Thread by @eugelin06

1/ Did y'all know you use a conventional HD machine to jerry-rig a CRRT machine? I don't usually do Tweetorials on clinical stuff, but saw some chatter on potential CRRT shortages #COVID19

https://twitter.com/roachjl/status/1243718645026639876

2/ Recall that there are two types of clearance: diffusion (when solutes randomly pass through the membrane) and convection (when hydrostatic pressure forces solutes across the membrane). nejm.org/doi/full/10.10…

3/ Your typical CRRT prescription has a diffusion component (dialysis) and a convection component (filtration). Remember the convection component is just ultrafiltration (UF)+replacement of urea-free fluid, so the patient doesn't become volume depleted

4/ Let's take a typical prescription of blood flow 220, dialysate 1200, replacement (filtration) 1200. Quick quiz, what is the total clearance of this patient?

5/ Don't forget the units! Blood flow is 220 cc/min, dialysate is 1200 cc/HR (20 cc/min) and replacement is 1200 cc/HR (20 cc/min). Dialysate is moving so slow it gets saturated, so total clearance is dialysate rate + UF (2400 cc/hr = 40 cc/min)

6/ Let's revisit that typical CRRT prescription in cc/min:

blood flow 220 cc/min
dialysate 20 cc/MIN
replacement 20 cc/MIN

7/ Remember with replacement fluid, an equal amount is being removed via UF. Here is the complete CRRT prescription:

blood: 220 cc/min
dialysate: 20 cc/min
UF: 20 cc/min (1200 cc/hr)
replacement: 20 cc/min

8/ How can we convert a CRRT prescription to a conventional HD machine? Blood flow is easy, just drop to 220 cc/min.

9/ What about dialysate? Most machines can't do 20 cc/min. However, they CAN do 0 cc/min (i.e., ultrafiltration without dialysis). To achieve the same clearance, we can use convection for the entire prescription.

10/ To achieve the same urea clearance using only convection, we use 40 cc/min (2400 cc/hr) of convection. Specifically:

2400 cc/hr UF (2.4 L / hr)
2400 cc/hr replacement

11/ Conventional HD doesn't do replacement, but... our patient is in the ICU and has IV access! So we can order 2400 cc/hr IV replacement

12/ What fluid should we use? You have 2 options:
- CRRT fluid (if there's no shortage)
- IVF (one option is isotonic bicarb + K added and a calcium drip)

13/ Putting it all together, the jerry-rigged CRRT prescription:

Dialysis nurse:
- UF only, 2.4 L/hr x 24 hrs (or 16-18 hrs if you have staffing issues)
- blood flow 220 cc/min

ICU nurse:
- IV replacement 2.4 L/hr (40 cc/min)

Pharmacy:
- Mix appropriate IV fluid

14/ Obviously, this is very resource intensive - you now have to coordinate with two nurses and the pharmacy. This is also VERY taxing on dialysis staff, when we already have personnel shortages

inquirer.com/health/coronav…

15/ Still, it's always good to know how to order CRRT when you have a machine shortage and an abundance of IVF and nurses

16/ To summarize, if no CRRT machines are available but dialysis nurses/machines available: order UF only + IV replacement

17/ Corollary: what if you don't have any CRRT dialysate / replacement fluid left? You have two options:

- Order the equivalent rx using a regular HD machine and coordinate IV replacement with pharmacy
- Use a CRRT machine and coordinate IV replacement with pharmacy