Intermittent Normobaric Hypoxia and Cardiac Autonomic Modulation in Adults: A Systematic Review

Intermittent normobaric hypoxia is used as a hypoxic-conditioning strategy in sport, exercise, and health settings, but its effects on cardiac autonomic modulation remain unclear. This review synthesised passive and exercise-combined protocols in adults. We searched 4 databases through March 2026 and reported the review according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 statement. Risk of bias was assessed using RoB 2 and ROBINS-I; findings were synthesised narratively, and certainty was graded for coherent evidence. Of 4,866 records, 18 studies were included, 12 arising from controlled parent designs. Controlled resting evidence for the root mean square of successive differences (RMSSD) comprised 3 comparisons involving 47 analysed participants. Of these, 2 multiweek comparisons (n = 34) showed numerically concordant between-group differences in RMSSD change, whereas 1 single-session crossover comparison (n = 13) did not establish a clear between-condition effect; certainty was very low in both strata. Frequency-domain heart rate variability evidence was sparse and methodologically unstable, baroreflex sensitivity findings were non-convergent, and exercise-combined evidence was insufficient for directional inference. Heterogeneous dosing, limited sham control, and limited respiratory monitoring restricted interpretation. Evidence remains insufficient to establish reproducible cardiac autonomic changes after intermittent normobaric hypoxia. Registration: PROSPERO CRD420261338890.