Understanding dialysis concepts can be challenging, not only for patients experiencing end-stage renal disease (ESRD), but also for the healthcare providers caring for them. There are two main types of dialysis: hemodialysis (HD), in which a dialyzer (artificial kidney) is used, and peritoneal dialysis (PD), in which the membrane in the patient's abdominal cavity is utilized. Each type works slightly different but operates on the same principle—removing waste products and excessive fluid from the blood. There are several factors that determine which type is used, including patient preferences, modality methods, daily life activities (work and family), and clinical contraindications. This article discusses dialysis concepts as they relate to HD treatments.
HD is used for patients whose kidneys aren't functioning properly, requiring long-term and/or permanent therapy. Additionally, it may be used for short-term treatment until kidney function resumes. Before HD treatments, patients will need an internal or external dialysis access placed by a vascular surgeon or a trained healthcare provider. This access allows blood to flow between the body and the dialyzer, pulling unfiltered blood from the body using negative pressure and returning the filtered blood back to the body using positive pressure.
For dialysis to be successful, dialysate and bicarbonate are necessary. Dialysate—a solution that contains all of the electrolytes similar in concentration to human plasma water—circulates outside the fibers within the dialyzer. Toxins, waste, and excess electrolytes and fluid from the blood shift into the dialysate solution. The dialyzer fibers act as gatekeepers (semipermeable membranes) by allowing certain particles from the blood to cross over into the dialysate. The dialysate compartment and the blood compartment are separated within the dialyzer and should never mix. The electrolyte levels in the patient's blood can be balanced by appropriately adjusting the electrolytes in the dialysate solution (see A closer look at the dialyzer).
Bicarbonate is a component in dialysate that's used to help the body neutralize acid. Often, renal failure can result in the patient's blood becoming too acidic; the use of bicarbonate during dialysis helps the blood maintain a normal acid-base balance.
HD treatment sessions occur three times a week and an average session can last between 3.5 and 4 hours. During this time, the patient may read, rest, or watch TV; however, limited movement is necessary to avoid complications.
What happens during HD?
There are three fundamental principles on which HD are based: diffusion, osmosis, and ultrafiltration.
Particles (toxins, wastes, electrolytes) and excessive fluids are removed from the blood by diffusion—a separation process in which the particles that are dissolved in a solution are relocated from an area of higher concentration in the blood to an area of lower concentration in the dialysate. Diffusion occurs through the random movement of forced pressure within the particles as they cross the semipermeable membrane that separates the blood compartment from the dialysate compartment.
Several factors affect the rate of diffusion: the temperature of dialysate and blood (the higher the temperature, the greater the removal of particles), the size of the particles (the larger the particles, the slower the rate), the size and number of fibers within the dialyzer (the greater the number of fibers, the faster the diffusion rate), the dialysate and blood pump rates (the greater the pump rates, the greater the removal of particles), and the concentration of blood particles compared with the dialysate particles (the greater the concentration, the greater the diffusion). When equilibrium of the particles within the solution on both sides (blood and dialysate) is reached, diffusion stops.
During diffusion, the particles mix with water; during osmosis, water mixes with the particles.
Osmosis is the movement of water across the dialyzer into the dialysate. Water moves from an area of lower concentration (within the blood) to an area of higher concentration (within the dialysate) to equalize the concentration between the compartments. The process of osmosis requires the application of two types of pressure: osmotic pressure (the pressure that prohibits the movement of water across the dialyzer) and hydraulic pressure (the actual pressure that forces water from the dialyzer into the dialysate).
During HD, proteins within the blood plasma maintain osmotic pressure in the blood and prohibit water movement out of the compartment. Thus, the impact of osmotic pressure during HD has little effect on water removal; it's the hydraulic pressure that forces water from the blood into the dialysate and has the greatest role in water removal during HD.
Ultrafiltration is the removal of plasma water from the blood as it moves into the dialysate. Ultrafiltration takes place between the membranes within the dialyzer. During HD, fluid moves from high pressure (within the blood) to low pressure (within the dialyzer). This is accomplished by energy derived from hydrostatic pressure creating a suction-like force.
Transmembrane pressure is the hydrostatic pressure that allows ultrafiltration across a dialyzer membrane by pulling water from the blood into the dialysate. This is achieved by applying a negative suction pressure in the dialysate compartment and a positive hydrostatic pressure in the blood compartment. The pressure in the blood compartment must exceed the pressure in the dialysate compartment for fluid to move from the blood into the dialysate. Because most patients with ESRD excrete little to no water, the force is necessary to remove excess fluid and achieve fluid balance.
Factors that can affect fluid removal include a drop in transmembrane pressure due to a leak or filter rupture, causing the blood and dialysate to mix, or an increase in transmembrane pressure due to filter clotting.
Patients are given scheduled times to arrive in dialysis settings. The patient's weight is obtained via scale to determine fluid status and estimate fluid removal during HD treatments. The patient is then assessed before beginning HD treatment, including vital signs, respiratory and cardiovascular status, essential lab values, and edema.
Fluid removal is determined by subtracting the patient's current weight from his or her dry weight. Patients are given a dry weight (determined by nephrology providers) early in the beginning of HD treatments. A dry weight is the amount of weight without excess fluid.
Once the patient has been assessed, the HD treatment is initiated. See Patient assessment and teaching for nursing care during and after treatment.
Educate for best outcomes
Although HD is one of the main treatment options for individuals experiencing ESRD, many patients who undergo HD aren't well educated on the process. That's why it's essential for healthcare providers to understand how HD works to provide safe, quality care and patient teaching.
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