Can someone please explain Acute Renal Failure table in FA?

[Saladin MD]

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My brain is just not working. Exam on Thursday. Can someone please explain the table on p.432 of the 2008 edition of FA? It's under the Acute Renal Failure section, and shows the difference between pre-renal, renal, and post-renal values for urine osmolality, Urine Na, etc...

For the life of me, I can't make any sense of it.

 

[rachmoninov3]

Some of the stuff on that table makes sense if you think about it, others you just need to go with, but I'll try the logical stuff first:

Overall, Pre-renal is the kidney sensing low volume. Renal is a filtration problem. And post renal is a mechanical obstruction problem.

Urine Osmolarity: Pre-renal will have a high osmolarity/cocacola coloured urine because very little is being filtered because there's a blockage before the kidney. If only small volumes of blood are being filtered by the kidney it will sense a low pressure and retain as much water as possible, making the urine hypertonic and coca cola coloured.

Renal and post renal ARF don't sense low volumes and therefore they have normal osmolarities.

Urine Na: Same concept, if there's low volume, Na will be reabsorbed to increase volume.

BUN/creat: BUN backs up faster than creat when there is little or no filtration. Prerenal will be greater than 20. Renal there's aberrant filtration so the BUN is more freely filtered than creat, so your ration is less than 15. In an obstruction, there's no problem with filtration so it's within the "normal" range of 15 to 20.

that's not everything on that table, but it's the stuff I can make sense of. Hope this helps!

Here's to short term memory!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[Vonsmack]

Prerenal- Think hypovolemia (hypoperfusion). Your body needs to retain water so it hangs on to sodium. Since its retaining water your urine is going to be concentrated (High osmolality), your hanging onto Na so low urine Na and low fractional excretion of Na. Your also reabsorbing all the urea so your BUN goes up (Cr isn't reabsorbed so the ratio goes up)

Renal- Think acute tubular necrosis or drug toxicity--> tubular dysfunction so you don't REABSORB as well. Sodium is not reabsorbed so it stays in the urine (high urine Na and higher FEna, remember your supposed to reabsorb 99% of the sodium). Urea is also not reabsorbed so your excreting urea, causing the BUN/Cr in your plasma to go down.

Postrenal- Think bilateral urinary obstruction. This gets tricky because it can change depending on the duration. Initially you have increased urinary space pressure from a back up of fluid that causes a back diffusion of urea increasing your BUN/Cr ratio. Eventually you'll get hydronephrosis and tubular dysfunction so lack of reabsorption (mimicking intrinsic renal) of sodium.

I think the chart is mostly correct except post-renal which mixes up acute/chronic obstruction. Also look for the urine sediments of cells and casts to differentiate ARF. I learned my path from RR Goljan so blame him if I'm wrong.

 

[Saladin MD]

Thanks a lot guys. Honestly I have two little RAM left in my brain now, so I hope I can retain this. My brain is reaching maximum capacity. Overload imminent.