Edema

Fluid Excess - Edema

IMBALANCES OF FLUIDS AND ELECTROLYTES:

Fluid and electrolyte disturbances are characteristically "multiple situations". Individual imbalances rarely exist alone. Imbalances may involve fluid, electrolytes, and pH simultaneously. Nonetheless this section will examine imbalances caused predominantly by a single factor since the "multiple situations" are quite difficult to understand.



           Abnormal Conditions which Affect the Fluid Balance

Exchange Process Effected	Clinical Condition
1. Increased blood pressure results in increased filtration from plasma.	Hypertension
2. Increased permeability of capillaries results in too large a loss of ions and protein from plasma.	Local inflammation caused by injury.
3. Loss of plasma protein results in lower osmotic pressure in plasma.	Kidney disease (cirrhosis); Malnutrition
4. Retention of salts especially total body sodium caused by decreased excretion in urine from low blood pressure prevents effective filtration. Increased water is retained to maintain osmotic pressure.	Kidney disease associated with low blood pressure; Congestive heart failure
5. High venular pressure prevents return of water by osmosis to plasma.	Phlebitis, obesity, varicose veins
6. Blockage of lymphatic drainage	Local inflammation.

Lab Tests for ECF Fluid Excess or Deficit:

An extracellular fluid (NaCl + water) excess causes the compartment to become expanded. This most commonly occurs when kidneys are not functioning properly as in renal disease or decreased renal blood flow. No electrolyte test will indicate this condition.

The hematocrit value may be used to indicate extracellular fluid excess and will be somewhat low.
(Hematocrit (HCT): Measures a volume percentage of red blood cells in the plasma.
Normal value : Females = 37-47%; Males = 40=54%)

Example:
Why should the HCT value be low? If the normal HCT value is 40% and the normal plasma fluid content is 3 liters, what is the HCT % if the plasma fluid is now expanded to 3.3 liters?

Solution:
First find the volume of red cells (40% of total plasma).
3 L x 40 / 100 = 1.7 L of red blood cells.
Find the new percentage of red blood cells at the increased volume.
% HCT = 1.7 L / 3.3 L x 100 = 36.3% HCT

Therefore, a lower percent value for HCT means an INCREASE in the fluid volume. Low %HCT means ECF excess. A height %HCT means an ECF Deficit.

  

INTRACELLULAR FLUID EXCESS (WATER INTOXICATION):

Cell volume is regulated by changes in the amount of total body water. All fluids taken into and excreted from the body pass through extracellular compartment. Osmosis and filtration distribute this in all compartments. For every liter taken in or excreted l/3 goes to or from the ECF compartment and 2/3 goes to or from the ICF compartment.

Intracellular fluid excess is caused primarily by excessive water intake or an increase in vasopressin hormone. Interestingly, most cases occur in the hospital following trauma, surgery, or anesthesia. These conditions stimulate the release of vasopressin which in turn decreases urine volume and increases retention of water. Forced fluids without salt could aggravate the situation.

Lab Test for ICF Fluid Excess - Measure the sodium concentration:

Serum sodium electrolyte values are the best indicators of ICF excess or deficit even though sodium is found and measured in the ECF compartment. For a condition with excess water the value is less than normal for much the same reason as the hematocrit values.

If the normal concentration of sodium is l40 meq/l or l40 mmoles/l and 17 liters is the volume in the ECF compartment, what is the new concentration of Na if one Liter of excess water is ingested? The calculation gives a value of 132 mmoles/L

Again a lower concentration of sodium means that it is present in a larger volume. Low sodium = ICF excess.