Academic Day Seminar #9: Fluids and Sodium

This session definitely helped clarify the different terminology used and how to assess and manage any abnormalities in volume status and sodium. I found it very useful how Dr. Loewen provided us cases and demonstrated that ICF and ECF should be thought of/assessed separately. I found figuring out the treatments (e.g. what bag to give, volume, infusion rates) quite challenging, but I think that as I continue to apply this in practice, I will be able to have a better understanding of it.


  • Hypo/hypervolemia: refers to ECF
  • Dehydration: refers to ICF
    • Hyponatremia: refers to EXPANDED ICF
    • Hypernatremia: refers to CONTRACTED ICF
    • Sodium is reflective of ICF status
      • does not indicate anything about ECF status
      • ↑↓ Na+ does not cause ↑↓ICF…instead ↑↓ICF affects Na+
  • Osmolality: # particles per kg of H2O (mOsm/kg), determined by # of particles in solution and NOT by particle size or valence
    (all body fluid compartments are iso-osmotic)

    • Flow from high osmolality to low osmolality
    • Therefore, ↑ plasma osmolality often ↓ intracellular volume, and
      ↓ plasma osmolality is associated with cellular hydration
      * ↑ plasma osmolality makes you thirsty (not ADH) *
  • Tonicity: the effective osmolality and is equal to the sum of the concentrations of the solutes which have the capacity to exert an osmotic force across the membrane
    • Effective osmoles (can NOT cross membrane freely): Na, K, glucose, mannitol (proteins is not a solute but an effective osmole!)
    • Ineffective osmoles: urea, ethanol, methanol

Maintain intracellular environment (pH, temperature, osmolality)

Distribution of water:

  • 60% of body weight: water
  • 2/3 Intracellular; 1/3 Extracellular
  • Within Extracellular:
    1/4 intravascular, 3/4 interstitial
  • Intravascular: 1/12 of total body water


Hypovolemia Hypervolemia Dehydration
  • Subjective: Thirst, fatigue, postural dizziness, sunken eyes, dry mucous membranes
  • Objective:
    Orthostatic hypotension (↑ HR > 30 bpm, ↓ SBP > 20, ↓ DBP >10)
    ↑ Urea: Creatinine Ratio
    SCr/BUN <~10

    ↓ JVP
    Urine Na+ < 30mmol/L
    ↓ urine output/↑urine concentration

    • Normal: 0.5-1mL/kg/hr
    • Obligatory water excretion = 20 mL/hr
  • edema (peripheral, pulmonary)
  • lung sounds – rales/crackles,
  • ascites
  • ↑ JVP
  • Generally represents HYPERNATREMIA
  • ↑ serum osmolality
  • ↑ serum [Na+]

s/s of hypernatremia:
Fatigue, muscle weakness, cramps, lethargy, irritability…if severe, confusion, paralysis, coma and seizures

s/s of hyponatremia: asymptomatic (usually if >125mmol/L), headache, nausea/vomiting, muscle cramps, lethargy, restlessness, disorientation…if severe, seizures, coma, brain damage, respiratory arrest, brain herniation, death



What’s available! 🙂

  • D5W= “free” water
    • Osmolality: 253 mOsm/L (similar to us)
    • 1L: 666mL in ICF, 333 mL in ECF (83mL in IV)
    • D5 prevents phlebitis (d/t cell lysis)
    • BEST for a contracted ICF (“Hypernatremia”)
  • NS (0.9% NaCl) = isotonic (154meq/L)
    • Osmolality: 308 mOsm/L (similar to us)
    • 1L: 0mL in ICF, 1000mL in ECF (250mL in IV)
    • BEST for a contracted ECF (“Hypovolemia”)
  • D5 1/2NS = Think of it like 500mL D5W + 500mL NS
    • 1L: 333mL in ICF, 666mL in ECF (166mL in IV)
    • BEST for balancing treatment of contracted ICF and ECf
  • 3% NaCl = super hypertonic (causes massive water shifts)
    • Osmolality: 1030
    • 1L: -2000mL in ICF, +3000mL in ECF (750mL in IV)
  • Mild (<5% of body weight)
    Oral therapy: water, juices, soft drinks, soup broth with extra salt, electrolyte solutions if tolerate PO
  • Moderate (5-10% of body weight) – Severe (10% of body weight)
    Crystalloid solutions (D5W, NS)
    Colloid solutions (albumin, etc)

Treating Hypovolemia and Hypernatremia:

  1. Treat hypovolemia FIRST (can be done quickly)
  2. Treat hypernatremia (sodium is slower to change)
  • Hypovolemia: NS 250-1000mL over 15-60 mins...reassess frequently. If infusing over a longer time, best to reassess prior to the infusion stopping to make sure we didn’t overshoot 
  • Hypernatremia: Best with D5W
    1. calculate the total water deficit (usually aim for Na of 140-145)
    [(Na actual/Na desired) – 1] x TBW
    2. Assess volume/ECF and whether ICF, ECF or both need to be repleted
    3. Choose a “free water” containing crystalloid. Infuse at 50-200mL/hour
    4. Choose a route and rate of administration.
    Do NOT reduce Na+ more than ~0.5 mmol/L/hr (max ~8-12 mmol/L/day)
    5. R/A frequently

Treating Hypovolemia and Hyponatremia (expanded ICF):

*Hypovolemia is CAUSING hyponatremia!*

  • How?
    Anti-diuretic hormone (vasopressin), which
    (1) aims to retain H2O during hypovolemia to maintain BP
    (2) prevent hyperosmolality during dehydration
    Causes free H2O reabsorption from the distal tubule “ADH = D5W
    *Think of ADH when thinking of hyponatremia!*
  • ADH is stimulated when:
    (a) increase serum osmolality (which…↓ intracellular volume)
    (b) significant ↓ in volume
  • When osmolality is low or normal and pt is euvolemic or hypervolemic, ADH turns OFF (When it is ON, it is in appropriate)
  • Can treat with NS 250-1000mL over 15-60 mins. Reassess frequently.
    (same as treating hypovolemia above)