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In this lesson, we’re going to talk about hypotonic solutions. What are they, how do they affect the body, and why do we use them?
Again, let’s quickly review what we mean when we talk about tonicity. Tonicity compares the osmolarity of two solutions. In these cases, we’re comparing an IV fluid to blood plasma. If we have a solution that is less concentrated than blood plasma, or has a lower osmolarity, it’s considered hypotonic. If the solution has a similar concentration, or osmolarity, we call it an isotonic solution – iso meaning ‘same’. If the solution has a higher concentration or osmolarity, we call it a hypertonic solution.
So, when we’re looking at a hypotonic solution – that means it has an osmolarity that is LOWER than the blood plasma, typically less than 250 mOsm/L
Some examples are ½ normal saline – which has an osmolarity of 154 mOsm/L, but also anything less than that like ⅓ or ¼ normal saline. We don’t see these used very often, though – usually it’s just ½ normal saline. Anything less than 5% Dextrose in Water, so D2.5W for example. But also remember that D5W in the body actually functions as a hypotonic solution because the Dextrose part gets used up and what you’re left with is just sterile water, which has an osmolarity of 0. So remember that with your 5% dextrose solutions that the dextrose will be used up, so whatever you’re left with is how it actually functions in the body.
As we already mentioned, the osmolarity of a hypotonic solution is less than 250 mOsm/L, remember that blood is about 275 – 295 mOsm/L. So we’re introducing a solution that is less concentrated into the blood vessels, and the blood plasma will now be less concentrated than it was before, compared to the cells. When you have a lower concentration on one side of a semipermeable membrane, which way is the fluid going to shift? The fluid will want to shift away from that side, right? So what we see is the fluid shifting out of the blood plasma and into the cells. That means the cells are going to swell. If they swell too much, they could even burst – that’s called cell lysis. We want to try to avoid shifting it that far.
So why would we use one of these solutions? Well the main reason would be some sort of cellular dehydration. Usually cellular dehydration is caused by a hyperosmolar state in the blood. That means there is more solute in the blood than there should be. The two best examples are hyperglycemia – like in DKA or HHNS – and hypernatremia. When the blood gets hyperosmolar – or super concentrated – the cells tend to shrink down because the fluid shifts out of them. So, to fix the problem, we want to reverse that process. We give a hypotonic solution and try to swell the cells back to normal so they can function properly. Now, it’s super important to note that there are situations when using a hypotonic solution is contraindicated. Remember, the fluid is going to shift out of the blood plasma – which will decrease our overall blood volume, right? So we would never want to give this type of solution to someone who is hypovolemic. They are already low on blood volume, so the last thing we want to do is drop it even more. Patients with burns are also at super high risk for hypovolemia because they lose a lot of fluid through their skin, so a hypotonic solution would only make it worse. The other thing is any patient with cerebral edema or increased intracranial pressure. Remember, these solutions would cause the cells to swell. When that happens in the brain it can be devastating – so if they are already experiencing this, we definitely don’t want to make it any worse by giving them a hypotonic solution. So if you are in a neuro unit or you have a patient with head trauma or a stroke – they should really NEVER receive a hypotonic solution. Always either isotonic or hypertonic.
Okay let’s recap – remember that a hypotonic solution has less solute than the blood plasma – so it’s going to make the blood more dilute than it was before. That will cause fluid to shift out of the blood plasma and into the cells, and sometimes even into the tissues. Examples are ½, ⅓, or ¼ normal saline, 2.5% dextrose in water, and sterile water. Remember also that 5% dextrose will actually be hypotonic in the body because the dextrose gets used up and what’s left is just sterile water. The main reason we use hypotonic solutions are for hyperosmolar states like hyperglycemia in DKA and HHNS and hypernatremia. The reason is because these conditions will cause cellular dehydration, so we want to restore the fluid in the cells and help them to work properly. And remember the contraindications for hypotonic solutions are hypovolemia (because we don’t want to shift even more fluid out of their cells) – which includes burn patients – and any kind of head injury or neuro issue like a stroke because of the high risk for cerebral edema.
So that’s it for hypotonic solutions – make sure you have also checked out the isotonic and hypertonic solutions lessons, as well as all the resources attached to this lesson. Now, go out and be your best selves today. And, as always, happy nursing!!