concentration of substance remaining in the patient's plasma =
= ((initial concentration of substance in plasma) * (2.71828 ^((-1) * (value) * t)))
where:
• (value) = ((volume of plasma exchanged) / (plasma volume of the patient))
• t is the duration of the exchange
Note:
This needs equation needs to be verified for 4 issues:
(1) the time factor t does not make sense unless the volume exchanged is in mL per minute rather than the actual volume removed. If this is the case, the amount of a substance after an exchange could be calculated by volume removed in mL, dropping the time;
(2) the original source uses "blood volume of the patient" but I think "plasma volume" is more accurate;
(3) the original source used concentration of substance in blood rather than plasma; this would assume whole blood calculations. I elected to use concentration in plasma;
(4) the equation would assume no other elimination except for pheresis, which would not be true for many drugs, etc.
Assumptions:
(1) There is no change in the patient's blood or plasma volume during procedure.
(2) Immediate mixing is achieved.
(3) There is no increase in production of the substance.
(4) There is no mobilization of substance from extravascular into vascular compartment.
(5) There is no accumulation of substance within red cells or transfer from within red cells into plasma.
|
Plasma Volume Exchanged
|
Percent of Substance Removed
|
|
1
|
63.2%
|
|
2
|
86.5%
|
|
3
|
95%
|
