oxygen toxicity symptoms

According to the USN Dive Manual sections and the suggested procedure for dealing with seizures is:

Management of Nonconvulsive Symptoms.

The stricken diver should alert his dive buddy and make a controlled ascent to the surface. The victim’s life preserver should be inflated (if necessary) with the dive buddy watching him closely for progression of symptoms.

Management of Underwater Convulsion.

The following steps should be taken when treating a convulsing diver:

  1. Assume a position behind the convulsing diver. Release the victim’s weight belt unless he is wearing a drysuit, in which case the weight belt should be left in place to prevent the diver from assuming a face-down position on the surface.
  2. Leave the victim’s mouthpiece in his mouth. If it is not in his mouth, do not attempt to replace it; however, if time permits, ensure that the mouthpiece is switched to the surface position.
  3. Grasp the victim around his chest above the underwater breathing apparatus (UBA) or between the UBA and his body. If difficulty is encountered in gaining control of the victim in this manner, the rescuer should use the best method possible to obtain control. The UBA waist or neck strap may be grasped if necessary.
  4. Make a controlled ascent to the surface, maintaining a slight pressure on the diver’s chest to assist exhalation.(see commentary below)
  5. If additional buoyancy is required, activate the victim’s life jacket. The rescuer should not release his own weight belt or inflate his own life jacket.
  6. Upon reaching the surface, inflate the victim’s life jacket if not previously done
  7. Remove the victim’s mouthpiece and switch the valve to SURFACE to prevent the possibility of the rig flooding and weighing down the victim.
  8. Signal for emergency pick-up.
  9. Once the convulsion has subsided, open the victim’s airway by tilting his head back slightly.
  10. Ensure the victim is breathing. Mouth-to-mouth breathing may be initiated if necessary.
  11. If an upward excursion occurred during the actual convulsion, transport to the nearest chamber and have the victim evaluated by an individual trained to recognize and treat diving-related illness.

Deciding whether to ascend with a diver who is convulsing can be tricky. In section 8-2.4 of Volume 1 of the U.S. Navy diving manual it states:

“If a diver convulses, the UBA should be ventilated immediately with a gas of lower oxygen content, if possible. If depth control is possible and gas supply is secure (helmet or full face mask), the diver’s depth should be kept constant until the convulsion subsides. If an ascent must take place, it should be done as slowly as possible. If a diver surfaces unconscious because of an oxygen convulsion or to avoid drowning, the diver must be treated as if suffering from arterial gas embolism.”

Obviously, a full face mask is the best way to perform diving with high oxygen mixes because the diver can be kept at depth until the convulsion subsides. If the diver is breathing from a mouthpiece and it comes out of his mouth, there is no option but to surface the diver, since when the convulsion stops he will try to take a breath. Training and practice are the only ways to ensure that divers will know how to bring a convulsing diver to the surface, using a slow, controlled ascent, if that becomes necessary.

In the section on the management of underwater convulsions, the reference to switching the mouthpiece to the surface position would refer only to rebreathers where an open mouthpiece which inadvertently becomes submerged can flood the UBA.

Also, step g should be modified if the victim is breathing nitrox using open-circuit scuba. If someone is convulsing, you won’t be able to remove the mouthpiece; and this should never be done by force. Once the convulsion subsides, if the mouthpiece is secure (or if the diver is wearing a full face mask) and if the diver is still in the water and breathing, then leave everything in place until you can get the injured diver out of the water. If he is not breathing, then remove the mouthpiece once on the surface and begin rescue breathing.

The main goal while the injured diver is in the water is to keep him from drowning. Next is to ensure that his airway is open after the convulsion stops by keeping the neck extended.

Finally, be on the lookout for foreign bodies in the trachea. It is possible to bite off the parts of the mouthpiece between the teeth during a convulsion, which can find their way into the trachea, blocking the airway. In these cases, the injured diver will begin coughing as he returns to consciousness, or he may try to breathe but not get any air into his lungs. Here you need to institute the standard procedures taught in CPR classes for foreign body obstruction of the trachea.

Continuous vs. Intermittent Oxygen Exposures

Remember that CNS oxygen toxicity symptoms are a time-duration phenomenon. They will not suddenly occur the minute a particular partial pressure is exceeded – it takes time. As you can see from the exposure limits in the tables (Table 4), as the inspired oxygen partial pressure increases, the exposure time decreases.

The U.S. Navy limit of 1.3 ata for continuous exposures reflects their desire to keep the risk of CNS symptoms essentially zero, no matter how long the dive.

In nitrox diving, however, divers breathe from open-circuit scuba with a fixed fraction of oxygen in the breathing mix. PADI has chosen 1.4 ata as the maximum open-circuit scuba limit; the limitations placed on duration by open-circuit scuba will ensure that the likelihood of CNS oxygen toxicity is no greater than would be experienced by the U.S. Navy closed-circuit divers.

When using open-circuit scuba, the 1.4 ata maximum oxygen partial pressure is reached only at the maximum depth, and for the vast majority of recreation divers, the time spent at this maximum depth will be limited to times where CNS oxygen toxicity is unlikely to be encountered. At all shallower depths, the oxygen partial pressure will be lower, and the overall exposure during the entire dive is unlikely to have physiological effects significantly different than a continuous 1.3 ata exposure. Be careful when extending this analogy to higher partial pressures, however. Formulas are available for integrating the exposures at various depths to predict overall exposure times when looking only at lung oxygen toxicity. This concept does have some support research done at Dr. C.J. Lambertsen’s laboratory at the Institute of Environmental Medicine in Philadelphia, Pa.

The case for CNS oxygen toxicity is much more complicated. Research done at the Navy Experimental Diving Unit (NEDU) in 1986 specifically looked at how brief exposures to oxygen partial pressures of 2.0 ata or greater would impact the overall exposure time at 20 feet / 6.1 meters of sea water (fsw). The results were not clear, and it was obvious that no formula could be developed which would allow integration of oxygen exposures at various depths into a single indicator which would help the diver avoid CNS oxygen toxicity. The best that could be said is that a single 15-minute excursion to 40 fsw/12 msw, or for five minutes at 50 fsw/15 msw, probably had no significant effect. This formed the basis of the current U.S. Navy recommendations. No such research has yet been carried out for high oxygen nitrox diving, to my knowledge.


Donald KM. Oxygen and the Diver. England: Images, 1993. Available through Best Publishing Co., Flagstaff, Ariz. (This reference also covers all of the NEDU studies mentioned and gives full citations for them.)

Harabin AL, Survanshi SS. A statistical analysis of recent Navy Experimental Diving Unit (NEDU) single-depth human exposures to 100-percent oxygen at pressure. Bethesda, M.D. Naval Medical Research Institute Report NMRI 93-59, 1993. Note: Both NEDU and NMRI Reports are available through: National Technical Information Service, 5385 Port Royal Road, Springfield VA 22161.

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