2. Normal Physiology
• Plasma osmolarity is about 300 of which Na+
is 145, combination of Cl- and HCO3- is 145.
• Fluids with electrolyte concentrations equal to
plasma are “isotonic” . Higher concentrations
than plasma are hypertonic (3% NS) and lower
concentrations are hypotonic (D5W, ½ NS)
• Infusion of very hypotonic fluid (pure water)
would cause cell lysis.
3. Crystalloids vs. Colloids
• NS, RL and modifications of these are
crystalloids. Colloids (always hypertonic) add
osmotically active substances that do not
easily cross the vascular space in order to
increase intravascular volume
(albumin, Hetastarch, pRBC). Fluids to
increase intravascular volume should be
isotonic or hypertonic.
4. Crystalloids
• Normal Saline (NS) has osmolarity of 300, of
which Na+ is 145, and Cl- is 145 (no Bicarb!)
• Ringer’s lactate osmolarity is 300, of which
Na+ is 145, Cl- is 110 and Lactate (metabolized
to Bicarbonate) is 28. Small amount of K+
• 5% D5W is also isotonic (osm 300), but is very
hypotonic once administered as glucose taken
up by cells
5. RL vs. NS
• Ringer’s lactate is designed to minimize Cl-
load. The lactate is usually metabolized to be
an acid buffer, so a good choice for
Hyperchloremic acidosis from NS. If patient
has lactic acidosis from volume depletion, RL
should actually help, but is technically
contraindicated
6. Colloids
• Expensive and require specific storage:
• Albumin
• Hetastarch
• Mannitol
• Colloids stay in the intravascular space and
usually pull fluid into that space from the
extravascular tissue that has a lower tonicity.
7. Distribution of IVF
• Isotonic fluids such as NS and RL distribute out
about 2/3rds to extravascular space and 1/3rd
to intravascular space. Someone getting
100cc/hour of IV NS is only getting about an
ounce an hour of intravascular fluid. Loss of
one unit of blood requires at least 3L of IVF to
replace the lost volume.
8. Complications of IVF
• Acidosis-NS has no Bicarb and too much Cl-.
Hyperchloremic acidosis most common
acidotic mechanism in the hospital (no anion
gap)
• Hyponatremia-hypotonic fluids
• Volume overload-rare in absence of impaired
cardiac or renal function
• Edema-cosmetic
9. Selecting IVF
• 1) Patient with volume depletion-thirsty, low
BP, tachycardic. Normal electrolytes?
• 2) SIADH patient (cancer)- Na+ 120, good
urine output, mild edema?
• 3) Head trauma or stroke with DI-Na+
165, volume depletion?
