Serum Na+ concentration (mEq/L) approximates the ratio of the total body Na+ (TBNa+) to
the TBW.
a. Serum Na+ ≈ TBNa+/TBW
(1) TBNa+ is the sum total of all ECF Na+ (vascular compartment + interstitial compartment), unlike serum Na+, which is the Na+ concentration that is limited to the vascular compartment (i.e., 136–145 mEq/L).
(2) Whenever there is an increase in fluid, the ECF compartment always expands, and whenever there is a decrease in fluid, the ECF compartment contracts.
Hypotonic disorders:
clinical findings that correlate with a decrease in TBNa+
(1) Definition: Hypotonic disorders are characterized by a decrease in POsm and serum Na+ and expansion of the ICF compartment.
(2) Decrease in TBNa+ produces clinical signs (physical exam findings) of volume depletion (Link 5-2). Another term for volume depletion is hypovolemia.
Note: some authors incorrectly use the term dehydration interchangeably with volume depletion. Definition: Dehydration refers to a loss of only water, whereas volume depletion refers to the loss of both water and Na+. Loss of pure water is present in diabetes insipidus and insensible water loss
Physical exam findings (signs) in patients with a decreased TBNa+ include dry mucous membranes and decreased skin turgor (i.e., skin tenting when the skin is pinched. Older adults commonly have decreased skin turgor as a result of a normal loss of subcutaneous connective tissue (not a decrease in TBNa+).
Blood pressure (BP) decreases when standing (postural hypotension), and pulse increases (tachycardia) when sitting/standing up from a supine position (i.e., positive tilt test).
Weight loss is due to a decrease in sodium-containing fluid.
Jugular venous pressure (JVP) is decreased (decreased prominence of the internal jugular vein in the lateral neck.
Gentle pressure with the index finger over the closed eye reveals a soft, sunken eyeball.
Additional findings include confusion and stupor, decreased urine output, lack of tears, and increased capillary filling time.
Increased capillary filling time is the most reliable test to use in children to detect volume depletion. In children, the normal capillary fill time after pinching the fingertip is <2 seconds. In children with volume depletion, the capillary fill time after pinching the fingertip is >2 seconds.
Symptoms (patient complaints) of a decrease in TBNa+ include thirst, dizziness on standing, and weakness.
Increased TBNa+ produces clinical signs of body cavity effusions (e.g., ascites, pulmonary edema, pleural cavity effusions) and dependent pitting edema which is called hypervolemia (fluid overload).
Dependent pitting edema is due to an excess of Na+-containing fluid in the interstitial space (>2–3 L). Because of the low protein content in edema fluid, the fluid obeys the law of gravity and moves to dependent portions of the body (e.g., ankles, if standing; sacral area, if supine). To test for pitting edema, apply gentle
pressure with the middle three fingers and note a temporary depression in the skin.
An alteration in Starling forces must be present to produce pitting edema and body cavity effusions
Increase in TBNa+ increases the plasma PH. Increase in plasma volume (more Na+-containing water is present) causes an increase in plasma PH. Increase in plasma PH is responsible for pitting edema and body cavity effusions (e.g., ascites, pleural effusions). It may also produce pulmonary edema (fluid in the alveolar sacs and interstitium of the distal airways) causing difficulty with breathing (called dyspnea).
Increase in TBNa+ increases the body weight. Increase in TBNa+ is the most common cause of weight gain in a hospitalized person; hence, it is important to weigh patients every morning. The most common causes of the increase in TBNa+ in a hospitalized patient are heart failure and/or infusion of a sodium-containing antibiotic.
Increase in TBNa+ increases the JVP, causing prominent distention of the internal jugular veins in the lateral neck.
Dyspnea (difficulty with breathing) is the most common symptom of an increase in TBNa+. It is most often due to pulmonary edema and/or excess fluid in the interstitial spaces in the lungs