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Flashcards in Fluids, Electrolytes, Nutrition Deck (73)
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Total Body Water

50-70% total body weight

Greater in lean individuals bc fat doesn't have much water. Avg 60%.

Newborns 70%. Decreases with age to around 50%.

1L water = 1kg

2 compartments. ICF and ECF



mostly in skeletal muscle mass, thus a little lower in females (50%) than males (60%)

Cell wall separates ICF and ECF and acts as semipermeable membrane



Plasma and interstitial fluid

Capillary membrane separates plasma and interstitial fluid and acts as a semipermeable membrane


Fluid totals

ICF = 67% Total body water

ECF = 33% Total body water
- Interstitial = 25% total body water (75% of ECF)
- Plasma = 8% Total body water (25% of ECF)


Normal plasma osmolality

285-295 mmol/L


Calculated plasma osmolality

2[Na] + [gluc]/18 + [BUN]/2.8

Na is mmol/L

The others are mg/dL


Osmolar gap

Measured osmolality - calculated plasma osmolality

Normal gap 10 = lactic acid, ketones, methanol, ethanol


How much fluid can you lose through a trach?

1500ml/day if unhumidified and hyperventilation


Renal control of fluids/lytes

Distal tubules - reabsorption of Na in exchange for K and H secretion

Affected by ACTH and aldosterone

Aldosterone directly stimulates K secretion and Na reabsorption from distal tubule

Low Extracellular volume leads to low renal perfusion. This increases renin from JGA. Angiotensin 1 increases. Then Angio 2. Then aldosterone.

Aldosterone also released when low volume receptors in R atrium activated or from ACTH which is released in response to high K.


Causes of volume deficit (dehydration)

Mimics ECF loss:


Loss of GI fluid - vomit, NG suction, diarrhea, fistula

PostOp fluid sequestration - 3rd spacing: intestinal obstruction

Intra-abdominal and retroperitoneal inflammation (pancreatitis, peritonitis)

SIRS, burns, sepsis, pancreatitis

Losses that are mostly water:
- Fever
- Osmotic diuresis
- DI
- Prolonged water deprivation
- Inadequate input during procedure


Judging degree of dehydration

Mild = 3% (adults) or 5% (kids) loss of body weight

Mod = 6% or 10%

Severe = 9% or 15%


Treatment of dehydration

Initial intervention is to give a large bolus as a volume expander: 20 ml/kg of NS or LR

During the next 8 hrs, expected maintenance fluid given plus 1/2 of remaining calculated loss.

Over next 16 hrs, the other 1/2 of remaining loss is given along with the assumed maintenance fluid



Dextrose is used to deliver free water to body (dextrose quickly metabolized)

0.9% NaCl quickly adds volume to intravascular space

Goal is to expand intravascular space



pRBCs, FFP, albumin

Stay mainly within intravascular space if the capillary membranes are intact

Possible increased incidence of pulm embolism and respiratory failure


- Patients with too much Na and water but hypovolemic - ascites, CHF, postcardiac bypass
- Patients unable to make enough albumin or other proteins to exert enough oncotic pressure - liver disease, transplant recipients, resections, malnutrition
- Severe hemorrhage or coagulopathy


Isotonic causes of volume excess

Iatrogenic - intravascular overload of IVFs with lytes

Increased ECF without equilibration with ICF - esp postop or trauma when hormonal responses to stress are to decrease Na and H2O excretion by kidney

Often secondary to renal insufficiency, cirrhosis, CHF


Hypotonic causes of volume excess

Inappropriate NaCl-poor solution as a replacement for GI losses (most common)

Third spacing (shift of ECF from plasma to elsewhere like interstitial or transcellular spaces)

Increased ADH with surgical stress, inappropriate ADH (SIADH)


Hypertonic causes of volume excess

#1 = excessive Na load without adequate water intake
- water moves out of cells bc of increased ECF osmolarity
- Causes increase in intravascular and interstitial fluid
- Worse when renal tubular excretion of water and/or Na is poor
- Can also be caused by rapid infusion of nonelectrolyte osmotically active solutes like glucose and mannitol


What can NS cause?

hypercholemic metabolic acidosis


What can LR cause?

When patient is hypovolemic and in metabolic alkalosis (from NG tube or vomiting), may worsen the alkalosis when lactate is metabolized


Treating hypervolemia

Restrict Na and fluids for isotonic

Free water replacement for hypertonic (will correct hypertonicity which should result in diuresis)

Saline for hypotonic (same as above)

Diuresis with furosemide 10-50mg
- replete K as needed
- don't overdiuresis

Cardiogenic drugs, O2, artificial vent as needed (heart failure or resp failure)


Causes of ongoing fluid loss

Fever - each degreeC above 37 adds 2-2.5ml/kg/day of insensible water loss

Loss of body fluids (vomit, NG suction, fistulas)

3rd space losses



How much fluid does an average adult patient need?

About 2.5L/day... about 100ml/hr

unless other factors warrant higher rate


Calculating free water deficit

FWD = normal body water - current body water

NBW = 0.6 x body weight in kg

CBW = NBW (normal serum Na/measured serum Na)


How can labs show hypovolemia?

BUN/Cr > 20



Working up hyponatremia

True hyponatremia = excess ingestion of water that overwhelms the kidneys (either normal or diseased) or due to increased ADH. It is NOT due to increased excretion of Na.

1) Determine plasma osmolality

- Normal: pseudohyponatremia. Lab artifact due to high lipids or plasma proteins. Check a lipid profile or possible Multiple Myeloma
- High - pseudohyponatremia. Due to increase of osmotically active molecules like glucose*** or mannitol
- Low - true hyponatremia

2) Assess volume status
- hypovolemia
- euvolemia
- hypervolemia


Account for glucose in hyponatremia

For every 100 mg/dl increment in plasma glucose above normal (normal = 100), plasma Na should decrease by 1.6 mEq/L

"sweet 16"

Glucose of 500 should have decrease of 6.4. 140-6.4 = 133.6


Hyponatremia with hypotonicity (true hyponatremia) and hypovolemia

- Renal cause = diuretics
- Extrarenal = vomit, diarrhea, burns, pancreatitis

Differentiate using urine Na. Urine Na 20 indicated renal cause.


True hyponatremia with euvolemia

SIADH = #1

Increased vasopressin release from posterior pit or ectopic source causes decreased renal free water excretion

- hypo-osmotic hyponatremia (hypotonicity)
- Inappropriately concentrated urine (urine osmolality > 100)
- Normal renal, adrenal, thyroid function

- neuropsych disorders, malignancies (esp lung), and head trauma
- glucocorticoid deficiency (Addison's) - cortisol deficiency causes hypersecretion of ADH
- hypothyroidism - causes decreased CO and GFR which leads to increased ADH release
- primary polydipsia - usually in psych patients who compulsively drink massive volumes of water


True hyponatremia with hypervolemia

May be from CHF, cirrhosis, nephrotic syndrome

Increased thirst and ADH

Edematous state


Signs/symptoms of true hyponatremia in general and tx

Decreased reflexes, respiratory depression, seizures, coma

N/v, HA, lethargy, muscle cramps

For hypovolemic: Give 0.9% NaCl. Na repletion with saline isotonic to patient in order to avoid rapid changes in ICF volume.

Major complication from rapid correction of chronic hyponatremia is central pontine myelinolysis

For hypervolemic: Correct underlying disorder - CHF, liver or renal failure

For euvolemic: Raise plasma Na (lower ICF volume) - restrict water intake