Hyperoxia: when oxygen becomes dangerous for health

Definition: what is hyperoxia?

Our body is generally able to withstand a partial pressure of breathed oxygen (PPO2) between 0.16 and 05 bar. We are talking about “normoxic” conditions. When the PPO2 is greater than 0.5 bar, it is called hyperoxia. In other words, we are subjected to an overdose of oxygen which can cause serious damage, even lead to death in certain circumstances, especially in scuba diving.

Acute hyperoxia attacks (Paul Bert effect)

The acute hyperoxia attacks can occur as soon as the PPO2 exceeds 1 bar. But the neurological toxicity of oxygen occurs from 1.6 bar. These seizures, similar to epileptic seizures, are also known as the “Paul Bert effect”.

The organism, when stressed, reacts in three stages: a tonic phase (characterized by muscle contractions), a clonic phase (characterized by softening) and a final depressive phase. The hyperoxic crisis eventually ends on its own when the victim stops inhaling too much oxygen.

Chronic hyperoxia attacks (Lorrain Smith effect)

L’chronic hyperoxia, it is triggered over the long term, by being regularly exposed to an oxygen level greater than 0.5 bar. This is the case, for example, of divers, who sometimes venture long hours underwater, being subjected to a PPO2 of between 0.5 and 1 bar. The crises that occur are also known as the “Lorrain Smith effect”.

What is the difference between hypoxia and hypoxemia?

We speak of hypoxia when the partial pressure of oxygen drops below 0.16 bar. The oxygen level in the tissues is therefore abnormally low. Hypoxemia, on the other hand, refers to a low level of oxygen in the blood.

Hyporexic crisis: what symptoms should alert?

Hyporexia attacks are a reaction of our organism to an aggression. Indeed, the oxygen superoxide anion has harmful chemical properties: it carries single electrons which constitute free radicals highly reactive and toxic. By dint of accumulation, free radials damage the membranes of our cells and alter the supply of our muscles.

Acute hyperoxia attacks (Paul Bert effect) can manifest as seizures of tetany and convulsion (hence the confusion with epilepsy). They most often occur without warning signs and cannot be stopped once they have started. The only thing to do is to wait while protecting the victim, until he finds a normal PPO2. Note: most often, these crises leave no sequelae or memories.

Chronic hyperoxia attacks (Lorrain Smith effect) are generally manifested by:

  • a pupil dilation,
  • teesvision rubles,
  • tachycardia,
  • a feeling of suffocation,
  • a burning and itchy throat sensation,
  • a cough
  • and difficulty breathing.

In the context of scuba diving, these symptoms are obviously difficult to detect. But the crisis can be manifested by slight tingling in the lips.

What are the causes of this phenomenon?

Hyperoxia is due to excess oxygen supply (an abnormally high and prolonged concentration). This type of contribution can be used in the context of:

  • of one emergency medical care,
  • in medical intensive care

It can also occur during a diving session.

Diving accidents, the first risk factors

Most hyperoxia attacks occur during too deep or too long dives which include gas mixtures such as nitrox or trimix. The diver(s) exceed(s) the tolerated oxygen partial pressure and expose(s) themselves to the Paul Bert effect. In France, the hyperoxic threshold is also set at 1.6 bar partial pressure of oxygen (PPO2) by article A322-92 of the Sports Code. Just like the World Confederation of Underwater Activities (CMAS) and the French Federation for Underwater Studies and Sports (FFESSM). Toxicity thresholds have therefore been established to limit the risks:

  • 6 meters for pure oxygen diving (O2)
  • and 30 meters for an air dive (depending on the gas mixture used).

Why is too much oxygen dangerous?

Air too rich in oxygen strongly disturbs the mechanism of regulation of the rate of free radicals. As indicated above, the toxic effects of oxygen arise from the chemical properties of the superoxide anion O2, carriers of electrons considered as free radicals, capable of damaging our cells when present in excessive amounts. Said damage depends however on the partial pressure of oxygen, but also on the time of exposure to this overflow of oxygen.

Consequences: what side effects to expect?

The accumulation of free radicals can cause functional impairment of nerve cells. We speak of a neurotoxic accident, characterized by confusion, visual disturbances, convulsive attacks and possible loss of consciousness.

If exposure is very long, it may also cause pulmonary edema Where a morphological alteration in the pulmonary alveoli.

Note: an acute or chronic hyperoxia crisis can lead to a drowning if it occurs while diving.

Excess oxygen can also be useful to our body

Despite its toxicity, hyperoxia can be used intentionally in the context of surgical treatment : the patient is placed under elevated PPO2 to prevent the risk of hypoxia. A higher than normal blood oxygen level therefore makes it possible to secure certain procedures.

It is also recommended in case ofcarbon monoxide poisoningofgas embolism ordecompression sickness, provided it is handled with care. However, several indications are discussed: acute coronary syndrome, cardiorespiratory arrest, head trauma, ischemic stroke, instability in intensive care, etc.

In addition, some patients may benefit from hyperoxia to heal quickly. Hyperbaric hyperoxia is, for example, used in the management of severe burns. It helps to destroy dead cells more quickly and to re-oxygenate new cells more quickly, which allows tissues to reconstitute themselves more quickly.

In the event of a seizure during a dive, the best thing to do is to wait for the post-convulsive phase before ascending to a suitable depth. Otherwise, the victim risks a pulmonary overpressure, therefore a possible ventilatory arrest. Once on the surface, any damage is taken care of (drowning, burning of the pulmonary alveoli, etc.). The victim must also be monitored for a few hours to manage the occurrence of a neurotoxic accident.

Reminder: to prevent this type of accident while scuba diving:

  • take care never to exceed the maximum authorized depth according to the proportion of oxygen measured in your gas;
  • do not perform excessively long dives with pure oxygen in too short intervals.

If the crisis occurs outside of a dive, it must be allowed to take its course while ensuring the safety of the victim (avoiding shocks, keeping potentially dangerous objects away, etc.).

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