Possible causes of narcosis-like symptoms in freedivers
Authors: Frédéric Lemaître, Guillaume Costalat, Jérémie Allinger, Costantino Balestra
DOI / Source: https://doi.org/10.22462/01.01.2023.38
Date: 2023
Reading level: Intermediate
Why This Matters for Freedivers
A lot of deep divers report “narcosis-like” moments—dreamy, confused, weird time perception, tingling, even mild panic—often on the ascent when you need clear thinking the most. This review lays out the most plausible reasons and (importantly) argues it’s probably not classic nitrogen narcosis. Understanding the likely causes helps you make safer choices around speed, workload, depth hangs, thermal stress, and how you respond if your head goes “foggy” on the way up.
Synopsis
Freedivers sometimes describe a very specific mental state during deep dives: it can feel dreamlike, time can warp, vision can blur, the mouth can go numb, fingers and toes tingle, and decision-making can get shaky. Some divers even report confusion and panic—often during the ascent or after surfacing. Because scuba divers get something that sounds similar, it’s tempting to call it “narcosis” and blame nitrogen. But this paper argues that the freediving version doesn’t fit the classic scuba story.
In scuba, inert gas narcosis is strongly linked to breathing nitrogen at higher pressure and tends to show up during descent/bottom time. In freediving, the reports often peak during ascent—when nitrogen pressure is dropping, not rising. That timing mismatch is one of the big reasons the authors think nitrogen alone is unlikely to explain what freedivers feel.
So what could be happening. The authors propose a “multifactor” explanation, with three main buckets:
1) Gas effects during a breath-hold
Unlike scuba, a freediver’s lung gas isn’t refreshed. Oxygen is being used up, carbon dioxide is building (though it can be buffered), and the whole gas mix is being squeezed and unsqueezed by depth. The descent can produce brief periods of high oxygen pressure, but the ascent is where things can get ugly fast: oxygen pressure can drop dramatically near the surface, creating severe hypoxia right when effort may rise. Meanwhile CO₂ and O₂ can both influence brain function and perception, especially toward the end of a hard dive.
2) The “hemodynamic hypothesis” — brain blood flow surges at the wrong time
This is the paper’s most interesting idea. Breath-holding triggers the diving reflex: heart rate drops and blood vessels in the limbs constrict, redirecting blood toward the heart and brain. At depth, lung compression also promotes “blood shift” toward the chest. On ascent, several forces collide: exercise/finning increases demand and CO₂ production, the diving reflex can still be strong, blood pressure can rise, and cerebral blood flow can increase (CO₂ is a powerful driver of cerebral vasodilation). The authors suggest that this combination can push brain blood flow and pressure regulation into an unusual state, potentially impairing normal brain function and producing those “narcosis-like” symptoms—especially on ascent.
3) Extra amplifiers that make it worse
Cold stress, age, fatigue, speed (especially fast or effort-heavy phases), and overall dive strategy may all shift the risk. The paper highlights that “speed” and workload patterns might matter a lot, because they change oxygen consumption and the tug-of-war between exercise physiology and the diving response.
Because the mechanism is probably not the same as scuba inert gas narcosis, the authors propose a new label: freediving transient cognitive impairment (FTCI)—a descriptive name that doesn’t assume nitrogen is the culprit. The big takeaway for divers is practical: if you get “foggy” on ascent, treat it as a warning sign of a brain under stress (oxygen, CO₂, and hemodynamics), and adjust training and strategy toward smoother pacing, lower effort spikes, and more conservative profiles—especially on deep dives and long hangs.
Abstract
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.