Transcript CVVH vs CVVHD Does it Matter? - Pediatric Continuous Renal

CVVH vs CVVHD
Does it Matter?
Patrick D. Brophy MD
University of Michigan
Pediatric Nephrology
OBJECTIVES
Definitions
– CVVH vs CVVHD
Mechanisms of action
– Convective vs Diffusive clearance
Other Issues & Selective data review
– Drug Clearance, membranes & patients, anticoag
Implementation of one modality over anotherRationale
– Sepsis vs ARF vs Toxic ingestions
– Advantages and Disadvantages, expertise
Definitions
 Continuous Venous Venous Hemofiltration
Mimics the process which occurs in the
mammalian kidney
Describes an almost exclusive convective
treatment with highly permeable membranes
Ultrafiltrate produced is replaced by a sterile
solution (High UF rates)
Patient weight loss results from the difference
between ultrafiltration and reinfusion rates
Definitions
 Continuous Venous Venous Hemodialysis
Describes a predominantly diffuse treatment
in which blood and dialysate are circulated
either side of the dialysis membrane in
countercurrent directions.
Dialysate may be custom or commercially
produced
The ultrafiltration rate is approximately equal
to the scheduled weight loss (lower UF rate).
Definitions
Post-Dilution CVVH Qr
Qb
CVVHD
Qb
Qef
Qef
f
Qd
f
Qr
Qr
Qb
Qb
Qef
Qef
f
Pre-Dilution CVVH
f
CVVHDF
Qd
Mechanisms of Action
 CVVH
Convection
Solute is removed by “Solvent Drag”. The solvent
carries the solute (plasma water) through a semipermeable membrane.
The Roller Pump creates Hydrostatic Pressure,
which drives the solvent through the membrane.
The membrane pore size limits molecular transfer
More efficient removal of larger molecules than
diffusion
Mechanisms of Action
 CVVH
Convection
Since it mimics the mammallian kidney its thought
to be more “physiologic” and provides better
removal of middle molecules (500-5000 Daltons)
thought to be responsible for uremia.
With the advent of highly porous membranes need
to use larger markers (500-50000 Daltons) to
determine “uremic clearance”.
Enhanced clearance of autologous cytokinesthought to be involved in Septic Inflammatory
Response Syndrome (SIRS).
Mechanisms of Action
 CVVH
Convection
Sieving Coefficient- clearance coefficient for
hemofiltration defined by UV/P
U= Filtrate Concentration
V= Volume
P= Mean plasma concentration over the clearance period
SC is 1 for molecules that pass through the
membrane easily & 0 for those that do not
Mechanisms of Action
 CVVHD
Diffusion (predominantly)
Solute diffuses down an electrochemical gradient
through a semi-permeable membrane in response
to an electrolyte solution running counter current to
the blood flow through the filter.
Diffusive movement occurs via Brownian motion of
the solute- smaller molecules (ie urea) have
greater kinetic energy and are preferentially
removed based on the size of the concentration
gradient
Mechanisms of Action
 CVVHD
Diffusion (predominantly)
Some convection occurs due to prescribed UF and
if High flux filters are utilized
Solute removal is proportional to the concentration
gradient and size of each molecule
Dialysate flow rate is slower than BFR and is the
limiting factor to solute removal
Solute removal is directly proportional to dialysate
flow rate
Mechanisms of Action
 CVVHD
Diffusion (predominantly)
Diffusion Coefficient- clearance coefficient for
hemodialysis defined by UV/P
U= Dialysate (+Filtrate) Concentration
V= Volume
P= Mean plasma concentration over the clearance period
Principle same as for SC with 1= to optimal
clearance and 0= to no (minimal clearance)
Other Issues
The greatest difference between
modalities is likely the impact of the
membrane utilized and their specific
characteristics.
There are no data available assessing
patient outcomes using diffusive (CVVHD)
and convective (CVVH) therapies
Other Issues
Low molecular weight solutes
Middle/High molecular weight solutes
Drug/Toxin Clearance
Impact on Adsorptive membrane
characteristics
Anticoagulation
Patient Characteristics
Low Molecular Weight Solutes
Relative equivalence of convective and
diffusive clearances (membrane variation
and design)
Solute Molecular Weight
and clearance
Jeffrey et al., Artif Organs 1994
Solute (MW)
Sieving Coefficient
Diffusion Coefficient
Urea (60)
1.01 ± 0.05
1.01 ± 0.07
Creatinine (113)
1.00 ± 0.09
1.01 ± 0.06
Uric Acid (168)
1.01 ± 0.04
0.97 ± 0.04*
Vancomycin (1448)
0.84 ± 0.10
0.74 ± 0.04**
*P<0.05 vs sieving coefficient
**P<0.01 vs sieving coefficient
Diffusive & Convective Solute
Clearances During CRRT
Brunet et.al AJKD 34:1999
Evaluated convective & dialysate
clearance of :
UREA
Creatinine
Phosphate
Urates
B2microglobulin
Variety of UF & Dialysate Flows with Multiflow60
&100 membranes
CVVH vs CVVHD continued
Conclusions:
At QUF with predilution (2L/hr) FRF 15-20%
reduction in urea, urates & creatinine
SC= 1 for all small molecules for CVVH-both filters
M100>M60 (QD 1.5-2.5L/hr) diffusive clearance
with the difference increasing as molecular weight
increased
QD > 1.5L/hr poor diffusive middle molecule
clearance (both membranes); whereas increasing
nonlinear clearance occurred with convection as
QUF increased for both filters
CVVH vs CVVHD continued
No additive effect with combination
dialysate & FRF therapy for middle
molecule clearance
Authors conclude:
– “Convection more efficient than diffusion in
removing mixed- molecular- weight solutes
during CRRT”
Drug & Toxin Clearance
Drug/Toxin Clearance
– Molecular Weight
– Protein Binding
– Vd
– Membrane composition
As MW increases diffusive drug clearance
declines more than convective clearance
Adsorptive Membrane
Characteristics
Biocompatible membranes appear to have
greater adsorptive properties than less
biocompatible membranes (PAN>Polysulfone)
Filter Characteristics for small molecule removal
include: pore size distribution & density and
surface area and at conventional flow rates (in
adults-2L or less) clearance is flow rate
dependent.
As molecular size increases: hydraulic
permeability & adsorption capacity become
important.
Adsorptive Membrane
Characteristics
No specific Membrane recommendations
as no studies to definitively prove superior
performance under specific modality
Anticoagulation
Citrate use- centers relatively confined to
diffusive therapy (works well with
CVVHDF)
– Citrate: multiple protocols for CVVHD
Few for CVVH (Niles et.al. 2002-CRRT abstract)
where citrate included in FRF
Heparin- both CVVH & CVVHD
Patient Characteristics
Etiology underlying the patient’s can help
determine choice of therapy
– Speculative benefit of CVVH in Sepsis, Toxin
removal (although filter impact very important)
– For ARF & Fluid overload little difference is
likely
No Definitive demonstration of superiority
of one over the other
Final Thoughts & Summary
Currently- no data to prove outcome
superior with either modality
Best to use what each center is most
comfortable with
Acute Dialysis Quality Initiative (ADQI)
Guidelines reflect these ongoing study
requirements and recommendations
Plenty of work to do!!!!
(p. brophy)
ACKNOWLEDGEMENTS
– MELISSA GREGORY
– ANDREE GARDNER
– JOHN GARDNER
– THERESA MOTTES
– TIM KUDELKA
– LAURA DORSEY & BETSY ADAMS