Explain hemodialysis
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hemodialysis mechanism procedure indications complications overview
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Hemodialysis
Hemodialysis (HD) is an extracorporeal blood purification technique used to replace the filtration, excretion, and regulatory functions of the failing kidneys. It is the most widely used form of kidney replacement therapy (KRT) worldwide, accounting for over 85% of all dialysis patients in countries like the United States and Japan.
1. Physiological Basis
The kidney normally removes uremic solutes, excess fluid, acids, and electrolytes from blood. In end-stage kidney disease (ESKD), these accumulate and cause the uremic syndrome. HD corrects this through two core transport mechanisms:
Diffusion
- The primary mechanism for small solute removal (urea, creatinine, potassium, phosphate).
- Solutes move down a concentration gradient from blood to dialysate across a semipermeable membrane.
- Efficacy depends on the concentration gradient, membrane surface area and permeability, blood and dialysate flow rates, and solute molecular weight.
- Higher molecular weight solutes diffuse more slowly.
Ultrafiltration (Convection)
- Fluid removal driven by a hydrostatic pressure gradient across the membrane (transmembrane pressure, TMP).
- Solutes are "dragged" along with water (solvent drag) - this is the convective mechanism.
- Particularly effective for removing middle molecules (molecular weight 500-60,000 Da), such as beta-2 microglobulin.
- Hemodiafiltration (HDF) combines diffusion and convection for enhanced middle-molecule clearance.
2. The Dialyzer (Artificial Kidney)
The dialyzer is the core component of the HD circuit. It consists of thousands of hollow fibers made of a semipermeable membrane. Blood flows through the inside of the fibers, while dialysate flows in the opposite direction (countercurrent) on the outside.
Membrane types:
- Low-flux membranes: good small-solute clearance; limited middle-molecule removal.
- High-flux membranes: enhanced middle-molecule clearance; recommended as standard of care.
- Medium cut-off (MCO) / high cut-off membranes: expanded middle-molecule clearance; pore size cut-off ~65 kDa.
- Modern membranes are predominantly made of synthetic materials (e.g., polysulfone), which are more biocompatible than older cellulosic membranes.
Key parameter: KoA (urea mass transfer area coefficient) - reflects the dialyzer's efficiency for urea removal.
3. The Extracorporeal Circuit
- Blood is drawn from the patient via vascular access.
- A blood pump drives blood through the dialyzer at 250-500 mL/min.
- Dialysate flows countercurrently at 500-800 mL/min.
- Purified blood returns to the patient.
- Heparin (or alternative anticoagulation) is infused to prevent clotting in the circuit.
- Air detectors and pressure monitors ensure safety.
4. Dialysate Composition
The dialysate is a carefully formulated electrolyte solution. Its composition directly determines what is removed from (or added to) the blood:
| Component | Typical Concentration | Purpose |
|---|---|---|
| Sodium | 135-145 mEq/L | Osmotic balance, blood pressure |
| Potassium | 1-4 mEq/L | Removal of excess K+ |
| Calcium | 2.5-3.5 mEq/L | Avoid hypo/hypercalcemia |
| Bicarbonate | 30-40 mEq/L | Correct metabolic acidosis |
| Glucose | ~100 mg/dL | Prevent hypoglycemia |
| Magnesium | ~0.5 mEq/L | Balance |
Bicarbonate-based dialysate is standard. Acetate-based dialysate is now rarely used.
Water treatment is mandatory - tap water must be purified (via reverse osmosis and deionization) to avoid contamination with heavy metals, chloramine, aluminum, bacteria, and endotoxins, which would freely cross the dialysis membrane.
5. Vascular Access
Good vascular access is essential for effective HD. The three main types:
| Access | Description | Preferred? |
|---|---|---|
| Arteriovenous Fistula (AVF) | Surgical anastomosis of artery to vein (usually radial artery + cephalic vein) | Yes - gold standard; best longevity and lowest infection risk |
| Arteriovenous Graft (AVG) | Synthetic tube connecting artery to vein; used when native vessels unsuitable | Second choice |
| Central Venous Catheter (CVC) | Tunneled or non-tunneled; placed in internal jugular, subclavian, or femoral vein | Last resort; highest infection and thrombosis risk |
For tunneled CVCs, the right internal jugular vein is the preferred site - it provides a straight route to the right atrium, minimizing central vein stenosis risk. Ultrasound guidance is used for cannulation. Fluoroscopy confirms correct tip placement at the level of the right atrium - Comprehensive Clinical Nephrology, 7th Ed.
6. HD Prescription and Dosing
Standard prescription:
- Frequency: 3 sessions per week (in-center conventional HD)
- Session duration: minimum 4 hours
- Blood flow rate: at least 250 mL/min (typically 300-500 mL/min)
- Dialysate flow rate: 500-800 mL/min
- Dialyzer: high-flux recommended
Dose measurement - Kt/V:
- The standard metric for HD adequacy.
- K = dialyzer urea clearance (mL/min), t = treatment time (min), V = urea distribution volume (patient body water, ~60% of body weight).
- Target: spKt/V ≥ 1.4 per session (minimum 1.2) for thrice-weekly HD per KDOQI guidelines.
- For other schedules: target stdKt/V ≥ 2.1 per week.
- Alternative metric: urea reduction ratio (URR) ≥ 65%.
Urea Reduction Ratio (URR):
URR = (pre-BUN - post-BUN) / pre-BUN × 100%
- Comprehensive Clinical Nephrology, 7th Ed., p. 1295
7. Indications for Hemodialysis
Chronic indications (ESKD):
- eGFR consistently <10-15 mL/min/1.73 m² with uremic symptoms
- The "AEIOU" emergency indications apply regardless of GFR:
- Acidosis (metabolic, refractory)
- Electrolyte abnormalities (hyperkalemia refractory to treatment)
- Ingestion/Intoxication (certain dialyzable poisons: lithium, salicylates, methanol, ethylene glycol, metformin)
- Overload (pulmonary edema refractory to diuretics)
- Uremia (pericarditis, encephalopathy, bleeding)
Acute indications (AKI):
- Oliguria/anuria refractory to fluid management
- Uremic complications
- Refractory hyperkalemia (K+ > 6.5 mEq/L)
- Severe metabolic acidosis (pH < 7.1)
- Pulmonary edema
8. Contraindications
Contraindications to HD are few (Comprehensive Clinical Nephrology, 7th Ed.):
| Contraindication | Type |
|---|---|
| No vascular access possible | Absolute |
| Difficult vascular access | Relative |
| Needle phobia | Relative |
| Cardiac failure | Relative |
| Severe coagulopathy | Relative |
9. Complications
Intradialytic (during the session)
| Complication | Frequency | Notes |
|---|---|---|
| Hypotension | 15-30% of sessions | Most common; due to rapid fluid removal, autonomic dysfunction, low osmolarity |
| Muscle cramps | Common | Related to fluid shifts and electrolyte changes |
| Dialysis disequilibrium syndrome | Rare | Cerebral edema from rapid urea/osmolarity shifts; risk in first sessions or severe uremia |
| Air embolism | Rare | Prevented by air detectors in circuit |
| Anaphylaxis / first-use syndrome | Uncommon | Reaction to membrane material or ethylene oxide sterilization |
| Arrhythmias | Not uncommon | Related to electrolyte shifts (especially K+) |
| Hemorrhage | Uncommon | From access sites or anticoagulation |
| Hypoxemia | Occurs | Associated with greater morbidity/mortality |
Dialysate cooling (setting dialysate temperature to 0.5°C below body temperature) significantly reduces intradialytic hypotension - an RCT showed cardiac and brain white matter protection with this approach. - Comprehensive Clinical Nephrology, 7th Ed., p. 1288
Chronic complications (long-term HD patients)
- Cardiovascular disease: leading cause of death in dialysis patients; accelerated atherosclerosis, LV hypertrophy, cardiomyopathy
- Anemia: treated with erythropoiesis-stimulating agents (ESAs) and IV iron; target Hb typically 10-12 g/dL
- Mineral bone disease (CKD-MBD): hyperphosphatemia, secondary hyperparathyroidism, vascular calcification
- Malnutrition: protein-energy wasting common; amino acids lost in dialysate
- Vascular access complications: thrombosis, infection, stenosis, aneurysm
- Infection: bacteremia (especially with CVCs); leading infectious complication
- Dialysis dementia: historically linked to aluminum toxicity in dialysis water (now rare with water purification)
- Beta-2 microglobulin amyloidosis: accumulation of beta-2 microglobulin in joints/tendons after years on dialysis (carpal tunnel syndrome, destructive arthropathy)
- Neurological: peripheral neuropathy, restless leg syndrome
10. Variants of HD
| Type | Description |
|---|---|
| Conventional in-center HD | 3×/week, 4 hours; most common globally |
| Home HD (HHD) | Performed by patient at home; better BP control, LV geometry, phosphate balance, quality of life |
| Short daily HD | 6×/week, 2-3 hours; improved uremic toxin removal |
| Nocturnal HD | 5-6 nights/week, 6-8 hours; best phosphate removal, may require phosphate supplementation |
| Hemodiafiltration (HDF) | Combines diffusion + high-volume convection; superior middle-molecule clearance |
| Continuous Kidney Replacement Therapy (CKRT) | Slower, continuous; used in hemodynamically unstable ICU patients with AKI |
| Intermittent HD for poisoning | Rapid elimination of small, water-soluble, low-protein-bound toxins |
11. HD vs. Peritoneal Dialysis
Most patients with kidney failure are suitable for either modality. No RCT has consistently shown survival advantage for either. PD is preferred in patients with cardiovascular instability or vascular access problems; HD is preferred when adequate dialysis cannot be achieved with PD or PD is contraindicated.
Summary
Hemodialysis works by circulating blood through an extracorporeal circuit where it comes into contact with a dialysate solution across a semipermeable membrane. Diffusion removes uremic solutes down their concentration gradient; ultrafiltration removes excess fluid. Adequate delivery (Kt/V ≥ 1.4 per session) requires appropriate vascular access, dialyzer selection, and blood/dialysate flow rates. The most common acute complication is intradialytic hypotension, while long-term complications center on cardiovascular disease and mineral metabolism.
Sources:
- Comprehensive Clinical Nephrology, 7th Edition (Chapters 95, 97, 98, 99, 103)
- Brenner and Rector's The Kidney, 2-Volume Set
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