,12]. Patients were subjected to pulse oximetry, noninvasive blood pressure monitoring (certain patients required invasive arterial monitoring), electrocardiography, and capnography. Before anaesthesia induction, the hyomental distance was measured by ultrasound (Navi series, Shenzhen Wisonic Medical Technologies,Shenzhen, China).
Patients were preoxygenated for 3 minutes through a facemask using 100% oxygen (5-6 L/min). Then, moderate doses of remifentanil (2 μg/kg) and propofol (2 mg/kg) were given through intravenous infusion until patients lost consciousness, then, subsequently administered muscle relaxants of vecuronium bromide for each participants (0.08-0.12 mg/kg). The ventilation condition was determined by observing the end-tidal CO2 (EtCO2) curve. Once patient’s condition was sufficient for LMA insertion (i.e., loss of eyelash reflex, jaw relaxation, or absence of movement), a selected lubricated LMA-classic (TUORen, HeNan, China) was inserted by an experienced doctor who had administered LMA-facilitated airway management in more than 1,000 prior cases. The LMA cuff was inflated with air until its pressure reached approximately 60 cmH2O [13]. A senior anaesthesiologist then confirmed whether the selected LMA was properly sized for patients who should meeting all following criteria: (1) observation of slight outward movement of the LMA tube upon full cuff inflation, (2) smooth and regular visualization of exhaled CO2 on the capnograph, (3) obvious chest undulation without air leakage or stomach undulation, and (4) a fiberscope grading of more than 2. Fiberscope grading was defined as: 4 = only vocal cords visible, 3 = vocal cords and posterior epiglottis visible, 2 = vocal cords plus anterior epiglottis visible, and 1 = vocal cords invisible [9,14,15].
Values of oropharyngeal leak pressure (OLP) after airway management, duration of operation (the time elapsed from LMA insertion to removal), and postoperative oral injury (defined by blood adhering to the LMA surface) were detected and recorded for each patient.
For patients’ safety, if the chosen LMA which was confirmed by the senior anaesthesiologist was not suitable for the patient, we would replaced a more fitting LMA or performed tracheal intubation; such cases would be recorded.
Ultrasonographic Measurement
All ultrasound measurements were performed by a single researcher after positioning patients supine with head and neck in neutral position. A low frequency probe was placed on patient’s neck perpendicular to skin to measure hyomental distance [8] (Figure 1). The researcher adjusted the ventilation according to the appropriate parameters for each patient. Patients were observed closely to prevent LMA displacement during the operation due to airway pressure. Anaesthesia was maintained with propofol and remifentanil.
Primary outcome was the correct rate of the predicted LMA size without needing for size adjustment or employing other devices. Secondary outcomes included injury rate (defined as visible blood stains on LMA upon removal), OLP value, and number of insertion attempts (defined as re-insertion after complete LMA removal from the oral cavity). The LMA adjustment process was not included as a measured secondary outcome.
Statistical Analysis
Based on results of previous studies [2,13], the minimum sample size required to detect a significant difference is at least 32 in each group (64 in total) for the first attempt between two groups, considering type I error (alpha) of 0.05, power (1-beta) of 0.8, we decided to include 70 participants in order to meet experimental requirements. The Kolmogorov-Smirnov test was used to detect the normality of quantitative data. Based on test results, data were displayed as mean ± standard deviation or median (P25-P75)—appropriately—while categorical data were presented as numbers. Pearson’s chi-squared test was used to compare success rates of LMA size-selection between weight group and hyomental-distance group. Wilcoxon rank test of two independent samples was used to examine OLP values and other outcomes. IBM SPSS Version 25.0 software was used for statistical analysis. A P value less than 0.05 was used to determine significance.