Often we get things mixed up a bit saying or documenting one thing but meaning another. In this KYJ ( #knowingyourjargon )we look at two old favourites.
Cells need oxygen to make energy (ATP) from Glucose. It is called aerobic metabolism; but when cells find themselves with less oxygen than they need, the cell is said to be hypoxic.
Hypoxia therefore means- reduced (low) oxygen. Ischaemia is slightly different, and implies causation. Ischaemia of tissue is Hypoxia but caused through a narrowing (constricted or spasming) blood vessel leading to said tissue, or a physical blockage in that vessel (clot/thrombus, air/fat/amnionic fluid bubble) Perfusion means blood flow.
So pulling this together, tissue hypoxia, is also called tissue ischaemia but only when the problem is due to poor perfusion.
Enter another important cause of hypoxia; that is, lack of oxygen being carried in the blood. This is called Hypoxaemia. It can occur for two main reasons, but when blood oxygen concentrations are low, hypoxaemia, then delivery of oxygen to cells is impaired causing hypoxia.
Technically hypoxaemia is oxygen pressures in arterial blood below 80mmHg breathing air, OR, oxygen saturations less than 94%.
Causes
Diffusion v anaemia v noxious atmosphere.
Respiratory diseases may reduce the ability of oxygen to diffuse into blood when there is fluid, inflammation or thickening(fibrosis) to the lungs, or if air flow is impeded. Blood oxygen levels may also be low if the blood has less than the adequate numbers of Red Blood cells (bleeding) or less haemoglobin than optimum (anaemia).
Finally, where a person is not breathing in adequate oxygen, or where there exists poisonous gases (carbon monoxide, cyanide, smoke, my brother after cabbage). In these situations, oxygen in our air may be in such low quantity (norm 21%), or unable to bind and be carried in blood, then hypoxaemia is diagnosed.
The take home here is Hypoxaemic blood always causes hypoxia, but hypoxia doesn't always mean that blood is hypoxaemic.
The classic example of this is seen in ischaemia like Strokes and MI (heart attacks). Blood oxygen levels are fine, but cells are hypoxic.
Phew!! Big topic.
It's more interesting with pics, diagrams, and my white board images, so why not jump to our page and look at one of our Acute Deteriorating Patient seminars coming up, or our newest course. Basic Nursing Assessment skills (Resp/cardiac/neuro) in December. Last seminar for the year.
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