CNS Oxygen Toxicity

When Oxygen Turns Against You

CNS oxygen toxicity is a condition where breathing oxygen at high partial pressure disrupts neuronal function. Paul Bert first described oxygen-induced convulsions in 1878 — the "Paul Bert effect." The mechanism is not simple suffocation; it involves free radical accumulation suppressing inhibitory neurotransmission, which triggers uncontrolled neuronal firing. Underwater, this is dangerous not because the seizure itself is lethal, but because it leads to loss of consciousness and mouthpiece expulsion.

At high PO₂, reactive oxygen species accumulate in the central nervous system faster than antioxidant defences can clear them. This suppresses GABA (the brain's main inhibitory neurotransmitter) and hyperactivates NMDA receptors, resulting in a cascade of uncontrolled neuronal firing. The endpoint is a grand mal convulsion. The convulsion itself is rarely lethal. The drowning or barotrauma from a breath-holding body ascending under buoyancy is.

This is distinct from pulmonary oxygen toxicity (the Lorrain Smith effect), which is a slower cumulative process involving lung inflammation — covered in the pulmonary oxygen toxicity article.

PO₂ Limits

The limits below reflect current technical diving standards:

The 1.4 bar working limit and 1.6 bar deco maximum are widely adopted across PADI TecRec, TDI, and IANTD standards. The original NOAA 180-minute limit at 1.3 bar was primarily a pulmonary toxicity precaution, not a CNS seizure threshold — a distinction that matters when planning long CCR dives.

Dive computers track CNS exposure as a running percentage using the NOAA exposure table:

CNS% for any segment = (time at PO₂) / (permitted exposure at that PO₂) × 100. A reading of 100% means the full recommended limit has been reached.

VENTID-C — Warning Signs and Their Limits

The VENTID-C mnemonic covers the recognised prodrome: Visual disturbances (tunnel vision, flashing lights); Ear symptoms (tinnitus); Nausea; Twitching (lip twitches, facial muscle spasms); Irritability or unusual anxiety; Dizziness; Convulsion.

Convulsion can and does occur with no prior warning. VENTID-C describes symptoms that may precede a seizure — it cannot be used as a reliable safety net. Prodromal signs are unreliable, especially in CCR where PO₂ can spike without a corresponding change in breathing feel.

If any of the first six signs appear during a dive: ascend immediately and switch to a lower-O₂ gas. Do not wait to see if symptoms resolve at depth.

Risk Potentiators

Individual susceptibility varies substantially, and the same diver can have different thresholds on different days. CO₂ retention is the most significant biochemical potentiator — hypercapnia and elevated PO₂ have a synergistic effect on seizure threshold. Any accumulation from skip breathing, high gas density, overexertion, or a poorly-scrubbed rebreather dramatically increases risk. Exercise increases O₂ demand, CO₂ production, and cerebral blood flow — it is the single largest modifiable risk factor during a dive. Cold water immersion, peripheral vasoconstriction, and elevated sympathetic tone reduce CNS O₂ tolerance. Stress, anxiety, illness, and fatigue all shift the threshold further.

Notably, Gur et al. (2024) found that methylphenidate (Ritalin/ADHD medication) does not increase COT risk and may be mildly protective in animal models.

Hypercarbia was documented in 14.7% of confirmed CNS oxygen toxicity cases in the Gur 2024 military rebreather dataset — a reminder that CO₂ is not a hypothetical risk.

Buddy Response to an Underwater Convulsion

1. Hold the diver — prevent uncontrolled ascent. Tonic muscle contraction during the seizure can cause rapid buoyant ascent.
2. Do not restrain convulsing limbs — this will not stop the seizure and risks injury to the rescuer.
3. Do not ascend during active convulsing — ascending a breath-holding body risks pulmonary barotrauma.
4. After the clonic phase ends: replace the mouthpiece and begin a slow, controlled ascent.

MOD Calculations

Maximum Operating Depth (MOD) is calculated from the oxygen fraction (FO₂) and the PO₂ limit:

MOD (m) = ((PO₂_limit / FO₂) − 1) × 10

Examples:

Verification: EAN50 at 22 m → PO₂ = 0.50 × (22/10 + 1) = 0.50 × 3.2 = 1.60 bar. This is the hard ceiling — 4 m below the 1.4 bar working MOD. There is no margin.

Differential Diagnosis

Not every seizure during a dive or HBOT session is CNS oxygen toxicity. Foley et al. (2021) reviewed 29 apparent O₂ toxicity seizures across two Australian hyperbaric units over 30 years — 4 of the 29 were mimics: one posterior reversible encephalopathy syndrome (PRES), one pethidine toxicity, one pre-existing epilepsy from prior subarachnoid haemorrhage, and one severe hypoglycaemia. All four had initially been attributed to O₂ toxicity.

For underwater incidents, immediate management is the same regardless of cause: ascent and gas switch. Post-incident investigation must consider the full differential: stroke, hypoglycaemia, cardiac dysrhythmia, drug interactions, and pre-existing seizure disorders.

Seizure Rates — Quantifying the Risk

Bonnington et al. (2021) conducted a retrospective review of 1,000 USN Treatment Table 6 sessions — a high-pressure, high-PO₂ clinical protocol — over 30 years at two Australian hyperbaric units. Total seizures: 4, all in multiplace chambers. Overall seizure rate: 0.40%. The rate in monoplace chambers was 0% (no statistical difference, p = 0.31).

For context, TT6 involves breathing 100% O₂ at 2.8 atm absolute — far above any recreational or technical diving exposure. The 0.40% rate represents a high-exposure medical environment, not diving.

Managing Oxygen Exposure

Keep working PO₂ ≤ 1.4 bar; use 1.3 bar as the standard CCR setpoint. Monitor CNS% throughout the dive, not just at the planning stage. Reduce exertion at high PO₂ — stop and hover rather than fighting current at depth. Manage CO₂ actively: maintain scrubber, avoid skip breathing, monitor gas density (helium helps). At shallow deco stops (3 m at 1.3 setpoint ≈ nearly 100% FO₂), consider reducing CCR setpoint to 0.7 bar — CNS clock runs fast at shallow depths with high PO₂. For long CCR dives, calculate CNS% before every dive. TDI recommends limiting single-dive CNS% to 80%.

Related Reading

• Oxygen Toxicity & ROS — What Happens Inside Your Body
• Gas Density & CO₂ Retention — The Silent Risk
• Pulmonary Oxygen Toxicity — What Long Exposure Does to Your Lungs

References

• Bonnington S, Banham N, Foley K, Gawthrope I. Oxygen toxicity seizures during United States Navy Treatment Table 6: An acceptable risk in monoplace chambers? Diving Hyperbaric Med. 2021;51(2):167–172. doi:10.28920/dhm51.2.167-172
• Foley K, Banham N, Bonnington S, Gawthrope I. Oxygen toxicity seizure mimics. Diving Hyperbaric Med. 2021;51(2):161–166. doi:10.28920/dhm51.2.161-166
• Gur I, Arieli Y, Matsliah Y. Methylphenidate and the risk of acute central nervous system oxygen toxicity: a rodent model and observational data in human divers. Diving Hyperbaric Med. 2024;54(3):168–175. doi:10.28920/dhm54.3.168-175
• NOAA Diving Program. NOAA Diving Standards and Safety Manual, Rev 05. 2020.
• Hamilton RW. REPEX: Development of Repetitive Exposure Tables for Oxygen. National Undersea Research Program, 1989.

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