Effect of the lateral positioning on upper-airway morphology in sedated adults: a paired-design study

Background Lateral positioning in sedated adults reduces the incidence and severity of respiratory adverse events. However, its mechanism in clinical practice is not yet clearly defined. The aim of this study is to explore anatomical differences between lateral and supine positioning through quantitatively comparing geometric parameters of the upper airway in sedated adults. Method This single-center, paired-design study was conducted between February and March 2024. Healthy volunteers aged ≥ 18 years were enrolled, and received Magnetic Resonance Imaging (MRI) of the upper airway in both lateral and supine positions. During MRI examination, volunteers received propofol-based sedation with Ramsay Sedation Scale of 2 to 5 while breathing spontaneously via a facemask with oxygen delivered at 5 L/min. Upper airway morphology was reconstructed using 3D Slicer software. The primary outcome was the narrowest cross-sectional area (CSA) of the retroglossal region. Secondary outcomes included the narrowest CSA of the retropalatal region, anteroposterior and transverse diameters in both regions, upper-airway length and volume, the distance from the hyoid bone to the posterior pharyngeal wall, and sedation related adverse events, including apnea and hypoxemia. Results Compared with the supine position, upper-airway dimensions were significantly greater in the lateral position. The mean narrowest CSA in the retroglossal region was larger in the lateral position than that in the supine position (lateral:169.6 (89.1) mm 2 vs. supine: 73.9 (56.5) mm 2 , mean difference 95.66 mm 2 , 95% CI: 63.80 to 127.53; P < 0.001). The mean narrowest CSA in the retropalatal region in the lateral position was 91.7 (55.3) mm 2 , which was larger than that in the supine position (58.2 (44.5) mm 2 , mean difference 33.53 mm 2 , 95% CI: 20.50 to 46.55; P < 0.001). The corresponding anteroposterior and transverse diameters at both retropalatal and retroglossal regions were also larger in the lateral position ( P < 0.01). Additionally, upper-airway length, volume and the distance from the hyoid bone to the posterior airway wall were markedly greater in the lateral position than in the supine position (all P < 0.01). The incidence of apnea was lower in the lateral position (lateral: 6.5% vs. supine: 45.2%; difference − 38.71%, 95% CI: -61.27 to -16.15; P = 0.002). Conclusion Among sedated adults, the lateral position was associated with larger upper-airway dimensions and a lower incidence of apnea compared with the supine position. Trial registration NCT06236971 (Registered on 13/01/2024 prior to volunteer recruitment): https://clinicaltrials.gov .