Dialysis serves two primary functions for patients with kidney failure: cleansing the blood of waste products and removing excess fluid from the body. This fluid removal, technically known as ultrafiltration, is a precisely calculated procedure that directly affects a person’s immediate health and long-term well-being. Determining the exact volume of fluid to be withdrawn during each session is a complex and highly individualized medical challenge. The volume removed must be enough to restore a healthy fluid balance without causing acute complications.
The Necessity of Fluid Removal in Dialysis
Failing kidneys lose the ability to regulate the body’s balance of water and sodium, which leads to a condition called volume overload. Since most patients on hemodialysis produce little to no urine, the fluid consumed between treatments accumulates in the body tissues and the bloodstream. This retention of excess fluid, or hypervolemia, has serious consequences for the patient’s health.
One visible sign of this is edema, or swelling, which commonly appears in the feet, ankles, and hands. This extra volume strains the circulatory system, often causing high blood pressure (hypertension). The accumulated fluid can also move into the lungs, leading to pulmonary edema, which results in severe shortness of breath and difficulty breathing.
Over time, the constant presence of volume overload forces the heart to work harder to pump the increased blood volume. This persistent strain can cause the heart muscle to weaken and enlarge, potentially leading to heart failure. Therefore, the ultrafiltration component of dialysis is a life-sustaining measure, directly addressing the fluid imbalance that could otherwise cause severe cardiovascular complications.
Defining the Target: The Concept of Dry Weight
To determine the correct amount of fluid to remove, medical teams rely on the concept of “Dry Weight” (DW), which is the patient’s weight when they have achieved a normal fluid balance. Dry weight represents the lowest weight a patient can reach after a dialysis session without experiencing symptoms related to low blood pressure or dehydration. At this weight, the patient should be free of edema, have normal blood pressure without medication, and be comfortable without shortness of breath.
The target fluid removal for any given dialysis session is simply the difference between the patient’s pre-dialysis weight and their established dry weight. For example, if a patient weighs 75 kilograms before a session and their dry weight is 72 kilograms, the goal is to remove three liters of fluid, since one liter of fluid weighs slightly more than one kilogram. This calculation is performed before every treatment to set the ultrafiltration goal.
Determining the precise dry weight is often a clinical estimate, relying heavily on the medical team’s experience and the patient’s symptoms. The team assesses the patient for signs like swelling, listens to the lungs for fluid crackles, and monitors blood pressure readings. If a patient consistently experiences symptoms of fluid overload, the medical team may gradually increase the dry weight target to remove more fluid over several sessions.
The dry weight is not a fixed number and must be constantly re-evaluated over time. Changes in body composition, such as weight loss or gain unrelated to fluid, or improvements in heart function, necessitate an adjustment of the target. Open communication between the patient and the care team about symptoms like dizziness, cramping, or persistent swelling is essential for accurately maintaining this personalized fluid target.
Risks of Removing Too Much or Too Little Fluid
The ultrafiltration process requires a delicate balance, as removing fluid too quickly or failing to meet the target volume carries distinct and serious risks. One immediate risk is removing fluid too rapidly, which can cause the blood volume to drop precipitously. This condition is called hypovolemia and often leads to acute symptoms during the dialysis session itself.
When blood volume drops too quickly, the patient may experience a sudden, severe drop in blood pressure, known as hypotension. This rapid fluid shift can also cause painful muscle cramping, nausea, vomiting, dizziness, and fainting. Furthermore, a high rate of fluid removal has been associated with long-term damage to the heart muscle.
Conversely, consistently removing too little fluid between sessions results in persistent volume overload, often called under-dialysis. This failure to reach the dry weight target means the patient remains hypertensive, placing chronic stress on the cardiovascular system. The long-term effects of this include an increased risk of stroke and sustained heart enlargement, which can worsen heart failure over time.
Missing the dry weight target means the patient enters their next session with a greater fluid excess, which then requires an even faster removal rate, perpetuating a dangerous cycle. Therefore, the goal of ultrafiltration is to remove the appropriate volume at a safe, gradual rate to protect the patient’s organs and prevent both acute and chronic complications.