Fluid & Electrolytes Flashcards
What 2 factors are manipulated by intravascular fluid therapy
Preload and hemoglobin
Total body water (TBW) percentage of body and compartments
60%, ~40L
- ICF (intracellular fluid) = 2/3 of TBW, ~25L
- ECF (extracellular fluid) = 1/3 of TBW ~15L
ECF compartments
IVF (intravascular fluid) -Blood and plasma -20%, ~3L ISF (interstitial fluid space) -Lymph and transcellular fluid -80%, ~12L ISF and IVF are separated by a capillary membrane
Fluid distribution percentages to remember
60/40/20 (15/5) 60% TBW 40L TBW 20L ICF 15L ECF 5L IVF
Major ICF Cations
Potassium (most abundant)
Magnesium
Major ICF Anions
Proteins
Phosphates
Organic ions
Major ECF Cations
Sodium (most abundant)
Calcium
Major ECF Anions
Chloride
Bicarbonate
How is intracellular osmolality maintained
Na/K/ATPase pump
- Exchanges 3 Na for 2 K, offsets tendency for Na to diffuse into the ICF
- In ischemia or hypoxia this is disrupted, causing swelling of cells
4 ways fluid moves between compartments
Diffusion
-Particles down concentration gradient through lipid bilayer or protein channels (facilitated)
Osmosis
-Water moving between cell compartments (intracellular and interstitial space)
Active Transport
-Water, ions, molecules pumped against concentration gradient on cell wall, requires enzymes and energy
-Controls cell volume and concentration
Filtration
-Pressure causes water, ions, molecules to move to a lower pressure area (edema)
Starlings Forces
Determines amount and direction of filtration
- Governed by differences in hydrostatic pressure (interstitial, capillary) in addition to osmotic forces (interstitial, plasma colloid)
- Fluid tends to move out of capillaries at arterial end (BP>osmotic pressure) and back in at venous end (BP
Hydrostatic pressure
Pressure that fluid exerts on the walls
-Interstitial or capillary
Osmotic pressure
Pressure against a semipermeable membrane that prevents water from diffusing across the membrane
- Interstitial or plasma colloid
- Based on osmotic particles not molecular weight
Plasma colloid osmotic pressure
Significant to anesthetic
- Can be increased or decreased depending on what we give to maintain circulating fluid volume in intravascular space
- Crystalloid increases loss of fluid from intravascular to interstitial space
- Replace 3:1 crystalloid to colloid
Osmolarity
Expression of the number of osmoles of solute in a liter of solution
Osmolality (and normal serum and urine osmo)
Number of osmoles of solute in a kilogram of solvent
- Serum osmo: 285-295 mOsm/kg
- Urine osmo: 500-800 mOsm/kg
Tonicity
How a solution affects cell volume
-Compares the osmolarity of solution to the plasma
How to estimate osmolarity
(Na x 2) + (Glucose / 18) + (BUN / 2.8)
-Na is the most important osmotically active substance influencing water in the brain tissue
Control of plasma osmolality
Closely regulated by osmoreceptors in the hypothalamus
- Control secretion of ADH and the thirst mechanism
- ADH is the most important regulator of serum osmolality
Surgery effects on fluid status (3)
Hemorrhage
Evaporative loss
-From exposed viscera (all water, no electrolytes)
Third spacing
-From manipulation of tissues
-Fluid redistributed from intravascular space to interstitial space
-Replacement best done with fluid composition close to ECF
Hypovolemia vs dehydration
Hypovolemia = ECF water deficit, blood volume Dehydration = low water relative to sodium level, TBW
Hypovolemia can result from ____ or ____ (and how to replace)
Absolute loss of fluid
-From GI tract, polyuria, diaphoresis, decreased intake
Relative loss of fluid
-Water redistributed in the body = reduced circulating volume
-Burns, third spacing form surgery
Weight doesn’t decrease
Replace with isotonic crystalloids
