Background
Methodology
Characteristics of the membrane and filter
Geometric characteristics
Multidimensional characteristic | Symbol | Formula |
---|---|---|
Surface area |
A
|
A = 2 ⋅ N
f
⋅ L ⋅ π ⋅ r
−
i
|
Filter priming volume |
V
b
F
|
V
b
F
= N
f
⋅ L ⋅ π r
−
i
2
|
Total priming volume |
V
b
TOT
|
V
b
TOT
= V
b
F
V
b
TOT
+ volume of tubes |
Membrane porosity |
ρ
|
ρ = N
p
⋅ π ⋅ r
−
p
2
|
Performance characteristics
Membrane ultrafiltration coefficient and filter ultrafiltration coefficient
Mass transfer area coefficient
Membrane sieving coefficient/rejection coefficient
Cut-Off
Mechanisms of solute and fluid transport
Ultrafiltration and convection
Flowrate | Symbol | Unit of measure | Definitions and comments |
---|---|---|---|
Blood flowrate | QB
| ml/min | Depends on: - modality - vascular access - hemodynamic stability of the patient |
Plasma flowrate | QP
| ml/min | Approximated as: QP = (1 – HCT) ∙ QB
where HCT = hematocrit |
Ultrafiltration flowrate | QUF
| ml/h | Total volume of fluid removed in the filter by positive TMP per unit of time: QUF = QUF
NET + QR.
Depends on: - blood flow rate - filter and membrane design - transmembrane pressure (TMP) - membrane ultrafiltration coefficient and surface area |
Net ultrafiltration flowrate (Δ weight flowrate) (weight loss flowrate) | QUF
NET
| ml/h | Net volume of fluid removed from the patient by the machine per unit of time |
Plasma ultrafiltration flow rate | QP-UF
| ml/h | Total volume of plasma removed in the plasma filter by TMP per unit of time |
Replacement flowrate (Substitution flow rate) (Infusion flowrate) | QR
PRE
QR
POST
QR
PRE/POST
| ml/h | Sterile fluid replacement can be: - upstream of filter (pre-replacement, pre-infusion or pre-dilution): reduced depurative efficiency but better filter life - downstream of filter (post-replacement, post-infusion or post-dilution): higher depurative efficiency but lower filter life - both upstream and downstream of filter (pre-post replacement, pre-post infusion or pre-post dilution): compromise between the two modalities |
Replacement plasma flow rate | QP-R
| ml/h | Replacement of plasma downstream of the plasma filter |
Dialysate flowrate | QD
| ml/h | Volume of dialysis fluid running into the circuit per unit of time |
Effluent flowrate | QEFF
| ml/h | Waste fluid per unit of time coming from the outflow port of the dialysate/ultrafiltrate compartment of the filter: QEFF = QUF + QD = QUF
NET + QR + QD
|
Transmembrane pressure
Diffusion
Adsorption
Modalities of extracorporeal RRT
Hemodialysis
Hemofiltration
Hemodiafiltration
Isolated ultrafiltration
Plasmapheresis
Hemoperfusion/plasmaperfusion
Fluids, volumes and flows
Filtration fraction and concentration ratio
Treatment evaluation methods: the “dose” of RRT
Target dose (prescribed)
Target machine dose (set)
Current dose (estimated from treatment parameters)
Average dose (measured/calculated)
Projected dose (calculated/estimated)
Current effective delivered dose (measured)
Average effective delivered dose (measured)
Efficiency, intensity and efficacy
Measurement | Name | Symbol | Unit of measure | Formula |
---|---|---|---|---|
Efficiency | Target (prescribed) |
K
T
| ml/kg/h | Assuming that the patient’s clinical condition does not change, KT is a constant value throughout the treatment |
Efficiency | Target machine |
K
Tm
| ml/kg/h | Considering the downtime and the reduction in clearance properties of the membranes during treatment, K
Tm
is usually set at a greater value than K
T
|
Efficiency | Current |
K
Cr
| ml/kg/h |
\( {K}_{Cr}=\frac{\left({Q}_R^{PRE}+{Q}_D+{Q}_{UF}^{NET}+{Q}_R^{POST}\right)}{B.W.}\cdot \frac{Q_B\ }{Q_B + {Q}_R^{PRE}} \)
|
Efficiency | Average |
K
Am
| ml/kg/h |
\( {K}_{Am}=\frac{1}{t_1}\cdot {\int}_0^{t_1} KCrdt \)
|
Efficiency | Projected |
K
Pr
| ml/kg/h |
\( {K}_{Pr}=\frac{{\displaystyle {\int}_0^{t_1}}{K}_{Cr}dt + \left({t}_{tot}-{t}_1\right) \cdot {K}_{Tm}^{\hbox{'}}}{t_{tot}} \)
where K
Tm
' is the new target machine efficiency set |
Efficiency | Current effective delivered |
K
Cd
| ml/kg/h |
\( {K}_{Cd}=\left({Q}_B\cdot \frac{C_{Bi}-{C}_{Bo}}{C_{Bi}}+{Q}_{UF}\cdot \frac{C_{Bo}}{C_{Bi}}\right)\cdot \frac{1}{B.W.} \)
|
Efficiency | Average effective delivered |
K
Aed
| ml/kg/h |
\( {K}_{Aed}=\frac{1}{t_1}\cdot {\displaystyle \underset{0}{\overset{t_1}{\int }}}{K}_{Cd}dt \)
|
Intensity | Target (prescribed) |
I
T
| ml/kg | Blood volume that should be cleared applying K
T
during the total time of treatment |
Intensity | Target machine |
I
Tm
| ml/kg | Blood volume that should be cleared applying K
Tm
during the total time of treatment |
Intensity | Current |
I
Cr
| ml/kg |
I
Cr
= K
Cr
⋅ t
tot
|
Intensity | Average |
I
Am
| ml/kg |
\( {I}_{Am}\kern0.5em =\kern0.5em {K}_{Cm}\cdot {t}_1\kern0.5em =\kern0.5em {\int}_0^{t_1}{K}_{Cr}dt \)
|
Intensity | Projected |
I
Pr
| ml/kg |
\( {I}_{Pr}={K}_{Pr}\cdot {t}_{tot}=\kern0.5em {\int}_0^{t_1}{K}_{Cr}dt + \left({t}_{tot}-{t}_1\right) \cdot {K}_{Tm}^{\hbox{'}} \)
|
Intensity | Current effective delivered |
I
Cd
| ml/kg |
I
Cd
= K
Cd
⋅ t
1
|
Intensity | Average effective delivered |
I
Aed
| ml/kg |
\( {I}_{Aed}={K}_{Ced}\cdot {t}_1\kern0.5em =\kern0.5em {\int}_0^{t_1}{K}_{Cd}dt \)
|
Efficacy | Target (prescribed) |
E
T
| Dimensionless | Solute removal obtained applying I
T
to the volume of distribution of the solute |
Efficacy | Target machine |
E
Tm
| Dimensionless | Solute removal obtained applying I
Tm
to the volume of distribution of the solute |
Efficacy | Current |
E
Cr
| Dimensionless |
\( {E}_{Cr}=\frac{I_{Cr}}{V}=\frac{K_{Cr} \cdot {t}_{tot}}{V} \)
|
Efficacy | Average |
E
Am
| Dimensionless |
\( {E}_{Am}=\frac{I_{Cm}}{V}=\frac{1}{V}{\int}_0^{t_1}{K}_{Cr}dt \)
|
Efficacy | Projected |
E
Pr
| Dimensionless |
\( {E}_{Pr}=\frac{I_{Pr}}{V}=\frac{1}{V}\cdot \left[{\int}_0^{t_1}{K}_{Cr}dt + \left({t}_{tot}-{t}_1\right) \cdot {K}_{Tm}^{\hbox{'}}\right] \)
|
Efficacy | Current effective delivered |
E
Cd
| Dimensionless |
\( {E}_{Cd}=\frac{I_{Cd}}{V}=\frac{K_{Cd}\cdot {t}_1}{V}=\frac{1}{V}\cdot \left({Q}_B\cdot \frac{C_{Bi}-{C}_{Bo}}{C_{Bi}}+{Q}_{UF}\cdot \frac{C_{Bo}}{C_{Bi}}\right)\cdot \frac{1}{B.W.} \cdot {t}_1 \)
|
Efficacy | Average effective delivered |
E
Aed
| Dimensionless |
\( {E}_{Aed}=\frac{I_{Ced}}{V}=\frac{K_{Ced}\cdot {t}_1}{V}=\frac{1}{V}\cdot {\int}_0^{t_1}{K}_{Cd}dt \)
|