Netherlands Journal

netherlands journal of critical care
Table 4. Modes of CRRT
Solute transport
Blood flow
(mL/min)
Ultrafiltrate
flow
(mL/min)
Dialysate
flow
(mL/min)
Clearance
(L/24h)
Slow continuous ultrafiltration No 50 – 100 1 - 2 No no
Continuous arteriovenous or venovenous haemofiltration (CAVH or CVVH) Convection 50 -200 8 - 66 No 12 - 96
Continuous arteriovenous or venovenous haemodialysis
Diffusion 50 -200 2 - 3 10 - 20 14 - 36
(CAVHD or CVVHD)
Continuous arteriovenous hemodiafiltration or venovenous
Convection and diffusion 50 -200 8 - 12 10 - 20 20 - 40
hemodiafiltration (CAVHDF or CVVHDF)
Continuous arteriovenous or venovenous high flux dialysis (HDF) Convection and diffusion 50 -200 2 - 8 50 - 200 40 - 60
[13,31]. Low-volume (2 L/h) CVVH [30] seems to have no positive effects
in patients with sepsis/SIRS and imminent ARF (level II).
Modes of acute renal replacement therapy
In the ICU, renal replacement therapies are primarily limited to conventional
IHD and CRRT. During IHD, intensive dialysis is performed
for 3-4 hours at variable intervals, whereas during CRRT, continuous
and gradual removal of fluid and toxins is provided at lower blood
flow. More recently several hybrid therapies [41] have been described,
with a treatment duration between CRRT and conventional IHD, (ie
extended dialysis [42], sustained low-efficiency dialysis [43], shortterm
HF [18] or pulse HF [35].
The nomenclature and definitions of the various CRRT techniques
are based on their operational characteristics [44] (Table 4).
Haemodialysis and haemofiltration are the two main principles of
solute transport of CRRT.
During haemodialysis, removal of solutes is driven by diffusion
(solute transport across a semi-permeable membrane generated by a
concentration gradient). During haemofiltration, removal of solutes
is based on convection (water and solute transport across a semipermeable
membrane generated by a pressure gradient). There are
no data showing any given modality as superior with regard to clinical
outcomes. Haemofiltration resembles most the principle of glomerular
filtration and increases the middle molecule clearances [45];
however, whether this is beneficial in ARF is unknown. Factors that
may affect current practice include local availability of equipment,
fluids and costs.
CRRT is applied either in the arteriovenous (driving force is patient’s
blood pressure and flow) or venovenous mode (driving force
is external pump). Advantages of the arteriovenous therapies include
ease of set-up and operation and low extracorporeal blood volumes.
Disadvantages of arteriovenous therapies include the prolonged arterial
cannulation, the requirement of a MAP of >60 mm Hg to maintain
circuit blood flow, and the low blood flows that can be achieved.
Advantages of the venovenous therapies are the decreased risk of vascular
damage as compared to the arteriovenous therapies, the ability
to maintain blood flow independent of MAP, the ability to achieve
higher blood flow rates and clearances (level III) [46,47]. The higher
clearances associated with better survival [26] cannot be achieved
without the introduction of a blood pump. The use of blood pumps
has increased the complexity of CRRT systems, but in clinical practice
this disadvantage does not counterbalance the advantages, and
there is general consensus that venovenous systems are the modality
of choice [46-49].
CRRT vs conventional IHD
One of the most pressing clinical questions regarding the use of CRRT
is whether CRRT offers an important advantage over IHD, regarding
survival and/or recovery of renal function. The effects of IHD versus
CRRT on survival and/or recovery of renal function were reported in
five prospective RCTs [50-54] and two meta-analyses [55,56].
1. In a large multicenter RCT (n=160) CVVHDF showed no survival
(ICU and hospital) advantage compared with alternate day IHD
after adjustment for severity of illness (level I) [52]. However,
CRRT was associated with a significantly higher rate of complete
renal recovery in surviving patients who received an adequate trial
of therapy, without crossover to IHD (CRRT 92.3% vs IHD 59.4%,
p < .01). Of notice, in this study patients were excluded when MAP
was