POST-TRANSFUSION HEMOGLOBIN LEVELS
A number of recent blood utilization studies suggest that post-transfusion hemoglobin target levels do not need to be as high as previously thought.
For many years, concern about transfusion transmitted infections and transfusion reactions provided the main impetus for limiting transfusion usage. Interestingly, the new data demonstrate acute outcomes improve when patients receive fewer transfusions for reasons unrelated to infectious risks.(1)
Attempts to define lower hemoglobin limits prior to exhausting compensatory mechanism include isovolemic hemodilution experiments and clinical observations. The hemodilution experiments involve healthy subjects undergoing repetitive phlebotomy and intravenous fluid replacement until the hemoglobin concentration approaches 5g/dL. In one report, 3 of 55 healthy adults had ECG ST-segment depression when the heart rate increased in response to acute anemia (hemoglobin concentration less than 6.7g/dL).(2) The ECG changes returned to baseline when the heart rate decreased; none of the subjects had cardiac symptoms. In another experiment, 9 healthy volunteers underwent hemodilution followed by memory and neuro-psychologic testing.(3) No detectable changes occurred with an acute hemoglobin reduction to 7g/dL. Increased reaction times and impaired immediate and delayed memory occurred at hemoglobin levels of 5 to 6 g/dL, changes that reversed following infusion of autologous blood.
Surgical patients refusing transfusion for religious reasons provide further information about the ability to tolerate low hemoglobin levels.(4) In an analysis of 2,083 consecutive patients, 307 had post-operative hemoglobin levels of 8g/dL or less (mean age = 56 years). The 30 day in-hospital mortality was 0% among patients with post-operative hemoglobin levels of 7.1-8.0g/dL, ~9% when the hemoglobin was 5.1-7.0g/dL, and >35% when the hemoglobin concentration dropped below 5g/dL.
In a landmark study, investigators in Canada randomly assigned ICU patients to receive red cell transfusions when hemoglobin levels fell below 7g/dL (hemoglobin concentration maintained 7-9 g/dL) or when the hemoglobin dropped below 10g/dL (hemoglobin concentration maintained Hb 10-12 g/dL).(1) The 30 day mortality was similar in both groups. However, mortality was significantly lower in less severely ill patients transfused at the 7g/dL threshold, than in patients transfused when the hemoglobin dropped below 10g/dL, 8.7%, vs 16.1%, p=0.03. Among patients younger than 55 years, the comparison was 5.7% vs 13.0%, p=0.02. The overall in-hospital mortality rate was lower in the patients transfused at the lower limit, 22.2% vs 28.1%, p=0.05.
In sum, physiologic and clinical studies demonstrate adverse outcomes when the hemoglobin concentration falls below 6g/dL. Clinical data suggest patients transfused to maintain the hemoglobin concentration between 7 and 9g/dL (threshold for transfusion, hemoglobin < 7g/dL) have similar or better outcomes than patients transfused at higher levels. Overall, it appears there is wisdom in restricting transfusion use and simultaneously pursuing careful clinical monitoring to achieve better outcomes.
· Patients transfused to maintain hemoglobin concentration between 7 and 9g/dL have similar or better outcomes than patients transfused at higher levels.
References:
1. Hèbert PC, Wells G, Blajchman MA, et al. A multicenter, randomized, controlled clinical trial of transfusion requirements in critical care. N Engl J Med 1999;340:409-17.
2. Leung JM, Weiskopf RB, Feiner J. Electrocardiographic ST-segment changes during acute, severe isovolemic hemodilution in humans. Anesthesiology 2000;93:1004-10.
3. Weiskopf RB, Kramer JH, Viele M. Acute severe isovolemic anemia impairs cognitive function and memory in humans. Anesthesiology 2000;92:1646-52.
4. Carson JL, Noveck H. Mortality in patients with very low blood counts who decline blood transfusion. Transfusion 2000;40 (Suppl):41S.