Physiological and autonomic responses to intermittent low-lung-volume apnoea and pain sensitivity: A randomized...

Intermittent apnoeas may induce hypoalgesia; however, the effects of low-lung-volume static apnoea have not been investigated. Eighty healthy participants were randomized to an apnoea group (AG) or a control group (CG). The AG performed a 6-min protocol of intermittent static apnoea at low lung volume (passive exhalation), whereas the CG breathed normally in the supine position. Pressure pain thresholds (PPTs) were assessed at the thumb, tibialis anterior and C7, together with heart rate (HR), oxygen saturation ( $S_{p{O}_2}$ ), and systolic and diastolic blood pressure (SBP and DBP). Open-ended questions explored sensations, emotions and adverse effects. ANCOVA (baseline pressure pain threshold) revealed no significant between-group differences in PPT at thumb (mean difference [MD] = -0.209 kg/cm²; 95% CI: -0.584, 0.158; P = 0.268), tibialis (MD = 0.014 kg/cm²; 95% CI: -0.341, 0.354; P = 0.939), or C7 (MD = -0.276 kg/cm²; 95% CI: -0.669, 0.102; P = 0.169). Intrasession analyses showed higher values in the AG for $S_{p{O}_2}$ (MD = 0.598%, P = 0.009), percentage maximum heart rate (MD = 4.344%, P < 0.0001), SBP (MD = 15.344 mmHg, P < 0.0001) and DBP (MD = 14.273 mmHg, P < 0.0001). The AG reported more distress-related sensations and emotions, whereas the CG described more comfort-related experiences. Adverse effects were mild. A 6-min intermittent low-lung-volume static apnoea did not induce hypoalgesia despite autonomic and cardiovascular activation and was well tolerated. These findings suggest respiratory-induced autonomic stress may be insufficient to modulate nociceptive processing, highlighting the importance of physiological conditions under which breath-hold interventions may influence pain. Trial registration: This study was prospectively registered on 27 November 2023, at ClinicalTrials.gov (Identifier: NCT06158256).