Oliguria (from Greek "oligo" meaning "small", aka hypouresis) is low output of urine.
Oliguria, what is it? I know it's related to urine, but what is "oligo"?
"Oligo" is like "oligopoly", where there are only a few competitors, but not yet a monopoly where there's 1 dominant company. So there's little urine output. But there's not no urine at all - that's what we call anuria.
Oliguria is where urine output is <50% of what we expect it to be, so that's:
In infants, <1mL/kg/hr, which is apprxoimately 4 wet nappies in 24 hours, as it should normally be >2mL/kg/hr
In children, <0.5mL/kg/hr - this value can also be used to define oliguria in adults as well, as it should normally be >1mL/kg/hr
In adults, <500mL/day (which is 0.24mL/kg/hr), as it should normally be >0.5mL/kg/hr
Anuria, is where <100mL/day, representing the non-passage of urine. It is often caused by failure in the function of the kidneys, but can also occur because of severe obstruction (kidney stones, tumors), or with ESRD (end stage renal disease)
How low is low urine output?
So we pee more the younger we are. In infants, it's <1mL/kg/hr. In children, it's <0.5mL/kg/hr. And in adults, it's about <0.25mL/kg/hr - we can just use <500mL/day though.
How about in kids, where they urinate into their nappies? Is there a better way to measure?
Yep, the number of wet nappies they have, from what is normal for them.
Prerenal, due to hypoperfusion of the kidney, including:
Dehydration by poor oral intake
HONK → dehydration
DKA → dehydration
Renal, due to kidney damage, including:
Pre-eclampsia → kidney damage
UTI → kidney infection
Postrenal, due to obstruction of the urine flow, including:
Tumor compression urinary outflow
What causes low urine output?
The problem can be before the kidney, in the kidney, or after the kidney. What does this mean? Perhaps not enough blood is reaching the kidney. The kidney could be damaged. Or there could be obstruction of urine flow.
So before the kidney, why might not enough blood be reading the kidney?
Not getting enough fluids. Low blood pressure. Or losing fluids through bleeding, or diarrhea.
How about at the kidneys, what can damage the kidney?
Urine infection. Renal failure. And things that cause problems for the kidney to process, like drugs, and muscle breakdown.
After the kidneys, what could obstruct urine flow?
Anything that could compress the tube, so a big prostate, cancer, a collection of dried blood.
Postoperative oliguria is where Pt's have decreased urine output after a major operation, that may a normal physiological response to:
Hypoperfusion of the kidney: fluid/blood loss, causing hypovolemia and/or hypotension, and thus decreased GFR
Obstruction of urine flow, due to urinary retention: increased aldosterone and ADH release, due to adrenal response to stress