Hemodialysis is a treatment that involves a pump that circulates a patient's blood through tubing to a semipermeable membrane and returns the blood to the patient. The patient's blood is on one side of the semipermeable membrane and the dialysate bath is on the other side. The dialysate both is made up of bicarbonate, treated water and acid concentrate. The acid concentrate contains specific concentrations of potassium, sodium, magnesium and other electrolytes as well as glucose.
At the semipermeable membrane, toxins are removed from the blood. This is accomplished through a concentration gradient that exists between the dialysate bath and the composition of the patient's blood. For instance, if a patient's blood has a potassium value of 6.5 mEq/l and the dialysate bath has a concentration of 2.0 mEq/l, then the potassium will diffuse to the lower concentration over time. This concentration exists with other electrolytes that are in the dialysis bath. Drugs weighing less than forty thousand Daltons and are not protein bound can also pass through the membrane. The typical dialysis treatment is four hours.
Water is removed by adding pressure to the membrane, this is called the transmembrane pressure. Dialysis machines today are very specific in the amounts of water that can be removed from a patient. The amount of water to be removed is ordered by the physician. This is determined in large part by the physical assessment of the patient. The “dry weight” of a patient is the weight the patient should weigh after the dialysis treatment. This is determined during the first few dialysis treatments.
An anticoagulant such as heparin is needed to reduce the possibility of clotting in the membrane or extracorporal circuit. If heparin is contraindicated, normal saline rinses, 200 cc every 15 minutes, is necessary to prevent clotting.
You cannot have a good dialysis treatment, if you do not have a good access! For adequate hemodialysis to take place, a sustained blood flow of 200–300 ml/min is necessary. The “gold standard” for dialysis accesses is the creation of a native fistula. This is a surgically created link between an artery and a vein. Not all patients have the anatomy that can support a native fistula. In these patients, a Gortex graft is the next option. A well cared for native fistula or Gortex graft can last for many years. Various large bore catheters are the final option. Catheters can be under the skin into a large vessel; this reduced the incidence of infection and can remain in place for up to a year. Catheters can be problematic due to clotting, kinking or both, and might not deliver the necessary blood flow of 200–300 ml/min.
In the chronic setting, dialysis treatments typically take place on a Mon-Wed-Fri or Tues-Thur-Sat schedule and are usually fours hours in length. In the hospital, all renal patients are visited by the hospital physicians and a determination of dialyze is made on a daily basis. Most chronic patients will remain on their usual schedule or dialyze an extra day to get on their usual schedule. However, certain conditions such us uremic pericarditis or fluid overload may require several days of consecutive dialysis treatments.