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.
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