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There are several possible causes of a low hemoglobin.
However, for someone that is critically ill, one of the more common causes are because of stress induced ulcers or stomach irritation causing GI bleeding. Some amount of stress induced ulcers or stomach irritation is very common in severely ill patients, occurring in most patients admitted to an intensive care unit, although having such severe disease that is caused significant GI bleeding only occurs in a fraction of these patients.
There are other possible causes. For example, it is also possible that whatever caused the respiratory failure could also have caused some hemolysis (break down of red blood cells). However, statistically, it is more likely that it was due to GI bleeding from stress induced ulcers or stomach irritation.
In this situation, it is appropriate to replace the red blood cells by transfusion and also to use medicines to inhibit stomach acid to promote healing of the stomach.
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You are very welcome.
I hope that he gets better soon.
Please let me know if I can provide any further assistance.