BRUSSELS--Tolerance to anemia differs in critically ill patients, depending on the degree of anemia and the adequacy of the cardiac response. By identifying which patients will have an inadequate response, it may be possible to avoid unnecessary and potentially risky blood transfusions.

A SIMPLE APPROACH

How can the adequacy of cardiac function be assessed in critically ill patients with anemia? A team of researchers in Belgium recently developed a simple algorithm to interpret the relationship between the cardiac index (CI) and the oxygen extraction ratio (O2ER), which they say can be used to identify some patients with an inadequate cardiac response to anemia (Figure 1).[1] "This simple and new approach can help the physician to rapidly assess the adequacy of the CI in the presence of anemia," said Daniel DeBacker, MD, PhD, in an interview with PULMONARY REVIEWS. He added, "The CI/O2ER ratio can easily be calculated at the bedside."

FIGURE 1 INTERPRETING THE ADEQUACY OF CARDIAC RESPONSE IN ANEMIC PATIENTS

*O2ER= (SaO2-SvO2)/SaO2; CI, cardiac index; O2ER, oxygen extraction ratio; SaO2, arterial oxygen saturation; SvO2, venous oxygen saturation. Adapted from Yalavatti et al. Chest. 2000. [1]

Subjects in the prospective observational study included 60 critically ill patients with anemia who were not actively bleeding and had arterial and pulmonary artery catheters in place. Patients who had any signs of compromised cardiac function (eg, congestive heart failure, recent history of significant arrhythmias, and electrocardiographic signs of ischemic heart disease) were assigned to one group; those with no history of cardiac disease were assigned to another group.

LOWER CI/O2ER RATIO SUGGESTS IMPAIRMENT

In all patients, hemodynamic measurements were obtained as soon as anemia was diagnosed. The specific variables measured were pulmonary artery occlusion pressure (PAOP), CI, and O2ER. Anemia was defined as a hemoglobin level below 10 g/dL; the patients' concentrations ranged between 6.5 and 9.9 g/dL.

The researchers found that patients with compromised cardiac function had a lower CI, a higher O2ER, and a lower CI/O2ER ratio than did patients with normal cardiac function: More than half (27 of 40) of the patients with a cardiac history had a CI/O2ER below 10. "This finding suggests that changes in O2ER were more important than changes in CI in determining oxygen consumption," said Dr. DeBacker, an Assistant Professor of Medicine in the Department of Intensive Care at Erasme University Hospital, Free University of Brussels, in Belgium.

In contrast, most of the patients (16 of 20) with normal cardiac function had a CI/ O2ER ratio greater than 10, suggesting an adequate cardiac response.

In the four patients who had a CI/O2ER ratio less than 10, inadequate cardiac response to anemia was attributed to either hypovolemia (in three cases) or to altered myocardial function secondary to sepsis (one case).

Among the 13 patients with a cardiac history who had a CI/O2ER ratio greater than 10, all but one had evidence of sepsis. Sepsis was also present in 12 of the 16 patients with normal cardiac function and a CI/O2ER ratio greater than 10.

FURTHER STUDY NEEDED

Pulmonary artery occlusion pressure did not differ significantly between the two groups. However, the authors cautioned that a normal PAOP may not rule out hypovolemia.

"A CI/O2ER ratio below 10 suggests an impaired cardiac response to anemia," said Dr. DeBacker. In this setting, additional studies are necessary to determine whether the inadequate response results from hypovolemia or altered myocardial function. For example, he said, an anemic patient with a low CI/O2ER ratio could be given a fluid challenge; if the result excluded underlying hypovolemia, it might be appropriate to consider transfusion more rapidly. He also noted that in the presence of anemia, CI values may be elevated even when cardiac response is inadequate; hence, sampling of mixed-venous blood is important for assessing the adequacy of response.

--Deborah L. O'Connor

Reference
1. Yalavatti GS, DeBacker D, Vincent J-L. Assessment of cardiac index in anemic patients. Chest. 2000;118:782-787.

Return to table of contents