Thread by @ssfarouk

1/

As kidney replacement therapies (KRT) have become a strained and limited resource during the #COVID19 pandemic, here is a quick #tweetorial on how these treatments work:

What is the main mechanism by which clearance is achieved during intermittent #hemodialysis?

2/

KRT has 2 powers:
1⃣Clearance (removal of solutes, like potassium 🍌)
2⃣Volume removal 💧

...and it comes in 2 flavors:
1⃣Intermittent
2⃣Continuous

3/ How does the potassium go from 7.2 meq/L to 4 after an intermittent #hemodialysis (iHD) session?

Chemistry 101: DIFFUSION!

Solutes are cleared via diffusion as they move ⬇️ their concentration gradient across a SEMIPERMEABLE membrane

DOI: 10.1056/NEJMc1301071

4/ A closer look at that dialysis filter...

Blood runs through the filter "straws", while the dialysate runs in the opposite direction 🔃 around them & does not come in direct contact with the 🩸

Larger filters with bigger pores can be used to further optimize clearance.

5/ Strategies to maximize clearance (particularly relevant now given the #COVID19-related strain on KRT):

1⃣Bigger filter size
2⃣Higher dialysis blood flow rates (usually, these are 300-400 cc/min)
3⃣Widen the concentration gradient and try a potassium-free dialysate

6/ So that's how we remove solute. What about the fluid?

Ultrafiltration (UF), or ISOTONIC fluid removal, is achieved via pressure that is exerted across the membrane that drives fluid out.

DOI: 10.1056/NEJMct1206045 @luck_urine

7/ Here's what goes into the iHD order:

1⃣Access, blood flow, + dialysate flow rates (standard 800 cc/min)
2⃣Dialysate composition
3⃣Ultrafiltration volume
4⃣Filter size
5⃣Duration (standard 3.5hrs - though #COVID19 has forced shorter treatment times, affecting clearance)

8/ What about SLED? (sustained low efficiency dialysis)

It's just like iHD, but...not as efficient...and should be done for longer durations (e.g. 8 hours) to obtain similar clearance to iHD

9/ Let's move on to the CONTINUOUS KRTs

The Players: CVVH, CVVHD, & CVVHDF

CVVH = continuous venovenous hemofiltration

CVVH is the only continuous modality that does NOT use a dialysate. So how do we get clearance?

💥REPLACEMENT FLUID💥

10/ If you've seen a pt connected to a #CVVH machine, you remember the huge bags hanging in the room - these are bags of replacement fluid that achieve clearance & are infused into the patient via the filter.

Hospitals can purchase pre-made replacement fluid (example below) 👇🏽

11/ Remember that replacement fluid is essentially infused unchanged into the pt, so use caution in those with hypoNa to avoid overcorrection

Of note: Therapy fluid can be infused either before or after the CVVH filter. If you do it before, may lose some solute (via convection)

12/ The graphic below illustrates the differences between #CVVH, #CVVHD, and #CVVHDF

CVVH: ✅Convection + replacement fluid; ⛔️dialysate
CVVHD✅Dialysate + convection; ⛔️ replacement fluid
CVVHDF: ✅Dialysate + convection + replacement fluid

@RenalFellowNtwk #FOAMed

13/ Say you only need volume removal, but no clearance:

Use your resources wisely and opt for a pure-#convection therapy:

Slow continuous ultrafiltration (#SCUF) AKA aquapheresis or "CHF Solutions"

Don't waste dialysate or replacement fluid if you don't need it!

#COVID19

14/ Compared to iHD, continuous KRT will achieve slower clearance & volume removal 🐢 (e.g. 100 cc/hr vs 1L/hr)

PIRRT = "middle ground" and uses higher RF rates to lower tx times & has been used during #COVID19 pandemic to maximize CKRT machine use 👇🏽

nxstage.com/hcp/therapies/…

15/ In summary:

1⃣ KRT = continuous/intermittent
2⃣Solute clearance: diffusion; UF: convection
3⃣Modality choice: Volume/solute removal needs + what's available
4⃣Optimize treatments that may be shorter by ⬆️ blood/RF flow rates, using larger filters, & tailoring your dialysate

16/

And did not cover this above - but acute #PeritonealDialysis is another innovative and effective way to relieve some of the strain on resources like iHD & CVVH

Will let @osamaelshamy88 tackle that #tweetorial...