CO2 retention: The key to stopping hiccups

Background While investigating the mechanisms behind hiccups, our team discovered what could be the sufficient physiological conditions for terminating even persistent cases. Methods To investigate the role of CO2 retention, a healthy male volunteer was asked to perform three kinds of rebreathing experiments using different materials: (I) a 20 L air‐filled plastic bag, (II) a 20 L air‐filled plastic bag with a 1.5 × 1.5 cm hole and (III) a 20 L oxygen‐filled plastic bag. During each experiment, CO2 level upon expiration (EtCO2) and inspiration (InspCO2) were measured until the volunteer gave up. Once the safety of this manoeuvre was demonstrated with the volunteer, we performed the technique using the materials from experiment (I) on two actual patients with persistent hiccups. Results In experiments (I) and (III), InspCO2 increased from the beginning and reached almost the same level as EtCO2 after 90 seconds. Both levels continued simultaneously increasing, finally reaching 56 mm Hg in (I) and 79 mm Hg in (III), respectively. In (II), both increased; however, after 120 seconds, EtCO2 plateaued at 47 mm Hg and InspCO2 at 37 mm Hg. In the actual patients, both CO2 levels reached the same value of 35.9 mm Hg at 60 seconds and 37.0 mm Hg at 90 seconds, and hiccups stopped at 195 seconds and at 359 seconds when EtCO2 reached 50 mm Hg and 53 mm Hg, respectively. Conclusion The study determined that to successfully obstruct the mechanisms causing hiccups, it is necessary that the level of InspCO2 not only increases at the same level as EtCO2, but also reaches approximately 50 mm Hg.