A wee #FOAMEd thought for today: Hb is utterly useless in acute bleeding.

Follow me for a thought experiement. I'm not going to actually do this, it would be difficult to get past ethics, and a touch messy.

The next person who tells me "they can't be bleeding, the Hb is fine", I am, in the interests of science, going to hang them from the ceiling and cut their throat.

When I do, I'm going to, in the interests of science, measure the Hb of the first jet of arterial spray. And, as they exsanguinate and have gone pale and limp, I will measure the Hb of the last drop. Anyone care to guess what the difference will be?

That's right, naff all. So this person has given their life for science, and I will be on a murder charge. I'm told a normal Hb is ok often enough for me to consider it worth the sacrifice.

Hb concentration is falsely reassuring in acute bleeding. A fall in Hb may be useful information, but not always. It can take hours to days for Hb to fall. It only occurs fast if you're using salty water for resuscitation.

If you want to know how your resus is going, look at skin colour, peripheral temperature and urine output. If you have to measure something, use a venous gas, and look at the BE and lactate.

But please don't tell me a normal Hb means everything is ok. I'll take that as you volunteering for my experiment.