Intermittent Hemodialysis (IHD) in Critical Care


πŸ“Œ What is IHD?

Intermittent Hemodialysis (IHD) is a renal replacement therapy that removes waste products, excess fluid, and electrolytes over a short period β€” typically 3–5 hours/session, 3–6 times per week.

It mimics native kidney function but in a non-continuous fashion, using the principles of diffusion, ultrafiltration, and convection.


πŸ§ͺ Core Principles

Principle

Description

Diffusion

Movement of solutes (e.g., urea, creatinine, K⁺) across semipermeable membrane down a concentration gradient

Ultrafiltration

Water removal via transmembrane pressure (controlled by dialyzer)

Convection (minor)

Solute dragged with water movement



🧠 Indications for IHD in ICU

Similar to AEIOU criteria but preferred in hemodynamically stable patients:


βš™οΈ Components of IHD Setup

Component

Function

Dialyzer (“artificial kidney”)

Hollow fiber membrane for solute/fluid exchange

Blood pump

Maintains flow rate (200–400 mL/min)

Dialysate pump

Delivers dialysate (500–800 mL/min)

Vascular access

Double-lumen catheter in large vein (e.g., R IJV, femoral)

Water purification system

Removes toxins and endotoxins from dialysate

Heparin pump (optional)

For anticoagulation during dialysis



πŸ“ Typical IHD Prescription in ICU

Parameter

Default/Adjustable

Duration

3–4 hours

Frequency

Daily or every other day (not fixed like CKD)

Blood flow rate (Qb)

200–300 mL/min

Dialysate flow rate (Qd)

500–800 mL/min

Dialyzer type

High-flux (better clearance of middle molecules)

UF goal

Based on fluid balance; usually 500–2000 mL/session

Anticoagulation

Unfractionated heparin bolus 500–1000 IU/h (omit if bleeding risk)



πŸ’‰ Dialysate Composition (Typical)

Component

Range

Na⁺

135–145 mEq/L

K⁺

1–4 mEq/L (lower in hyperkalemia)

Ca²⁺

1.25–1.75 mmol/L

HCO₃⁻

32–35 mEq/L

Glucose

~100 mg/dL


πŸ“Œ Modify K⁺ and HCO₃⁻ depending on patient’s labs and clinical need.

πŸ’‰ Anticoagulation in IHD

  • Unfractionated heparin: bolus 1000–2000 IU, then 500 IU/h
  • No anticoagulation: if active bleeding or high-risk (short sessions)
  • Citrate anticoagulation: rarely used in IHD, more for CRRT



πŸ”¬ Solute Clearance in IHD

  • Urea Reduction Ratio (URR) and Kt/V are standard metrics in CKD


πŸ“‰ 1. Urea Reduction Ratio (URR)

πŸ”Ή Definition:

URR measures how much blood urea nitrogen (BUN) is reduced by a single dialysis session.

πŸ”Ή Formula:

URR=(BUNpre BUNpre βˆ’BUNpost )Γ—100

  • BUNβ‚šα΅£β‚‘: blood urea nitrogen before dialysis
  • BUNβ‚šβ‚’β‚›β‚œ: blood urea nitrogen after dialysis

πŸ”Ή Interpretation:

  • URR > 65% is generally considered adequate for thrice-weekly HD
  • Example:
    Pre-dialysis BUN = 80 mg/dL
    Post-dialysis BUN = 30 mg/dL
    URR = (80 – 30) / 80 Γ— 100 = 62.5%


πŸ”¬ 2. Kt/V

πŸ”Ή Definition:

Kt/V is a dimensionless number that reflects the fraction of total body water (V) cleared of urea 

  • K = urea clearance (mL/min, based on dialyzer and flow rates)
  • t = duration of dialysis (minutes)
  • V = volume of distribution of urea β‰ˆ total body water (in mL)


πŸ”Ή Interpretation of Kt/V:

  • Kt/V β‰₯ 1.2 per session (3x/week) = adequate
  • Kt/V 1.4–1.6 = more optimal
  • Low Kt/V means inadequate dialysis β†’ leads to uremia, poor outcomes


🧠 URR vs Kt/V – Quick Comparison:

Feature

URR

Kt/V

What it measures

% fall in BUN

Volume of urea cleared per TBW

Simplicity

Easier to calculate

More accurate but complex

Includes time?

No

Yes

Adjusts for body size

No

Yes (via V)

Adequacy target

>65%

β‰₯1.2 per session



❌ Complications of IHD

Type

Examples

Hemodynamic

Hypotension (most common), arrhythmia

Electrolyte

Hypokalemia, hypocalcemia, hypophosphatemia

Dialysis disequilibrium syndrome

Cerebral edema, confusion, seizures

Infection

Catheter-related bloodstream infection (CRBSI)

Bleeding

From anticoagulation

Thrombosis

Clotting of dialyzer or access