A 16-Year-Old with Hypokalemia
31 seconds CATHY
[00:03] CATHY Hi, welcome to Harrison's Podclass where we discuss important concepts in internal medicine. I'm Cathy Handy. And I'm Charlie Wiener, and we're coming to you from the Johns Hopkins School of Medicine. [music] It is hospital day five for a 65-year-old patient with prerenal azotemia secondary to dehydration. His creatinine was initially 3.6 milligrams per deciliter on admission, but it has improved, today to 2.1 milligrams per deciliter.
31 seconds CATHY
[00:31] CATHY He complains of mild lower back pain and you prescribe naproxen to be taken intermittently. By what mechanism might this drug further impair his renal function? So, Cathy, let's talk about this case.
43 seconds CATHY
[00:43] CATHY Alright, so the only patient factors that we get in this case are that it's a 65-year-old man, and the symptoms that came on are low back pain. And we don't know whether or not they're acute or chronic, but we're treating with naproxen for those symptoms. The injury that he already has, which also contributes to the host factors are that he has acute kidney injury thought to be attributed to dehydration which is a reversible cause, and we see that there's already been some improvement from 3.6 serum creatinine down to 2.1. So, now we need to talk about naproxen which is an NSAID, so a non-steroidal anti-inflammatory drug, and that does have an impact on the kidney.
1 minute 21 seconds CHARLIE
[01:21] CHARLIE So, let's read over the possible options here that are listed in the question. And remember the stem says, "By what mechanism might this drug further impair his renal function?" Option A is afferent arteriolar vasoconstriction, option B is afferent arteriolar vasodilation, option C is efferent arteriolar vasoconstriction, option D is proximal tubular toxicity, and option E is ureteral obstruction.
1 minute 53 seconds CHARLIE
[01:53] CHARLIE So Cathy, let's review briefly what is meant by afferent and efferent arteriolar vasoactivity in the kidney and how is that relevant.
2 minutes 2 seconds CATHY
[02:02] CATHY Okay, so the afferent arteriole is upstream of the glomerulus and feeds the glomerulus and it can really either constrict or vasodilate. If you constrict the afferent arteriole then you decrease the blood supply and the results in GFR. And if you vasodilate the afferent arteriole, then you increase the blood supply and the result in GFR. The efferent arteriole is downstream and the exact opposite happens, so constriction will increase the GFR and vasodilation of the efferent arteriole will decrease the GFR.
2 minutes 33 seconds CATHY
[02:33] CATHY Now, in the setting of kidney injury, the response is naturally that the afferent arteriole will vasodilate to preserve GFR.
2 minutes 41 seconds CHARLIE
[02:41] CHARLIE So based on that, we can already eliminate two of the choices. Because answer B, vasodilation of the afferent arteriole, and answer C, vasoconstriction of the efferent arteriole, will both increase GFR, and therefore would not impair renal function. They would actually help it, right?
2 minutes 57 seconds CATHY
[02:57] CATHY Right, that's exactly right. So, now if we go back to the impact that NSAIDs would have on renal perfusion, this would sort of fall under the category of prerenal causes of acute kidney injury. And NSAIDs act on prostaglandins which exert vasodilatory action on the afferent arterioles. So, when you block the prostaglandins like PG and prostacyclin, which happens with NSAIDs, then you get loss of vasodilation and therefore constriction of the afferent arteriole, and that results in a decrease in GFR, so the answer is A.
3 minutes 29 seconds CHARLIE
[03:29] CHARLIESo the answer is A: constriction of the afferent arteriole due to inhibition of prostaglandin synthesis by the naproxen could actually exacerbate his acute renal injury or his acute kidney injury by making the kidney think that the glomerulus is being under profuse.
3 minutes 46 seconds CHARLIE
[03:46] CHARLIE Is that correct?
CATHY: That's correct.
3 minutes 47 seconds CATHY
[03:47] CATHY So B and C we already said were wrong, and then just to go over the other choices. D, which talks about proximal tubular toxicity can happen if NSAIDs are taken for a really long periods of time or other drugs like antibiotics can also cause that, but that's not the case here. And then obstruction is usually caused by mass effect which wouldn't be caused by NSAIDs, and it also shouldn't really cause acute kidney injury unless there's pre-existing disease.
4 minutes 13 seconds CHARLIE
[04:13] CHARLIE So, in summary this question highlights the impact of NSAIDs on renal perfusion, such that they should be used in caution in patients with possible impaired renal perfusion, such as elderly patients or other patients with chronic kidney injury, and probably are not the best choice for post-operative pain in this demographic.
4 minutes 30 seconds CHARLIE
[04:30] CHARLIE Is that true?
CATHY: I agree completely.
4 minutes 32 seconds CHARLIE
[04:32] CHARLIE So, in this case, Cathy, if this man were having pain that was symptomatic enough such that you did want to give him something what agents might you consider given that we've already decided that non-steroidals are probably a bad choice.
4 minutes 44 seconds CATHY
[04:44] CATHY So, you can try something topical like a lidocaine patch if it was something focal, or if you wanted to use a systemic drug, you could try something like acetaminophen. Or if that wasn't controlling the pain, then you could also do opioids or opiate-like medications.
4 minutes 59 seconds CHARLIE
[04:59] CHARLIE With the understanding that you'd be judicious similarly in an elderly patient with chronic underlying comorbidities.
5 minutes 5 seconds CATHY
[05:05] CATHY Absolutely.
5 minutes 6 seconds CHARLIE
[05:06] CHARLIE Great. Okay, thank you! [outro music]
