What is ultrafiltration in hemodialysis?

Ultrafiltration in Hemodialysis

Ultrafiltration in hemodialysis is the process of removing excess fluid from the blood across a semipermeable membrane to reduce volume overload, with fluid being removed almost exclusively from the extracellular fluid space. 1

Definition and Mechanism

Ultrafiltration is a fundamental component of hemodialysis therapy that specifically addresses fluid removal. Key characteristics include:

• It involves the movement of water and small-to-medium weight solutes across a semipermeable membrane 2
• The source of net fluid loss during ultrafiltration is almost exclusively the extracellular fluid space, with the intracellular fluid space remaining relatively unaffected immediately after treatment 1
• Because the electrolyte concentration in the ultrafiltrate is similar to plasma, relatively more sodium can be removed than by diuretics alone 2

Clinical Applications

Volume Management

• Ultrafiltration is essential for removing excess fluid accumulated between dialysis sessions
• It helps achieve the patient's "dry weight" - the weight at which a patient has normal extracellular fluid volume without signs of fluid overload or depletion 2
• Approaching dry weight should be done gradually over 4-12 weeks (may require up to 6-12 months in some patients) 2, 3

Considerations for Ultrafiltration Rate

• Ultrafiltration rate (UFR) should be carefully managed to avoid complications
• High UFR (>13 ml/kg per hour) is associated with increased mortality 4
• Patients who experience frequent episodes of high UFR have significantly lower survival rates compared to those who don't 4
• Some evidence suggests UFR might be better scaled to body surface area rather than body weight 5

Ultrafiltration Techniques

Standard Ultrafiltration

• Occurs simultaneously with diffusive clearance during regular hemodialysis
• Rate is determined by the patient's interdialytic weight gain and treatment time

Isolated/Sequential Ultrafiltration

• Separates ultrafiltration temporally from diffusive clearance 2
• Can be beneficial for patients who experience hypotension during combined ultrafiltration and dialysis
• Results in prompt and appropriate increases in stroke index, cardiac index, and mean arterial pressure 2
• May require extending total treatment time to compensate for time lost for diffusive clearance 2

Ultrafiltration Profiling

• Involves altering the ultrafiltration rate during the course of the dialysis session 6
• Can decrease subclinical repeated end-organ ischemia during dialysis 6
• Often used in conjunction with sodium profiling to improve hemodynamic stability 6

Managing Complications

Preventing Hypotension

When patients experience symptoms during ultrafiltration, consider:

1. Slowing the ultrafiltration rate, especially for patients with diabetes or cardiomyopathy 2, 3
2. Performing isolated ultrafiltration 2
3. Increasing dialysate sodium concentration (148 mEq/L) or implementing sodium modeling 2
4. Reducing dialysate temperature from 37°C to 34-35°C to increase peripheral vasoconstriction and cardiac output 2, 3
5. Administering midodrine predialysis 2
6. Correcting anemia to the recommended range 2

Monitoring During Ultrafiltration

• Regular assessment of pre- and post-dialysis weight
• Blood pressure monitoring
• Evaluation of clinical signs of fluid overload
• Monitoring for symptoms of hypotension during treatment 3

Special Considerations

Patients with Heart Failure

• Ultrafiltration may be considered for patients with obvious volume overload to alleviate congestive symptoms 2
• It can be considered for patients with refractory congestion not responding to medical therapy 2
• Ultrafiltration in heart failure has been shown to reduce neurohormone levels and increase diuretic responsiveness 2

Dietary Sodium Restriction

• Hemodialysis patients should limit sodium intake to 1.8-2.3g (80-100 mmol) per day 3
• Recommended daily sodium chloride intake is 4.7-5.8g to reduce interdialytic weight gain and blood pressure 3
• Water restriction alone is ineffective and may cause unnecessary thirst 3

Practical Implications

• Consultation with a nephrologist is appropriate before initiating ultrafiltration, especially when the provider lacks sufficient experience 2
• The need for veno-venous access, provider experience, and nursing support remain concerns about routine use of ultrafiltration outside of standard hemodialysis 2
• Extended dialysis sessions (5 hours per session, 3 times weekly) or alternative regimens may be considered for patients with persistent volume overload 3

By properly managing ultrafiltration, clinicians can help hemodialysis patients achieve optimal fluid balance while minimizing complications, ultimately improving morbidity, mortality, and quality of life.