Dialyzer Math

The Math Terms of Hemodialysis Kuf Efficiency Flux Performance KoA Kt/V URR Recirculation

KUf Permeability of Water mL of water per hour per mm Hg KUf = Volume/hrs / (transmembrane gradient)

KUf Questions What Transmembrane Pressure (TMP) is needed to ultrafilter 3 L of fluid over 3 hours with an F-40 filter? 50 mmHg What Transmembrane Pressure (TMP) is needed to ultrafilter 3.5 L of fluid over 3 hours with an 8L filter? 103 mmHg

Self-Assessment KUf Question Pre-HD weight in a patient on chronic HD is 2.4 kg above dry weight. He is usually dialyzed for 4 hours. If the KUf of the dialyzer is 3, what is the TMP in mmHg required to bring his weight down to the dry weight? 100 200 300 400

Self-Assessment KUf Question Pre-HD weight in a patient on chronic HD is 2.4 kg above dry weight. He is usually dialyzed for 4 hours. If the KUf of the dialyzer is 3, what is the TMP in mmHg required to bring his weight down to the dry weight? 100 200 300 400

Clearance K K = f (KoA, QD, QB)

Self-Assessment Question The solute transport of a dialyzer is dependent on all of the following except: KoA QB QD Priming volume of the dialyzer

Self-Assessment Question The solute transport of a dialyzer is dependent on all of the following except: KoA QB QD Priming volume of the dialyzer

Efficiency - KoA Dialyzer Mass Transfer Area Coefficient KoA = f (Ko, surface area) Ko = f (pore size, membrane thickness) mL/min Usual efficiency: 500-700 High efficiency: > 700

Using KoA Once KoA of a dialyzer is known, the clearance (K) of urea can be calculated given the QD and the QB Charts or Graphs available for each dialyzer

Getting the K from Graphs

Using the dialyzer with KoA of 400 shown below, what’s the clearance of urea at a QD of 500 mL/min and a QB of 350 mL/min?

So What’s Flux, Performance & Efficiency? Efficiency: KoA Performance: Clearance Flux: KUf

Flux Refers to size of pores High Flux is KUf > 20 mL/hr/mmHg Urea Clearance Same for high-flux vs high efficiency Beta-2 Microglobulin More clearance with high-flux

Ways to Measure Adequacy URR Kt/V

URR The Urea Reduction Ratio (Pre BUN – Post BUN ) / Pre BUN R = 1- URR

Simple Kt/V R = e – Kt/V R = 1 – URR

Kt/V Calculations Good: Kt/V = -ln(R) Better: Kt/V = -ln(R-0.008 * t) Better Yet: Kt/V = -ln (R - 0.008 * t) (spKt/V) + (4 - 3.5 * R) * UF/W Best: In-line monitoring

spKt/V Formula Breakdown Urea generation during dialysis -ln (R - 0.008 * t) + (4 - 3.5 * R) * UF/W Raw Urea Reduction Adjustment For Volume Removed

Problem A new patient weighing 60 kg comes in with newly discovered ESRD. The BUN is 120. For the first run, you choose to drop the BUN no more than 20% using a small surface area dialyzer with K of 250 mL/min. A: How long should the treatment be? B: What’s the URR? 33 min

Tougher Problem A new patient weighing 25 kg comes in with newly discovered ESRD. The BUN is 100. For the first run, you choose to drop the BUN no more than 20% using an F-40 filter at a QB of 200 mL/min and QD of 500 mL/min. A: What’s the Urea Clearance (K)? B. How long should the treatment be? 20 min

All the following are true of high/low flux hemodialyzers except: A. Conventional HD uses high-flux membranes B. High efficiency HD refers to a rapid removal of small solutes C. Low flux HD is good for urea clearance D. High flux HD uses membranes with large pore size

All the following are true of high/low flux hemodialyzers except: A. Conventional HD uses high-flux membranes B. High efficiency HD refers to a rapid removal of small solutes C. Low flux HD is good for urea clearance D. High flux HD uses membranes with large pore size

Recirculation Running the same blood through the dialyzer over and over Overestimates adequacy Easiest Measure: 3-Sample Method Peripheral (P) Arterial from Dialyzer (A) Venous from Dialyzer (V) (P-A) / (P-V) = Recirculation Accepted Recirc % is ~ 12%

Recirculation Question A 16 year old being dialyzed daily in the PICU via a Shiley in the PICU has pre-dialysis BUN of 120 mg/dL and post-HD BUN of 30 mg/dL despite restriction of protein and no evidence of catabolism. You suspect recirculation. What samples do you ask to be drawn?

Recirculation Question Pre-Dialyzer BUN: 100 Post-Dialyzer BUN: 65 Peripheral Venous BUN: 115 (115 – 100 ) 15 ---------------- = ---------- = 30 % (115 - 65) 50