Hemodialysis is used for patients who are acutely ill and require short-term dialysis (days to weeks) and for patients with advanced CKD and ESRD who require long-term or permanent renal replacement therapy. Hemodialysis prevents death but does not cure renal disease and does not compensate for the loss of endocrine or metabolic activities of the kidneys. More than 90% of patients requiring longterm renal replacement therapy are on chronic hemodialysis (USRDS, 2007). Most patients receive intermittent hemodialysis that involves treatments three times a week with the average treatment duration of 3 to 4 hours in an outpatient setting. Hemodialysis can also be performed at home by the patient and a caregiver. With home dialysis, treatmenttime and frequency can be adjusted to meet optimal patient needs.The objectives of hemodialysis are to extract toxic nitrogenous substances from the blood and to remove excess water. A dialyzer (also referred to as an artificial kidney) serves as a synthetic semipermeable membrane, replacing the renal glomeruli and tubules as the filter for the impaired kidneys. In hemodialysis, the blood, laden with toxins and nitrogenous wastes, is diverted from the patient to a machine, a dialyzer, where toxins are filtered out and removed and the blood is returned to the patient.
Diffusion, osmosis, and ultrafiltration are the principles on which hemodialysis is based. The toxins and wastes in the blood are removed by diffusion - that is, they move from an area of higher concentration in the blood to an area of lower concentration in the dialysate. The dialysate is a solution made up of all the important electrolytes in their ideal extracellular concentrations. The electrolyte level in the patient’s blood can be brought under control by properly adjusting the dialysate bath. The semipermeable membrane impedes the diffusion of large molecules, such as RBCs and proteins.
Excess water is removed from the blood by osmosis, in which water moves from an area of low concentration potential (the blood) to an area of high concentration potential (the dialysate bath). In ultrafiltration, water moves under high pressure to an area of lower pressure. This process is much more efficient than osmosis at water removal and is accomplished by applying negative pressure or a suctioning force to the dialysis membrane. Because patients with renal disease usually cannot excrete water, this force is necessary to remove fluid to achieve fluid balance.
The body’s buffer system is maintained using a dialysate bath made up of bicarbonate (most common) or acetate, which is metabolized to form bicarbonate. The anticoagulant heparin is administered to keep blood from clotting in the dialysis circuit. Cleansed blood is returned to the body. By the end of the dialysis treatment, many waste products have been removed, the electrolyte balance has been restored to normal, and the buffer system has been replenished.
Dialyzers
Dialyzers are hollow-fiber devices containing thousands of tiny strawlike tubes that carry the blood through the dialyzer. The tubes are porous and act as a semipermeable membrane allowing toxins, fluid, and electrolytes to pass through. The constant flow of the solution maintains the concentration gradient to facilitate the exchange of wastes from the blood
through the semipermeable membrane into the dialysate solution, where they are removed and discarded.
Dialyzers are hollow-fiber devices containing thousands of tiny strawlike tubes that carry the blood through the dialyzer. The tubes are porous and act as a semipermeable membrane allowing toxins, fluid, and electrolytes to pass through. The constant flow of the solution maintains the concentration gradient to facilitate the exchange of wastes from the blood
through the semipermeable membrane into the dialysate solution, where they are removed and discarded.
Dialyzers have undergone many technologic changes in performance and biocompatibility. High-flux dialysis uses highly permeable membranes to increase the clearance of low- and mid-molecular-weight molecules. These special membranes are used with higher than traditional rates of flow for the blood entering and exiting the dialyzer (500 to 550 mL/min). High-flux dialysis increases the efficiency of treatments while shortening their duration and reducing the need for heparin.
Vascular Access
Access to the patient’s vascular system must be established to allow blood to be removed, cleansed, and returned to the patient’s vascular system at rates between 300 and 800
mL/min. Several types of access are available.
Access to the patient’s vascular system must be established to allow blood to be removed, cleansed, and returned to the patient’s vascular system at rates between 300 and 800
mL/min. Several types of access are available.
Vascular Access Devices
Immediate access to the patient’s circulation for acute hemodialysis is achieved by inserting a double-lumen, noncuffed, large-bore catheter into the subclavian, internal jugular, or femoral vein by the physician. This method of vascular access involves some risk (eg, hematoma, pneumothorax, infection, thrombosis of the subclavian vein, inadequate flow). The catheter is removed when no longer needed (eg, because the patient’s condition has improved or another type of access has been established). Double-lumen, cuffed catheters may also be inserted, usually by either a surgeon or interventional radiologist, into the internal jugular vein of the patient. Since these catheters have cuffs under the skin, the insertion site heals, sealing the wound and reducing the risk for ascending infection. This feature makes these catheters safe for longer-term use. Infection rates, however, remain high and septicemia continues to be a common cause for hospital admission.
Immediate access to the patient’s circulation for acute hemodialysis is achieved by inserting a double-lumen, noncuffed, large-bore catheter into the subclavian, internal jugular, or femoral vein by the physician. This method of vascular access involves some risk (eg, hematoma, pneumothorax, infection, thrombosis of the subclavian vein, inadequate flow). The catheter is removed when no longer needed (eg, because the patient’s condition has improved or another type of access has been established). Double-lumen, cuffed catheters may also be inserted, usually by either a surgeon or interventional radiologist, into the internal jugular vein of the patient. Since these catheters have cuffs under the skin, the insertion site heals, sealing the wound and reducing the risk for ascending infection. This feature makes these catheters safe for longer-term use. Infection rates, however, remain high and septicemia continues to be a common cause for hospital admission.
Arteriovenous Fistula
The preferred method of permanent access is an arteriovenous fistula (AVF) that is created surgically (usually in the forearm) by joining (anastomosing) an artery to a vein, either side to side or end to side. Needles are inserted into the vessel to obtain blood flow adequate
to pass through the dialyzer. The arterial segment of the fistula is used for arterial flow to the dialyzer and the venous segment for reinfusion of the dialyzed blood. This access will need time, (2 to 3 months) to “mature” before it can be used. As the AVF matures, the venous segment dilates due to the increased blood flow coming directly from the artery. Once sufficiently dilated it will then accommodate two large-bore (14-, 15-, or 16-gauge) needles
that are inserted for each dialysis treatment. The patient is encouraged to perform hand exercises to increase the size of these vessels (ie, squeezing a rubber ball for forearm fistulas) to accommodate the large-bore needles. Once established, this access has the longest useful life and thus is the best option for vascular access for the chronic hemodialysis patient.
The preferred method of permanent access is an arteriovenous fistula (AVF) that is created surgically (usually in the forearm) by joining (anastomosing) an artery to a vein, either side to side or end to side. Needles are inserted into the vessel to obtain blood flow adequate
to pass through the dialyzer. The arterial segment of the fistula is used for arterial flow to the dialyzer and the venous segment for reinfusion of the dialyzed blood. This access will need time, (2 to 3 months) to “mature” before it can be used. As the AVF matures, the venous segment dilates due to the increased blood flow coming directly from the artery. Once sufficiently dilated it will then accommodate two large-bore (14-, 15-, or 16-gauge) needles
that are inserted for each dialysis treatment. The patient is encouraged to perform hand exercises to increase the size of these vessels (ie, squeezing a rubber ball for forearm fistulas) to accommodate the large-bore needles. Once established, this access has the longest useful life and thus is the best option for vascular access for the chronic hemodialysis patient.
Arteriovenous Graft
An arteriovenous graft can be created by subcutaneously interposing a biologic, semibiologic, or synthetic graft material between an artery and vein. Usually a graft is created when the patient’s vessels are not suitable for creation of an AV fistula. Patients with compromised vascular systems (eg, from diabetes) will require a graft because their native vessels are not suitable for creation of an AV Fistula. Grafts are usually placed in the arm but may be placed in the thigh or chest area. Stenosis, infection, and thrombosis are the most common complications that result in loss of this access. It is not at all uncommon to see a dialysis patient with numerous “old” or “nonfunctioning” accesses present on their arms. The patient is asked to identify which is the current access in use and it is checked carefully for the presence of a bruit and thrill.
An arteriovenous graft can be created by subcutaneously interposing a biologic, semibiologic, or synthetic graft material between an artery and vein. Usually a graft is created when the patient’s vessels are not suitable for creation of an AV fistula. Patients with compromised vascular systems (eg, from diabetes) will require a graft because their native vessels are not suitable for creation of an AV Fistula. Grafts are usually placed in the arm but may be placed in the thigh or chest area. Stenosis, infection, and thrombosis are the most common complications that result in loss of this access. It is not at all uncommon to see a dialysis patient with numerous “old” or “nonfunctioning” accesses present on their arms. The patient is asked to identify which is the current access in use and it is checked carefully for the presence of a bruit and thrill.
Complications
While hemodialysis can prolong life indefinitely, it does not alter the natural course of the underlying CKD, nor does it completely replace kidney function. The CKD complications
previously discussed will continue to worsen and require more aggressive treatment. With the initiation of dialysis, disturbances of lipid metabolism (hypertriglyceridemia) are accentuated and contribute to cardiovascular complications. Heart failure, coronary heart disease, angina, stroke, and peripheral vascular insufficiency may occur and can incapacitate the patient. Cardiovascular disease remains the leading cause of death in patients receiving dialysis (Burrows & Muller, 2007).
While hemodialysis can prolong life indefinitely, it does not alter the natural course of the underlying CKD, nor does it completely replace kidney function. The CKD complications
previously discussed will continue to worsen and require more aggressive treatment. With the initiation of dialysis, disturbances of lipid metabolism (hypertriglyceridemia) are accentuated and contribute to cardiovascular complications. Heart failure, coronary heart disease, angina, stroke, and peripheral vascular insufficiency may occur and can incapacitate the patient. Cardiovascular disease remains the leading cause of death in patients receiving dialysis (Burrows & Muller, 2007).
Anemia is compounded by blood lost during hemodialysis. Gastric ulcers may result from the physiologic stress of chronic illness, medication, and preexisting medical conditions (eg, diabetes). Patients with uremia report a metallic taste and nausea when they require dialysis. Vomiting may occur during the hemodialysis treatment when rapid fluid shifts and hypotension occur. These contribute to the malnutrition seen in patients on dialysis. Worsening calcium metabolism and renal osteodystrophy can result in bone pain and fractures, interfering with mobility. As time on dialysis continues, calcification of major blood vessels has been reported and linked to hypertension and other vascular complications. Phosphorus deposits in the skin can occur and cause itching.
Up to 85% of people undergoing hemodialysis experience major sleep problems that further complicate their overall health status (Phillips & Ryr, 2005). Early-morning or late-afternoon dialysis may be a risk factor for developing sleep disturbances.
Other complications of dialysis treatment may include the following:
- Episodes of shortness of breath often occur as fluid accumulates between dialysis treatments.
- Hypotension may occur during the treatment as fluid is removed. Nausea and vomiting, diaphoresis, tachycardia, and dizziness are common signs of hypotension.
- Painful muscle cramping may occur, usually late in dialysis as fluid and electrolytes rapidly leave the extracellular space.
- Exsanguination may occur if blood lines separate or dialysis needles become dislodged.
- Dysrhythmias may result from electrolyte and pH changes or from removal of antiarrhythmic medications during dialysis.
- Air embolism is rare but can occur if air enters the vascular system.
- Chest pain may occur in patients with anemia or arteriosclerotic heart disease.
- Dialysis disequilibrium results from cerebral fluid shifts. Signs and symptoms include headache, nausea and vomiting, restlessness, decreased level of consciousness, and seizures. It is more likely to occur in acute renal failure or when blood urea nitrogen levels are very high (exceeding 150 mg/dL).
