If I spill some coffee from my cup while rushing to a Code blue event, does the concentration of caffeine in the rest of my coffee change? If you think “Of course not”, you are right ! Concentration stays the same – it’s the volume that has reduced !
So why does a patient’s hemoglobin concentration drop when they bleed?
Our body is super-engineered to maintain adequate blood volume to maintain perfusion pressure to vital organs. Any acute bleed triggers a volume-expanding response including ADH-Renin-Aldosterone secretion to retain more fluid from renal excretion besides vascular redistribution. In other words, our Extracellullar fluid (ECF) is used to slowly replace lost blood volume which then dilutes the Hemoglobin concentration and drops it . But this process of course takes time…
Two clinically pertinent things to take-home from this :
- Checking Hemoglobin immediately after a bleed might show a minimal to no drop in Hemoglobin concentration, just like the spilled coffee. So if a patient has acute hypotension, do not rule out acute bleed just cause the stat hemoglobin check came back minimally changed. If bleed is clinically suspected (pallor, tachycardia, on anticoagulants), check once again after 2-4 hours
- Same reason why when u trend hemoglobin levels in a patient with a known bleed that was fixed, you may not want to do it too frequently. Every 6-8 hours is usually adequate in this scenario initially
P.S. Coffee is too precious to spill 🙁
