What is the minimum acceptable hemoglobin concen-tration (transfusion trigger)?
If blood loss continues during surgery, even if intravas-cular volume is maintained, oxygen-carrying capacity will eventually fall too low to meet metabolic demands, and red cell transfusion will be required. The minimum safe level of Hb, or transfusion trigger, is a question on which much attention has been focused. Awareness of acquired immuno-deficiency syndrome (AIDS) and other transfusion-related diseases has led to the desire to withhold blood transfusion until absolutely necessary.
From animal models, and from experience with other-wise healthy Jehovah’s Witnesses, it is known that survival is possible down to a Hct of 5–6% (Hb 2 g/dL) if normo-volemia is maintained. Experience with other chronically anemic patients, such as renal failure patients, has shown that Hcts in the low 20s are routinely tolerated. From these data, it is apparent that previously recommended transfusion triggers of Hb 10 g/dL and a Hct of 30% are unnecessarily restrictive.
A theoretical model has been developed to determine the critical Hct, below which oxygen delivery is inadequate to meet metabolic needs. In conditions of normal systemic oxygen consumption in an otherwise healthy patient, the critical Hct is 14% (Hb 4.7 g/dL). Increasing systemic oxygen consumption by a factor of 3, which is typical for the postsurgical patient, increases the critical Hct to 21%. Based on such data, as well as clinical studies, the current US National Institutes of Health recommended trigger for transfusion is a Hb of 7 g/dL (Hct approximately 21%). Many clinicians will accept Hcts in the low 20s in otherwise healthy patients.
The transfusion trigger may differ in patients with cardiac disease. Maximal stress on oxygen delivery occurs in the heart, where 70% of available oxygen is normally extracted, as opposed to 25% for the body as a whole. If CaO2 drops, the reserve for increased extraction is low. The only available compensatory mechanism is to increase coronary blood flow.
In patients with coronary artery disease, ability to increase coronary blood flow may be compromised, and the critical Hct level may be much higher. Therefore, patients with coronary artery disease should probably receive blood to maintain the Hct at approximately 30%. Similarly, patients with significant valvular heart disease or poor ventricular function, as well as those in whom CaO2 is limited by pulmonary disease or who are in hypermeta-bolic states with large oxygen extractions, should have high transfusion triggers.
In summary, although it may not be possible to deter-mine with certainty the minimum safe Hb level for a given patient, there are guidelines on which to base transfusion therapy. Healthy patients seem to tolerate Hcts in the low 20s. Patients with cardiopulmonary disease may require Hcts of 30%. Other criteria, such as overall medical condi-tion or likelihood of continued blood loss, may be used to modify the transfusion trigger. Transfusion of red blood cells should be undertaken only to increase the oxygen-carrying capacity and never for volume expansion alone. As with other medical procedures, the benefits of transfu-sion should always be weighed against the risks.