Hgb vs Hct

[amberrn]

Do you use hgb or hct when figuring your ABL? I have been told the hgb will be approximately 1/3 of the hct so it really doesn't matter. My brain is programmed to think about hgb- 10 for a fragile elderly person with history of heart problems, 8 for mechanically ventilated pts, ect. I know the answer begins with the basic thought about what do the hemoglobin and hematocrit really measure and I know the crit is a measurement of volume of RBC in a given amount of blood. What I don't remember is how do we measure hgb? What are the factors or disease states that would make one more reliable than the other? Or does it really not matter? I'll admit it I haven't really searched for an answer- I should be working on careplans but it was just a little question that popped into my head and I thought I really should know the rationale to this.

[athomas91]

hct is a % and therefore swayed by fluid status -

[brownie1129]

hct is a % and therefore swayed by fluid status -

If a patient recieves lots of fluids and then a low Hbg is resulted without clinical s/s of bleeding it is often blamed on hemodilution...How does this work? It seems like hemodilution should effect Hct and not Hbg.

Any ideas?

[charles-thor]

Hemodilution affects both hgb and hct. Imagine a set volume (the patient's blood volume) in which hgb is in solution - say a blood volume of 50 dl and a hgb concentration of 14 g/dl. If you add volume to the solution (giving crystalloid), but the amount of red cells is unchanged, then both the hgb and hct will decrease.

In the case of surgical blood loss, you're giving fluid to maintain volume status, however this is doing nothing to preserve oxygen content. In essence, you can attempt to maintain the patient's volume status roughly similar to how they started out, but you've now diluted them with additional crystalloid. Transfusion is then considered when the patient's risk for ischemia outweighs risks of transfusion. I hope this helps.

[sproutsfriend]

In the case of surgical blood loss, you're giving fluid to maintain volume status, however this is doing nothing to preserve oxygen content.

I agree, its the oxygen carrying capacity of the intravascular volume that we need to worry about. If we dilute the grams of hgb per deciliter down too much with excessive fluid administration our oxygen delivery will be insufficient to meet oxygen consumption by the tissues. The critical hgb where compensatory mechanisms are exhausted and metabolism switches from aerobic to anaerobic is typically between 3-4.5 g/dl This makes sense mathmatically because if you calculate the oxygen content based on a hgb of 3 g/dl and then multiply it by your cardiac output your delivery will be usually under typical oxygen consumption of 3 ml of oxygen per kg of body weight per minute and ischemic tissue will result.