What are the mechanics of hemodialysis in adults with end-stage renal disease, particularly those with a history of chronic conditions like diabetes or hypertension?

Mechanics of Hemodialysis

Core Transport Mechanisms

Hemodialysis removes uremic toxins and excess fluid through two fundamental physical processes: diffusion and convection, both occurring across a semipermeable membrane that separates blood from dialysate. 1, 2

Diffusion (Primary Mechanism for Small Solutes)

• Diffusion operates according to Fick's law, where solutes move down their concentration gradient from blood (high concentration) to dialysate (low concentration) across the semipermeable membrane 1, 3
• Small molecular weight solutes (urea, creatinine, potassium) are primarily removed by diffusion 1, 2
• Diffusive transport efficiency depends on three critical compartments: the blood compartment, the membrane itself, and the dialysate compartment 3
• The dialysate composition resembles plasma water with electrolytes adjusted to compensate for abnormalities of end-stage renal disease 4

Convection (Solvent Drag)

• Convection removes larger molecular-weight solutes through ultrafiltration and solvent drag, where solutes are carried along with plasma water as it moves across the membrane 1, 2
• Ultrafiltration is the primary mechanism for removing excess plasma water and achieving volume control 1, 5
• Internal filtration within high-flux dialyzers significantly enhances convective solute removal even during standard hemodialysis 3, 2

Membrane Technology and Filter Design

Membrane Characteristics

• Modern hollow fiber membranes dominate current practice, containing thousands of tiny capillary-like fibers through which blood flows 3, 2
• Membranes are classified by water permeability (low-flux vs. high-flux) and composition (cellulosic vs. synthetic) 2
• High-flux membranes with enhanced permeability allow greater removal of middle molecules while maintaining acceptable albumin retention 2, 3
• Newer membrane properties include hydrophilicity, adsorption capacity, and electrical potential, which affect biocompatibility and solute removal 2

Blood-Membrane-Dialysate Interface

• Blood flows through the inside of hollow fibers at 200-500 mL/min while dialysate flows countercurrent on the outside at 500-800 mL/min 3
• The countercurrent flow maximizes the concentration gradient across the membrane, optimizing diffusive clearance 1, 3
• Blood/membrane interactions critically influence filtration-based therapies and can affect biocompatibility 3

Ultrafiltration and Volume Management

Ultrafiltration Mechanics

• Ultrafiltration removes fluid by applying transmembrane pressure, forcing plasma water across the membrane while retaining blood cells and proteins 1, 5
• The ultrafiltration rate must not exceed the plasma refill rate from interstitial compartments to avoid intravascular volume depletion 6, 7
• Ultrafiltration rates exceeding 13 mL/kg/hour exceed plasma refill capacity and cause hypotension, myocardial stunning, and loss of residual kidney function 7

Critical Volume Control Principles

• Volume overload from sodium and water retention is the primary cause of hypertension in hemodialysis patients 5
• Achievement of dry weight through adequate ultrafiltration and sodium restriction (≤5.8 g sodium chloride daily) is essential for blood pressure control 5
• Excessive ultrafiltration is the leading cause of intradialytic hypotension, occurring in 20-30% of treatments 7, 6

Dialysis Prescription Parameters

Standard Thrice-Weekly Hemodialysis

• Target single-pool Kt/V of 1.4 per session with minimum delivered dose of 1.2 for patients treated three times weekly 5
• Minimum treatment time of 3 hours per session for patients with minimal residual kidney function (<2 mL/min) 5
• Blood flow rates typically 200-500 mL/min and dialysate flow rates 500-800 mL/min optimize clearance 3

Dialysate Composition

• Dialysate bicarbonate concentration should be 35-38 mmol/L to prevent metabolic acidosis, with target predialysis serum bicarbonate ≥22 mmol/L 8, 5
• Dialysate glucose concentration affects post-dialysis glycemic control; glucose-free dialysates are now preferred to avoid hyperglycemia 5
• Dialysate temperature reduction improves hemodynamic stability during ultrafiltration 6, 7

Special Populations and Complications

Diabetes and Hemodialysis

• Patients with diabetes on hemodialysis experience frequent hypoglycemia requiring insulin dose reduction of approximately 40-50% due to decreased renal insulin clearance and gluconeogenesis 5
• Insulin is the preferred antihyperglycemic agent for dialysis patients with diabetes requiring medication 9
• Glycemic variability increases during and after hemodialysis due to glucose removal, altered insulin clearance, and dialysate glucose concentration 5

Hemodynamic Management

• Intradialytic hypotension prevention requires limiting ultrafiltration rate, increasing treatment time, reducing dialysate temperature, and avoiding food during dialysis 7, 6
• Paradoxical intradialytic hypertension occurs in 5-15% of patients and requires strict sodium restriction and dry weight reassessment 7
• Maintaining mean arterial pressure in the normal range is critical to optimize forward flow and prevent end-organ damage 5

Critical Pitfalls to Avoid

• Never use aggressive ultrafiltration rates >13 mL/kg/hour, as this causes severe hypotension and accelerates loss of residual kidney function 7
• Do not ignore bicarbonate levels between 18-22 mmol/L—this range causes protein catabolism and bone disease requiring intervention 8
• Never delay emergency dialysis for life-threatening indications (uremic pericarditis, encephalopathy, refractory hyperkalemia) to achieve "optimal" vascular access; place temporary catheter immediately 7
• Preserve peripheral veins in patients with stage III-V chronic kidney disease for future arteriovenous access creation 9
• Avoid citrate-containing alkali preparations in dialysis patients, as they dramatically increase aluminum absorption 8