5.03 Approach to Common Water and Electrolytes Flashcards
In diarrhea, a common accompaniment is loss of which electrolyte?
K+ so expect possible hypokalemia
Water vs Volume balance
Volume/Saline Balance - that means px is either vol deficit or volume excess.
When we talk abt vol, it is synonymous to talking about saline bc the total body Na determines the amount of volume in your body so you can also use saline balance or vol balance
Water balance - only talking abt osmolality or whether px’s plasma is diluted or concentrated. Talking abt water deficit or xs.
THERE IS A DIFFERENCE BETWEEN VOL BALANCE AND WATER BALANCE!! Easiest analogy for this: vol balance - talking about analogy to coffee: small cup or big mug of coffee of the same size but diff volume. For water balance, two cups of coffee of the same size but one is v concentrated (like espresso) and one is cafe americano (w/c is more or less a little diluted) so talking abt osmolality or conc not how much volume there is.
Percentage of water in body weight
Body Fluid Compartments
55-60% of body weight is water
Fluid compartments 40% Intracellular Fluid 20% Extracellular fluid >5% Intravascular (Plasma) >15% Interstitial
Major cation in the ECF? ICF
PISO
Potassium In, Sodium Out
ECF: Na+
ICF: K+
Daily water losses/Daily water output
Total output: 2.5 L
Urine: 1 - 1.5 L
Skin: 0.7 L
Feces: 0.15
Sweat: 0.15
Example you have 70 kg male so body water what is the water composition?
If you add 1.5 L of D5W I, how will water be distributed?
60% of body weight (70 kg) = 42 L
>ICF: ⅔ = 28 L
>ECF: ⅓ = 14 L
The 1.5 L D5W will be distributed across all compartments of the body. Because freely permeable, water given in the intravascular space is eventually distributed all throughout so osmolality becomes lower because you added.
1.5 divided by ⅔ for ICF and ⅓ for ECF. Remember EC is further divided into intravasc and interstitial. 125 mL remaining in intravascular space. Showing you that you cannot expand effectively your intravascular space if you’ll just give D5Water or water.
Thus, we do NOT use D5W or water in to expand intravasc space in px.
After receiving 1.5 L of plain NSS IV, the ECF compartment of a person is expected to expand by how many mL?
How many mL will remain in the intravascular space?
1500 mL.
Inject 1.5 L NSS IV, inject in EC, and it’ll be distributed in the extracellular space only.
Why?
Na+ although can pass through PM and go into the cell, will just be pumped out again by your Na/K pump so it’ll go out again, and effectively remain in the extracellular space.
For the intravascular space, 375 mL (¼ of 1.5 L) will remain there and the rest goes into the interstitial space.
SO BETTER FLUID CHOICE TO EXPAND INTRAVASCULAR VOLUME rather than D5W or water.
Control of ECF
- Afferent limb
- Efferent limb
-Afferent limb:
HOW BODY DETECTS:
Remember, the priority of your body to maintain an effective circulating volume that means the intravascular space is the one most affected by the body so there are several sensors for your body to know how much is in the intravascular space. The sensors are found in the heart - in the atrium, ventricles; lungs, big arteries, big arteries (carotid, aortic arch baroreceptorsZ), kidneys brain, and even in the liver so there are many sensors that will tell your body what’s happening to the intravascular volume and they usually sense if there is a decrease in intravasc volume or effective circulating volume.
-Efferent limb:
HOW IS THE CONTROL AFFECTED: All of the main way that the body corrects fluid volume is through the kidneys. It will adjust your GFR (so that means if you have to reduce intravasc volume, the renal blood flow will go down and the filtration rate will go down so that you will not be losing fluid into your kidneys). It will also adjust the absorption factors that will allow greater absorption of your fluids and solutes into the tubules. And there’s activation of your hormones - your RAAS, ADH, PG, Natriuretic. Your renal nerves are also involved. But all of them basically ctrl your fluid retention in the kidneys.
Which of the ff laboratory results is compatible with volume depletion?
A. Low serum albumin
B. Hemoconcentration
C. BUN/Creatinine ratio of 10:1
d. Urine sodium > 40 meq/L
B. Hemoconcentration
Low serum albumin
-Increases because you have dec volume = concentration of plasma solutes
BUN/Creatinine ratio normal is 10-20:1.
-When you have vol depletion, both of them will go up bc you have dec RBF, dec GFR so both of them will go up. But you will notice that your urea (BUN), goes higher than crea making the ratio greater than 20:1 in px w/ vol depletion and explanation. When you have reduced blood flow to the kidneys due to vol depletion, you will also have reduced urine flow in the tubules and that will trigger the reabsorption of urea from the tubules back to the blood so your urea in the blood goes up, higher, since it is reabsorbed in states of deficit. But your crea is not reabsorbed so crea does not go up as high as urea.
Urine sodium
- Reflection of how kidneys are expanding to the volume status
- How kidneys are expanding to the vol status
- If low, px has dec RBF = tubules will reabsorb as much as it can to the sodium filtered. Thus, urine sodium is expected to go down and the cut off is 20. Urine sodium of less than 20 meq/L is what’s expected in a px w/ volume depletion and w/ a fxning kidney tubules.
Best way to assess volume status?
Physical Examination
-Most importantly: Body Weight
Every 1 kg change in body weight = how much fluid change?
1L of saline either retained or lost
T or F. If px does not eat anything, she may lose more than 0.5 kg in a day.
F F F!!!
If px did not eat anything, the weight will not change by more than 1⁄2 a kilo a day. If the change is greater than that, then that means those changes are due to fluid retention or fluid losses so body wt is very impt.
Clinical Dx for Volume Status
- Weight
- BP
- Pulse
- CVP
- Skin turgor
- Buccal mucosa
- Hands, axillary region
- Edema
- Hgb/Hct
Clinical Dx for Volume Status
- Weight
- Deficit: Dec
- Excess: Inc - BP
- Deficit: Dec or orthostatic drop depending on severity
- Excess: Inc - Pulse
- Deficit: Inc
- Excess: Normal or Inc(?) - CVP
- Deficit: Dec
- Excess: Inc
- Measured via jugular venous distension; not all the time correlated
Other parameters that aren’t as helpful but may give a clue:
- Skin turgor
- Deficit: Poor
- Excess: Normal - Buccal mucosa
- Deficit: Dry
- Excess: Moist - Hands, axillary region
- Deficit: Dry
- Excess: Moist - Edema
- Deficit: (-)
- Excess: (+) - Hgb/Hct
- Deficit: Inc (concentrated)
- Excess: Dec
How to determine severity of the deficit
Mild
BP: Normal BP
Estimated amount of fluid deficit: 3% of BW (1.5 - 2 L)
Moderate
BP: with postural hypotension
Estimated amount of fluid deficit: 6% (3-4 L)
Severe
BP: with frank hypotension
Estimated amount of fluid deficit: 9% (x > 5 L)
How do you replace fluids?
Replace only half of the estimated fluid deficit, then re-evaluate. Reason for this: we don’t want to create iatrogenic problems. We don’t want to immediately give back the amt that is lost and then cause volume overload on the px because estimate is wrong. So better to correct only half then re-eval then give again half until you’ve corrected everything. Then the initial fluid deficit is suggested to be corrected w/in 1st 6 hrs.
○ So in px w/ mild - probably give abt a liter in the 1st 6-8 hrs, going to give half of estimated (2L).
Limitation of using PE to assess volume depletion
○ As said earlier, in assessing the vol status, use PE. V useful esp in the presence of mod to severe vol depletion. However in cases of mild vol depletion or mild vol excess, the PE would have significant limitation so much so that in some studies, clinicians can only correctly predict vol status 50% of the time using PE technique.
○ Even your CVP measurement and your PAWP measurement are not really that accurate as found out by newer studies so current recommendations or parameters we could use to assess vol status actually is recommending the use of dynamic variables such as measuring CO before and after volume challenge or doing leg racing test and measuring CO.
Treatment of Volume Deficit
- Quantify total deficit
- Quantify volume losses
- Estimate basic daily maintenance requirement
- Identify concomitant electrolyte and water imbalance
- Formulate replacement plan w/c should include the ff:
a. Quantify replacement = deficit + active losses + basic daily requirement
b. Replacement fluid = plain NSS or LR or balanced crystalloids; modify accdgly to concomitant electrolyte imbalance
c. Rate of replacement = dependent on severity and rate of onset of the deficit
Basic daily requirement (Maintenance) of:
a. Water
b. Sodium
c. Potassium
a. Water: 2000 - 2500 mL
b. Sodium: 50 - 150 meq
c. Potassium: 40 - 80 meq
Q: Why do we have to give D5 together with water and not just plain or sterile water IV?
If you give a hypotonic fluid anything below the osmolality of the plasma, you will have intravascular hemolysis bc your RBC will burst in the presence of hypotonic fluid that you are infusing into the veins so D5 there will make fluid isoosmotic to plasma.
What will happen if you give a lot of NSS?
Cl lvls go up
Then we have your plain NSS or D5 NSS (normal saline soln) and that is 0.9% NaCl or 0.9 g NaCl per 100 mL = 154 meq/L Na and 154 meq/L Cl, you remember your serum Cl has an upper limit of abt 150 so giving a lot of NSS would make the Cl lvls in the blood higher.
What is your “balanced crystalloids” soln?
Lactated Ringer’s
-come also in D5 LR of just plain LR, it has 130 meq/L Na and 4 meq/L K and some other small amts of other electrolytes. You can see this is more balanced hence called balanced crystalloids. Lvl of Na and K is v similar to blood.
