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Overview :
Bourke and Smith developed 2 equations to estimate the
allowable blood loss for a patient who is bleeding and who is to be replaced
only with colloid solutions. This can help identify patients who might not need
blood transfusion, as well as to decide when to start using blood products. This
allows for better use of blood inventory.
allowable blood loss in mL =
= (blood volume in mL) * LN ((initial hematocrit) / (final
hematocrit)) =
= (blood volume in mL) * ((LN(initial hematrocrit)) - (LN(final hematrocrit)))
where:
• Hematocrits may be either as percents (from 0 to 100) or as
decimal fractions (0 to 1)
• The final hematocrit is the lowest allowable hematocrit for
the patient.
Since they felt that natural logarithm tables might not be
available to clinicians, they developed a second equation that they thought
would be easier to calculate.
allowable blood loss in mL =
= (blood volume in mL) * ((initial hematocrit) - (final
hematocrit)) * (3 – (average of the 2 hematocrits))
average of the 2 hematocrits =
= ((initial hematocrit) – (final hematocrit)) / 2
where:
• Hematocrits are in decimal fractions
NOTE: Chapter 2 gives several ways of estimating blood volume
from anthropometric data. The simple method based on body weight was used in the
spreadsheet. This method seems to slightly overestimate the blood volume as
compared to other equations.
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