Of course, Scotch is a colloid

In discussing blood pressures, and fluid resuscitation, my current precept asked a simple question, “When would you use crystalloids over colloids, assuming both were available?”.  Further discussion led to a narrowing of the question to, “When would you use crystalloids and pressors, rather than simply switching to colloids?”

Since there are a variety of reasons to use any fluid resuscitation, let’s make this a trauma patient.  Mr. Smith was using his chainsaw to remove a fallen tree on his property when – whoops! – chainsaw slips and he has a deep cut on his anterior thigh.  EMS arrives, bleeding is controlled to an oozing wound.  Mr. Smith is ashen, tachycardic and hypotensive.  The nearest ED is a 20-minute response.

Now, to understand why the precept posed this question, you need to know that it takes far less of a colloid solution to produce the same effect on blood pressure as lots of crystalloid.  Generally, 250ml of colloid has the same BP effect as 4 liters of crystalloid.  That is, 1/16th the amount of colloids does the work of crystalloids.

“What!”  you exclaim.  “DTs, this means that…  hmmm, 4/4 = 1 liter, therefore 250/4 = 62.5… this means that instead of hanging a liter of saline wide open to raise a blood pressure, I can draw up a 50ml syringe of this ‘colloid’ of which you speak and bolus a nice big systolic BP almost immediately!  Why, this revolutionizes EMS!  A guaranteed systolic in my pocket!”

Not so fast!  There is, as you have probably guessed, quite a bit more to it, and as you might also have guessed, we’ll start at the very beginning of the subject.

We first need to discuss pressures.  Any fluid, in any container, exerts hydrostatic pressure.  This is the pressure the fluid (hydro) exerts on the container walls due to gravity, when the fluid is at rest (static).

If we were dumping fluid into a metal bucket, or a Styrofoam cup, we’d pretty much know all we needed to.  Since we’re putting fluid into living things (patients), we need to first explore a couple of concepts.

To get fluids into our patient, we generally introduce the fluids via the vascular bed – which includes the veins, arteries, and capillaries.  That’s usually where we want it to stay, too, if we’re trying to raise the BP.  And we also know that the vascular bed is made up of cells, which have cell walls.  If these walls just allowed anything in to or out of the cell, they’d be pretty worthless.  To work well, they need to be semi-permeable, and to selectively allow the admission or expulsion of fluids or chemicals.

Another important pressure is osmotic pressure.  It’s called “osmotic pressure” because it deals with osmoles, which is the number of osmotically active particles in a kg of solution (there can be non-osmotically active particles in a solution, but we don’t care about those right now).  By osmosis, fluids move from one side of a semi-permeable membrane to the other,  based on which side has the most solutes.   Imagine a solute as acting like a small sponge;  if cell A has a sponge inside, and the blood vessel outside the cell has a fluid which has 20 times the number of solutes (sponges), fluid will flow out of the cell and into the vessel.

Osmotic pressure is used in a lot of different fields, but in the medical field, we’re talking about cell membranes (and no other kind), and usually as it relates to existing blood plasma (your patient wasn’t completely empty, was he?) so we get to have our own term for it – tonicity – which completely ignores a lot of other ugly stuff about osmolality, osmolarity, and other junk that biologists have to worry about.

Our fluids are either hypotonic – containing fewer solutes than surrounding tissue; isotonic – containing the same number of solutes; or hypertonic – containing more solutes.

There are dozens if not hundreds of different IV solutions in existence.  We are ignoring here whole blood, blood plasma, packed red cells, and other mainstays of ED life, and concerned only with crystalloids and colloids.

What is a crystalloid?

A crystalloid is a fluid in which the solutes are dissolved.   If the particles in a fluid do not dissolve, then that fluid is not a crystalloid.  Two very common crystals also happen to make up our two most common crystalloid solutions.  Salt crystals are added to water to make Normal Saline.  Sugar crystals are added to water to make D5W.

Isotonic ( from the Greek isos, meaning “equal”) crystalloids are those fluids which have roughly the same tonicity as blood plasma.  These include:

  • Normal Saline 0.9%.  This fluid is the most widely used in EMS for volume expansion.  It has no red blood cells, hence no oxygen carrying capability, and includes no electrolytes.  Its administration is purely to increase the hydrostatic pressure in the vessels.  However, it has been noted that about 75% of a saline bolus leaves the vascular bed almost immediately, leaving 25% in circulation.  That 75% can contribute to edema and wet lung sounds if the patient is over-hydrated;
  • Lactated ringers.  This fluid contains a bit more dissolved in it – sodium, chloride, lactate, potassium, and calcium.  It is useful in resuscitation because, as the liver metabolizes lactate, the by-products of that metabolism help to counteract acidosis.  For resuscitation the usual dosing is 20-30ml/kg of body weight.  Ringers is not, however, used for long-term drips since the electrolytes sodium (130 mEq/L) and potassium (4 mEq/L) are respectively too high and too low for homeostasis.  That is, while the tonicity of the fluid is the same as the body, the electrolyte balance is not;
  • D5W.  This fluid is not used in resuscitation.   Dextrose (the D in D5W) is metabolized by the body and leaves plain water (the W in D5W) behind.  Plain water is hypotonic, containing fewer solutes than blood plasma.  Remembering the “solutes = sponges” concept, if the vascular bed has plain water (fewer solutes) and the surrounding cells have more solutes, fluid will shift OUT of the vascular bed and INTO the cells, resulting in a drop in BP.   Since this is occurring wherever a cell contacts the vascular bed, e.g. everywhere, it happens with all cells.  A common complication is that brain cells may swell, causing headache, weakness, nervousness, vomiting, tremors, convulsions, coma, and dilated pupils.  These are not good things.

Hypertonic (from the Greek prefix hyper-, “over, or excessive”) crystalloids are those whose tonicity exceeds that of plasma.  Again, if the solutes can be thought of as little sponges, this means there are more little sponges going in to the vascular bed than currently exist in the cells.  This results in water being drawn out of the cells and into the vasculature.  The cells shrink, which is called crenation, and this cell-shrinkage is exactly what is sometimes needed:

  • 7% hypertonic saline is considered “mucoactive” and is used to hydrate thick secretions to assist in expectoration;
  • 7% can be administered via central line for traumatic brain injury;
  • 3% hypertonic saline can be used for hemorrhagic shock (drawing water into the vasculature to increase BP), but no other kinds of shock;
  • 3% may be used for acute intracranial pressure (this lowers ICP by shrinking the brain cells);
  • 3% may be used for severe hyponatremia, but this is controversial

Hypotonic (from the Greek prefix hypo-, “under”) crystalloids are those where tonicity is below that of plasma.  Since surrounding cells will contain more solutes, the fluid is drawn immediately into the cells.  This is, in part, why you get so wrinkly in the bathtub – cells contain more solutes than the surrounding fresh water.   We’ve mentioned what happens with over-zealous administration of D5W.  When cells swell to bursting (which they can), the process is known as osmotic lysis or  cytolysis.  There is currently no out-of-hospital use that I know of for hypotonic fluids.

What is a colloid?

A colloid is a fluid which has something in it which is not dissolved.   The particles in colloids are larger, and do not fit through the vascular pores, and so they tend to stay in the vascular bed.  None of the particles in a colloid are osmotically active, and so we don’t have “hypotonic colloids” or “isotonic colloids”.

Colloids are used mainly for fluid expansion, and since it doesn’t leak as readily from the vascular bed smaller amounts (1/16 by volume) can be used to achieve the same results as crystalloids.  However, as hydrostatic pressure increases, the vascular pores “stretch” and allow the larger colloid particles to migrate out of the vascular bed, into cells and interstitial spaces.  When the hydrostatic pressure lowers, those pores “snap shut” and the colloidal particles are trapped outside of the vascular bed.  Therefore, edema caused by colloid administration takes much longer to resolve than edema caused by over-hydrating with crystalloids.

Examples of colloid fluids include:

  • Human albumin, used for trauma, burns, surgeries, and liver disease with ascites;
  • Hetastarch, a synthetic starch used for hemorrhage, burns, surgery, sepsis, and trauma.  Hetastarch has no O2 carrying capabilities or plasma proteins, and a couple of important contraindications

So there we have it, crystalloids and colloids, and completely ignoring Hartmann’s solution, blood plasma, PRBC, and about 99 other IV fluids that a patient can receive.  And we can probably, at this point, answer the initial question:  “What is best for our Mr. Smith?”

Of course, Answer #1 is, “always follow your local protocols”.  But presuming we had, say, normal saline 0.9%; D5W; Lactated Ringer’s solution, and for some reason human albumin on hand, which would be better?

Mr. Smith exhibits ashen skin and tachycardia, and a wound that bled heavily prior to EMS arrival.  He clearly needs fluids.

  • Hypotonic fluids are right out – they would, as we’ve seen, speed in through our IV and straight into cells and just make everything worse;
  • Hypertonic fluids *might* make some sense – they would draw fluid from the cells and interstitial spaces and into the vasculature.  But Mr. Smith has an overall deficit of fluid and needs more added, not just what he has shifted around

Isotonic or colloid it is.  Of the isotonic, D5W is right out – the dextrose will be metabolized and the water will enter cellular space, not stay in vascular space to help with BP.  Of the two remaining, Lactated Ringer’s solution might do well for a bolus, and may correct some of the acidosis we might expect from his initial trauma, but Ringers might not work well as an ongoing drip (due to electrolyte imbalance)

So, we’re down to Normal Saline, and a colloid (we’re pretending we have albumin).  And there, sorry to wuss out on you, is where the jury is still out.  Studies are being done all the time, coming to one conclusion (“Yay Saline!”) or another (“Yay Colloids!”), and the next study claims to shoot that idea down.  We just don’t know which is better.  We do know that colloids are much more expensive than crystalloids.  It would seem that Mr. Smith is getting saline today.

But at least we know why, right?

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