How to predict Hematocrit before starting Cardiopulmonary bypass (CPB)

You should know patient's blood volume:

VpB = 1000 *f*wt

where f is the proportion of body weight attributable to
blood volume; f = 0.08 for infants and children up to 12
years of age, f = 0.065 for older patients.

For example VpB = 1000 *0.065*80 = 5200 ml.
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Than you shoud calculate the volume of patient's red cells:

VpRBC = VpB * HCTp (patient's hematocrit)

so :

VpRBC = 5200 ml * 0.35 = 1820 ml
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And finally calculation of hematocrit at the beginning of CPB:

HCTpm = VpRBC / (VpB + VmB)

where VmB is priming volume of CPB machine:

HCT pm = 1820 / (5200 + 1300) = 0.28