Fluids and Electrolytes

What % of the body is total body water?

60%

ICF makes up __ of TBW

2/3 (40%/60%)

ECF makes up __ of TBW

1/3 (20%/60%)

What makes up the ECF?

• Interstitial fluid (15%)

• Plasma (5%)

Of all the TBW how much of it can we actually assess?

5% (plasma)

Osmotically active particles predominantly drive osmotic equilibrium between the ICF and ECF, what are the 2 most important particles for this? Where are they most commonly found?

• ECF = Na

• ICF = K

_______ is the most important in the maintenance of plasma volume

Sodium (Na)

What are the 3 different Abnormal Body Water Distributions?

1. Movement of water from IV→ Interstitial compartment (Edema)

2. Intravascular fluid compartment pooling (Shock)

3. Movement of water from IV→Body Cavity (Ascites, pleural effusion, uro-ab)

What are commonly used in practice to estimate fluid compartments of a patient?

1. Estimate volume of ECF compartment

   1. Impractical tests

2. PCV or Hct and Plasma total protein (TP)

   1. If there is no change in the quantity of erythrocytes intravascularly, then an alteration of the HCT is most likely due to changes in plasma volume.

3. Weighing the patient regularly

4. Using serum urea, creatinine, Na & Cl concentrations

5. Clinical signs of the patient

   1. Pale MM

   2. Tenting of the skin

What effect does dehydration have on hematocrit?

• It causes Hct to increase

  • Fluid loss reduces blood plasma volume, creating a higher concentration of red blood cells

How is Osmolality assessed?

Measured by freezing point depression osmometry

____, ______, and ____ are the most important determinations of osmolality

• Sodium

• Urea

• Glucose

What is the Osmolal Gap? What is the normal range for it?

• The difference between Measured Osmolality and Calculated Osmolality

  • Measured Osmolality (via freezing point depression osmometry) - Calculated osmolality (2Na + Urea + Glucose)

• Should not be >10

Define Osmolality

Osmolality is the number of particles of solute per kg of water (solvent) or the concentration of osmotically active particles in the solution (mOsm/kg)

About one-half to two-thirds of sodium within the body is present in the ______, it is integral for the maintenance of fluid here

ECF

Briefly explain the 2 main mechanisms that control serum Na?

1. The Renin-Angiotensin-Aldosterone System (RAAS) (Volume Regulation)

   1. While ADH controls the concentration of sodium, the RAAS pathway regulates the total amount of sodium (and consequently, extracellular fluid volume and blood pressure).

   2. Regulation of blood volume, such as when a patient is hypovolemic

2. The ADH and Thirst Mechanism (Osmoregulation)

   1. This is the primary defense mechanism for maintaining normal serum sodium concentrations by controlling the amount of free water in the body.

What is the #1 cause of Hypernatremia?

Decreased total body water (aka dehydration)

What are the 2 different causes of dehydration-induced hypernatremia?

1. Decreased water intake

2. Solute-poor water loss (Water without solutes, therefore the # of solutes stays the same but the concentration inc)

   1. Water moves from ECF→ICF

What is another possible cause of hypernatremia?

• Increased total body sodium

  • Hypertonic fluids have been given

  • Salt poisoning

There are 6 potential causes of Hyponatremia, describe at least 4

1. “Sodium deficit”

   1. Loss of sodium (GI, renal, cutaneous, 3rd space)

2. Water excess

   1. Edematous disorders (CHF, nephrotic syndrome)

3. Movement of water from ICF→ECF

   1. Hyperglycemia (more of another solute in ECF draws fluid from ICF→ECF)

4. Movement of Sodium from ECF→ ICF

   1. Acute myocyte injury?

5. Movement of Na from intravascular to extravascular fluid

6. Potassium depletion causing shifts in sodium and water

______ is the primary intracellular cation present in cells

Potassium

Potassium, unlike _______, is not secreted by the way of feces or sweat

Sodium

There are 2 general ways that Hyperkalemia can occur, what are they?

1. Movement of K from ICF → ECF

2. Increased total body K+

What are the 6 ddx for movement of K from ICF→ECF?

1. Metabolic acidosis

   1. Drives hydrogen ions into cells, forcing potassium ions out into the extracellular fluid (plasma) to maintain electrical neutrality

2. Insulin deficiency

   1. Prevents potassium from entering cells and promotes its shift out of cells into the bloodstream.

   2. Insulin normally acts as a key to drive potassium into cells along with glucose

3. Muscle Damage

   1. Muscles are a large reservoir of potassium

   2. When they get damaged the potassium leaks (along with creatinine)

4. Massive tissue necrosis

   1. K+ is stored in the cells (ICF), so when they rupture it is brought out of the cell

5. Pseudohyperkalemia

   1. Sample was contaminated with EDTA

What are the ddxs for Increased total body K+ → Hyperkalemia?

1. Decreased renal excretion of K+

   1. Can be caused by renal failure, urinary tract obstruction, uroabdomen, hypoadrenocrotisism

2. Increased intake of K+

What are the 3 main causes of Hypokalemia? Provide the ddxs for each cause

1. Movement of K+ from ECF→ICF

   1. Metabolic alkalosis

   2. Increased insulin activity

2. Decreased total body K

   1. Decreased K intake

   2. Increased K excretion

      1. Renal, GI, cutaneous

3. Renal failure in cats

__ and _ are commonly compared to one another

Na, K

What are some causes of a decreased Na:K ratio, <27 (22 or 25 depending on the reference)?

• Hypoadrenocorticism

• Renal Failure

• Urinary tract obstruction

• Uroabdomen

• D.M with ketonuria

• Pleural or peritoneal effusions

• Diarrhea (severe)

• Severe myopathy

Chloride is primarily present in the ___

ECF

What two molecules greatly influence chloride? How does each molecule affect chloride?

1. Na⁺

   1. Direct correlation, when Na is increased, Cl is also expected to be increased

2. HCO₃^- (Bicarbonate)

   1. Indirect Correlation, when bicarbonate is increased, Cl is expected to be decreased

What are the 3 most common causes of Hyperchloremia?

1. Concurrent hypernatremia

2. Hyperchloremic metabolic acidosis (Metabolic acidosis = decreased Bicarbonate)

   1. Loss of HCO3 in the GI or Renal causes an increase in Cl^- → causes acidosis

3. Chronic respiratory alkalosis

   1. Through renal compensation to manage the excess base (high pH) resulting from decreased carbon dioxide

What are the 4 main causes of Hypochloremia?

1. Chloride deficiency

   1. Concurrent hyponatremia

   2. Metabolic alkalosis (HCl loss or sequestration)

   3. Metabolic acidosis with an increased anion gap

      1. A blood test calculation measuring the difference between negatively charged (anions) and positively charged (cations) electrolytes

      2. It is calculated as Sodium minus the sum of Chloride and Bicarbonate: Na - (Cl + HCO3)

2. Water excess

   1. Water retention > Cl retention

3. Movement of water from ICF → ECF

4. Movement of chloride from Intravascular (plasma) fluid → Extravascular fluid