Anemia and Erythrocyte Transfusions
I. Anemia in the Icu
A. Definition
Anemia is defined as a decrease in the oxygen carrying capacity of blood. The most accurate measure of this is the red cell mass, which is not easily obtained. As a result, the hemoglobin (Hb) and hematocrit (Hct) are used as surrogate measures of the O2 carrying capacity of blood. (The reference ranges for Hb, Hct, and other erythrocyte-related measurements are shown in Table 11.1.)
There is one problem with Hb and Hct as measures of O2 carrying capacity; i.e., they are influenced by the plasma volume. For example, an increase in plasma volume will decrease the Hb and Hct (dilution effect), thereby creating the false impression of a drop in the O2 carrying capacity of blood (pseudoanemia). Clinical studies have confirmed that the Hb and Hct are unreliable as markers of anemia in critically ill patients (4,5,6).
Table 11.1 Reference Ranges for Red Cell Parameters in Adults | ||||||||||
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B. Contributing Factors
ICU-associated anemia is attributed to two conditions: systemic inflammation, and repeated phlebotomy for laboratory studies.
1. Inflammation
Inflammation is responsible for the anemia of chronic disease, which is now called the anemia of inflammation (3,4,5,6).
The hematologic effects of inflammation include inhibition of erythropoietin release from the kidneys, reduced marrow responsiveness to erythropoietin, iron sequestration in macrophages, and increased destruction of RBCs (3,7).
The resulting anemia is hypochromic and microcytic, with a low plasma iron level. Inflammatory anemia can be confused with iron-deficiency anemia. How-ever, the plasma ferritin level (a marker of tissue iron stores) is increased in inflammatory anemia, and decreased in iron-deficiency anemia.
2. Phlebotomy
The volume of blood withdrawn for laboratory tests averages 40–70 mL daily in ICU patients (8), and the cumulative loss of blood in one week can reach 500 mL (one unit of whole blood).
A significant percentage of blood loss for laboratory testing is related to technique; i.e., when a blood sample is obtained from a vascular catheter, an initial aliquot of blood is withdrawn first and discarded, to eliminate interference from intravenous fluids in the catheter. The discarded volume is about 5 mL per blood draw, and returning this blood to the patient can reduce daily phlebotomy loss by 50% (9).
C. Physiological Effects of Anemia
Anemia elicits two responses that help to preserve tissue oxygenation: (a) an increase in cardiac output, and (b) an increase in O2 extraction from capillary blood.
1. Cardiac Output
The influence of anemia on cardiac output is explained by the Hagen-Poiseuille equation (see Equation 7.1 in Chapter 7), which shows that the flow rate of a fluid is inversely related to the viscosity of the fluid. Since Hct is the principal determinant of blood viscosity, a decrease in Hct will decrease blood viscosity, which will result in an increase in blood flow (cardiac output).
 FIGURE 11.1 The influence of progressive isovolemic anemia on measures of systemic oxygenation. DO2 = O2 delivery, VO2 = O2 consumption. Data from Reference 10. |
2. O2 Extraction
As described in Chapter 6, Section I-D, O2 extraction is the ratio of O2 consumption (VO2) to O2 delivery (DO2); i.e.,
Rearranging terms in this relationship yields the following:
This relationship predicts that a decrease in O2 delivery (e.g., from anemia), will not impair aerobic metabolism (VO2) if there is a proportional increase in O2 extraction. This type of response is demonstrated in Figure 11.1 (10); i.e.,
The progressive decrease in Hct is accompanied by a similar decrease in O2 delivery (DO2). However, the decrease in DO2 is initially accompanied by an increase in O2 extraction, and this keeps the O2 consumption (VO2) constant.
When the Hct falls below 10%, the increase in O2 extraction is no longer able to match the decrease in DO2, and the VO2 begins to fall. This is the anaerobic threshold.
Thus, aerobic metabolism is maintained during progressive anemia because of an increase in O2 extraction, and the Hct and Hb must fall to extremely low levels before aerobic metabolism is affected.
3. Tolerance to Anemia
II. Transfusion Triggers
A. Hemoglobin
Surveys indicate that 90% of erythrocyte transfusions in ICU patients are given to alleviate anemia (13), and thus are guided by the Hb concentration in blood.
The first transfusion trigger, which dates back to 1942, was a Hb concentration of 10 g/dL and a corresponding
Hct of 30% (14). This “10/30” rule became the standard for over half a century.
More recent clinical studies have demonstrated that adopting a lower transfusion trigger (i.e., Hb <7 g/dL) has no adverse consequences, and decreases the transfusion burden considerably (13,15).
However, the use of the Hb concentration in blood as a “transfusion trigger” is a flawed practice for two reasons:
The Hb concentration in blood provides no information about the state of tissue oxygenation.
The Hb concentration in blood is influenced by changes in plasma volume, which means that changes in the Hb concentration in blood do not always reflect changes in the O2 carrying capacity of blood.
The most recent guidelines on erythrocyte transfusions in critically ill patients state that the use of the Hb concentration as a “trigger” for transfusions should be avoided (4). Despite this statement, the guidelines recommend that transfusion should be considered when the Hb is <7 g/dL in critically ill patients, and <8 g/dL in acute coronary syndromes (4).
B. Oxygen Extraction
As described earlier (and shown in Figure 11.1), anemia elicits a compensatory increase in O2 extraction from capillary blood, which serves to maintain a constant rate of aerobic metabolism. However, the increase in O2 extraction reaches a maximum at about 50%, whereupon further decreases in Hb are accompanied by proportional decreases in O2 consumption.
Therefore, an O2 extraction of 50% identifies the anaerobic threshold, and can serve as a physiologic transfusion trigger (16,17).Related posts:
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