One of the definitive conditions for ceasing hiccups is CO2 retention in the body, the target of which is around 60 mmHg of CO2 in venous blood. The fight-or-flight response induced by acute hypercapnia can effectively suppress hiccups because their original nature is highly likely to be aberrations in the swallowing reflex, which is associated with the parasympathetic nervous system.

https://journal.chestnet.org/article/S0012-3692(20)31301-5/fulltext