Water constitutes 50 - 60 % of total body weight: ~ 60 % in males, ~ 50% in females
Lower percentage in females correlates well with a relatively larger amount of adipose tissue and
smaller muscle mass
Intracellular Fluid
~ 40% of body weight
Potassium and magnesium are the principal cations
Phosphates and proteins are the principal anions
Separated from the extracellular fluid by the semipermeable cell membrane
Water is freely diffusible across the cell membrane
Sodium and potassium concentrations are maintained by the energy dependent sodium-potassium
pump
Extracellular Fluid
~ 20% of body weight
Consists of the plasma volume (5%) and interstitial fluid (15%)
Sodium is the principal cation
Chloride and bicarbonate are the principal anions
Capillary membrane separates the plasma from the interstitial fluid, and is freely permeable
to charged ions, glucose, and small plasma proteins
Large plasma proteins cannot cross the capillary wall and are responsible for the plasma
osmotic pressure
Electrolyte Composition of Body Fluid Compartments
Some Basic Chemistry Terms
Mole: molecular weight of a substance in grams
Gives no direct information about the number of osmotically active ions or the
electric charges that they carry
Equivalent: atomic weight in grams divided by valence
Number of electric charges per unit volume
Represents chemical combining activity
Univalent ions: 1 mmol = 1 mEq
Divalent ions: 1 mmol = 2 mEq
Number of mEq of cations = number of mEq of anions
Osmoles: actual number of osmotically active particles in solution (mOsm/l)
Not dependent on chemical combining capacities of substances
Not dependent on the size of the particles
Composition of Fluid Compartments
Differences in ionic composition between the intracellular and extracellular fluid are
maintained by the semipermeable cell membrane
Total number of osmotically active particles is 290 - 310 mOsm in each compartment
Total osmotic pressure of a fluid is the sum of the partial pressures of each solute in that
fluid
Effective osmotic pressure is dependent on those substances that fail to pass through the
pores of the semi-permeable membrane
Dissolved proteins in plasma are responsible for the effective osmotic pressure between the
plasma and interstitial fluid compartments (colloid osmotic pressure)
Sodium contributes the majority of the effective osmotic pressure between the intracellular
and extracellular compartments
Other substances that do not cross the plasma membrane freely, such as glucose and urea, also contribute to
the effective osmotic pressure
Because cell membranes are freely permeable to water, the effective osmotic pressure in the
two compartments is equal
Any condition that alters the effective osmotic pressure in either compartment results in
redistribution of water between the compartments
Normal Exchange of Water and Salt
Water Exchange
Normal individuals consume 2000 - 2500 ml water/day, 25% - 40% of which comes from solid foods
Daily losses are from urine, stool, and insensible (skin 75%, lungs 25%)
Insensible losses are increased by hypermetabolism, hyperventilation, fever
Fever causes loss of 150 ml/day per degree of fever
During a laparotomy or thoracotomy, insensible losses from the operative field can be as high as 1L/hr
To clear the products of metabolism, the kidneys must excrete ~ 500 ml of urine/day,
which means that the minimum water intake is 500 mL/day
Free water deficit calculation (liters):
Salt Exchange
Recommended daily salt intake varies from 50 - 90 mEq/day (3 to 5 gr)
Balance is maintained primarily by the kidneys, which can reduce excretion of sodium to as little as 1 mEq/day
Sodium deficit calculation:
Assessment of Volume Disorders
Hypovolemia
Most common fluid disorder in surgical patients
GI losses: N-G suction, vomiting, diarrhea, enterocutaneous fistulas
FeNa < 1%: FeNa = (urine Na x plasma Cr) / (plasma Na x urine Cr)
↑ Base deficit
↓ CVP, stroke volume, cardiac output
Daily weights
Management
Fluid boluses or an increased maintenance IV rate to achieve a urine output of 0.5 mL/kg/hr
Under resuscitation leads to renal insufficiency/failure
Over resuscitation leads to pulmonary insufficiency and increased time on the ventilator
Diuretics worsen an intravascular volume deficit
Hypervolemia
Iatrogenic from overresuscitation
CHF, renal failure, cirrhosis
Clinical Assessment
Peripheral edema
Increased CVP
Pulmonary edema
Weight gain
Management
Decrease maintenance IV rate
Diuretics if normal renal function
Dialysis if renal failure
IV Fluid Therapy
IV Solutions
Isotonic Fluids
LR and 0.9% NaCl (NS)
Used to correct volume deficits because they are limited to the extracellular space
(but only 25% of the infused volume remains in the intravascular compartment of the extracellular space)
LR is an extracellular fluid mimic
Lactate in LR is converted to bicarbonate in the liver, and may be the better choice in
acidotic patients
Avoid LR in renal failure patients (contains 4 mEq K+)
Large volume NS resuscitation can cause a hyperchloremic metabolic acidosis
Hypertonic Saline
Increases serum osmolality, which allows fluid to move from the intracellular space into the
interstitial and intravascular spaces
Used in massive burn resuscitations to reduce the total amount of resuscitation volume required
Also used in traumatic brain injury patients with elevated intracranial pressure
May cause a hyperchloremic metabolic acidosis as well as hypernatremia
Colloids
5% albumin, Hespan
Used to preferentially expand the intravascular volume since the large molecules do not pass
through the capillary pores into the interstitial space
In inflammatory states, the pore size increases and allows large molecules to migrate into the
interstitial space, negating the value of this therapy
Numerous randomized trials have failed to demonstrate that colloid use improves patient outcomes
Maintenance Fluids
Usually given as D5.45% NaCl
Provides sufficient free water for insensible losses from the skin and lungs
Provides sufficient Na
5% dextrose provides 170 kcal, which may limit protein catabolism in noninjured patients
5% dextrose is always added to D5W and D5.2NS to prevent red cell lysis from rapid infusion
of hypotonic fluids
