CO 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 CO 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, CO level upon expiration (EtCO ) and inspiration (InspCO ) 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), InspCO increased from the beginning and reached almost the same level as EtCO 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, EtCO plateaued at 47 mm Hg and InspCO at 37 mm Hg. In the actual patients, both CO 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 EtCO 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 InspCO not only increases at the same level as EtCO , but also reaches approximately 50 mm Hg.