Dialysis is a medical procedure that takes over the function of failing kidneys. Yes, dialysis is an effective and necessary process for removing excess sodium from the bloodstream. When the kidneys fail, waste products and electrolytes, including sodium, accumulate in the body. Dialysis serves as a substitute, using principles of chemistry and physics to extract retained sodium and the fluid it holds, preventing complications.
Why Sodium Regulation is Necessary
The healthy kidney regulates the amount of sodium and water in the body to maintain stability. Sodium is the main determinant of the fluid volume outside of the cells, including the plasma in the blood vessels. Kidneys continuously adjust the reabsorption and excretion of sodium to ensure this volume remains within a narrow range.
When kidney function declines, the ability to excrete sodium and water is impaired, leading to positive sodium balance. Because water follows sodium through osmosis, this retention causes fluid overload (hypervolemia). This excess volume places a burden on the cardiovascular system, contributing directly to high blood pressure.
Uncontrolled hypertension accelerates damage to blood vessels. The volume overload can also stretch the heart muscle, leading to enlargement and heart failure. Dialysis must regularly remove accumulated sodium and fluid to mitigate this cardiovascular strain.
The Principles of Sodium Removal
Dialysis machines rely on two physical processes to remove sodium from the blood: diffusion and ultrafiltration. Both utilize a semipermeable membrane that separates the patient’s blood from the dialysate solution. The membrane permits small molecules like sodium to pass through but blocks large components like blood cells.
Diffusion removes sodium based on a concentration gradient. Sodium moves from the blood, where its concentration is higher, into the dialysate, where the concentration is lower. The dialysate fluid is purposefully mixed to ensure a net movement of sodium ions out of the blood.
Ultrafiltration involves the mechanical removal of excess fluid driven by a pressure gradient across the membrane. Pressure is lowered on the dialysate side, forcing water and dissolved small solutes, including sodium, out of the blood. This process, where solutes are carried along with the bulk movement of fluid, is called convection.
Ultrafiltration removes the accumulated fluid volume (excess water caused by retained sodium). Diffusion fine-tunes the concentration of sodium in the patient’s blood plasma. Controlling the dialysate sodium concentration prevents rapid shifts in blood sodium levels that could cause complications like cramping or low blood pressure.
Dialysis Modalities and Sodium Management
Hemodialysis (HD) and peritoneal dialysis (PD) apply diffusion and ultrafiltration differently to manage sodium. In hemodialysis, sodium removal is precisely controlled by the dialysate solution composition. The dialysate sodium concentration is prescribed by a physician, often set between 135 and 145 milliequivalents per liter (mEq/L), to create the necessary diffusion gradient.
A lower dialysate sodium concentration promotes greater diffusive removal, helping control long-term blood pressure. However, removing sodium too quickly can destabilize the patient, causing symptoms like muscle cramps and a sudden drop in blood pressure. Some centers use individualized sodium prescriptions, tailoring the dialysate concentration to the patient’s pre-dialysis blood sodium level to optimize fluid balance.
Peritoneal dialysis uses the patient’s own peritoneal membrane as the filter, relying primarily on ultrafiltration. The dialysate fluid contains osmotic agents, usually dextrose, which creates a strong osmotic pressure gradient. This gradient pulls excess water from the blood across the peritoneal membrane into the fluid dwelling in the abdominal cavity.
As this fluid is removed, it carries sodium along with it, linking sodium removal directly to the volume of fluid removed. A phenomenon known as “sodium sieving” can occur early in the dwell time, where free water is pulled rapidly, leaving some sodium behind. To maximize sodium removal in PD, the prescription can be adjusted by changing the osmotic agent concentration, the solution volume, or the dwell time duration.
Patient Experience and Sodium Management
Sodium control during dialysis is challenged by the patient’s intake between treatments. Excess dietary sodium leads to increased thirst, causing patients to drink more fluid, resulting in greater fluid and sodium accumulation between sessions. This interdialytic weight gain forces the dialysis machine to work harder to remove the fluid during the next treatment.
When the body retains too much sodium and fluid, patients experience physical symptoms. These include swelling (edema), often seen in the feet, ankles, and face. Fluid overload also causes shortness of breath, as excess fluid backs up into the lungs, and contributes to high blood pressure.
If dialysis attempts to remove a large amount of fluid too rapidly, the patient may experience muscle cramping and a drop in blood pressure during the session. These acute symptoms result from the body reacting to the rapid volume shift necessary to correct pre-treatment fluid overload. Patients are advised to adhere to a low-sodium diet, often restricting intake to less than 2,000 milligrams per day.
Limiting dietary sodium intake helps manage thirst and minimizes the total fluid the dialysis machine must remove. A lower sodium intake results in a smaller interdialytic weight gain, allowing for a gentler, safer, and more comfortable fluid removal process. Successful sodium management requires a partnership between the dialysis prescription and the patient’s lifestyle choices.