Okay, this is called Think Like a Nephrologist, and we're going to talk about combined water and salt disorders. So what I do to make these complicated disorders more easy to think about, I actually create this kind of a grid format. So I put total body salt on the x-axis, and I put total body water on the y-axis.
And so for total body salt, remember what we're really talking about is extracellular fluid volume, because sodium is excluded from cells because of the sodium potassium ATPase, which works... tirelessly to pump sodium outside of cells, we're really talking about extracellular fluid. And so when we have too much salt in our body, when we have excess extracellular fluid, we call that volume overload. And we can detect that on physical exam through detecting peripheral edema.
We can see that the patient has high blood pressure. And if the fluid accumulates in the lungs, we call that pulmonary edema. If the patient has lost too much salt, they have extracellular fluid volume depletion or.
just volume depletion. And we can detect that because they'll typically have hypotension. They'll have orthostasis, which just means they feel kind of dizzy and weak when they sit up or stand.
And we should not detect edema or fluid on their body. So I do the same thing for total body water. Let's say the patient has lost a lot of water.
So if someone has lost a lot of water, that is dehydration. And the diagnosis is pretty easy. All you have to do is look at the serum sodium concentration. If that is greater than 145 millicolvents per liter, that person is dehydrated because there is not a lot of water relative to the other solutes in the body, and so they have a total loss of water. You can also have too much water.
So if you have too much water relative to all the solutes in your body, we call that hyponatremia. And so let's assume that we've excluded hyperglycemia and pseudohyponatremia, and so we're only talking about hypotonic hyponatremia. And so again, the diagnosis is pretty easy.
All you have to do is look at the serum sodium concentration. If it's low, let's say less than 135 millicovins per liter, there is too much water in the body. Now again, before we move on, we're at this sort of complex idea in that to actually measure total body salt in a person, in a patient, you are assessing the extracellular fluid volume through things like their weight, their blood pressure, and the physical exam looking for the lack of edema or presence of edema. To measure the water balance in a patient's body, you're actually measuring tonicity. So you're looking at the plasma osmolarity.
You're looking at the serum sodium concentration. So serum sodium concentration, whether it's dilute or concentrated, tells you about water balance. The patient's physical exam and vital signs and weight tells you about salt balance.
This is really important. So with regards to total body water, I actually like to imagine what the cells look like because plasma tonicity and total body water balance determine cell size. And obviously, we care about cell size because of what it can do to the brain with regards to shrinking or swelling of cells. So I do like to imagine what the cells look like. And so when we're in perfect water balance, I imagine the cells are of normal size and shape and they're really happy.
When... were dehydrated, I'd imagine the cells are shrunken and shriveled and they're really unhappy. And when there's excess water, I imagine the cells are sort of bloated and very unhappy.
So now let's go through each quadrant. So first look at the lower left. So let's consider a patient with both hypovolemia and dehydration.
And so the common admission would be a patient who has a source of salt loss, whether it's diarrhea or some other illness. But they've also had inadequate access to water. And so they may feel thirsty, but let's say they were, you know, hiking or in a desert.
I'd say the most common admission would be a patient who is a resident of a nursing facility and they've developed sepsis or illness and altered mental status and they haven't been eating or drinking normally. So this kind of a patient gets admitted to the hospital very frequently. And so the... Treatment here, you'd want to correct the volume depletion first, perhaps with isotonic fluid, maybe a couple of boluses of fluid.
And then once you feel like you've improved volume status, then you can start correcting the dehydration by administering water in the form of D5 water. And so this is the kind of situation where you're typically running fluid in two different IVs. Maybe you're running crystalloid in one IV and water in the other IV.
Next, you can have hypovolemia and hyponatremia. And so think about it. This is a patient who's had total body salt depletion, so they've volume depletion, but they had... excess water in relation to all the solute in their body.
And so this is another common emission. And this tends to be, you know, any patient who's had some issue with total body salt depletion, let's again say someone with diarrhea in the community. And, you know, maybe they know that in the setting of diarrhea, they should increase fluid intake.
And so maybe their relative told them to, well, drink more water, just drink more water. And so unfortunately, that doesn't really fix the problem because it's not that they need. more water, they actually need more salt. And so what they should be drinking is like broth or bullion cubes dissolved in a cup of water.
What they were doing was just drinking pure water. So if they're losing salt water and drinking pure water, what happens in the setting of volume depletion is that AVP or ADH is secreted. And in the setting of high ADH secretion due to the volume depletion, they will retain water.
And so if they're drinking lots of water and they're retaining lots of water, they're going to develop hyponatremia. And so this is another common emission, hypovolemia and hyponatremia. And in the setting, you fix both problems by just fixing the hypovolemia. So if you administer crystalloid like normal saline or LR, theoretically the signal, right, the signal to release ADH or AVP will all of a sudden be taken away. and the kidney will start making dilute urine, ridding the excess water that is present.
And so this is a common emission, and you should be able to know how to handle it. So next, let's consider hypervolemia and hyponatremia. And this is a patient who has excess total body salt.
They have extracellular fluid volume expansion. They have volume overload, but they also have hyponatremia. They have too much water in their body. And the classic examples here would be patients with cirrhosis or heart failure. Remember, in the setting of cirrhosis or heart failure, there's low perfusion, and that's really it.
Low perfusion leads to increasing renin, right? And then it also increases sympathetic nervous system activity. Oh, and of course, low perfusion also increases ADH or AVP secretion. So the net effect of all these things is to retain sodium. and retain water.
And so when all those hormones are active, the kidney's going to be retaining sodium and water, and whatever the patient eats or drinks, right? So whatever salt they ingest is going to be retained, and the water that they ingest is going to be retained as well. What happens as the disease process worsens?
They can develop both hypervolemia and hyponatremia. So it's really a retention of both salt and water. Therefore, the hyponatremia in this case is when...
perhaps they ingest more water in relation to the salt or other solutes. And so the fix here is using diuretics or removing the fluid in some way. And using loop diuretics will help remove the excess salt from the body. And the way it interacts with the kidney, it can also lead to water excretion. Think about the concentrated medullary interstitium.
Well, the channel that loop diuretics block. is the channel that actually pumps sodium and chloride into that medullary interstitium that we use to help concentrate our urine. So if you block that long enough, it does impair the kidney's ability to concentrate the urine, and that will help with water excretion as well. Of course, sometimes AVP or ADH secretion is so high, we can't treat it alone with loop diuretics. We have to use other medicines like AVP receptor antagonists.
like Tolvaptan, for instance. Finally, you can have hypervolemia and hypernatremia. This doesn't happen too often, but I've seen it a few times. I'd say the prototypical example is the patient who is in the hospital being treated for a heart failure exacerbation. So let's say a patient is admitted with fluid overload and they have a normal serum sodium.
They do not have... any water disturbance, and they've been treated with loop diuretics, you know, day after day after day for many days in a row, and let's say that perhaps they've been placed on a fluid restriction and they're not feeling very well, not drinking a lot, they can actually develop hypernatremia, but also still have hypervolemia as they've been, you know, urinating two or three or four liters a day on the loop diuretics. There's been excessive water losses, and you can actually have water loss in... excess of the sodium loss, you know, and then that can actually lead to sort of an acquired hypernatremia that occurs in the hospital. But of course, as you examine the patient, they could still have hypertension.
edema, peripheral pitting edema, and that tells you that they're still hypervolemic. So like I said, this doesn't happen too often, but it is one of the rare situations where you actually continue the loop diuretics, but you also administer water. So you're administering D5 water to correct the hypernatremia, remember, because I look over at the left and think about what the cells are doing.
And so if the cells are shrunken and unhappy, I don't like to leave a patient like that. I know the brain doesn't enjoy it. So I correct the hypernatremia with water, and I continue the loop diuretics to try and correct the hypervolemia. And you actually may have to give more water than otherwise, because if you keep using the loop diuretics, you're going to have continued water losses in the urine.
So for the patients on the right with hypervolemia, I want to make our lives easier, both theirs and ours, by restricting how much salt goes back into the body. Because remember, we're using... loop diuretics to try and remove salt from the body.
So they need to be on a salt restriction, like a sodium restriction in their diet. Okay, I mean, typically two grams a day or less. The patient at the top right, right, this is the patient with hypervolemia and hyponatremia.
Typically, it's a patient with heart failure and or cirrhosis. They should also be on a water restriction. So a pure water restriction, because we know that they have hyponatremia, we know that they're going to hold on to water. like crazy that they have a hard time diluting the urine getting rid of water and so whatever water is added to the system will be retained and so they should also be on a water restriction now you may be noticing these sort of columns in the middle well you know i left room for you know like pure hypervolemia right so if there's just excess sodium without any changes in body water and there can also be pure hypovolemia all right just a typical volume depletion And then you can also have pure dehydration, right, with normal blood pressure, for instance, normal volume status. And then you can also have pure hyponatremia with normal volume status.
And sometimes we call that SIADH or, you know, syndrome of inappropriate ADH secretion. So obviously in the middle, this is where, you know, everyone's happy and there's no salt or water disorder. And so I'll put the smiley face there. But I'd say typically most hospital admissions are going to be in one of the four quadrants. I would say the patients with hypovolemic hyponatremia and patients with hypovolemic dehydration tend to be the most common.
But also for hepatologists and heart failure physicians, hypervolemia and hyponatremia is very common. And then hypervolemia and hypernatremia, not as common, but can develop typically maybe a days or weeks after you're treating this patient, they can actually become hypernatremic. And so thinking about everything in this graph format allows me to separate the two issues in my head, and I think it works really well.
Hopefully it works well for you.