Active blood warming is a recent
practice and arises out of conflicting needs. On the one hand, the safety and
preservation of blood require refrigerated storage and delivery up to the
moment of transfusion. On the other hand, modern methods of very rapid
transfusion in resuscitation would cause clinically dangerous hypothermia if
unmodified, ice-cold blood were to be so transfused. These needs must be
reconciled in the interest of adequate patient care--hence the need for blood
warming. Nevertheless, blood warming creates risks of its own and should not be
used without justifying clinical indications. Within limits that extend
somewhat above normal body temperature, the application of heat does no harm to
stored RBC, a fact that is not reflected in current standards for blood
warmers. Bearing in mind the human tendency to "stretch" standards
and the fallibility of mechanical devices, caution is always wise. But perhaps
the time has come for reconsideration of the present upper limit of 38 degrees
C. Blood warming is seldom necessary or desirable for elective transfusions at
conventional rates, even for patients with cold autoagglutinins.
Hypothermia is a
frequent occurrence after trauma. In the natural history of the prehospital and
hospital course, especially the initial hours, patients can experience a large
heat loss. Administration of cold
intravenous fluids and blood can produce substantial hypothermia, although the
net effect of infusing cold solutions into the body depends on many factors
such as tissue blood flow, rate of body heat generation, rate of heat loss to
the outside environment, and
temperature gradients within the body. A
variety of commercial devices are available for warming intravenous fluids and
blood. Many of these fluid warming devices do not deliver fluids at
normothermia over a wide range of flows because of inefficient heat transfer of
the warmer and heat loss along the length of the administration tubing after
the fluid exits the heat exchanger.
Hypothermia occurs frequently in trauma
patients because of environmental exposure, infusion of cold fluids and blood,
opening of body cavities, decreased heat production, and impaired
thermoregulatory control. Although hypothermia decreases metabolic function of
the body and is neuroprotective, hypothermia is deleterious in traumatized
patients because of coagulopathy,
metabolic acidosis, and impaired immune response. Injured patients with hypothermia
are more likely to die than normothermic patients with a similar injury
severity score.
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