Dialysis - A Complete Guide

Why is Dialysis Needed?

How Dialysis Works - Core Mechanisms

1. Diffusion

2. Ultrafiltration / Convection

The Two Main Types of Dialysis

1. Hemodialysis (HD)

• Arteriovenous (AV) fistula - surgically joining an artery and vein, usually in the forearm. This is the preferred access as it is durable and has the lowest infection risk, but requires weeks to months to mature after creation.
• AV graft - a synthetic tube connecting artery to vein; used when fistula creation is not possible.
• Tunneled dialysis catheter - placed in the internal jugular or subclavian vein; used as a bridge while fistula matures (up to 6 months), but carries higher infection risk.
• Non-tunneled catheter - temporary access in femoral or jugular vein, used short-term (1-2 weeks) in the inpatient setting.

• 3-4 hours per session, three times per week for outpatient ESRD management
• Can remove approximately 3-4 L of fluid per session safely in hemodynamically stable patients
• Blood and dialysis solution flow in opposite directions (countercurrent) to maximize the concentration gradient and efficiency

URR = [(Pre-BUN - Post-BUN) / Pre-BUN] × 100

2. Peritoneal Dialysis (PD)

• The osmotic agent in most PD solutions is dextrose (1.5 to 4.25 g/dL), which draws fluid from blood vessels into the peritoneal cavity.
• Icodextrin is a non-glucose alternative that avoids hyperglycemia and offers better ultrafiltration, though it can interfere with blood glucose meter readings.
• Medications like insulin and antibiotics can be given directly via the intraperitoneal route for smoother absorption.

• Greater patient independence (done at home)
• No anticoagulation required
• Smoother, continuous solute removal without the rapid fluid and electrolyte shifts of HD
• Better for hemodynamically unstable patients

Continuous Renal Replacement Therapy (CRRT)

• CVVHD (continuous venovenous hemodialysis): diffusive clearance only
• CVVHDF (continuous venovenous hemodiafiltration): both diffusive and convective clearance
• SLED (sustained low-efficiency dialysis): 8-12 hours, a middle ground allowing some off-machine time for procedures

Indications for Emergency/Urgent Dialysis

Hemodialysis vs Peritoneal Dialysis - Comparison

Complications of Dialysis

• Hypotension during treatment - the most common; can worsen AKI by causing ischemic injury
• Access infections - fever and rigors during dialysis warrant empiric anti-staphylococcal coverage
• Disequilibrium syndrome - neurological symptoms from rapid osmotic shifts, especially in new patients
• Air embolism - rare but potentially fatal
• Clotting of the circuit - requires anticoagulation

• Peritonitis - abdominal pain, cloudy effluent; usually treated with intraperitoneal antibiotics
• Catheter-related infections - exit-site or tunnel infections
• Ultrafiltration failure - membrane loses efficiency over time
• Hernia formation - from raised intraabdominal pressure

Key Points Summary

• Dialysis replaces the filtration function of failed kidneys but not their hormonal functions (EPO production, vitamin D activation)
• Transport mechanisms: diffusion (waste removal down gradient) + ultrafiltration (fluid removal by pressure)
• Hemodialysis is more efficient but requires vascular access and intermittent sessions
• Peritoneal dialysis is gentler, home-based, and continuous, but risks peritonitis
• CRRT is reserved for hemodynamically unstable ICU patients
• Transplantation remains the best long-term treatment for ESRD when feasible

Adjusting the Hemodialysis Machine - The Prescription

The Components of a Hemodialysis Prescription

1. Treatment duration
2. Frequency
3. Vascular access
4. Dialyzer selection
5. Blood flow rate (Qb)
6. Dialysate flow rate (Qd)
7. Ultrafiltration rate / target
8. Dialysate composition
9. Anticoagulation
10. Dialysate temperature
11. Intradialytic medications

1. Treatment Duration (Time on Machine)

• Never prescribe less than 3 hours - shorter sessions cause rapid electrolyte shifts, increased UF rate, hypotension, and myocardial stunning.
• Reducing session length below 3 hours reduces clearance of larger molecules (e.g., beta-2 microglobulin), increases cardiovascular risk, and increases intradialytic symptoms (nausea, cramps).
• Observational data show strong survival benefit for sessions ≥ 4 hours, partly because a lower UF rate (below 10 mL/kg/hour) improves cardiovascular stability.
• Efficiency declines after 4-5 hours (urea rebounds), so sessions >5 hours show diminishing returns for small-molecule clearance.
• Longer/more frequent sessions are used for: larger patients, severe volume overload, hyperphosphatemia, or when a higher Kt/V target is needed. - NKF Primer on Kidney Diseases, p. 600-601

2. Dialyzer Selection

• Membrane surface area - larger = higher clearance. Adjust up for larger patients or when Kt/V is inadequate.
• High-flux vs low-flux - high-flux membranes are now standard; they remove larger middle molecules (beta-2 microglobulin, cytokines) that low-flux membranes cannot.
• KoA (mass transfer coefficient) - a fixed property of the dialyzer indicating urea clearance capacity; choose a dialyzer with higher KoA when blood or dialysate flow rates cannot be increased further. - Comprehensive Clinical Nephrology 7th Ed., p. 1295

3. Blood Flow Rate (Qb)

• Increasing Qb directly increases solute clearance and dialysis efficiency (Kt/V).
• Minimum: 250 mL/min for adequate therapy (KDOQI).
• Typical target: 300-400 mL/min for most patients.
• Limited by the vascular access: AV fistulas can usually support 400+ mL/min; catheters are often limited to 300-350 mL/min due to flow resistance.
• When to increase Qb: Inadequate Kt/V or URR, large patient (high urea distribution volume V), or when Kt/V target is not being met.
• When to reduce Qb: Access dysfunction (poor arterial inflow), recirculation, or catheter malfunction.

Higher Qb = better clearance, but the gain diminishes at very high flow rates because the blood-dialysate concentration gradient falls as dialysate becomes saturated. - NKF Primer, p. 600

4. Dialysate Flow Rate (Qd)

• Counter-current flow maintains the concentration gradient between blood and dialysate throughout the dialyzer, maximizing diffusive clearance.
• At a Qb of 300 mL/min, increasing Qd from 500 to 800 mL/min raises urea clearance by approximately 10-15%.
• Adjustment: When Kt/V remains low despite adequate Qb and treatment time, increasing Qd to 800 mL/min is a simple intervention.
• At very high Qb (>400 mL/min), Qd may need to match proportionally to maintain gradient efficiency. - Tintinalli's Emergency Medicine

5. Ultrafiltration (UF) Rate and Target

Setting the UF Target:

1. Weigh the patient on arrival.
2. Compare to their dry weight (euvolemic target weight, assessed clinically).
3. The difference = fluid to remove. E.g., if a patient gained 3 kg since last session, set UF target = 3 L.

UF Rate:

• UF target is spread evenly over the treatment time.
• Safe limit: ≤ 10 mL/kg/hour - exceeding this is associated with intradialytic hypotension, myocardial stunning, and increased mortality.
• Example: 70 kg patient needing 3 L removed over 4 hours = 3000/4 = 750 mL/hr = 10.7 mL/kg/hr - slightly high, so consider extending session to 4.5 hours.

When UF rate is too high:

• Extend treatment time - the primary intervention.
• Reduce interdialytic weight gain through sodium/fluid restriction counseling.
• Use sodium modeling (higher dialysate Na early in session) cautiously - maintains intravascular volume but increases thirst and interdialytic weight gain.
• Lower dialysate temperature (see below) to improve hemodynamic tolerance. - Washington Manual of Medical Therapeutics, p. 503-504

6. Dialysate Composition

A. Sodium (Standard: 138-140 mEq/L)

• Standard practice: match dialysate Na to patient's predialysis serum Na to avoid osmotic stress.
• Sodium modeling (ramping): Starting with higher Na (e.g., 145 mEq/L) and tapering down during the session - helps hemodynamic stability but increases weight gain and BP; now largely abandoned except in select patients.

B. Potassium (Individualized: 1-4 mEq/L)

• Optimal predialysis serum K: 4.6-5.3 mEq/L.
• Potassium modeling (gradual step-down during session) may reduce arrhythmia risk by keeping the blood-to-dialysate gradient constant. - Brenner and Rector's The Kidney, p. 2734

C. Calcium (Standard: 1.25-1.5 mmol/L)

D. Bicarbonate (Standard: 35-40 mEq/L)

• Target: predialysis serum bicarbonate 22-26 mEq/L.
• Excessively high dialysate bicarb causes post-session alkalosis, which paradoxically increases mortality.

E. Glucose (Standard: 5.5 mmol/L, ~100 mg/dL)

• Matched to plasma glucose to avoid osmotic-driven fluid shifts.
• Lower in diabetics if intradialytic hyperglycemia is a concern.
• Higher may reduce disequilibrium risk in new patients.

F. Dialysate Temperature (Standard: 36.5-37°C)

• Lowering to 35-36°C significantly reduces intradialytic hypotension by causing peripheral vasoconstriction and improving cardiac output.
• Used routinely for hypotension-prone patients - a simple, effective adjustment that costs nothing. - Brenner and Rector's The Kidney, p. 2733

7. Anticoagulation

• Standard: Unfractionated heparin (UFH)
  • Loading dose: 1,000-5,000 units IV bolus at start
  • Maintenance: 500-1,500 units/hour during session
  • Stopped 30-60 minutes before end to allow clotting at needle sites
• Heparin-free dialysis: For patients with active bleeding or post-surgery - rinse circuit with saline periodically; higher flow rates to prevent clotting
• Regional citrate anticoagulation: Used in CRRT; citrate chelates calcium in the circuit; ionized calcium must be monitored closely
• LMWH: Single pre-session dose; popular in Europe; not easily reversible
• Alternatives for HIT: Argatroban, bivalirudin, lepirudin - Brenner and Rector's The Kidney, p. 2733

8. Measuring Adequacy and Adjusting the Prescription

1. First, check if the session was representative - was it cut short? Was there access recirculation?
2. Check for vascular access problems (most common cause of low Kt/V).
3. Check for blood sampling errors (delayed post-HD sampling gives falsely low Kt/V).
4. Then adjust:
   • Increase treatment time (most effective)
   • Increase blood flow rate (Qb)
   • Increase dialysate flow rate (Qd)
   • Switch to a larger or more efficient dialyzer (higher KoA, higher surface area)

• Comprehensive Clinical Nephrology 7th Ed., p. 1295-1296

Quick Reference: Adjustment Summary