Hypertension due to Renal Artery Stenosis

Note: As a rule, I'm trying to write this blog so that you don't need to read a previous entry to understand a new one. So if you're reading all of them, there's a good chance you'll end up seeing a bunch of the old anatomy/physiology repeated. Feel free to read just what you need to remember/appreciate the really cool ways things work. . .  . .and fail :).

On to our disease of the day!

Short 'n Sweet:
A whiny self-centered kid(ney) in the car insists on cranking the A/C  BP (blood pressure) when it's already too high.

Anatomy & Physiology:
The kidneys really are a wonderful pair of organs. They self-regulate to make sure they keep working, have at least a 75% reserve capacity at baseline performance, and purge toxins with incredible selectivity. You can think of the kidneys as a filter for the bloodstream - they sample the bloodstream, filter out toxins, and retain useful levels of useful nutrients. The rest goes to the ureter, then the bladder, and finally to our lovely friends in the plumbing department :).

In order to do this job, it should be obvious that the kidneys need to sample the blood, right? Roughly 30% of the heart's output is sent towards the kidneys through a blood vessel (artery) known as the Renal Artery. After passing through the renal artery, blood flows into a million tiny vessels and then glomeruli. The glomerulous is where the blood is "squeezed" and little particles filtered out into the urine along with fluid. Not 100% of it, mind, but certainly most of it. The kidney then reabsorbs teh stuff it doesn't want to get rid of (>90% of filtrate) and sends the rest on to the bladder. Special sensors through the tubules sense the flow-speed of filtrate. If it goes down, the sensors triggers the release of hormones (chemical signals) including eventually Aldosterone and Angiotensin II (Ald and AngII).
Ald makes the body retain extra sodium (and thus water). This causes the fluid-volume in the blood space to go up, which causes blood *pressure* to go up, which means: more blood gets to the kidney - so flow goes to normal! (self-regulating)
AngII does lots of things - but one thing it does is to constrict (tighten) arteries throughout the body. Imagine squeezing down on a small volume of fluid in a tube - doing this leads to increased *pressure*, which again means more kidney blood flow - which helps filtrate-flow go to normal.

The last thing you need to know to appreciate Renal Artery Stenosis is hypertension. Hypertension means that your blood is at too high of a pressure, that either the blood vessels are "squeezing" the blood too tightly, or the blood is pushing out against the blood vessels too much (since there's too much blood). It's almost always the latter, as blood vessels don't have the muscle strength to tighten that much to the point of hurting you. Hypertension isn't painful - but it is dangerous. Left unmanaged, it can steadily cause damage to the smaller more vulnerable sized arteries throughout your body. It also accelerates the process of artery narrowing (stenosis) (described later) The #1 organ for blood-pressure maintenance, though, is the kidney - since the #1 way to drop or increase blood pressure is to hold more fluid or get rid of it, respectively.



How it goes wrong!:

As good as this system is - there *is* an underlying issue - it's reliant on the idea that low *kidney* blood pressure means low *body* blood pressure! Now remember what I said earlier - the heart sends 30% of its output to the kidney almost directly: (Aorta -> Abdominal Aorta -> Renal Artery). 99% of the time, if the kidney sees low blood pressure that means there really is low blood pressure. But most the time isn't the same as always :).

In greek, stenosis means "narrowing". For whatever reason (usually plaque buildup) our patient gets a buildup of plaque in the renal artery . . . blocking the kidney. This blockage starts off small - <10%, and so blood flow is only marginally lower and the kidney doesn't have to compensate. But then it gets more and more blocked as the stenosis gets bigger - 20%, 30%, 50%. Now stenosis like this isn't exactly a rare thing, and the normal result is that the end-organ just suffers from lack of blood and oxygen. Like, say the end blood vessel in my left toe got blocked; it won't get any blood, will cry for a while, then just die. Tragic, sure, but you can survive without a left toe.

But the kidney is very different from other organs! Firstly, it doesn't need much of that blood supply to survive - 30% of heart output is a TON of blood, it only takes so much to filter it, it could survive with a lot less. Secondly, even if the kidney dies, there's a second one to keep you alive. The problem is that the kidney doesn't KNOW this! And it's not able to tell the difference between low blood delivery from - say - someone getting dehydrated and losing blood pressure and from it *personally* not getting any blood. So the kidney does what it knows best: increase blood pressure to increase its blood supply.

So picture this: you've got the whole body sitting at a nice, healthy blood pressure (120). Your renal artery is blocked though, and so the kidney gets much less blood = less pressure = (80).

The kidney freaks out and starts shouting at the rest of the body - "Emergency, Blood

pressure collapsing! Raise the blood pressure! All hands to water-retention stations

and blood vessel constriction! This is NOT a drill people, this is NOT a drill!!!"

The body responds. . . and body's blood pressure flies up to 150!. But this only increases the kidney blood pressure to 82, since the blockage is still in place!!! What's worse, one of the major factors that helps a stenosis get bigger is - get this - high blood pressure :)! The block gets worse, the kidney shouts even louder, and the hypertension gets even WORSE.

It's like having a thermostat switch outdoors (in winter) with a building full of people indoors. The guy outside keeps cranking the heat up higher and higher since he's still freezing. He's oblivious to the fact that he's making everyone inside melt with the heat! And what was supposed to regulate blood pressure now becomes responsible for ruining it.

Treatment:

Treatment is actually really straightforward: just remove the block. We've got an awesome procedure called 'stenting' where we literally just pass a wire through the bloodstream with a balloon attached to the end. when the ballon gets "stuck" in the blocked artery, we blow it up - which smushes the plaque out of the way and opens the artery! Then we add a metal sheath to hold the artery open. That's a bit of an oversimplification, but it's the basic outline of a stenting procedure.

How bad is it really?:

So there's two major factors affecting outcome. First is whether it's one renal artery or both that are blocked. If just one is blocked, sure it's bad. But it's a tug-of-war situation. One kidney is trying to increase blood pressure as hard as it can, the other ends up growing extra big (since it needs to handle 2x the filtered amount of toxins) and tries to stabilize blood pressure. If it's both blocked, then nobody is keeping the blood pressure down and treatment is URGENT.

Second factor is how long it's lasted. Remember how hypertension increases the risk of damage and stenosis? Well the longer you leave this type of condition untreated the less likely it is you've got a healthy patient when all is said and done. He'll be at greater risk (due to damage of the hypertension) of getting fresh problems and stenoses.

And that's Renal Artery Stenosis!