Anemia and blood transfusions

Acute anemia is a common reason for transfusion of blood products. The hemoglobin (Hb) threshold for correcting anemia has been a hot topic of discussions for a long time. Here I would like to share my experience and thoughts about this very surgical issue.

About 15 years ago I listened to a talk that was given by a German gastroenterologist who worked at that time in Malawi, a small country in the southern part of Africa. Among other interesting stories, he said that they did not even think about blood transfusion unless there is the number 4 (four!) before the dot/comma in the Hb g/dl level. This was due to a relative lack of blood products but he said also that they never had any problems related to uncorrected acute anemia.
We should assume though, that most of the patients there are generally in a better health that the average patient in the Western world, mostly because of the short life expectancy and very low rate of chronic diseases like Diabetes or Hypertension. Also, as one of my current mentors noted, the rate of patients with sickle cell anemia might be significantly higher there. These patients tend to tolerate much lower Hb levels.

In Medical School I was trained that Hb < 10 g/dl should be corrected, esp. in patients with cardiac history. Later, during the residency, I was told that tolerating Hb as low as 8 g/dl is acceptable if the patient is otherwise stable and has a healthy heart.

The controversy is very well discussed in Marino's The ICU Book (a real medical "thriller"!). The chapter about blood transfusion is highly recommended for every surgeon. Recently I had a very interesting discussion with two transplant surgeons during a teaching class for students. Here are the interesting papers which we discussed.

The TRICC trial showed that going down to Hb 7 g/dl is generally safe in patients without active coronary ischemic syndromes. Nine months later Bracey et al. published a study proposing a threshold of Hb 8 g/dl in patients undergoing coronary artery bypass procedures. The paper of Bracey is important in terms of transfusions in patients with cardiac problems which is considered always as a special situation requiring more aggressive Hb resuscitation.

Blood transfusions have some downsides which represent additional burdens in indicating them. Acute hemolytic reactions and infections are rare. But allergic reactions could occur in up to 1/200 cases. Blood transfusions lead to increased rate in nosocomial infections and are considered to be immunosuppressive even though the mechanism for this is not completely clear. Amato and Pescatori published 2006 a meta-analysis of the effect of blood transfusion on recurrence of colon cancer and concluded:

All analyses support the hypothesis that perioperative blood transfusions have a detrimental effect on the recurrence of curable colorectal cancers. However, since heterogeneity was detected and conclusions on the effect of surgical technique could not be drawn, a causal relationship cannot still be claimed. Carefully restricted indications for perioperative blood transfusions seems necessary.

Last but definitely not least, Viele and Weiskopf (1995) and later Weiskopf et al. (1998) published results in patients with even lower Hb and found that anemia associated complications occur in Hb < 5 g/dl. Both studies have clearly their downsides. Nevertheless, these are evidences that support the practice of the above mentioned gastroenterologist. In The ICU Book Marino goes even lower - 3 g/dl (see his "Final Word" below) but he supports this with some animal testing that I refused to consider further since I am interested in human and not in veterinary medicine.

I have to admit that my personal feeling goes with the number of 5 g/dl as the lowest tolerable Hb-limit for (esp. oncological) otherwise healthy and normovolemic patients.

As always, the clinical judgment of the current patient's condition considering their history should be leading in making the decision for (or against) blood transfusion. Still, we need evidences to know when less is more and I think that the mentioned publications show where we are today.

Here is Marino's "Final Word" on this topic:

The single most important point to remember from this chapter is that anemia is well tolerated as long as intravascular volume is maintained. When blood volume is normal, hemoglobin levels have to drop to 3 g/dL to demonstrate evidence of impaired tissue oxygenation. What this means to me is that cardiac output is more important for tissue oxygenation than the hemoglobin level in blood. The importance of cardiac output over hemoglobin is evident when you consider that hypovolemic shock and cardiogenic shock are recognized entities, while "anemic shock" is not. Since shock is a condition of impaired tissue oxygenation, the fact that anemia is not a recognized cause of shock is proof that anemia does not impair tissue oxygenation (at least not until the anemia reaches dangerously low levels). If this is the case, then most RBC transfusions to correct anemia will not improve tissue oxygenation (and thus are not warranted).
The second point that deserves emphasis is the nonvalue of the hemoglobin for assessing transfusion needs. A better measurement for evaluating systemic oxygenation is the peripheral O₂ extraction, which can be measured as the (SaO₂ - SvO₂) difference. All you need is an indwelling central venous catheter to measure SvO₂, and a pulse oximeter to measure SaO₂. When the (SaO₂ - SvO₂) approaches 50%, and there is no reason to suspect a low cardiac output (which can also increase peripheral O₂ extraction), then transfusion of RBCs is a reasonable consideration.