Fluid overload, medically termed hypervolemia, is a common and serious complication for individuals living with End-Stage Renal Disease (ESRD). Healthy kidneys regulate fluid balance by filtering blood and excreting excess water as urine. When kidney function fails, this regulatory system breaks down, and the body cannot remove sufficient water. Dialysis is then required to artificially take over the kidney’s role and prevent fluid accumulation between treatments.
Causes and Symptoms of Excess Fluid
The primary reason for fluid accumulation is the kidneys’ inability to produce enough urine to match fluid intake, often resulting in little to no urine output. Dietary consumption of sodium (salt) is a significant contributing factor, as it causes the body to retain water to dilute the increased sodium concentration. This retained water quickly builds up in the tissues and circulatory system, necessitating removal during dialysis sessions.
This excess fluid manifests most commonly as peripheral edema, which is swelling in the extremities like the feet, ankles, and hands. Swelling can also occur in the face and around the eyes, signaling a significant increase in the body’s overall fluid volume. Internally, fluid accumulation poses a substantial threat to the cardiovascular system.
As fluid volume rises in the blood vessels, it causes hypertension (high blood pressure), forcing the heart to work harder to circulate the larger volume. This constant strain can lead to the enlargement and weakening of the heart muscle, potentially resulting in heart failure. Excess fluid can also back up into the lungs, causing pulmonary edema, which presents as shortness of breath, especially when lying flat. Managing fluid gain between treatments is important for the long-term health and survival of a dialysis patient.
Measuring and Defining Target Weight
The goal of every dialysis session is to remove accumulated fluid and return the patient to their optimal fluid status, defined by “Target Weight.” Target Weight (formerly “Dry Weight”) represents the patient’s weight when they have a normal fluid volume, are free of fluid-related symptoms, and maintain healthy blood pressure without medication.
Clinicians determine Target Weight through an assessment combining physical examination and clinical data. The physical exam checks for the absence of edema, clear breath sounds in the lungs, and stable blood pressure readings. If blood pressure is stable and healthy immediately following dialysis, and the patient has no symptoms like cramping or dizziness, the Target Weight has likely been accurately achieved.
Some dialysis centers may use advanced non-invasive tools, such as Bioimpedance Spectroscopy (BIS), to help estimate the patient’s fluid status. BIS works by measuring the body’s electrical resistance, which allows for a calculation of total body water, providing a more objective measure of fluid volume. The Target Weight is not fixed and must be regularly reassessed and adjusted by the care team, as a patient’s true body weight can change over time due to muscle mass or fat loss.
How Dialysis Removes Excess Fluid
Excess fluid removal during hemodialysis relies on ultrafiltration. This process is driven by the dialysis machine, which creates a precise pressure difference across the semipermeable membrane within the dialyzer (artificial kidney). The membrane separates the patient’s blood from the dialysate, a specialized fluid flowing on the other side.
By controlling the pressure in the dialysate compartment, a pressure gradient is established that is lower than the pressure in the blood compartment. This pressure difference forces water, along with some dissolved solutes, out of the blood and across the membrane into the dialysate, effectively mimicking the kidney’s filtration process. The amount of fluid removed during a session is calculated based on the difference between the patient’s pre-dialysis weight and their established Target Weight.
For patients on peritoneal dialysis (PD), fluid removal relies on osmosis. The dialysate fluid used in PD contains a high concentration of sugar, typically dextrose. This concentration creates an osmotic pressure gradient across the peritoneal membrane, which acts as the filter inside the abdomen. This gradient draws excess water from the circulation into the dialysate solution, which is then drained.
Patient Strategies for Managing Fluid Intake
Managing fluid intake between dialysis treatments is a primary patient responsibility that directly impacts health outcomes and treatment effectiveness. Fluid restriction guidelines are often limited to 32 ounces or less per day, personalized based on residual urine output. All liquids, including soups, ice cream, gelatin, and moisture in certain foods, count toward the daily fluid limit.
A highly effective strategy for controlling fluid gain is to severely restrict sodium intake, as salt is the main driver of thirst and subsequent fluid consumption. Patients should limit processed foods, canned goods, and restaurant meals, which are often high in hidden sodium. Using herbs, spices, and non-salt seasonings can enhance flavor without stimulating thirst.
To combat the persistent sensation of thirst, patients can employ several practical techniques. Sucking on sugar-free hard candies, chewing gum, or using sour foods stimulates saliva production and moistens the mouth. Freezing allowed fluids into ice chips or freezing fruit like grapes provides a slower-melting option that prolongs the sensation of drinking while using less total volume. Using small cups or glasses for beverages helps manage the restricted volume, allowing for more frequent but smaller sips throughout the day.