General principles of ultrafiltration on CPB
Define transmembrane pressure (TMP)
Co-efficient of ultrafiltration
Factors affecting ultrafiltration
rate
• An effective method for blood conservation
through the preservation of platelets and coagulation factors
• Selective separation of plasma water and low molecular weight
solutes from the intravascular cellular components and plasma proteins of blood
using a semipermeable membrane filter
• Driving force for ultrafiltration is provided by the pressure
differential occurring across the ultra filtration membrane
• Primary factor determining filtration rate
• TMP = PA
+ PV + PS
2
• TMP: transmembrane pressure gradient
PA: arterial (or inlet) blood pressure into
ultrafilter [mmHg]
PV: venous (or outlet) blood pressure from
ultrafilter [mmHg]
PS: negative pressure applied to effluent
side of ultrafilter [mmHg]
• The efficacy of a particular ultrafiltration device in producing
ultrafiltrate is expressed as a ultrafltration coefficient
• QF = UC x [TMP — IP]
• UC: ultrafiltration coefficient
QF: efficiency of the ultrafilter's ability
to remove fluid
IP: Protein oncotic pressure in blood
TMP:
transmembrane pressure gradient
• TMP = PA + PV + PS QF = UC x [TMP — IP]
2
1) TMP (Transmembrane pressure gradient)
• Usual range 100 - 500 mmHg
• Increased TMP associated with increased ultrafiltration rate
• Application of a negative pressure on the effluent side of
the membrane or the use of increased
perfusion pressure (eg by partially clamping
venous outflow) applied to the blood side of the membrane results in improved solute and fluid
filtration
2) Blood Flow
• Higher blood flows are associated with higher arterial and
venous pressures —> increased
TMP —> increased filtration rate
• Also, see an improvement in filtration at higher flow rates
due to the more rapid removal of the
accumulated proteins on the membrane surface
that obstruct blood flow through the pores
3) Blood Temperature
• Cooling of blood strikingly increases
viscosity
• Increased viscosity increases resistance to blood flow
through ultrafilter
4) Shear rate
• At equal Hb levels, the resistance to blood flow through a
filter is disproportionately greater when low flow is used as opposed to high flow
• Due to the fact that the viscosity of blood under high shear
circumstances (high flows) will be lower than in low flow conditions
5) Hct
• Higher Hct is associated with higher blood
viscosity
6) Plasma proteins
• The higher the serum protein concentration, the higher the
protein oncotic pressure in blood and the slower the filtration rate
• Also serum proteins accumulate on the membrane surface and
occlude pores
7) Pore size, number
(surface area of membrane) & pore length
• Determines ultrafiltration coefficient
• Pore size normally ranges between 10 - 35 angstroms —
allowing molecules of up to 25,000 daltons to pass (eg
electrolytes, creatinine & glucose)
• Note that heparin (6000-25000 daltons) is removed during ultrafiltration
and should be monitored more vigorously, however, ultrafiltration may have
little effect on anticoagulation as the heparin plasma concentration remains
the same