HB – HOW LOW CAN YOU GO?

Bleeding down in hemoglobin is a kind of limbo dance for the patient involved. How low can you go? Here’s a case report on a trauma patient with a hemoglobin of 0,7 g/dL(!) yielding a hematocrit of 2.2%(!). O neg blood wasn’t available, and for some reason, blood matching wasn’t ready before 10 hours into his surgery before finally getting blood transfusions. Not only did he survive those 10 hours – his arterial blood gas numbers remained largely normal, with a base excess of -4.

This remarkable survival and numbers were achieved with crystalloids and colloids to keep the patient near normovolemic, and oxygen delivery was achieved through 100% O2. The patient was 34 degrees celcius. Anaesthesia helped lower the metabolism and oxygen demand. The body’s physiological responses also helped keeping the patient alive: hemoglobin doesn’t just transport O2, it’s also the most important carrier of CO2, so the low hemoglobin level also led to CO2 accumulation in peripheral tissue and was likely helping with tissue oxygenation by shifting the oxygen disassociation curve. He did also receive some matched plasma to help him coagulate, and a great plasma expander. Still, the main points is keeping intravascular volume adequate and giving 100% oxygen if you don’t have blood available. This patient survived and is doing well. The case report is well worth a read.

So, we don’t need blood transfusions anymore?
Well, obviously, this was a special case. But, yes, often patients can be kept alive at a very low Hb. For hours, maybe days. But eventually, it will kill them. They get organ failure. Of course, it’s hard to make good studies on extreme anaemia, but we do have one group that guinea pigs for this stuff. Jehova’s Witnesses. So some smart guys pooled all Jehova Witness patients in the States over the past years and looked at the ones with a post operative Hb<8 g/dL. post OP. 300 patients in all. How did they do? After trying to level the field by taking age, heart disease and APACHEII score into account, they found this relationship:

Hb 1-2: Mortality 100%
Hb 2-3: Mortality 54%
Hb 3-4: Mortality 27%
Hb 4-5: Mortality 34%
Hb 5-6: Mortality 9%
Hb 7-8: Mortality 0%

It was not a surprising finding, but interesting to get an idea of where the cut-off is. Below 5 g/dL is not where you want to be. And above 7 g/dL is looking really good. This fits pretty well with standard transfusion practice with a cut-off for healthy people of 8 g/dL and patients with heart disease 10 g/dL. Even if these levels are also subject to continuous debate.

Another interesting finding with this study was that the anaemic patients didn’t die straight away. I would have though they would end up with acute organ failure and/or cardiac arrest. But most of them managed to stay alive for days, or even over a week, with extremely low Hb levels. So anaemia isn’t the biggest emergency – hypovolemia is. And coagulopathy might also be a more pressing danger than low hemoglobin. But to secure a good outcome in the long run, it is important to keep hemoglobin levels adequate. So, for massive, uncontrollable bleeding the saying still holds true: If they’re bleeding – give blood.

Case report on the trauma patient with Hb 0.7 g/dL:
Intraoperative management of extreme hemodilution in a patient with a severed axillary artery, Anesth Analg, 2010.

Morbidity and mortality from Jehovas Witness patients with low Hb:
Mortality and morbidity in patients with very low postoperative Hb levels who decline blood transfusion, Transfusion, 2002.

Time course and etiology of death in patients with severe anemia, Transfusion, 2009.