Overview and Types of Dialysis

In the context of acute or chronic kidney failure, renal function may diminish to a point at which it is no longer able to adequately support life. When this happens, renal replacement therapy is indicated. Renal replacement therapy refers to dialysis and/or kidney transplantation. Dialysis is a procedure by which toxins and excess water are removed from the circulation. Hemodialysis and peritoneal dialysis (PD) are the two types of dialysis, and their primary difference is the location of the filtration process (external to the body in hemodialysis versus inside the body for PD).

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Overview

Definition

Dialysis is a form of renal replacement therapy (RRT) that is used to perform the blood-filtering role of the kidneys when the kidneys are not functioning.

  • The conditions generally include:
    • AKI
    • CKD
  • Types:
    • Hemodialysis
    • Peritoneal dialysis

Indications for dialysis in acute kidney injury

Acute kidney injury is sometimes so severe that dialysis is needed as a life-support measure while waiting for possible renal recovery.

  • Dialysis is a temporizing measure that is used with the hope that the patient will recover enough renal function to allow dialysis to be discontinued permanently.
  • Some patients with AKI do not recover renal function and require long-term RRT.

Indications:

  • Signs of uremia
  • Severe hyperkalemia
  • Severe metabolic acidosis
  • Volume overload refractory to diuretics
  • Acute poisonings: 
    • Lithium
    • Toxic alcohols (ethylene glycol, methanol)

Indications for dialysis in chronic kidney disease

  • Chronic kidney disease is often progressive, such that patients will reach end-stage renal disease (ESRD) and then require dialysis or kidney transplantation.
  • The decision to start chronic dialysis is individualized based on uremic symptoms and lab results.  
  • Consensus statements from Kidney Disease: Improving Global Outcomes (KDIGO) and Kidney Disease Outcomes Quality Initiative (KDOQI) help guide these decisions, considering the following factors:  
    • GFR < 15 mL/min
    • Uremic symptoms:
      • Encephalopathy
      • Serositis (pericarditis/pleuritis)
      • Worsening nutritional status
    • Hypervolemia that is uncontrollable with diuretics
    • Electrolyte abnormalities (primarily hyperkalemia) that are uncontrollable with medications
    • Hypertension that is uncontrollable with medications

Principles

Dialysis, which depends on the principles of diffusion and ultrafiltration through a semipermeable membrane, can be administered through two distinct mechanisms:

  • Hemodialysis: The semipermeable membrane is the synthetic dialysis filter.
  • Peritoneal dialysis (PD): The semipermeable membrane is the patient’s peritoneal membrane.

Definitions:

  • Semipermeable membrane:
    • A membrane that is permeable to some, but not all, solutes
    • Permeability may be determined by size and/or charge of the solutes.
    • Necessary for the concept of concentration gradients 
  • Diffusion: movement of solutes across a semipermeable membrane from areas of high concentration to areas of low concentration
  • Ultrafiltration: 
    • Movement of water across a semipermeable membrane
    • In dialysis, this occurs with water moving from a place of higher pressure (the blood) to a place of lower pressure (the dialysate).

Introduction to Hemodialysis

Overview

Hemodialysis is a procedure by which waste products and excess water are removed from a patient’s blood. This is done by directly removing blood from the patient’s circulation, passing it through the dialysis filter, and then returning it directly back into the circulation.
Apparatus needed:

  • Dialyzer or dialysis filter
  • Dialysate (dialysis solution)
  • Tubing for transport of blood and dialysate
  • Machine that powers and monitors the filtration

Hemodialysis access

  • Provides direct access to the patient’s circulatory system via the large central veins 
  • Hemodialysis cannot be performed without an adequately functioning access point. 
  • Two basic types: 
    • Central venous catheters 
    • Permanent access points (i.e., arteriovenous fistula)

Central venous catheters

  • Large bore (usually 11–13 French) allows for the faster flow rates needed for dialysis.
    • Usually inserted into the internal jugular vein or femoral vein
    • Can be nontunneled or tunneled (underneath the skin)
  • Nontunneled dialysis catheters:
    • Commonly referred to as “Quinton catheters” or “temporary dialysis catheters”
    • Most commonly placed in hospital settings for AKI
    • Secured to the patient only with sutures and surgical dressing
    • Intended to stay in place for only a period of days:
      • High risk of infection over time
      • Significant bleeding risk if inadvertently dislodged
    • Patient may not be discharged from the hospital with this type of catheter.
  • Tunneled dialysis catheters (TDC):
    • Also known as “Hickman catheters” or “permacaths”
    • Not considered permanent access
    • Used for patients who need chronic dialysis started urgently (serving as a “bridge”), but do not yet have a permanent access (i.e., arteriovenous fistula)
    • Usually inserted into internal jugular vein 
    • Catheter is tunneled subcutaneously and exits the skin underneath the clavicle.
    • Secured to patient via a “cuff”:
      • Synthetic material around the part of the catheter that rests underneath the skin near the exit site
      • Fibrosis develops around the cuff and surrounding subcutaneous tissue, holding the catheter in place and serving as a barrier to infection.
    • May be left in place for a period of several weeks.
      • Patient may be discharged from the hospital with this type of catheter.
      • Less risk for infection as compared with nontunneled catheters
      • Overall risk for infection is still significant, particularly if left in place for long periods.
      • Removed once permanent dialysis access is functional
Nontunneled versus tunneled dialysis catheters

Central venous catheters for dialysis:
A: Nontunneled dialysis catheter
B: Tunneled (underneath the skin) dialysis catheter

Image by Lecturio.

Permanent access

Permanent dialysis access points allow dialysis for the long term (i.e., years) and include arteriovenous fistulas (AVFs) and arteriovenous grafts (AVGs).

Arteriovenous fistula:

  • Ideal type of access for chronic hemodialysis:
    • Least risk of infection
    • Longest lasting
  • Direct surgical connection (i.e., anastomosis) of an artery and a vein, most commonly: 
    • Radial artery to cephalic vein
    • Brachial artery to cephalic vein
    • Brachial artery to basilic vein
    • Lower-extremity AVFs are also possible.
  • High pressure from the artery is transmitted directly into the vein, instead of being dispersed throughout a capillary bed.
    • Results in vein changing anatomically (i.e., thickening, or “arterializing”) to resemble an artery, which takes weeks to occur
    • This process is known as “maturation” of the fistula; AVF cannot be used during this period.
    • Necessary because normal veins will either collapse or blow out under the high fluid flow rates needed for dialysis
  • Poor candidates for AVF:
    • Inadequate or small veins
    • History of AVFs that do not mature adequately
    • Patients with multiple failed AVFs and no other locations available for AVF

Arteriovenous graft:

  • Better for chronic dialysis than TDC, but not as good as AVF
  • Indirect surgical connection of an artery and vein using a tube of prosthetic material
  • Many possible locations: forearm, upper arm, chest, thigh
  • Advantages compared with AVF:
    • Much shorter maturation period (can hypothetically be used immediately, though usually wait approximately 2 weeks before initial use)
    • Dialysis needles are placed directly into the synthetic graft material.
    • Can be placed in patients who are poor candidates for AVF
    • Can be placed in more anatomic locations than AVFs (i.e., chest)
  • Disadvantages compared with AVF:
    • Higher risk of infection owing to synthetic material residing inside the body
    • Higher risk of clotting

The Process of Hemodialysis

Hemodialysis procedure

  1. Two sets of tubing are connected to the patient’s dialysis access:
    • Connected directly to central venous catheter
    • Two needles inserted into AVF/AVG and taped down
  2. Azotemic blood pumped from patient into dialysis filter 
  3. Dialysis filter removes toxins primarily through diffusion:  
    • Dialysis filter is a plastic cylinder filled with thousands of tiny individual tubes composed of the filtering material.
    • Blood flows through the inside of the tiny tubes in one direction.
    • Dialysis fluid (dialysate) flows on the outside of the tiny tubes (but still within the single plastic cylinder that contains them) in the opposite direction.
    • The opposing directions of blood and dialysate result in maximal concentration gradients that drive the diffusion of toxins:
      • Known as “countercurrent” mechanism
      • Also results in correction of electrolyte/acid–base abnormalities via diffusion 
  4. Dialysis filter removes excess water from the blood through ultrafiltration.
    • Suction force is applied by the dialysis machine across the dialysis filter.
    • Water is pulled from the blood side into the dialysate side. 
  5. Clean blood and waste-filled dialysate exit the dialysis filter.
    • Clean blood is pumped back into the patient’s circulation.
    • Waste-filled dialysate is disposed of (including the excess water from the patient’s body that was removed during ultrafiltration).
  6. Overall process continues until the end of the treatment session.
    • Chronic dialysis
      • 3–4 hours each session
      • 3 times a week (Monday/Wednesday/Friday or Tuesday/Thursday/Saturday)
    • Acute dialysis: Treatment duration and daily schedule are variable.
Hemodialysis setup

Circuit setup for hemodialysis:

Two sets of tubing are connected to the patient’s dialysis access (central venous catheter or two needles inserted into AVF/AVG). Blue tubing in the illustration represents the azotemic blood. This blood is pumped from the patient into the dialysis filter, which then removes toxins primarily through diffusion. Inside the dialyzer or filter are tubes composed of the filtering material. Blood flows through the inside of the tiny tubes in one direction.
Dialysis fluid, or dialysate (which enters and leaves the dialyzer via the tubing shown in yellow in the image), on the other hand, flows on the outside of the tiny tubes (but within the single plastic cylinder that contains them) in the opposite direction. The opposing directions of blood and dialysate result in maximal concentration gradients that drive the diffusion of toxins. The filter removes excess water from the blood through ultrafiltration via a suction force/pressure applied by the machine across the filter. Water is pulled from the blood side into the dialysate side.
Clean blood (through the red tubing in the image) is pumped back into the patient’s circulation. Waste-filled dialysate (through the yellow tubing) exits the dialysis filter and is disposed of (including the excess water/fluid).

Image: “Hemodialysis” by Yassine Mabret. License: CC BY 3.0
Hemodialysis filter

Schematic of hemodialysis filter/dialyzer showing the blood (coming into the filter) flowing in the direction opposite to that of the dialysate:
The process of filtering fluid (from the blood to the dialysate) is ultrafiltration. Pressure is generated by the dialysis machine, and the transmembrane pressure between the blood (high pressure) to the dialysate (low pressure) allows fluid to be removed. Toxins are also removed from the azotemic blood. Toxins from the blood move across the dialysis membrane by the process of diffusion.

Image by Lecturio.

Hemodialysis prescription

The nephrologist may control many variables within the dialysis procedure:

  • Duration of treatment 
  • Ultrafiltration goal
  • Anticoagulation:
    • Blood is prone to clotting as it travels through the plastic tubing and dialysis membrane.
    • When cloting occurs, the treatment must be stopped and reset, which limits the overall treatment time and effectiveness.
    • IV heparin is often used to help prevent this clotting of the dialysis circuit.
  • Electrolyte composition of the dialysate: 
    • Potassium can be set between 0 and 4 mEq/L.
    • Sodium can be set between 130 and 145 mEq/L.
    • Calcium can be set between 2.5 and 3.5 mEq/L.
    • Bicarbonate can be set between 30 and 40 mEq/L.
  • Speed of blood flow and dialysate flow:
    • Blood flow ranges from 250 to 450 mL/min.
    • Dialysate flow ranges from 500 to 800 mL/min.

Introduction to Peritoneal Dialysis

Overview

  • Peritoneal dialysis achieves the same net result as hemodialysis (the removal of toxins and excess water); however, the process is completely different.  
  • Instead of removing blood from the patient, filtering it externally, and then returning it to the patient, the filtration occurs within the patient’s abdominal cavity.
  • The only fluid moving in and out of the patient’s body is the dialysate.

Anatomical considerations

Anatomy of the abdomen can be simply thought of as including:

  • Intraperitoneal space
  • Abdominal organs
  • The peritoneal membrane, which lines the intraperitoneal space and abdominal organs, functions as the dialysis filter:
    • Semipermeable membrane
    • Capillaries on one side
    • Dialysate inside the intraperitoneal space on the other side
  • Peritoneal dialysis (PD) catheter:
    • Surgically inserted through the abdomen
    • Allows direct access for the insertion and drainage of dialysis fluid
    • Inside the body, the catheter tip is curled and rests in the intraperitoneal space.
    • Outside the body, the catheter extends several inches from the skin.
Image by lecturio.

The peritoneal membrane serves as the semipermeable membrane (between the dialysate and the blood/capillary side) and filter in PD.

Image by Lecturio.

Peritoneal dialysis access

  • Provides direct access to the peritoneal space via a surgically implanted catheter
  • Unlike hemodialysis, there is only one basic type of PD access: the PD catheter:
    • Surgically implanted through the abdominal wall
    • Utilizes a cuff system to hold it in place, similar to TDC
    • Can be used immediately if urgently needed, but usually wait approximately 2 weeks after surgery

Peritoneal dialysis fluid

  • Not the same as is used for hemodialysis!
  • The dialysate from PD is very hypertonic to the patient’s blood (the dialysate from hemodialysis is isotonic).
    • Hypertonicity is the result of very high glucose concentrations.
    • Creates a concentration gradient for ultrafiltration

Process of Peritoneal Dialysis

Peritoneal dialysis procedure

  1. Dialysate is inserted into the peritoneal space through the PD catheter.
  2. Dialysate and blood interact with the peritoneal membrane. 
  3. Toxins and water move from the blood side to the dialysate side via diffusion:
    • Toxin concentrations are high in the blood and zero in the dialysate. 
    • Tonicity of blood (i.e., water concentration) is lower than that of the high-tonicity dialysate. 
    • Electrolyte/acid–base abnormalities also are corrected via diffusion. 
    • After several hours, the two sides equilibrate and no further net transport occurs.
  4. Equilibrated dialysate (including the toxins and excess water from the blood) is removed from the peritoneal space through the PD catheter.
  5. The process is repeated several times per session.

Methods of peritoneal dialysis

There are two basic methods to perform peritoneal dialysis:

  • Continuous ambulatory peritoneal dialysis (CAPD)
  • Automated peritoneal dialysis

Both methods utilize the same catheter and generally have the same clinical results, with selection dependent on patient preference.

The primary difference is the use of a machine (called a “cycler”) in automated peritoneal dialysis to automatically pump the dialysis fluid into and out of the body.

Continuous ambulatory peritoneal dialysis:

  • Does not use a machine
  • Gravity is used to allow dialysate to flow into and out of the intraperitoneal space:
    • Bag of fresh dialysate is hung above the patient.
    • Drainage bag for spent dialysate rests below the patient.
  • Treatment is done throughout the day:
    • Patient uses sterile technique to connect PD catheter tubing to dialysate bag.
    • Approximately 2 L of dialysate are inserted into the abdomen.
    • Patient uses sterile technique to disconnect from the dialysate bag.
    • Patient can ambulate and go about their normal day while the fluid acts.
    • Several hours later (dwell time), the patient uses sterile technique to connect to the drainage bag and dialysate bag.
    • Patient drains the spent dialysate from the abdomen into the drainage bag.
    • Patient refills the abdomen with fresh dialysate (process known as an “exchange”).
  • Pros:
    • No machine needed
    • Patient not continuously attached to apparatus for many hours
    • Patient less likely to have sleep disturbed by dialysis (treatment is during the day)
  • Cons: 
    • More work for the patient
    • Must connect and disconnect multiple times per day, increasing the chance for touch contamination
    • Patient must monitor the dwell time.
    • Fluid that goes in and out of the abdomen must be measured; too much fluid in will be uncomfortable (net fluid removed is the ultrafiltration volume).
Continuous ambulatory peritoneal dialysis

Continuous ambulatory peritoneal dialysis:
A bag of fresh dialysate is hung above the patient and a drainage bag for spent dialysate rests below the patient. The treatment is done throughout the day, with approximately 2 L of dialysate inserted into the abdomen. The patient can ambulate and go about their normal day while the fluid acts. Several hours later (dwell time), the patient uses sterile technique to connect to the drainage bag and dialysate bag. The patient drains the spent dialysate from the abdomen into the drainage bag.

Left image by Lecturio. Right image: “Our index patient with peritoneal dialysis catheter” by Faculty of Medicine, “Ovidius” University, 145 Tomis Blvd, Constanta 900591, Romania. License: CC BY 2.0

Automated peritoneal dialysis:

  • Uses a machine (cycler) to transfer fluid into and out of the peritoneal space
  • Treatment is done primarily at night.
    • Patient uses sterile technique to connect PD catheter to the cycler.
    • Cycler is turned on, and patient goes to sleep.
    • Cycler automatically pumps approximately 2 L of dialysate into the abdomen.
    • Dialysate dwells for several hours.
    • Cycler automatically pumps the spent dialysate out.
    • Process repeats with fresh dialysate.
    • When the patient wakes up, they detach from the cycler and the process is finished.
  • Pros:
    • Less work for patient, as the cycler measures fluid volumes in and out 
    • Patient can review data (ultrafiltration volume) on the cycler after treatment.
    • Patient connects/disconnects only two times per session, so less chance for touch contamination.
    • Can function during daytime without having to stop for drain/fill procedure (i.e., exchanges)
  • Cons:
    • Cost and maintenance of the cycler
    • Must be attached to cycler for 8–10 hours consecutively
    • May interrupt sleep

Peritoneal dialysis prescription

Considerations:

  • The variables for the PD prescription are much different from those for the hemodialysis prescription.  
  • After choosing between automated peritoneal dialysis and CAPD, the primary variables are:
    • Strength of dialysate used
    • Number of exchanges per session
    • Length of each exchange
  • It takes several weeks after changing the PD prescription to determine its effect, unlike the hemodialysis prescription, which can be monitored after each session.

Prescription contents:

  • Automated peritoneal dialysis versus CAPD
  • Number of exchanges per session (usually 4–5)
  • Dwell time per exchange: 
    • 4–6 hours for CAPD
    • 1–3 hours for automated peritoneal dialysis
  • Dwell volume (usually approximately 2 L, but can be increased)
  • Dialysate strength:
    • 3 options if dextrose is used as the osmotic agent:
      • 1.25% dextrose, 2.5% dextrose, 4.25% dextrose
      • Osmolality is 346, 396, and 485, respectively.
      • Higher strength results in more ultrafiltration.
    • 1 option if dextrose is not used as the osmotic agent: icodextrin
      • Colloid
      • Less commonly used than dextrose solutions
    • Bags are color coded to help in patient/physician communication.
      • 1.25% dextrose (yellow), 2.5% dextrose (green), 4.25% dextrose (red)
      • Icodextrin (purple)
      • Example: Physician may instruct the patient to switch from green bag to red bag if more ultrafiltration is needed.
    • Other components (i.e., electrolytes, base buffer) are standardized and not commonly manipulated.

Choice of Method

Hemodialysis versus peritoneal dialysis

  • Studies have failed to show overall, long-term differences in outcomes (including mortality) between hemodialysis and peritoneal dialysis.  
  • The decision between the two methods comes down to patient preference, based on the factors below.

Pros and cons of hemodialysis

Pros:

  • Not work-intensive for the patient: 
    • Patients do not have to connect themselves to the dialysis machine.
    • Patients do not have to operate or monitor the dialysis machine.
  • Changes to dialysis prescription can be assessed for quickly.
    • Ultrafiltration goal can be changed every session if extra fluid weight is gained.
    • Time can be added to a session, if needed.
    • Electrolytes can be manipulated each session.

Cons:

  • Some patients have long-term issues with hemodialysis access:
    • Arteriovenous fistulas can fail to mature and require multiple surgeries.
    • Arteriovenous fistulas/arteriovenous grafts can clot and require declotting procedures.
    • Tunneled dialysis catheter can get infected.
  • Must spend considerable time in dialysis unit (approximately 9–12 hours) 
  • Must have adequate transportation to and from the dialysis unit
  • Residual renal function is lost more quickly than with PD.
  • Must be stricter with oral fluid restriction:
    • Hemodialysis patients tend to have minimal residual urine output.
    • Hemodialysis patients tend to have minimal response to oral diuretics.
    • All fluid that the patient drinks between dialysis treatments (approximately 48 hours) must be removed during the following treatment.
  • Some have significant issues with adverse effects:
    • Intradialytic hypotension
    • Muscle cramps

Pros and cons of peritoneal dialysis

Pros:

  • Occurs at home
  • Can adjust life around dialysis more easily than with hemodialysis:
    • Many patients on PD continue to work, which is less common for those on hemodialysis.
    • Dialysis takes place only at night with cycler
  • Residual renal function is lost more slowly than with hemodialysis.
  • Can be more liberal with oral fluid restriction:
    • With PD, patients tend to have significant urine output and will respond to oral diuretics.
    • With PD, patients dialyze 7 days a week, with the fluid from only the previous 24 hours needing to be removed with each treatment.

Cons:

  • Very work-intensive for the patient:
    • Patients must connect themselves to and disconnect themselves from the dialysis apparatus.
    • Patients must always use strict sterile technique and avoid touch contamination.
    • Patients must monitor dialysis process (dwell time, ultrafiltration).
    • Patients must make sure they do not run out of supplies. 
  • Peritonitis:
    • Infection of the peritoneal space
    • Very painful
    • Usually due to touch contamination during the connect/disconnect process
    • Treated with antibiotics that can be added to the dialysate bags
  • Hyperglycemia:
    • Dextrose solutions cause varying degrees of hyperglycemia, depending on strength.
    • Can limit the utility of PD if patient is diabetic (insulin requirements will increase)
  • Changes to dialysis prescription take several weeks to take effect.

References

  1. Alam M, Krause M. (2021). Peritoneal dialysis solutions. UpToDate. Retrieved February 26, 2021, from https://www.uptodate.com/contents/peritoneal-dialysis-solutions
  2. Bleyer A. (2020). Indications for initiation of dialysis in chronic kidney disease. UpToDate. Retrieved February 26, 2021, from https://www.uptodate.com/contents/indications-for-initiation-of-dialysis-in-chronic-kidney-disease
  3. Bleyer A. (2020). Urine output and residual kidney function in kidney failure. UpToDate. Retrieved February 26, 2021, from https://www.uptodate.com/contents/urine-output-and-residual-kidney-function-in-kidney-failure
  4. Kidney Disease: Improving Global Outcomes (KDIGO) Chronic Kidney Disease Work Group. (2013). KDIGO Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International Suppl.; 2:1–163.
  5. National Kidney Foundation. (2015). KDOQI clinical practice guideline for hemodialysis adequacy: 2015 update. American Journal of Kidney Diseases 66(5), 884–930. https://doi.org/10.1053/j.ajkd.2015.07.015
  6. Pirkle J. (2020). Prescribing peritoneal dialysis. UpToDate. Retrieved February 26, 2021, from https://www.uptodate.com/contents/prescribing-peritoneal-dialysis
  7. Pirkle J. (2019). Evaluating patients for chronic peritoneal dialysis and selection of modality. UpToDate. Retrieved February 26, 2021, from https://www.uptodate.com/contents/evaluating-patients-for-chronic-peritoneal-dialysis-and-selection-of-modality
  8. Schmidt R. (2020). Overview of the hemodialysis apparatus. UpToDate. Retrieved February 26, 2021, from https://www.uptodate.com/contents/overview-of-the-hemodialysis-apparatus

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