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:
- 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.
- Signs of uremia
- Severe hyperkalemia
- Severe metabolic acidosis
- Volume overload refractory to diuretics
- Acute poisonings:
- 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:
- 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
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.
- 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
- 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
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.
- Dialyzer or dialysis filter
- Dialysate (dialysis solution)
- Tubing for transport of blood and dialysate
- Machine that powers and monitors the filtration
- 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
Permanent dialysis access points allow dialysis for the long term (i.e., years) and include arteriovenous fistulas (AVFs) and arteriovenous grafts (AVGs).
- 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
- 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
- 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
- Azotemic blood pumped from patient into dialysis filter
- 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
- 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.
- 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).
- 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.
- Chronic dialysis
The nephrologist may control many variables within the dialysis procedure:
- Duration of treatment
- Ultrafiltration goal
- 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
- 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.
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.
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
- Dialysate is inserted into the peritoneal space through the PD catheter.
- Dialysate and blood interact with the peritoneal membrane.
- 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.
- Equilibrated dialysate (including the toxins and excess water from the blood) is removed from the peritoneal space through the PD catheter.
- 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”).
- 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)
- 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).
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.
- 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)
- Cost and maintenance of the cycler
- Must be attached to cycler for 8–10 hours consecutively
- May interrupt sleep
Peritoneal dialysis prescription
- 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.
- 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
- 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.
- 3 options if dextrose is used as the osmotic agent:
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
- 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.
- 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
- 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.
- 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.
- 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
- 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.
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