Isosmolar Fluid Infusion
So we know the body is full of fluids. Aside from the water in dense connective tissue and bone (making up 15% of total body water), there are 3 main compartments where we consider the movement of water.
- Intracellular Fluid (55%)
- Interstitial Fluid (20%)
- Intravascular Fluid (7.5%)
When calculating the effect of isosmolar fluid infusion, we take into account the following assumptions:
Total Body Water = 1/3 ECF and 2/3 ICF
ECF = 1/4 Plasma and 3/4 ISF
And of course, we remember that total blood volume is about 5L.
So we’ll look at 3 types of rapid IV infusion. Dextrose 5%, Normal saline, and Plasma Protein Solution.
Dextrose 5%
Since glucose is very rapidly taken up by the cells of the body, we can more or less treat this as pure water. A large percentage of this would remain intracellular. Thus, this is useful for replenishing intracellular fluids
For every 1000mls of Dextrose 5% infused,
ICF = 1000 X 2/3 = 666.67 mls
ECF = 1000 X 1/3 = 333.33 mls
Therefore,
Volume going into plasma = 1/4 X 333.33 = 83.33 mls
This increase in plasma volume is barely enough to trigger the volume receptors (which only fire off at 7-10% increases). However, plasma osmolarity decreases and this triggers osmoreceptors. This in turn decreases ADH (anti-diuretic hormone) secretion, and there is an increase in water excretion.
Normal Saline
Normal saline has the same concentration of sodium as ECF and therefore it sort of sticks around in the ECF. This makes is very useful for replacing ECF without mucking up your ICF too much.
For every 1000mls of normal saline infused,
ISF = 1000 X 3/4 = 750 mls
Plasma = 1000 X 1/4 = 250 mls
Therefore, we can see that if we want to replace plasma volume with this, we need to multiply our normal saline volume by 3-4 (since only 1/4 of it goes to plasma). However, as you can imagine, this would inevitably mess with something.
Since [Na+] of normal saline and the plasma are about the same, they are isosmotic. Therefore there is no marked change in plasma osmolarity and tonicity. The only thing that matters is this decrease in oncotic pressure.
Oncotic pressure is the pressure exerted by plasma proteins. Since plasma proteins are generally too large to pass through capillary walls, they exert oncotic pressure which draws water into the capillaries, reducing “leaky” capillaries. In patients with proteinuria (loss of proteins through urine), there is a fall of oncotic pressure, leading to oedema of the tissues.
So, back to normal saline. Fall in oncotic pressure triggers an increase in glomerular filtration rate in the kidneys, causing a decrease in water reabsorption. Therefore we get increased excretion.
Plasma Protein Solution
This is really fancy (and expensive) stuff. Examples of plasma protein solutions would be something like Human Albumin 5%. Since this is a colloid, it sticks pretty strictly intravascularly. So…
For every 1000mls of plasma protein solution infused,
1000mls goes to plasma.
This would definitely trigger the volume receptors, which causes a drop in ADH secretion, increasing the excretion of water.
Having the excretion of water, oncotic pressure would then increase. This draws more water from the ISF into the intravascular space. This whole process is pretty slow and slows the excretion of water as well.
But, since albumin is slow to excrete, the excretion of the transfused fluid is slow as well. This allows the effects of the transfusion to linger around longer.
It’s all a vicious, slow slow slow osmotic cycle really.
