IV Fluids

Steven Podnos MD

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.

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.

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

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

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.

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.

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

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?