High-Flow Nasal Oxygen and the Risk of Gastric Insufflation: A Systematic Review and Meta-Analysis Supplemented by Narrative Synthesis

High-flow nasal oxygen (HFNO) generates positive airway pressure, raising concerns about gastric insufflation and aspiration risk. Although most studies report minimal or no gastric distension, some suggest significant changes. This systematic review and meta-analysis applied the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) framework to evaluate the effect of HFNO on gastric insufflation and related markers across clinical settings. We searched Medline, Embase, Emcare, and CINAHL through August 2025 for studies reporting qualitative or quantitative markers of gastric insufflation during HFNO use, including comet-tail artifacts, antral cross-sectional area, and gastric volume. Eligible designs included randomized trials, observational and volunteer studies, and case reports. Methodological quality was evaluated using the Mixed Methods Appraisal Tool, and certainty of evidence was rated with GRADE. Meta-analysis was performed for outcomes reported in two or more studies. Six randomized trials, five observational studies, two volunteer studies, one case series, and two case reports were included. Observational studies primarily assessed outcomes before and after HFNO intervention. Pooled analysis of four randomized controlled trials (RCTs; n = 375) showed HFNO significantly reduced gastric insufflation compared with face-mask ventilation during elective peri-intubation (risk ratios [RR] = 0.32; 95% confidence interval [CI], 0.19-0.52; P < .00001; I2 = 0%), rated moderate-certainty. For antral cross-sectional area, pooled analysis of three RCTs (n = 318) found no significant difference between HFNO and face-mask ventilation (MD -0.33 cm2; 95% CI, -0.72 to 0.05; P = .09; I2 = 74%), rated moderate-certainty. Observational studies assessing pre- and post-HFNO changes showed no significant increase in antral cross-sectional area (MD 0.08 cm2; 95% CI, -0.29 to 0.45; P = .67; I2 = 0%) and no significant change in gastric liquid volume (MD -0.01 ml/kg; 95% CI, -0.07 to 0.06; P = .80; I2 = 0%), both rated low certainty. Nonpooled data suggested possible increases in critically ill patients, but the evidence was of very low certainty. A single study assessing microaspiration found HFNO reduced gastroesophageal reflux and prevented microaspiration compared with face-mask ventilation. No clinically significant aspiration events were reported across studies. Moderate-certainty evidence supports HFNO as safe regarding gastric insufflation and antral cross-sectional area in most elective and procedural contexts. Low-certainty evidence suggests no increase in gastric volume. Caution is warranted due to limited, low-to-very-low-certainty evidence at higher flow rates and among critically ill patients. Larger multicenter trials and robust observational studies are necessary to confirm safety in these settings.