HP&L Question

[Flaperon75]

Hi,

I have just sat the HP&L paper this morning and there was a question I was not sure of. It was along these lines:

Pilots should not fly after donating blood because of:

a) can't remember but was obviously wrong
b) low levels of haemoglobin
c) an increased chance of low blood pressure due to low blood volume
d) an open needle woulnd that may bleed during flying

Now I went for c as I was thinking low blood pressure may lead to fainting and I had seen this somewhere before, but I was torn between c and b, because with less blood the haemoglobin level would be lower and therefore more chance of Hypoxia.

I am hoping that someone of a more medical bent could provide some clarification on this. Maybe 'low levels of haemoglobin' is a biologically incorrect statement?

Cheers

[paco]

A donation may lead to a reduced tolerance of altitude, making you more susceptible to hypoxia, in which case b) might be a more direct choice - the low blood pressure bit kinda reminds me of Tony Hancock's empty arm!

Phil

[littco]

Tricky one and may be worth an appeal. However, I think it would be answer B. Less blood = less haemoglobin = less oxygen = increased susceptibility to hypoxia . You can have low blood pressure and normal haemoglobin levels.

[Grasscarp]

The correct answer is the low levels of haemoglobin. For your blood to regain the same level of platelets/haemoglobin etc after donating blood takes about 3 months. As a pilot you need as much as possible for oxygen carriage, and for this reason pilots are usually discouraged from blood donation.

[Flaperon75]

b*g**r!!

Is the low blood pressure optin not a valid one though?? For increased risk of fainting etc? Think it may be contestable?

[Grasscarp]

No, it isnt a valid one I'm afraid. I use to teach HPL for many years and can be fairly certain that it would be a waste of time to contest, as there is no real risk of fainting after your cup of tea etc. Even if it is possible to argue that you might, the much bigger risk is to your ability to carry oxygen around your system and in all these exams only the most correct answer is right. Hope you got enough others correct that this one wont affect a good result overall.

[Flaperon75]

ok - well thanks for clearing it up......:bored:

[BHenderson]

I'm guessing but will your system not restore blood pressure by increasing the water content, so you'll just have the blood diluted?

Bobby

[Windy Pants]

Well let's just throw a spanner in the works...

After blood donation people can suffer from what is termed hypovolemia (also hypovolaemia), which is a state of decreased blood volume; more specifically, decrease in volume of blood plasma. When arterial pressure and blood flow decrease beyond a certain point, the functions of the brain becomes critically decreased (i.e., the blood supply is not sufficient), causing lightheadedness, dizziness, weakness and fainting.

If you think about it logically the amount of O2 carrying ability has not been decreased in the red blood cells (on a per cell basis) they still have the same ability to carry oxygen, this has not been diminished by a blood donation. The percentage of red blood cells is still the same per unit volume of plasma, the donation doesn't selectively only extract the red blood cells! Therefore the percentge of red blood cells per unit volume of plamsa remains unchanged, what has chnaged though is the volume of blood in the circulatory system and hence the pressure also.

If they are claiming that answer "b" is the right answer than this could only be true for anemic hypoxia; now in relation to blood donation this doesn't ring true either as anemic hypoxia (a result of being anemic) can be caused by excessive blood loss; now seeing that the average blood donation is 450ml (around 8-10%) of your total blood volume and most people can easily donate 10-15% without any side effects, I would hardly call a donation "excessive" blood loss! To add to this if the suggestion is that the reduced red blood cell count due to the donation would lead to anemic hypoxia then we would have to ground pilots for 3-5 weeks as that is what it takes for the bone marrow to replace the lost red blood cells, whereas it only takes 24hrs for plasma volumes to return to normal and hence re-establish previous blood pressure levels.

So in summary I would go with reduced blood volume, leading to lower blood pressure, as the answer over a reduced level of haemoglobin.

[Grasscarp]

Windy Pants made me think again and go back to the CAA guidelines. The guidance is that there is no flying for 24 hours due to slight risk of fainting. The reason that pilots are discouraged from donating blood is that they will have for around 3 months, less red blood cells overall. I have sent Flaperon a message as he may not look back at this thread. Sorry if I misled anybody first time round, as I didnt RTFQ.

[Flaperon75]

Ahaaa!! I am a genius after all! :} Thanks for the update grasscarp!