IV FLUID CONSTITUENTS
Dr. David Lyness
@Gas_Craicpropofology.com
5% Dextrose/Glucose
Rapidly lost intravascularly. Glucose is rapidly taken up by the cells.Hence, the distribution of 1 litre of normal saline is approximately: (extracellular) = 1000 ml (ISF 750 ml, IVF 250 ml)
Starling forces:
Starling equation illustrates movement of fluid across capillaries depending on three factors:
-Hydrostatic forces that push fluid
-Oncotic pressure exerted by proteins in fluid that pulls water
-Permeability of endothelium between plasma and interstitial fluid
On the arterial side of capillary bed, intravascular fluid moves into interstitial space (higher intravascular hydrostatic force). On the venous side, fluid is reabsorbed into plasma (lower intravascular hydrostatic force).
When fluid is infused into plasma, hydrostatic forces increase and oncotic pressure decrease (dilution effect) until fluid is evenly distributed in ECF and Starling forces are in equilibrium.
A consideration of the ionic contents, osmolality and pH in IV Crystalloids
The OsmolaRity of a solution expressed as the total number of solute particles per litre.
OsmolaLity is proportional to the number of particles per kilogram of solvent.
Cell membranes are freely permeable to water, the osmolality of ECF is approximately equal to that of the intracellular fluid (ICF).
Therefore, plasma osmolality is a guide to intracellular osmolality.- In normal people, increased osmolality in the blood stimulates antidiuretic hormone (ADH).
- A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more conc plasma.
- An increase of only 2% to 3% in plasma osmolality will produce a strong desire to drink.
OSMOLALITY
pH is clearly an important consideration when giving
IV fluids, most pertinently in significant volumes.
Be wary of the ion content of IV fluids in patients with imbalance in such electrolytes. Consideration needs to be given to hypo/hypernatraemia, potassium and chloride particularly.
