Possible causes of narcosis-like symptoms in freedivers

During deep-sea freediving, many freedivers describe symptoms fairly similar to what has been related to inert gas narcosis in scuba divers. This manuscript aims to present the potential mechanisms underlying these symptoms. First, known mechanisms of narcosis are summarized while scuba diving. Then, potential underlying mechanisms involving the toxicity of gases (nitrogen, carbon dioxide and oxygen) are presented in freedivers. As the symptoms are felt during ascent, nitrogen is likely not the only gas involved. Since freedivers are frequently exposed to hypercapnic hypoxia toward the end of the dive, it is proposed that carbon dioxide and oxygen gases both play a major role. Finally, a new "hemodynamic hypothesis" based on the diving reflex is proposed in freedivers. The underlying mechanisms are undoubtedly multifactorial and therefore require further research and a new descriptive name. We propose a new term for these types of symptoms: freediving transient cognitive impairment.