Page 174 - Biomedical Engineering and Design Handbook Volume 2, Applications

DESIGN OF ARTIFICIAL KIDNEYS 153

Essential nutrients

Purified water

Ion exchanger/ Mixing Heater Deaerator Conductivity pH Temp
filter chamber cell/ probe sensor
concentration Pressure
Heparin monitor Pressure
pump control control
Flow
Pump
Pressure meter
measurement Fluid replacement
in high flux dialysis

Flow meter
Active clamp

Arterial
pressure Roller Dialyzer
measurement pump
Flow meter
Flow meter
Pressure
Pump probe Bypass
Blood leak
To drain detector

FIGURE 5.6 The hemodialysis system for high flux dialysis. Replacement fluid is added to the blood before returning to the
patient’s vein.

access to prevent clotting. Heparin is an anticlotting hormone. The treatment regimes vary from
clinic to clinic and from patient to patient.

5.5 TREATMENT PROTOCOL AND ADEQUACY OF DIALYSIS

Let us consider a simple one-compartmental model for the prescription of treatment protocols for
dialysis using an artificial kidney device (Fig. 5.7). While the blood urea concentration (BUN) in the
normal individual is usually 15 mg% (mg% = milligrams of the substance per 100 mL of blood), the
BUN in uremic patients could reach 50 mg%. The purpose of the dialysis is to bring the BUN level
closer to the normal. During the dialysis, some hormones also diffuse out of the dialyzer membrane
along with the urea molecule. Too rapid dialysis often may lead to depression in the individual due
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to the rapid loss of hormones. On the other hand, too slow dialysis may lead to unreasonable time
required at the hospital. Simple modeling can be used to calculate the treatment protocols. Let us
consider a one-compartmental model of the tissue where we assume that the blood and tissue are
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well mixed, and that the concentration of urea is uniform throughout the body. Let C be the con-
Bi
centration of urea at the inlet of the dialyzer in the arterial line which takes blood into the dialyzer,
that is, at the outlet of the body. Let C be the concentration of urea at the exit of the dialyzer in the
Bo
venous line which brings the blood back to the body, that is, at the inlet of the body compartment.
Mass balance demands that the rate of change of mass in the body be equal to the net rate of mass
coming into the body from the dialyzer, plus the metabolic production rate G:
(VdC/dt) = Q (C – C ) + G = –Q C (1 – C /C ) + G (5.11)
B Bo Bi B Bo Bi