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In hemodialysis, a patient’s blood is pumped into a dialyzer containing 2 fluid compartments configured as bundles of hollow fiber capillary tubes or as parallel, sandwiched sheets of semipermeable membranes. In either configuration, blood in the first compartment is pumped along one side of a semipermeable membrane while a crystalloid solution (dialysate) is pumped along the other side, in a separate compartment, in the opposite direction. Concentration gradients of solute between blood and dialysate lead to desired changes in the patient’s serum solutes, such as a reduction in urea nitrogen and creatinine; an increase in HCO3; and equilibration of Na, Cl, K, and Mg. The dialysate compartment is under negative pressure relative to the blood compartment and has a higher osmolality to prevent filtration of dialysate into the bloodstream and to remove the excess fluid from the patient. The dialyzed blood is then returned to the patient.
The patient is usually systemically anticoagulated during hemodialysis to prevent blood from clotting in the dialysis machine. However, hemodialysis treatment may also be done with regional anticoagulation of the dialysis circuit (using heparin or trisodium citrate) or with saline flush, in which 50 to 100 mL of saline every 15 to 30 min clears the dialysis circuit of any blood clots.
The immediate objectives of hemodialysis are to correct electrolyte and fluid imbalances and remove toxins. Longer term objectives in patients with renal failure are to
The optimal “dose” of hemodialysis is uncertain, but most patients do well with 3 to 5 h of hemodialysis 3 times/wk. One way to assess the adequacy of each session is by measuring BUN before and after each session. A ≥ 65% decrease of BUN from predialysis level ([predialysis BUN − postdialysis BUN]/predialysis BUN × 100% is ≥ 65%) indicates an adequate session. Specialists may use other, more calculation-intensive formulas, such as KT/V ≥ 1.2 (where K is the urea clearance of the dialyzer in mL/min, T is dialysis time in minutes, and V is volume of distribution of urea [which is about equal to total body water] in mL). Hemodialysis dose can be increased by increasing time on dialysis, blood flow, membrane surface area, and membrane porosity. Nightly hemodialysis sessions (6 to 8 h, 5 to 6 days/wk) and short (1.5 to 2.5 h) daily sessions, when available, are used selectively for patients who have any of the following:
These daily sessions are most economically feasible if patients can do hemodialysis at home.
Hemodialysis is usually done through a surgically created arteriovenous fistula. However, dialysis can be done through a central vein catheter if an arteriovenous fistula has not yet been created or is not ready for use or if creation of an arteriovenous fistula is impossible. The primary disadvantages of central vein catheters are a relatively narrow caliber that does not allow for blood flow high enough to achieve optimal clearance and a high risk of catheter site infection and thrombosis. Central venous catheterization for hemodialysis is best done by using the right internal jugular vein. Most internal jugular vein catheters remain useful for 2 to 6 wk if strict aseptic skin care is practiced and if the catheter is used only for hemodialysis. Catheters with a subcutaneous tunnel and fabric cuff have a longer life span (50% functional at 1 yr) and may be useful for patients in whom creation of an arteriovenous fistula is impossible.
Surgically created arteriovenous fistulas are better than central venous catheters because they are more durable and less likely to become infected. But they are also prone to complications (thrombosis, infection, aneurysm or pseudoaneurysm). A newly created fistula may take 3 to 6 mo to mature and become usable, so in patients with chronic kidney disease, the fistula is best created at least 6 mo before the anticipated need for dialysis. The surgical procedure anastomoses the radial, brachial, or femoral artery to an adjacent vein in an end-of-the-vein to the side-of-the-artery fashion. When the adjacent vein is not suitable for access creation, a piece of prosthetic graft is used. For patients who have poor veins, an autogenous saphenous vein graft is also an option.
Complications such as infection, stenosis, thrombosis (often in a stenotic passage), and pseudoaneurysm or aneurysm, significantly limit the quality of hemodialysis that can be delivered, increase long-term morbidity and mortality, and are common enough that patients and practitioners should be vigilant for suggestive changes. These changes include pain, edema, erythema, breaks in the skin overlying the access, absence of bruit and pulse in the access, hematoma around the access, and prolonged bleeding from the dialysis cannula puncture site. Infection is treated with antibiotics, surgery, or both.
The fistula may be monitored for signs of impending failure by serial Doppler dilution blood flow measurements, thermal or urea dilution techniques, or by measurement of the static venous chamber pressures. One of these tests is usually recommended at least monthly. Treatment of stenosis, thrombosis, pseudoaneurysm, or aneurysm may involve angioplasty, stenting, or surgery.
Complications are listed in Complications of Renal Replacement Therapy.
Hypotension is most common and has multiple causes, including too-rapid water removal, osmotic fluid shifts across cell membranes, acetate in the dialysate, heat-related vasodilation, allergic reactions, sepsis, and underlying conditions (eg, autonomic neuropathy, cardiomyopathy with poor ejection fraction, myocardial ischemia, arrhythmias).
Many patients also have restless leg syndrome, cramps, pruritus, nausea and vomiting, headache, and chest and back pain. In most cases, these complications occur for unknown reasons, but some may be part of a first-use syndrome (when the patient’s blood is exposed to cuprophane or cellulose membranes in the dialyzer) or dialysis dysequilibrium syndrome, a syndrome thought to be caused by too rapid removal of urea and other osmolytes from the serum, causing osmotic movement of fluid into the brain. More severe cases of dialysis dysequilibrium manifest as disorientation, restlessness, blurred vision, confusion, seizures, and even death.
Dialysis-related amyloidosis affects patients who have been on hemodialysis for years and manifests as carpal tunnel syndrome, bone cysts, arthritis, and cervical spondyloarthropathy. Dialysis-related amyloidosis is believed to be less common with the high-flux dialyzers in wide use today because ß2-microglobulin (the protein causing the amyloidosis) is removed more effectively with these dialyzers.
Complications of Renal Replacement Therapy
Overall adjusted annual mortality in hemodialysis-dependent patients tends to be about 20%. The 5-yr survival rate is lower for patients with diabetes than for patients with glomerulonephritis. Death is generally mostly attributable to cardiovascular disease, followed by infection and withdrawal from hemodialysis. Blacks have usually had a higher survival rate in all age groups. Nonhemodialysis contributors to mortality include comorbidities (eg, hyperparathyroidism, diabetes, undernutrition, other chronic disorders), older age, and late referral for dialysis.
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