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In this lesson we’re going to talk about fluid shifts. What does it mean for fluid to shift and why do we care?
So, first, let’s understand what we’re talking about when we say fluid shifts. Remember we have multiple fluid compartments. There’s the space inside the cells – the intracellular space – the space in the blood vessels – the intravascular space – and the space in the tissues – the interstitial space. As we mentioned in the fluid compartments lesson, there’s a certain percentage of our body fluids contained in each space and our body prefers it to stay that way. Any time you have portions of our body fluids shifting from one space to another, it’s going to cause a lot of problems in our system. If the cells don’t have just the right amount of fluid, they’re not going to function correctly. If our blood vessels don’t have the right amount of volume, we’re going to struggle to get the blood out to our organs, right? So, let’s look at what causes these shifts.
So, there two main culprits for fluid shifts. These aren’t the ONLY reasons, but they’re the most common and the ones you really need to be aware of. One is changes in osmolarity. What happens is that the blood becomes more or less concentrated for some reason and it means the osmotic pressure changes. As you remember from the last lesson, if you have one solution that is more concentrated than another, meaning it has more solutes – or particles dissolved in it – then you will see water shifting towards it to try to balance out the concentrations. And vice versa if you have a solution that is more dilute – water is shifting away from it. The second general cause of fluid shifts is what is known as a capillary leak. Remember that the capillary walls are a semipermeable membrane – that means that they’re selective – only certain things and certain amounts of water can get through. In a capillary leak – something affects the permeability of the capillary, therefore allowing more water to escape. Think of it like a dam on a river – there might be one or two channels that allow water through to hold the river back. If we punch a bunch more holes in the dam, the water is going to rush through and it’s going to cause flooding on the other side of the dam, right? This is what can happen in a capillary leak. Water is allowed to escape the capillaries and floods into the tissues, organs, and body cavities. Let’s look at some examples of how each of these things happens.
So let’s talk about Osmolarity changes. If we see that the osmolarity of the blood is Increased – that means it’s more concentrated than normal… then fluid is going to want to shift towards the blood, right? It will shift into the intravascular space. So – what happens to the cells? The fluid comes out of them so they will shrink and become very dehydrated. Cells that are super dehydrated cannot work correctly! Some examples of this are hyperglycemia and hypernatremia – that’s excessive blood sugar or excessive sodium in the blood. That makes it more concentrated and causes severe cellular dehydration. The other thing could be Diabetes Insipidus. We talk about this in the Metabolic/Endocrine course – it causes massive water loss from the system. So rather than adding more solutes, we took away the solvent or the fluid – so that’s what makes it super concentrated. None of these conditions are good and we need to fix the problem asap.
Now, if we see that the osmolarity is decreased – that means the blood is less concentrated or more diluted than normal – then fluid is going to want to shift OUT of the intravascular space. So where would it go? It would go into the cells and into the tissues. What happens when we add more fluid somewhere? It swells, right? So we may see swollen cells – which also can’t work right – and edema. This could be peripheral edema, pulmonary edema, cerebral edema, or even something called “Third Spacing” which we’ll talk about in just a minute. Examples of this are SIADH, which we also talk about in the Metabolic/Endocrine course – it causes us to hold onto massive amounts of water. We could also see it with Water Intoxication, which is exactly what it sounds like – the patient consumes too much water. There’s even a really interesting condition called Neurogenic Polydipsia – it’s a neurological disorder that causes people to drink a TON of water – I had a patient once who literally went into the bathroom and put her mouth under the tub faucet because she was trying to drink as much water as she could. This excessive amount of water causes diluted blood, which leads to this fluid shifting out of the vessels and it is very dangerous. And then, of course, if we over-resuscitate someone, especially with hypotonic fluids, we can cause this same problem.
When we’re talking about a capillary leak, remember we’re talking about a physiologic change in the capillary permeability. It isn’t fully understood, but the thought is that some sort of inflammatory process and the chemicals released (called cytokines) are involved. Because of this change – the fluid can leak out of the capillaries into the tissues, organs, or body cavities. Common causes here are burns, sepsis, and snake bites because of the venom. Rarely we may also see this with autoimmune inflammatory conditions as well. The most common times you’ll see this are in burns and sepsis. The fluid is all shifting out of their vessels – usually third spacing, which we’ll look at in just a sec. So if the fluid isn’t in their vessels, they’re at risk for hypovolemic shock. If you want to learn more about that, check out the hypovolemic shock lesson in the Cardiac Course.
So let me clear up what Third Spacing is. I’ve mentioned it a couple times, and you may have heard it during school. Think about it this way – you have blood vessels and you have cells. In the cells is “space 1”, in the vessels is “space 2”. So when fluid shifts anywhere else – like in the tissues, for example – it’s called “third spacing”. It’s not the first or second space, it’s the third space. The two best examples of this are ascites like you see at the top here and peripheral edema at the bottom. Ascites is when the fluid ends up in the peritoneal cavity and happens a lot in liver failure and pancreatitis. Peripheral edema happens when the fluid collects in the tissues in the extremities, usually in the subcutaneous tissue. We could also see pulmonary edema, pleural effusions, or even weeping wounds – again any leakage of fluid into somewhere that isn’t the vessels or cells – a third space. This can happen for any number of reasons, including the two major things we already talked about. Another reason is a loss of oncotic pressure. Remember oncotic pressure is when the proteins pull fluid in and hold it there. If you have a loss of protein – like in liver failure – you could see the fluid leaking out into these spaces.
As I mentioned before – the fluid going anywhere it doesn’t belong is never a good thing. Let’s just quickly review some of the major complications of fluid shifts. Here are our main compartments – the vessels, the cells, and the tissues. Intravascular, Intracellular, and Interstitial. If the fluid all shifts out of the vessels – we lose blood flow and can have hypotension and even signs of shock. If the fluid all shifts out of cells, we see cellular dehydration – which means the cells will shrink up and can’t work properly. Symptoms of this, of course, depend on which cells are involved. If the fluid shifts into the tissues, we see significant edema, and we could possibly even see compartment syndrome if the pressure builds up enough to cut off circulation. And I want to specifically point out the complications if the fluid shifts around the brain cells and tissues. Remember the brain is super sensitive to these changes – we could see increased intracranial pressure and even seizures – so keeping an appropriate fluid balance is so important.
Priority nursing concepts are going to be fluid & electrolyte balance and perfusion. Remember that our #1 goal will always be to treat the cause – but we’ve got to make sure that the patient is adequately fluid resuscitated and has good perfusion. How’s their blood pressure? What about peripheral pulses? All important things to look at. We can even do circumference measurements on their extremities or abdomen to see if the fluid is still accumulating. So, look at the specific condition your patient is experiencing and prioritize your care based on that, but keep these two general priorities in mind.
So, let’s recap. We know that fluids in the body have a place where they are supposed to be and in certain proportions. If fluid shifts to a place where it doesn’t belong, it can cause quite a few issues in the body. Some common causes of fluid shifts are changes in osmolarity – because the fluid tries to shift towards the more concentrated area – and capillary leaks – because it allows more fluid to leak out of the vessels into the tissues. This can cause what’s known as third spacing, when fluid accumulates in spaces other than the cells or the vessels – spaces like tissues, organs, or body cavities. Our goal with these situations is going to be to treat the cause and try to prevent complications. Fluid shifting out of blood vessels can cause perfusion issues and fluid shifting into or out of cells will cause them to not work properly – especially in the brain.
So those are the basic important points for fluid shifts. Make sure you check out the three lessons on the types of IV fluid solutions we would use (hypotonic, isotonic, and hypertonic) and how they affect the fluid balance and the cells in the body. And don’t forget to check out all the resources attached to this lesson. Now, go out and be your best selves today. And, as always, happy nursing!!