Fluid & Electrolyte Imbalances: Na & Water (Exam 1)

how is total body water (TBW) estimated in healthy adults?

-men: ~60% body weight ---> 0.6 L/kg

-women: ~50% body weight ---> 0.5 L/kg

-in obesity (>20% above IBW), use IBW to estimate TBW

how is total body water distributed between the ICF and ECF?

-ICF = 2/3 of TBW

-ECF = 1/3 of TBW

what are the major electrolytes in the ECF versus ICF?

-ECF: Na, Cl, and HCO3

-ICF: K, Mg, phosphate, proteins

how is the extracellular fluid (ECF) distributed?

-interstitial fluid = 3/4 of ECF

-intravascular fluid = 1/4 of ECF

what is the normal serum osmolarity/osmolality?

275-190 mOsm/L

what is the difference between osmolarity and osmolality?

-osmolarity = Osm/L

-osmolality = Osm/kg

what is the equation for calculated serum osmolarity?

seurm osmolarity = (Na * 2) + (glucose/18) + (BUN/2.8)

what is tonicity?

the concentration of impermeable/effective solutes in the ECF that influence water movement across cell membranes

what is the equation for serum tonicity?

tonicity = (Na * 2) + (glucose/18)

what is the key difference between osmolarity and tonicity?

-osmolarity: all solutes ---> Na + glucose + BUN

-tonicity: only effective/impermeable osmoles ---> Na + glucose

what happens when serum osmolarity increases?

hypothalamic osmoreceptors detect the increase ---> increased ADH release and increased thirst

what does ADH do?

increases water reabsorption by the kidneys ---> decreased water excretion ---> decreased serum osmolarity

what happens when blood volume/arterial pressure decreases?

baroreceptors detect decreased pressure ---> activation of RAAS ---> angiotensin II ---> increased ADH and increased renal Na/water retention

what is the major difference between osmotic and non-osmotic regulation of water?

-increased osmolarity ---> osmoreceptors ---> ADH + thirst

-decreased volume/BP ---> baroreceptors/RAAS ---> Na + water retention

what is the different between dehydration and hypovolemia?

-dehydration: loss of TBW/free water ---> increase serum osmolality ---> often hypernatremia

-hypovolemia: loss of both Na and water from ECF ---> volume depletion, usually without a major osmolality disturbance

what findings suggest dehydration versus hypovolemia?

-dehydration: thirst, dry mucous membranes, decreased skin turgor, CNS symptoms, hypernatremia

-hypovolemia: dizziness, orthostasis, tachycardia, hypotension; severe cases ---> shock

how is the total fluid needed to correct dehydration, established?

basal fluid requirement + fluid deficit + ongoing losses

what is the difference between crystalloids and colloids?

-crystalloids: water + electrolytes; commonly used for fluid/electrolyte replacement

-colloids: contain larger molecules that initially pull/retain more fluid intravascularly but have little to no therapeutic advantage over crystalloids for routine resuscitation

what are the major physiologic effects of hypotonic, isotonic, and hypertonic fluids?

-hypotonic: provides free water ---> water moves into cells ---> increase ICF

-isotonic: expands ECF without major shift between ECF/ICF

-hypertonic: pulls water out of cells into ECF ---> increase ECF, decrease ICF

what common IV fluids should you associate with each tonicity?

-hypotonic: D5W after glucose metabolism; solutions < 0.9% NaCl such as 0.45% NaCl

-isotonic: 0.9% NaCl, LR

-hypertonic: >0.9% NaCl, especially 3% NaCl

how is hyponatremia defined?

serum Na

what is the stepwise approach to evaluatin hyponatremia?

1. confirm Na

what are the options for serum osmolarity? (step 2.)

-isotonic: 275-290

-hypertonic: >290

-hypotonic:

if hypotonic, what are the options for volume status? (step 3.)

-hypovolemic

-euvolemic

-hypervolemic

what is isotonic hyponatremia?

Na

what is hypertonic hyponatremia and what is its most common cause?

Na

what is the osmolar gap?

-osmolar gap = measured osmolality - calculated osmolality

-a gap >10 mOsm/L suggests an unmeasured osmotically active substrate

what is the most common form of hyponatremia?

hypotonic hyponatremia

how can you distinguish the three types of hypotonic hyponatremia clinically?

-hypovolemic: decreased BP, tachycardia, dry mucosa, poor skin turgor

-euvolemic: normal BP/HR, normal skin, no edema

-hypervolemic: pitting edema, weight gain, dyspnea/swelling

what causes hypovolemic hypotonic hypernatremia?

Na loss > water loss (burns, vomiting, diarrhea)

why does volume loss worsen hyponatremia?

volume loss stimulates ADG, causing the kidney to retain water and further dilute serum Na

how is non-severe hypovolemic hypotonic hyponatremia treated?

0.9% NaCl or Lactated Ringer's to restore ECF volume

what causes hypervolemic hypotonic hypernatremia?

water gain > Na gain (heart failure, cirrhosis, kidney failure)

how is non-severe hypervolemic hypotonic hyponatremia treated?

-treat underlying cause

-loop diuretic

-water restriction

-Na restriction

-may need to add a "vaptan" in selected patients

what is the most common cause of euvolemic hypotonic hyponatremia?

SIADH

what happens in SAIDH?

excessive ADH activity ---> increased collecting-duct water reabsorption ---> water gain without proportional Na gain ---> dilutional hyponatremia. urine remains inappropriately concentrated

how is non-severe SIADH treated?

-water restriction

-correct underlying cause

-stop causative medications

-other options include NaCl tablets with/without a loop diuretic or "vaptan"

how do "vaptans" help treat hyponatremia?

block ADH activity at renal V2 receptors ---> decreased aquaporin-2 insertion ---> increase free-water excretion ---> decrease urine osmolality and increase serum Na. they are also called aquaretics

what findings indicate severe hyponatremia?

-seizures

-coma

-respiratory arrest

why does severe hypotonic hyponatremia cause neurologic symptoms?

the hypotonic ECF causes water to move into brain cells ---> cerebral edema

why must chronic hyponatremia not be corrected too rapidly?

rapid correction causes water to rapidly leave adapted brain cells ---> osmotic demyelination syndrome (ODS)

what sodium correction rate is recommended?

generally 6-12 mEq/L per 24 hours to reduce the risk of ODS

what is the general treatment for severe hypotonic hyponatremia?

-hypertonic 3% NaCl is central to treatment*

-hypovolemic: 3% NaCl initially

-hypervolemic: 3% NaCl + loop + fluid restriction

-euvolemic/SIADH: 3% NaCl with/without loop

what is the sodium deficit equation?

Na deficit = (desired Na - serum Na) * TBW

what is the basic pathophysiology of hypernatremia?

hypernatremia generally occurs when there is too little water relative to Na, often from impaired access to water, excessive water loss, or diabetes insipidus

why does hypernatremia cause neurologic symptoms?

hypertonic ECF pulls water out of brain cells ---> decreased neuronal cell volume, which can cause confusion, weakness, seizures, and potentially death

what is the difference between central and nephrogenic diabetes insipidus (DI)?

-central DI: decreased ADH secretion

-nephrogenic DI: kidneys have decreased response to ADH

what causes hypovolemic hypernatremia?

water loss > Na loss (GI losses, osmotic diuresis, poor access to water)

how is hypovolemic hypernatremia treated?

-first: 0.9% NaCl to restore perfusion/ECF volume

-once hemodynamically stable ---> switch to free-water/hypotonic replacement, such as D5W OR 0.45% NaCl

what causes hypervolemic hypernatremia?

caused by Na gain > water gain, such as excessive hypertonic/Na-containing fluids

how do you treat hypervolemic hypernatremia?

-loop diuretic ---> remove excess Na/fluid

-D5W ---> replace free water

how is hypernatremia causes by central diabetes insipidus, treated?

desmopressin (DDAVP) + free-water replacement such as D5W