Oxygen mask phenomenon

[punkalouver]

This was in a safety report I recently read. Quite interesting and perhaps something everyone should know.

"When a pilot first dons an oxygen mask, the added oxygen can initially make him feel worse because of a sudden decrease in blood pressure due to momentary dilation of capillaries. Do not take off the oxygen mask. Trust your oxygen system"

Never heard of that before.

[keithl]

I never heard of it either! And I'm not convinced, having spent many years of flying with an oxy mask as standard apparel. I'd be surprised if any mil guys recognise this "phenomenon".

[PENKO]

Don't dismiss this too quickly, I've come across it before. I'm no authority on the subject,this is what I vaguely remember reading (ATPL human factors). When you are in an oxygen deprived environment, your body adapts as best as it can. When suddenly there is 100% oxygen available in the lungs, all the cells in the body open up to accept this 100%, even when this oxygen hasn't reached them yet. This causes a momentary worsening of the hypoxia symptoms, rapidly followed by recovery.


Those mil guys will not notice this 'phenomenon' if they are flying on 100% oxygen all the time, correct me if I'm wrong.

[keithl]

Oh, yes, I recognise that. If you are hypoxic then the initial gulp of oxy can make you worse. I've even experienced it in the decomp. chamber.

The initial post said "When a pilot first dons an oxy mask", it didn't say "after becoming hypoxic". So I read it as when you put it on in response to, say, a decompression - i.e. before suffering any effects.

[SRB]

Punkalouver is correct. The report is probably referring to a phenomenon known as the oxygen paradox. This is well known in avmed circles.

Punkalouver’s description is slightly misleading through no fault of his. The oxygen paradox can only occur when a mask is donned after someone has been hypoxic. It does not occur if an oxygen mask is donned for other reasons such as smoke, fumes or training purposes.

From Ernsting & King’s Textbook of Aviation Medicine (Vol 2):

“The administration of oxygen to a hypoxic subject usually results in rapid and complete recovery: . . .
In some subjects, however, sudden restoration of the alveolar oxygen tension to normal may cause a transient increase in the severity of the symptoms and signs of hypoxia for 15-60 seconds. This oxygen paradox is usually mild and is manifest only by flushing of the face and hands and perhaps a worsening of performance of complex tasks over the immediate period following restoration of the oxygen supply. . .

The mechanisms responsible for the phenomenon are uncertain.”

It is thought that it is more likely to occur in subjects who have become hypocapnic (ie have lower blood CO2 levels – perhaps due to a degree of hyperventilation which hypoxia can cause). The paradox is accompanied by a periodic drop in arterial blood pressure, but this drop in blood pressure is not necessarily the cause of the oxygen paradox as Punkalouver’s sources suggest. It is thought that this transient drop in blood pressure, accompanied by the constriction of cerebral blood vessels caused by the hypocapnia, restricts cerebral blood flow for a short while even after the restoration of the arterial oxygen levels.

As Punkalouver correctly goes on to say, do not remove the oxygen mask despite any worsening of symptoms, which may even include unconsciousness. This will be temporary and the subject will eventually recover.