Page 1924 - Hematology_ Basic Principles and Practice ( PDFDrive )

1704 Part XI Transfusion Medicine

concentration of glycerol depends on the rate and the temperature of impact of anemia varies depending on its pathogenesis, rate of onset,
freezing. The freezing process destroys other blood constituents, the presence or absence of accompanying hypovolemia, and, most
except for a small percentage of immunocompetent lymphocytes. importantly, the individual patient. The Hb level at which a given
RBCs are prepared for transfusion by thawing and washing away the individual manifests the signs and symptoms of anemia relates, in
glycerol using a series of progressively less hypertonic crystalloid part, to underlying health status, cardiorespiratory reserve, and tissue
solutions, allowing glycerol to diffuse gradually from the cells to oxygen demand.
prevent hemolysis. The cells are resuspended in an isotonic saline
solution containing glucose. The extensive washing removes approxi-
mately 99.9% of the plasma as well as cellular debris. Perioperative Period
RBCs can be stored in the frozen state up to 10 years with good
viability. After thawing and washing, storage is typically limited to Many generalizations have been made about the appropriate transfu-
24 hours because of the open system. Frozen cells have been shown sion management of acute blood loss, often with little hard data to
to maintain prefreezing adenosine triphosphate (ATP) and support the arguments. One rule of thumb is that blood loss of 10%
2,3-diphosphoglycerate (DPG) levels. To maintain these factors at or less of total blood volume requires no replacement therapy at all;
high levels, the standard is to freeze within 6 days of collection. When loss of up to 20% can be replaced exclusively with crystalloid solu-
it is necessary to freeze older units, rejuvenation with a solution tions; and loss of greater than 25% generally requires RBC transfusion
containing pyruvate, glucose, phosphate, and adenine has provided to restore oxygen-carrying capacity, along with crystalloid and some-
excellent results. The major indication for frozen RBCs is the stock- times colloid solutions to restore intravascular oncotic pressure to
piling of rare donor units for patients who have developed alloanti- achieve adequate perfusion. For years, the threshold of 10 g/dL of
bodies. Some patients with rare phenotypes can make autologous Hb had been used as the gold standard for the RBC transfusion
donations that can be frozen for later use. Cells from autologous trigger during the perioperative period, but 7 g/dL is now more
donors can be frozen if more units are required than can be collected commonly used. Each case must be evaluated individually on the
in the 42-day liquid storage period or if surgery is postponed. Because basis of clinical signs and symptoms, rather than on the basis of labo-
of the high cost and cumbersome nature of freeze-thaw procedures, ratory values. If the cardiovascular system is healthy and the degree
other uses of frozen RBCs are somewhat difficult to justify. of hypoperfusion is not significant, good tissue oxygenation can be
maintained at much lower Hb levels. A National Institutes of Health
APPROPRIATE TRANSFUSION PRACTICE IN VARIOUS consensus conference suggested that many surgical patients do not
need transfusion unless the Hb level falls to less than 7 g/dL. Given
CLINICAL SETTINGS that RBC transfusion should be tailored to individual needs, the
question arises as to whether there is any readily available, objective
The response to RBC transfusion varies from patient to patient. In measurement that can be used to determine how low the Hb level
the absence of increased red cell destruction or sequestration, one can safely be allowed to fall before RBC transfusion is initiated.
unit of RBCs can be expected to increase the Hb level by 1 g/dL or Global hemodynamic parameters do not always correlate with
the hematocrit level by approximately 3%. This rise is usually not microvascular perfusion. Assessment of tissue oxygenation at the
fully realized until approximately 24 hours after transfusion, when microvascular level would help evaluate the effectiveness of a red
the plasma volume has had time to return to normal. On the basis cell transfusion, evaluate the effect of red cell storage on end-organ
of a half-life of approximately 57.7 days for donor red cells, Mollison perfusion, and provide data about when to transfuse. Several general
and associates calculated that an average-sized adult requires 24 mL methods are available to evaluate the microcirculation and include
of RBCs per day to maintain a given hematocrit level, assuming no direct assessment using image techniques and indirect methods of
red cell production. Patients with red cell aplasia require approxi- assessment, such as measures of microvascular oxygen availability and
mately 2 units of RBCs every 2 weeks. function. Direct assessment can be performed using laser Doppler
Several factors can adversely affect the survival of transfused red flowmetry, imaging of the microcirculation, intravital microscopy,
cells. Hemolysis, caused by either immune-mediated red cell damage orthogonal polarization spectral imaging, and sidestream dark-field
or mechanical trauma, shortens the survival of transfused cells, much imaging. Assessments of oxygen availability include oxygen electrodes,
as it shortens the survival of the patient’s own cells. Hypersplenism reflectance spectrophotometry, and near-infrared spectroscopy. The
can lead to initial sequestration as well as increased destruction of red techniques described are currently considered research tools and are
cells. Continued blood loss is another obvious cause of suboptimal not available in routine clinical practice. Many have yet to prove
response to transfusion. It should also be emphasized that transfusion reliable and reproducible in the clinical setting. Further, some are
suppresses erythropoiesis, so that the net result of transfusion may be only useful in specific organ systems and do not reflect the global
less than expected if transfusions are administered on a chronic basis. oxygenation of the patient. To be useful at the bedside, a tech-
nique must be technically simple, rapid and noninvasive without
large interoperator variation. Such a device has yet to become
Chronic Anemia available.
The Society of Thoracic Surgeons and the Society of Cardiovas-
As a rule, signs and symptoms attributable to anemia are unlikely to cular Anesthesiologists published clinical practice guidelines that
develop at a Hb level of greater than 7 or 8 g/dL. When the anemia identified six variables that increased a patient’s risk of postoperative
is of gradual onset, the body’s compensatory mechanisms for main- blood transfusion: advanced age, low preoperative red cell volume,
taining oxygen delivery to the tissues come into play. Both cardiac preoperative antiplatelet or antithrombotic drugs, reoperative or
output and intracellular 2,3-DPG increase, and thus, oxygen unloads complex procedures, emergency operations, and noncardiac patient
at a lower oxygen saturation of Hb. When chronic anemia is due to comorbidities. The report recommended developing institution-
red cell destruction, the healthy bone marrow responds by increasing specific protocols to screen for high risk patients and apply blood
erythropoiesis up to sixfold. conservation interventions, such as erythropoietin or antifibrinolytic
RBC transfusion more commonly provides symptomatic support administration, intraoperative blood salvage or normovolemic hemo-
rather than definitive therapy for anemia. Transfusion should be used dilution, and institution-specific blood transfusion algorithms supple-
only when there is no definitive treatment for the underlying cause, mented with point-of-care testing. In 2015, the American Society of
or when the severity of the anemia and the clinical manifestations in Anesthesiologists published a practice guideline for perioperative
the patient make it impossible to wait for the effects of the treatment blood management that emphasizes the preoperative patient assess-
to be realized. ment and encourages greater utilization of pharmacologic agents and
Generalizations about RBC transfusion indications and practices point of care testing-directed transfusion algorithms to minimize
are difficult to make and are usually inappropriate. The clinical blood transfusion.