Fluids and Electrolytes

Foundations

Study Guides

Some very general fluid and electrolyte help (don’t think there aren’t exceptions lol):

o   General tip: Read the Med-Surg textbook (I think it’s chapter 10). First study each electrolyte separately. Maybe one or two per day. Then make a chart and compare.

o   Na+ tends to move opposite K+ (remember the sodium/potassium pump?)

o   Low Na+: think symptoms of fluid volume overload. Check the lungs and mental status!

o   Ca+ tends to move opposite Phosphate

o   In cases of toxicity, Ca+ antagonizes Mg and vice versa

o   Ca+, Mg+, and K+ are all closely related. This relationship is NOT easily simplified. You would do well to research it.

o   You will never give K+ via IV push

o   On Calcium, especially for fourth semester students:

Look at Calcium especially as a cation, it is positively charged (+). Ca+ moves inversely to blood pH (thus, blood CO2). When pH is low (acidic), bound (inactive) Ca+ is released from proteins in the blood, to raise the pH. Conversely, when blood pH is too high (alkaline), Ca+ is bound to protein to lower blood pH. To illustrate the importance of this:

1.    A treatment is for hypoparathyroidism (thus, low blood Calcium) is rebreathing. Why? Because “rebreathing” air you have already “breathed,” will increase blood CO2.  Thus the higher blood CO2 will cause a low pH (acidic, or “negative” charge from excess hydrogen)à Serum Calcium increases.

2.    That tingling in the lips when you are hyperventilating? It is at least in part due to calcium. Why? Because when you are hyperventilating, you are blowing off excess CO2, even more than is necessary (you’re losing CO2 faster than the body can produce it from metabolism). When you lose CO2, the pH increasesà Ca+ is bound to proteinà risk of hypocalcemiaà tingling.

o   An excess or deficit in Ca+ and Mg shows similar manifestations in the body. VERY generally speaking, too much of either has an “inhibiting” or “slowing” effect, whereas too much does the opposite. These are guides, not steadfast rules.

§  Case in point: Too little Ca+ causes muscle-twitchingà tetany, and too little Mg causes hyperactive reflexes. On the other hand, too much Ca+ causes muscle weakness, and Mg will cause depressed RR and reflexes.

o   For fluids, always keep in mind and visualize three main areas of the body:

§  The blood vessels (the vascular space)

§  The Cells (The “end of the line,” which is the prime target of nearly everything you intentionally put into the body)

§  The “empty” space between all of that. More accurately, the “interstitial (or third) space.” Edema? Think excess fluid. Excess fluid will move to spaces outside of where it should be (cells or vessels)… to the interstitial space.

o   *Know fluid volume overload and deficit like the back of your hand!

§  Know that they can BOTH affect your mental status and blood pressure

o   Know what “the dilutional effect” is. Basically, this means that fluid volume excess will tend to “dilute” the relative amount of a given substance. So when a blood lab is drawn, the substance may appear lower than normal. In other words, though excess salt will cause you to retain fluid, eventually your serum sodium will appear low, because the sodium level is measured relative to the total amount of serum (fluid) in the blood. This applies especially to:

§  BUN

§  H&H (Hemoglobin and Hematocrit)

§  Sodium

o   *Fluid Volume excess? Check the LUNGS! Why? Because excess fluid is travelling to areas it shouldn’t be. Your physiologic priority will always be airway. You will likely hear crackles. This is BAD.

o   Fluid in the vascular system (blood vessels) moves toward the area of more solutes. Don’t let this confuse you. Just remember it. This is the foundation of osmosis, if you recall, and is the basic principle behind fluid administration… easily remembered by two main ideas:

§  If you are dehydrated (cells are lacking water), the most appropriate fluid will ultimately be hypotonic, because fluid will move from the vascular system to the cells. You will switch to isotonic NS or LR once hydration is established.

§  If you have excess fluid (retaining or third spacing), the IV fluid to treat it with will be Hypertonic. Hypertonic PULLS fluid from the third space, and brings it BACK to the blood vessels. The excess fluid is then carried via the blood vessels to the kidneys to be excreted out via the uretersàbladderàurethra.

o   Lactated ringers is basically an isotonic fluid with electrolytes added to correct fluid and electrolyte deficits. It contains water, Sodium Chloride (saline), Calcium, Potassium, and Lactate.

§  Ringer’s: refers to the “saline” portion of the fluid

§  Lactate: is lactate. Lactate is metabolized by the liver to create bicarbonate, which will help correct acidosis.

o   Need to replace or maintain fluids? You will generally start with NS (Normal Saline) or LR (Lactated ringers). Why? Because these fluids are the most similar in consistency to the blood, or Isotonic. “iso-” = same, “-tonic= tonicity. For “tonicity” think: “tone” or “consistency.” So, if it has the same tonicity as the normal fluid (blood) in the vessels, it will tend to stay in the vessels.

o   Daily weights will pretty much always be the best indicator of fluid volume status (AH2: CVP may be used to determine fluid volume status)

o   Fluid volume excess = hypertension; fluid volume deficit = hypotension

o   Potassium (K+): Think “heart and kidneys” first.

§  Potassium is primarily excreted by the kidneys. You will never give K+ (KCl) to a patient if you do not know their kidney function status (BUN, creatinine, and URINE OUTPUT). “No pee, No K+!” Why? Because if the kidneys can’t excrete it, it can build up and become toxic.

§  Too much OR too little K+ can have devastating effects on the heart muscle.

o   For Those in OB:

§  MgSO4: (aka “mag”) obviously contains Magnesium. Too much? Look to the CNS: RR depression and deep tendon reflex depression (see above). Since Ca+ inhibits Mg in the body, Calcium gluconate may be a treatment.

o   Watch for electrolytes and drugs.

§  Example: Thankfully, tests, especially HESIs, will ask you over and over and over and over and over and over what drugs you need to know Potassium levels for. The answer will be likely be most diuretics and Digitalis (Digoxin). Why? Because when diuretics induce diuresis (pee pee), which cause you to lose potassium (again, potassium is primarily excreted from the body via the kidneys). With digitalis, low levels of Potassium can cause toxic levels of Digitalis. So, if the patient is on a diuretic and digitalis, which is very likely in a heart failure patient, the diuretic can cause hypokalemia, which can cause digitalis toxicity.