Why is blood volume high in a patient with end-stage renal disease (ESRD) or acute kidney injury (AKI) undergoing dialysis despite a high ultrafiltration (UF) rate?

Why Blood Volume Remains High Despite High Ultrafiltration Rates

The most common reason blood volume remains elevated despite aggressive ultrafiltration is that the rate of plasma refilling from the interstitial space cannot keep pace with the speed of fluid removal from the vascular compartment, triggering compensatory mechanisms that prevent further volume depletion and often necessitating premature termination of ultrafiltration. 1

Primary Physiological Mechanisms

Plasma Refilling Rate Limitation

• When ultrafiltration rates exceed approximately 10 ml/h/kg, the vascular compartment depletes faster than fluid can shift from the interstitial space back into the bloodstream, causing intradialytic hypotension that forces clinicians to slow or stop ultrafiltration before adequate total body volume is removed 1, 2
• This mismatch between ultrafiltration speed and plasma refilling capacity means the patient experiences intravascular volume depletion (causing hypotension) while still maintaining excess total body fluid in the interstitial compartment 3, 4
• The result is a paradoxical situation: high ultrafiltration rates cause hemodynamic instability that prevents achievement of true dry weight, leaving the patient volume overloaded despite aggressive attempts at fluid removal 5

Inadequate Cardiovascular Compensation

• Insufficient cardiovascular compensatory responses (particularly in patients with cardiomyopathy, diabetes with autonomic dysfunction, or left ventricular hypertrophy) prevent adequate vasoconstriction and cardiac output augmentation during rapid fluid removal 1
• These patients develop hypotension at ultrafiltration rates that would be tolerated by those with intact cardiovascular reflexes, forcing premature cessation of fluid removal 5, 4

Common Clinical Scenarios

Saline Administration During Dialysis

• When hypotension occurs during dialysis, clinicians frequently administer normal saline and slow or discontinue ultrafiltration temporarily 1
• This intervention not only fails to remove the originally targeted fluid excess, but the infused saline further expands extracellular volume, worsening the volume overload 1
• The patient ends the session with higher total body volume than the pre-dialysis target 1

Insufficient Dialysis Time

• Conventional dialysis time (typically 3-4 hours, three times weekly) is too short for patients with large interdialytic weight gains to achieve adequate ultrafiltration without exceeding safe ultrafiltration rates 1
• Attempts to accelerate ultrafiltration to compensate for limited time precipitate hypovolemia and hypotension, preventing adequate total volume removal 1

Excessive Interdialytic Sodium and Water Intake

• High sodium intake (>5.8 g sodium chloride daily) stimulates thirst and promotes fluid consumption, resulting in interdialytic weight gains >4.8% of body weight, which are associated with increased mortality 1
• These large interdialytic gains require ultrafiltration rates that exceed physiologically tolerable limits, creating the cycle described above 1

Critical Management Errors to Avoid

Attempting Rapid Dry Weight Achievement

• The most dangerous pitfall is attempting to achieve true dry weight in a single dialysis session through aggressive ultrafiltration 5
• True dry weight reduction must occur gradually over 4-12 weeks (or even 6-12 months in some patients), reducing dry weight by only 0.1 kg per 10 kg body weight per dialysis session 5
• Rapid dry weight reduction with frequent hypotensive episodes causes seizures, myocardial stunning, and accelerated loss of residual kidney function 5, 4

Misinterpreting Hypotension as Achievement of Dry Weight

• Hypotension during dialysis does not necessarily indicate the patient has reached true dry weight; it often indicates the ultrafiltration rate exceeds the plasma refilling rate 5
• If the patient has clear signs of volume overload (hypertension, edema, elevated interdialytic weight gains) but develops hypotension during dialysis, the issue is ultrafiltration rate tolerance, not total volume status 5

Evidence-Based Solutions

Extend Dialysis Time

• The most effective strategy is lengthening dialysis sessions or adding additional treatments to lower ultrafiltration rates below 10 ml/h/kg while achieving the same total volume removal 1, 6
• Ultrafiltration rates >10 ml/h/kg are associated with 59% increased all-cause mortality and 71% increased cardiovascular mortality compared to rates <10 ml/h/kg 2
• Slow continuous ultrafiltration produces significantly smaller variations in blood pressure and blood volume compared to rapid intermittent ultrafiltration, even with similar total fluid removal 3

Sodium Restriction

• Implement strict dietary sodium restriction to <2 g/day (equivalent to <5.8 g sodium chloride or <100 mmol sodium) to reduce interdialytic weight gains 1, 6
• Lower dialysate sodium concentration to 135-138 mmol/L to facilitate sodium removal without stimulating thirst 6

Immediate Response to Intradialytic Hypotension

• When hypotension occurs, increase the dry weight target by 0.3-0.5 kg and reduce ultrafiltration rate for the remainder of the current session 5
• Reassess volume status between sessions looking for clinical signs of fluid overload (edema, hypertension, elevated jugular venous pressure) before the next dry weight adjustment 5

Special Populations Requiring Slower Approach

Patients with diabetes mellitus (autonomic dysfunction), cardiomyopathy, or congestive heart failure require even slower dry weight reduction (potentially 6-12 months) due to severely impaired compensatory mechanisms 5, 4, 2