L2 - Fluid and Electrolyte Disorders pt 1

60% of total body water is in the ________ compartment

intracellular (within cells)

40% of total body water is in the _________ compartment

extracellular

What are the two components of the extracellular fluid?

Interstitial fluid and intravascular fluid

Define interstitial fluid

Fluid that surrounds cells and is outside of blood vessels

Define intravascular fluid

Fluid inside blood vessels

What is osmotic pressure?

The pressure applied by the solution to prevent the movement of water across the membrane

What is tonicity?

The measure of osmotic pressure. Only includes solutes that cannot freely cross the membrane

What is osmolarity?

Measure of the total concentration of solute particles

Units: mOsm/L (calculated at bedside)

What is osmolality?

Measure of the total concentration of solute particles

Units: mOsm/kg (derived in lab)

Describe sodium's relation to osmolarity

Sodium is an effective osmolyte and is a main determinant in osmolarity.

How is sodium's concentration regulated within cells?

Through the Na+/K+ ATPase

Define the Na+/K+ ATPase

Pumps 3 Na⁺ out of the cell and 2 K⁺ into the cell

Keeps potassium high in the cell and sodium high outside the cell in order to maintain the resting membrane potential

What is the equation to find osmolarity?

[2 x sodium] + [glucose/18] + [BUN/2.8]

What is the Antidiuretic Hormone (ADH)?

A hormone released from the posterior pituitary gland when osmolarity changes that increases the reabsorption of water and fluid retention

When is ADH secreted?

When plasma osmolality increases or circulating blood volume decreases (volume decrease, increase osmolarity)

How does ADH work in regard to osmolarity change?

It binds to vasopressin-2 receptors in the kidneys, which results in water reabsorption and fluid retention

How does ADH work in regard to fluid loss?

It activates arterial baroreceptors which activates angiotensin II. This increases thirst, ADH secretion, and water reabsoprtion

Inhibiting ADH results in?

Diuresis (excessive urination)

What are the two types of IV access?

Peripheral and Central

Describe Peripheral IV access

An IV that can be placed in various areas, that is low-cost, easy to insert, and has minimal complications

What are some disadvantages of peripheral IV access?

Too high a tonicity can be damaging to the patient, tissue damage is possible, and easily occluded

Describe Central IV access

An IV is inserted directly into a large central vein with the tip near the heart. It can be used for more hypertonic solutions and is less likely to occlude due to dilution

What are some disadvantages of central IV access?

Increased cost, higher infection risk, and some may need to be surgically inserted

What is Bolus administration?

Large dose given all at once → quickly raises drug/fluid levels.

What is maintenance administration?

Continuous or repeated small doses → keeps drug/fluid at a steady level over time

Describe a hypotonic solution

Solution with a lower solute concentration than ICF. Causes water to move into the intracellular compartment and swell cells

Describe a hypertonic solution

Solution with a higher solute concentration than ICF. Causes water to move out of the intracellular compartment and shrink cells (lyse)

Describe an isotonic solution

Solution with an equal solute concentration as ICF. Causes water to remain in place.

Name common IV fluids that are considered hypotonic

0.225% (1/4) NaCl, 0.45% (1/2) NaCl, and D5W

What is the sodium content of 0.225% NaCl? 0.45% NaCl? D5W?

0.225% (1/4) NaCl: 38 mEq/L

0.45% (1/2) NaCl: 77 mEq/L

D5W: 0 mEq/L

Name common IV fluids that are considered hypertonic

3% NaCl, 7% NaCl, D10W, D51/4NS, D51/2NS, and D5NS

What is the sodium content of 3% NaCl? 7% NaCl? D10W? D51/4NS? D51/2NS? D5NS?

3% NaCl: 513 mEq/L

7% NaCl: 1198 mEq/L

D10W: 0 mEq/L

D51/4NS: 38 mEq/L

D51/2NS: 77 mEq/L

D5NS: 154 mEq/L

(HINT: know that NS is 154 and set up a proportion to solve for others!!)

Name common IV fluids that are considered isotonic

0.9% NaCl, Lactated Ringer's, and Plamalyte A

What is the sodium content of 0.9% NaCl? Lactated Ringer's? Plamalyte A?

0.9% NaCl: 154 mEq/L

Lactated Ringer's: 130 mEq/L

Plamalyte A: 140 mEq/L

Why does dextrose increase tonicity?

It is a form of glucose that adds osmotic pressure. It increases tonicity temporarily because it raises extracellular osmoles that cells cannot immediately equilibrate with

Define sensible fluid loss

Fluid loss that is measurable

Gastrointestinal, urine, sweating, etc.

Define insensible fluid loss

Fluid loss that is unmeasurable

Increased respiratory rate, thermal/chemical burns, phototherapy, fever, etc.

What are maintenance fluids?

Fluids given to patients who cannot take enteral fluids. They contain water and electrolytes to replace sensible and insensible losses.

How are maintenance IV fluids administered?

Given over a long period of time at a fixed rate to maintain hydration and electrolyte balance.

What are some reasons for using dextrose in maintenance fluids?

To prevent catabolism (breaking down complex molecules into simpler ones) with prolonged NPO

What are some reasons for not using dextrose in maintenance fluids?

If the patient has diabetes or there is a concern for hyperglycemia

Dextrose is often administered with what?

Potassium of 20-40 meq/L if urine output is good

Why is potassium given with dextrose if urine output is good?

Dextrose increases insulin production; insulin drives potassium into cells (along with glucose); add K⁺ if kidneys can safely excrete it.

What maintenance fluid types are you responsible for knowing?

0.45% (1/2) NaCl, 0.9% NaCl, D51/4NS, D51/2Ns, D5NS, Lactated Ringer's, and D5LR

Why do maintenance fluids not have to be isotonic?

Because they are given at a fixed, slow rate, the body can adjust fluid shifts gradually.

What are bolus (replacement) IV fluids indicated for?

For patients with significant fluid deficits, electrolyte abnormalities, and/or severe acid-base disturbances.

When are bolus fluids given?

Before maintenance fluids in patients with fluid deficits to quickly restore volume and stabilize electrolytes.

What tonicity must bolus fluids have?

Isotonic

Why must bolus fluids be isotonic?

Rapid volume replacement requires isotonic fluids to avoid water shifting into or out of cells.

Define Lines of Therapy

The sequence of treatments given for a disease.

Define hyponatremia

Sodium < 135 mEq/L

The most common electrolyte abnormality

Give the symptoms of severe (

Seizures, coma, brainstem herniation, death, etc.

What is a common adverse effect of trying to rapidly correct hyponatremia (> 12 mEq/L per day)?

Osmotic demyelination syndrome

Osmotic demyelination syndrome

A neurologic disorder caused by overly rapid correction of chronic hyponatremia.

When sodium is raised too quickly, extracellular osmolality increases suddenly → water leaves brain cells → they shrink → oligodendrocyte injury → demyelination.

Define hypovolemic hypotonic hyponatremia

A condition where total body sodium and water are both decreased, but sodium loss is greater than water loss.

What is the clinical volume status in hypovolemic hypotonic hyponatremia?

Signs of volume depletion

What is the typical urine osmolality in hypovolemic hypotonic hyponatremia?

> 450 mOsm/kg

What is the typical urine sodium in hypovolemic hypotonic hyponatremia?

Why can urine sodium be

If sodium loss is non-renal in origin, the kidneys are receiving low arterial blood volume and are trying to conserve water by reabsorbing sodium

Why can urine sodium be >20 mEq/L in hypovolemic hypotonic hyponatremia?

If the sodium loss is renal in origin, the kidneys can't conserve sodium properly

What are some causes of non-renal loss hypovolemic hypotonic hyponatremia?

GI fluid loss, skin loss (sweating/burns), lung loss (hyperventilation), etc.

Sodium is lost, and water volume decreases.

How is non-renal loss hypovolemic hypotonic hyponatremia treated?

Fluid bolus with isotonic fluid (0.9% NS, LR, Plamalyte A)

What are some causes of renal loss hypovolemic hypotonic hyponatremia (can't hold sodium properly)?

Mineralocorticoid deficiency (Addison's disease, pituitary failure, hypothalamic failure)

OR

Diuretics (thiazide, etc.)

How does mineralocorticoid deficiency cause renal loss hypovolemic hypotonic hyponatremia?

Without aldosterone, the kidney cannot reabsorb sodium in the distal nephron, leading to sodium loss in urine and hypovolemia.

How do diuretics cause renal loss hypovolemic hypotonic hyponatremia?

Diuretics make the kidneys lose sodium in urine → hypovolemia. The low volume triggers ADH, causing water retention and dilutional hyponatremia

What is the first-line treatment for renal loss hypovolemic hypotonic hyponatremia due to mineralocorticoid deficiency?

Initiate steroid replacement with hydrocortisone (patient lacks aldosterone and has low cortisol concentrations)

What is the second-line treatment for renal loss hypovolemic hypotonic hyponatremia due to mineralocorticoid deficiency?

Fluid bolus with isotonic fluid (0.9% NS, LR, Plasmalyte A)

What is the first-line treatment for renal loss hypovolemic hypotonic hyponatremia due to diuretics?

Stop or decrease diuretic therapy

What is the second-line treatment for renal loss hypovolemic hypotonic hyponatremia due to diuretics?

Initiate enteral sodium replacement

What is the third-line treatment for renal loss hypovolemic hypotonic hyponatremia due to diuretics?

Fluid bolus with isotonic fluid (0.9% NS, LR, Plasmalyte A)

Why can hypovolemic hypotonic hyponatremia be treated with an isotonic fluid bolus?

An isotonic saline bolus restores intravascular volume → shuts off ADH → allows water excretion.

Serum sodium then corrects naturally and safely.

Reminder: What diuretic works more on proximal convoluted tubule (PCT)?

Carbonic anhydrase inhibitors

What is the mechanism of carbonic anhydrase inhibitors?

Inhibits carbonic anhydrase, which reduces hydrogen ion secretion and increases secretion of sodium, potassium, bicarbonate, and water.

Reminder: What diuretic works on the loop of Henle?

Loop diuretics

What is the mechanism of loop diuretics?

Decrease sodium and chloride reabsorption in the loop of Henle and the distal renal tubule

Allows sodium and water loss

Reminder: What diuretic works on the distal convoluted tubule?

Thiazide diuretics

What is the mechanism of thiazide diuretics?

Decrease sodium and chloride reabsorption in the distal renal tubules

Allows sodium and water loss

Reminder: What diuretic works in the collecting duct?

K+ sparing diuretics (aldosterone antagonist diuretics)

What is the mechanism of K+ sparing diuretics?

They are aldosterone antagonist and decrease sodium and chloride reabsorption. Also prevent K+ excretion

Allows sodium and water loss

When should 0.9% NS not be used in hypovolemic hypotonic hyponatremia? Why?

When the patient is in metabolic acidosis

NS has a high chloride content → large volumes of 0.9% can cause hyperchloremic metabolic acidosis by lowering plasma bicarbonate

Define hypervolemic hypotonic hyponatremia

Both sodium and water increased, but water increased more than sodium. (Na+ conc. is low due to dilution)

What is the clinical volume status in hypervolemic hypotonic hyponatremia?

Signs of fluid overload

What is the typical urine osmolality and sodium in hypervolemic hypotonic hyponatremia?

Urine osmolality: >100 mOSm/L (ADH active)

Urine sodium:

Why do the kidneys keep preserving sodium and water in hypervolemic hypotonic hyponatremia despite total fluid overload?

The body senses low effective arterial blood volume due to large ECF volume and edema. This activates RAAS to conserve sodium and ADH to conserve water.

What are some causes of hypervolemic hypotonic hyponatremia?

Heart Failure, Cirrhosis, and Nephrotic Syndrome

What is the first-line treatment for hypervolemic hypotonic hyponatremia due to heart failure?

Water and sodium restriction

What is the second-line treatment for hypervolemic hypotonic hyponatremia due to heart failure?

Diuretics (loop, thiazide, and/or K+ sparing)

What is the third-line treatment for hypervolemic hypotonic hyponatremia due to heart failure?

ACE inhibitors and/or beta blockers

What is some alternative treatment for hypervolemic hypotonic hyponatremia due to heart failure?

VRAs such as Tolvaptan (chronic treatment)

(stop ADH from reabsorbing water)

What is the first-line treatment for hypervolemic hypotonic hyponatremia due to cirrhosis?

Water and sodium restriction

What is the second-line treatment for hypervolemic hypotonic hyponatremia due to cirrhosis?

Diuretics (loop and/or potassium-sparing)

What is some alternative treatment for hypervolemic hypotonic hyponatremia due to cirrhosis?

Liver transplant

What is the first-line treatment for hypervolemic hypotonic hyponatremia due to nephrotic syndrome?

Water and sodium restriction

What is the second-line treatment for hypervolemic hypotonic hyponatremia due to nephrotic syndrome?

Treatment of the underlying cause

Define euvolemic hypotonic hyponatremia

Total body water is increased relative to sodium, but the increase is small enough that there is no significant edema, and the patient appears clinically euvolemic

What is the clinical volume status in euvolemic hypotonic hyponatremia?

Appears normal (no edema, no dehydration).

In euvolemic hypotonic hyponatremia, what does urine osmolality

< 100 mOsm/L indicates dilute urine and ADH is not on

< 20 mEq/L sodium indicates non-renal sodium issues

The problem is excess water intake or low sodium intake

In euvolemic hypotonic hyponatremia, what does urine osmolality >100 mOsm/kg and urine sodium >20-30 mEq/L suggest?

> 100 mOsm/L indicates concentrated urine, and ADH is activated

> 20 mEq/L sodium excretion is normal but concentrated

The problem is impaired water excretion (e.g., SIADH, adrenal insufficiency, renal failure).

What is SIADH (In-hospital drowning)?

Syndrome of inappropriate ADH. Where too much ADH is secreted or has increased sensitivity to it.

What are some non-medication etiologies of SIADH?

Infections, post-op surgery, stressful states, cancer, etc.