Thread by @mcstarr1

1/Why do we consider 1cc/kg/hr normal urine output in pediatrics?

After recent discussion by @drjosflynn and others, I decided to try and uncover why this number has been long used.

However, the explanation is frustrating, fraught with challenged math, and not very satisfying.

2/ Lets start with a question.

How do we define normal urine output in pediatric patients?

3/ Normal urine output being 1cc/kg/hr is ubiquitous. It's everywhere. When you search, the top 20 hits are related to urine output in pediatric patients.

4/ Low urine output or oliguria has long been a concept in nephrology as a reflection of kidney function. The first to describe it was Galen (100 AD), where it was considered a general tool to guide the evaluation of a sick adult.

ncbi.nlm.nih.gov/pubmed/2655450

5/ How much urine do children need to make to stay healthy? The offhand answer is enough to stay in balance. Enough that every drop of water that is ingested is then excreted. Enough that every milligram of solute is excreted.

6/ Obviously, accumulation of solutes is the factor which better correlates to impaired function, so oliguria should be defined as the urine output, below which, children begin to accumulate solute. In health, this volume is highly dependent on the diet.

7/ So let’s do some math. Assuming normal diet (10mOsm/kg) + tubular function (concentration 1200 mOsm/L)

Minimal UO for 10kg child = 83cc (0.3cc/kg/hr)
Minimal UO for 30kg child = 250cc (0.35cc/kg/hr)
Minimal UO for 50kg child = 416cc (0.4cc/kg/hr)

So... why 1cc/kg/hr?

8/ The best (completely non-satisfying) answer is a little less rigorous. It appears to be drawn from Holliday-Seger's estimated needs (1cc/kg/hr), studies of urine output in children, and adult physiologic norms of 800cc - 2L/day. jbc.org/content/57/3/6…

9/ Holliday has written candidly on this in the years since Holliday-Seager estimating equation was published. He notes that the average physiological (insensible plus urinary) losses "conveniently worked out" to this metric. VOILA - 1cc/kg/hr! ncbi.nlm.nih.gov/pmc/articles/P…

10/ HOWEVER, it's important to remember that urine output is the first (and some would say best) biomarker of AKI. And in acute settings, a rapid reduction of urine output may be an early indication of decreased function. Urine output has several advantages over serum creatinine

11/ The Risk, Injury Failure, Loss, End stage (RIFLE) consensus from ADQI used urine output <0.5 ml/kg/hour to define AKI and all subsequent definitions have retained this criteria.

ccforum.biomedcentral.com/articles/10.11…

However, urine output volume was defined by consensus not by physiology.

12/ Yes the story of how we got to 1cc/kg/hr may be a little unsatisfying. Should rethink "normal"?

The bottom line: remember that urine output is a piece of the puzzle and that it's also about the balance, fluid overload, ins/outs, and patient status.

So what do you think?