Principles of Dialysis Content
The dialysis process Physical principles of dialysis Diffusion Ultrafiltration. The dialysis process The concept of dialysis, that is partition (separation) of substance between two solutions by use of semipermeable membrane, is quite simple. Extracorporeal dialysis employs the artificial kidney (dialyzer) as semipermeable membrane, while Intracorporeal dialysis employs the peritoneal membrane.
Physical principles of dialysis Diffusion, Ultrafiltration and Osmosis are the basic physical principles of dialysis. Diffusion is the net directional movement of molecules occurring from a solution of higher concentration to a solution of low concentration. Ultrafiltrationis the movement of solvent across a semipermeable membrane in response to a pressure difference applied across the membrane.If the solutes dissolved in the solvent is small enough to permeate the membrane, they are dragged along with the solvent and cross over to the other side, and this called Convection. Osmosis the movement of the solvent (e.g. water) from the side of low concentration to the side of higher concentration.
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Diffusion If two solution of different composition are placed on different side of a semipermeable membrane, solutes will move from the solution with the highest concentration to the solution with the lowest concentration.The rate of movement will depend on:-
B. the Gibbs-Donnan effect: Blood proteins are dialysis membrane impermeable, negatively charged and tend to accumulate at the membrane surface during dialysis. Coresponding numbers of membrane permeable cations such as sodium, calcium, magnesium must then retain in the blood to preserve elecroneutrality. This results in imbalance in the concentration of ions across the dialysis membrane. The protein-induced ion transport asymmetry is called the “Gibbs-Donnan effect”. It indirectly affects the magnitude of the concentration gradients required to drive diffusion across dialysis membrane.
As solute movement continues over a peroid of time, the concentration falls in the solution of higher concentration, rises in the solution with the lower concentration, and the two solutions approach each other in composition i.e Equilibrium. As a result of this dissipation of the concentration gradient, the transfer of solute slows with the time.The maximum rate of solute transfer occurs initially when the concentration gradient is greatest.
Schematic diagram of diffusion
No hydrostatic pressure is applied
[Van Stone et al 1994, Physiology of peritoneal dialysis in handbook of dialysis 2nd edition]
The dissipation of the concentration gradient can be minimized and the transfer of solute optimized by increasing the volume of the fluids.
a) In a static system (comparable to peritoneal dialysis) this is accomplished by replacing the recipient fluid with fresh solution at periodic intervals.
b) In a flowing system (comparable to hemodialysis) this accomplished by increasing the flow rate of parent fluid (blood) or recipient fluid (dialysate).
Both artificial and natural membranes are more permeable to small solutes than large solutes. Thus, dialysis is most effective in removing small solutes and less effective in removing larger solutes, particularly those over 1000 daltons.The surface area of the membrane available for diffusion affects the amount of solute transferred.
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Ultrafiltration The process of water removal from the blood stream is called ultrafiltration; the fluid removed is the ultrafitrate. The UF during dialysis is performed for the purpose of removing water accumulated by ingestion of fluid or by metabolism of food during the interdialytic period. It is essential to prescribe and control the fluid removal rate so that total fluid removed during dialysis will be equal to the total fluid gained since the previous dialysis or from the dry weight.
Schematic diagram of osmotic ultrafiltration
No hydrostatic pressure is applied. Triangles represent osmotic agent introduced to right compartment. This causes an early water shift to the right (ultrafitration), but this is later reversed if the osmotic agent is also small enough to diffuse along the concentration gradient from right to left. Thus only solutes of such size that they do not easily permeate the semipermeable membrane are capable of exerting a sustained osmotic force. [Sorkin MI et al 1994, Physiology of peritoneal dialysis in handbook of dialysis 2nd edition]
Schematic diagram of hydrostatic ultrafiltration
The application of external pressure forces movement of water from left to right. Low molecular weight constituents will be swept through the membrane with this water (solvent drag.). In dialysis setting the pressure gradient is generated by manipulation if dialysis fluid parameters such as pressure volume or flow.
[Von Stone MI et al 1994, Physiology of peritoneal dialysis in handbook of dialysis 2nd edition]
Mechanism of Ultrafiltration
A) In Hemodialysis:
1-Hydrostatic pressure
The primary driving force for ultrafiltration is the hydrostatic pressure difference across the membrane, which is the Transmembrane Pressure (TMP), expressed in millimeters mercury (mmHg). The TMP is determined by the average or mean pressure in the blood compartment minus the mean dialysate compartment pressure.The relationship of ultrafiltrate to TMP is entirely dependent on the membrane (Dialyzer) properties. The permeability of dialyzer membranes to water is high, varies consonsiderably, and is a function of membrane thickness and pore size.The total capacity of the dialyzer for ultrafiltration is given by the Ultrafiltration Coefficient (KUF).
The KUF is defined as the number of milliliters of fluid per hour that will be transferred across the membrane per mmHg pressure gradient across the membrane The KUF of most dialyzer ranges from 2 to 6 ml/hour. The relationship between ultrafiltration, KUF and TMP is expressed as:
Ultrafiltration rate (ml/hr) = KUF X TMP
How do you calculate UF rate from KUF?
If one needs to remove 2 kg during a period of 4 hours
1- Add the volume of saline that will be given at the end of dialysis (usually 300 ml) and the amount of ingested fluid during dialysis (e.g., 100ml).
2- This means that 2.4 L will have to be removed during 4 hours dialysis i.e.2.4 X 1000 /4 = 600 ml/hour.
3- When using a dialyzer with KUF value of 4.0 ml/hour, the TMP will need to be set at 600/4 =150 mm Hg. N.B. (If the dialyzer KUF is 6 the TMP should be = 100 mm Hg).
2-Osmotic Ultrafiltration:
Osmotic ultrafiltration does play an indirect role in total ultrafiltration; water shifts from intracellular to the extracellular compartment which occur during hemodialysis (so-called plasma refilling) can be optimized by introduction of an effective concentration of an osmotic agent into the extracellular space. Sodium is employed in some dialysis practice especially during sodium profiling
B) In peritoneal dialysis:
1-Osmotic Ultrafiltration
The primary driving force for ultrafiltraion in peritoneal dialysis is the Osmotic gradient (Osmotic Ultrafiltration). Osmosis isthe movement of the solvent (eg water) from the side of low concentration to the side of higher concentration through a semipermeable membrane.The result of this movement of water will be equalization of total solute concentration on both sides of the membrane.
Osmotic ultrafiltraion during peritoneal dialysis is achieved by adding large amount of glucose to dialysis solution. Peritoneal dialysis solution ordinarily contain 1.36 % (1.5), 2.27 % (2.5), 3.86 % (4.25). The osmotic pressure generated by glucose will draw water from the blood and tissues into the dialysate.
2-Hydrostatic pressure
The hydrostatic ultrafiltraion effect is of minor importance
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Physical principles of dialysis Diffusion, Ultrafiltration and Osmosis are the basic physical principles of dialysis. Diffusion is the net directional movement of molecules occurring from a solution of higher concentration to a solution of low concentration. Ultrafiltrationis the movement of solvent across a semipermeable membrane in response to a pressure difference applied across the membrane.If the solutes dissolved in the solvent is small enough to permeate the membrane, they are dragged along with the solvent and cross over to the other side, and this called Convection. Osmosis the movement of the solvent (e.g. water) from the side of low concentration to the side of higher concentration.
Top
Diffusion If two solution of different composition are placed on different side of a semipermeable membrane, solutes will move from the solution with the highest concentration to the solution with the lowest concentration.The rate of movement will depend on:-
- The concentration gradient for the solute between the two solutions
- Permeability of the membrane to the solute
- Surface area of the membrane.
- The size of the solute is highly correlated with its molecular weight. The heavier, larger solute moves more slowly along the concentration gradient than smaller lighter solutes. Thus, dialysis is most effective in removing small solutes and less effective in removing larger solutes, particularly those over 1000 Dalton.
- Blood protein affects the diffusive transport of solute across the membrane by two different mechanisms:
B. the Gibbs-Donnan effect: Blood proteins are dialysis membrane impermeable, negatively charged and tend to accumulate at the membrane surface during dialysis. Coresponding numbers of membrane permeable cations such as sodium, calcium, magnesium must then retain in the blood to preserve elecroneutrality. This results in imbalance in the concentration of ions across the dialysis membrane. The protein-induced ion transport asymmetry is called the “Gibbs-Donnan effect”. It indirectly affects the magnitude of the concentration gradients required to drive diffusion across dialysis membrane.
As solute movement continues over a peroid of time, the concentration falls in the solution of higher concentration, rises in the solution with the lower concentration, and the two solutions approach each other in composition i.e Equilibrium. As a result of this dissipation of the concentration gradient, the transfer of solute slows with the time.The maximum rate of solute transfer occurs initially when the concentration gradient is greatest.
Schematic diagram of diffusion
No hydrostatic pressure is applied
[Van Stone et al 1994, Physiology of peritoneal dialysis in handbook of dialysis 2nd edition]
The dissipation of the concentration gradient can be minimized and the transfer of solute optimized by increasing the volume of the fluids.
a) In a static system (comparable to peritoneal dialysis) this is accomplished by replacing the recipient fluid with fresh solution at periodic intervals.
b) In a flowing system (comparable to hemodialysis) this accomplished by increasing the flow rate of parent fluid (blood) or recipient fluid (dialysate).
Both artificial and natural membranes are more permeable to small solutes than large solutes. Thus, dialysis is most effective in removing small solutes and less effective in removing larger solutes, particularly those over 1000 daltons.The surface area of the membrane available for diffusion affects the amount of solute transferred.
Top
Ultrafiltration The process of water removal from the blood stream is called ultrafiltration; the fluid removed is the ultrafitrate. The UF during dialysis is performed for the purpose of removing water accumulated by ingestion of fluid or by metabolism of food during the interdialytic period. It is essential to prescribe and control the fluid removal rate so that total fluid removed during dialysis will be equal to the total fluid gained since the previous dialysis or from the dry weight.
Schematic diagram of osmotic ultrafiltration
No hydrostatic pressure is applied. Triangles represent osmotic agent introduced to right compartment. This causes an early water shift to the right (ultrafitration), but this is later reversed if the osmotic agent is also small enough to diffuse along the concentration gradient from right to left. Thus only solutes of such size that they do not easily permeate the semipermeable membrane are capable of exerting a sustained osmotic force. [Sorkin MI et al 1994, Physiology of peritoneal dialysis in handbook of dialysis 2nd edition]
Schematic diagram of hydrostatic ultrafiltration
The application of external pressure forces movement of water from left to right. Low molecular weight constituents will be swept through the membrane with this water (solvent drag.). In dialysis setting the pressure gradient is generated by manipulation if dialysis fluid parameters such as pressure volume or flow.
[Von Stone MI et al 1994, Physiology of peritoneal dialysis in handbook of dialysis 2nd edition]
Mechanism of Ultrafiltration
A) In Hemodialysis:
1-Hydrostatic pressure
The primary driving force for ultrafiltration is the hydrostatic pressure difference across the membrane, which is the Transmembrane Pressure (TMP), expressed in millimeters mercury (mmHg). The TMP is determined by the average or mean pressure in the blood compartment minus the mean dialysate compartment pressure.The relationship of ultrafiltrate to TMP is entirely dependent on the membrane (Dialyzer) properties. The permeability of dialyzer membranes to water is high, varies consonsiderably, and is a function of membrane thickness and pore size.The total capacity of the dialyzer for ultrafiltration is given by the Ultrafiltration Coefficient (KUF).
The KUF is defined as the number of milliliters of fluid per hour that will be transferred across the membrane per mmHg pressure gradient across the membrane The KUF of most dialyzer ranges from 2 to 6 ml/hour. The relationship between ultrafiltration, KUF and TMP is expressed as:
Ultrafiltration rate (ml/hr) = KUF X TMP
How do you calculate UF rate from KUF?
If one needs to remove 2 kg during a period of 4 hours
1- Add the volume of saline that will be given at the end of dialysis (usually 300 ml) and the amount of ingested fluid during dialysis (e.g., 100ml).
2- This means that 2.4 L will have to be removed during 4 hours dialysis i.e.2.4 X 1000 /4 = 600 ml/hour.
3- When using a dialyzer with KUF value of 4.0 ml/hour, the TMP will need to be set at 600/4 =150 mm Hg. N.B. (If the dialyzer KUF is 6 the TMP should be = 100 mm Hg).
2-Osmotic Ultrafiltration:
Osmotic ultrafiltration does play an indirect role in total ultrafiltration; water shifts from intracellular to the extracellular compartment which occur during hemodialysis (so-called plasma refilling) can be optimized by introduction of an effective concentration of an osmotic agent into the extracellular space. Sodium is employed in some dialysis practice especially during sodium profiling
B) In peritoneal dialysis:
1-Osmotic Ultrafiltration
The primary driving force for ultrafiltraion in peritoneal dialysis is the Osmotic gradient (Osmotic Ultrafiltration). Osmosis isthe movement of the solvent (eg water) from the side of low concentration to the side of higher concentration through a semipermeable membrane.The result of this movement of water will be equalization of total solute concentration on both sides of the membrane.
Osmotic ultrafiltraion during peritoneal dialysis is achieved by adding large amount of glucose to dialysis solution. Peritoneal dialysis solution ordinarily contain 1.36 % (1.5), 2.27 % (2.5), 3.86 % (4.25). The osmotic pressure generated by glucose will draw water from the blood and tissues into the dialysate.
2-Hydrostatic pressure
The hydrostatic ultrafiltraion effect is of minor importance
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