All about that fluid exchange machine

Most EDs get a steady pool of patients who are on, going to be or need urgent Renal Replacement Therapy (RRT). Here is a simplified and concise run down on RRT by one of our hospital's intensivists - Dr Vijo Poulose. Only slight editing done on headings, punctuation and context.

What is RRT?

RRT - Renal Replacement Therapy can be:
  • Acute (called Continuous RRT or CRRT) or
  • Chronic RRT (like IHD – intermittent hemodialysis or CAPD – continuous Ambulatory Peritoneal dialysis)
For the purpose of this talk, I am going to call plasma of renal failure patients (which is full of toxins like urea, creatinine, K) as "bad fluid" & I will call the fluid used for CRRT as "clean fluid" which comes in bags. Clean fluid is mainly water with some sodium, Ca, Mg, HCO3 and no toxins.
Also, we talk about chronic RRT, we will be referring only to IHD

CRRT - Continuous Renal Replacement Therapy may use:
  • Dialysis mode (CVVHD – Continuous Veno-venous HemoDialysis)  or
  • Filtration mode (CVVH - Continuous Veno-venous Hemofiltration) or
  • Both (CVVHDF Continuous Veno-venous HemoDiaFiltration)
So what is dialysis?

Sometimes we use the term ‘dialysis’ loosely (especially in the context of CRRT); whether the mode uses Filtration process or whether it uses a Dialysis process. But strictly speaking, dialysis is when we use a concentration (Diffusion) driven process. Basically in dialysis, you run blood with bad fluid through inner compartment of the filter (dialyser) and you run clean fluid along the outer compartment. Because the bad fluid has much more toxins (higher concentration) , it diffuses into the clean fluid and thus making the blood cleaner.

Where do we use dialysis (D) mode then?

Two common areas where we use the concentration (or diffusion) process are :
  • IHD uses this process and it is done 2-3 sessions a week for ESRF patients (example in NKF centers or CGH renal unit)
IHD uses fairly high blood flow rates and very high amounts of dialysis fluid are run along the outer compartment, so that you clear maximum toxins in that 3-4 hour period while he is in a dialysis center. Because of the osmotic shifts created in the intravascular space due to rapid removal of urea, the BP can fall. Also in normal circumstances, the removal of excess body water (negative balance) has to be achieved in that 4 hour period (2-3 liters over 4 hours) and hence that fast removal can also bring down the BP. IHD runs over 4 hour sessions, 3 times a week
  • CVVHD uses this process for AKI pts or AoCKD or ESRF pts in the ICU
CVVHD uses slower rates for a continuous period (1-2 days), is ‘gentler’ and hence better for shocked patients.

What is filtration (F) or haemofiltration (HF) mode then?

This is another mode where the machine creates a pressure difference between the inner and out compartment of the filter, thus pushing the bad fluid into the outer compartment via the pressure gradient and the bad fluid is taken away and disposed of.

Also, after the blood comes out of the filter and before it (with less plasma volume) returns to the body, you push in some clean fluid into it, which dilutes the blood toxin level and also preserves the blood volume from going too low.

So which mode?
Dialysis (D) mode is much better than Filtration (F) for removing small molecules like urea, creatinine and hence we use IHD For ESRF patients to remove large amount of toxins over a short period of time. For CRRT pts, both D and F modes are believed to be kind of equal in efficacy when given at the same flows (or dose). If you combine both (CVVHDF), the clearance is much better, but we rarely have a scenario when we need both (no clear recommendation when to use both).

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