Hemodialysis is a life-sustaining treatment for individuals with kidney failure, replacing the kidneys’ function of filtering blood and balancing fluids. Ultrafiltration is the controlled removal of excess fluid that accumulates between treatments. This fluid removal is calculated to bring the patient to their “dry weight,” representing the body’s optimal fluid status without causing low blood pressure or fluid overload.
Determining this target dry weight is complex, often relying on clinical judgment, as it is the lowest weight a patient can tolerate without becoming hypotensive. If the ultrafiltration rate is set too aggressively based on an incorrect weight assessment or high fluid intake, it rapidly depletes the body’s circulating blood volume. This rapid removal causes Intradialytic Hypotension (IDH), which has immediate and long-term consequences for the patient’s health.
Acute Drop in Blood Pressure and Physical Symptoms
Intradialytic Hypotension is the most frequent complication during hemodialysis sessions. Clinically, IDH is defined as a rapid drop in systolic blood pressure of 20 mmHg or more, or a drop in mean arterial pressure of at least 10 mmHg. The rapid fall in blood pressure triggers acute physical symptoms as the body’s tissues are temporarily starved of adequate blood flow.
Patients commonly report lightheadedness or dizziness, which can progress to fainting in severe cases. Severe muscle cramping, particularly in the legs and abdomen, results from temporary lack of oxygen to the muscle tissue. A sharp drop in blood pressure often activates the autonomic nervous system, leading to uncomfortable symptoms like nausea, vomiting, anxiety, and excessive yawning or sighing.
The Physiological Mismatch Causing Complications
The underlying mechanism for these acute symptoms is a failure of the body’s internal fluid dynamics to keep pace with the ultrafiltration rate. Total body fluid is distributed across several compartments; only a fraction exists as circulating plasma volume within the blood vessels, while the majority is stored in the interstitial space. During dialysis, fluid is removed directly from the plasma compartment.
To maintain circulatory stability, fluid from the interstitial space must shift, or “refill,” back into the blood vessels to replenish the plasma volume. This process, known as the vascular refill rate, is relatively slow and limited, typically allowing the body to tolerate an ultrafiltration rate of no more than 10 mL per kilogram of body weight per hour. When the dialysis machine removes fluid faster than the vascular refill rate can compensate, the total circulating blood volume drops precipitously. This acute hypovolemia reduces the blood returning to the heart, leading to a sharp decrease in cardiac output and a rapid fall in systemic blood pressure.
Systemic Consequences of Repeated Fluid Stress
Beyond the immediate discomfort of low blood pressure, repeated episodes of aggressive fluid removal cause long-term damage to vital organs. The most studied consequence is cardiac stunning, which refers to a transient reduction in the heart muscle’s ability to contract effectively. Each time the blood pressure drops severely during dialysis, the resulting hypovolemia reduces blood flow to the heart muscle, causing temporary ischemia.
This reduced oxygen supply temporarily “stuns” the heart muscle, leading to regional wall motion abnormalities. While heart function may recover immediately after the session, repeated stunning episodes drive irreversible changes, including myocardial fibrosis and long-term loss of left ventricular function. This recurrent injury increases the patient’s risk of developing chronic heart failure, cardiovascular events, and overall mortality. Ultrafiltration rates exceeding 13 mL/kg/hour are associated with higher long-term mortality. Recurrent blood flow disruption also affects other organs, including the gut, which can lead to ischemic colitis, and the brain, increasing the risk of silent strokes due to temporary cerebral hypoperfusion.
Management and Prevention Strategies
When Intradialytic Hypotension occurs, the immediate goal is to stabilize blood pressure and restore circulating volume. Medical staff first cease or significantly reduce the ultrafiltration rate to stop further fluid depletion. They may also place the patient in the Trendelenburg position to promote blood flow back toward the heart and increase venous return. If these measures are insufficient, rapid administration of isotonic saline or other intravenous fluid is used to quickly restore the intravascular volume.
For long-term prevention, the primary strategy is accurately determining and regularly reassessing the patient’s dry weight. The care team can also modify the dialysis prescription by extending treatment time or increasing session frequency to allow for a slower, more tolerable ultrafiltration rate. Using features like ultrafiltration profiling or biofeedback monitoring helps avoid aggressive fluid shifts. Patient adherence to fluid and dietary sodium restrictions between sessions is essential to limit the interdialytic weight gain that necessitates high ultrafiltration rates.