Hypovolemia Definition, Causes, Symptoms and Signs

What is hypovolemia?

The medical definition of hypovolemia is a decrease of the volume of circulating blood [Greek hypo = less; vol = volume; emia refers to blood] ¹.

Symptoms and Signs

Symptoms and signs of hypovolemia (pale, cool and moist skin, increased heart rate, weak pulse) may not be present until the blood volume decreases for more than 10-20%, when hypovolemic shock starts to develop.

Absolute vs Relative Hypovolemia

Absolute hypovolemia refers to a decreased volume of the fluid (blood) within the circulatory system in which the fluid leaves the body, for example in bleeding.

In relative hypovolemia, the fluid from the circulatory system does not leave the body and it can refer to 2 different situations:

The volume of the circulatory system increases, for example due to widespread vasodilation in neurogenic shock, and the volume of the fluid (blood) within the circulatory system remains the same but becomes insufficient.
Some fluid leaves the circulatory system, but not the body, for example in ascites, where the fluid accumulates in the abdominal cavity.

Causes of Absolute Hypovolemia

Loss of blood (external bleeding)
Loss of plasma (burns)
Loss of water from the body (severe diarrhea or vomiting, excessive sweating or excessive urination) ⁸

Causes of Relative Hypovolemia

Relative or distributive hypovolemia can occur in the following situations:

Vasodilation with an increase in the volume of the intravascular space with insufficient blood volume to fill this space and therefore a drop of blood pressure (orthostatic hypotension ², vasovagal syncope ⁴, neurogenic shock due to spinal cord injury, septic shock, anaphylaxis, side effect of barbiturates and antihypertensives)
Internal bleeding ³
Blood trapped within the circulatory system (aortic dissection)
Decreased blood volume in the arteries–arterial hypovolemia–in heart failure ¹⁰
Decreased oncotic pressure of the blood plasma due to low blood protein levels and hypoalbuminemia (in nephrotic syndrome with anasarca ^3,8, protein malnutrition [Kwashiorkor] or ascites — accumulation of the fluid in the abdominal cavity) ⁸
Peritonitis with a loss of fluid into the abdominal cavity
Shift of water from the blood into the body cells (in hyponatremia).

Dehydration

Hypovolemia and dehydration are closely related, but are not synonyms.

Dehydration means a decrease in total body water. Dehydration can be associated with hypovolemia, normovolemia or even hypervolemia.

Hypovolemia means a decrease in the volume of fluid within the vascular system with or without whole body water depletion. Hypovolemia can be caused by dehydration.

Hypotension

Hypovolemia causes hypotension when the blood volume loss is greater than 30%; this is called decompensated hypovolemic shock ⁷.

Hypotension can occur in hypovolemia but also without blood volume loss (due to vasodilation).

Hypovolemic Shock

Hypovolemia means any decrease in the blood volume and does not already mean hypovolemic shock. For example, donation of 500 mL blood causes hypovolemia, but usually not shock ⁶. Hypovolemic shock may start to develop at 10-20% blood loss (0.5-1 liter blood ) because of inadequate perfusion of the body tissues.

Until blood pressure does not drop, the hypovolemic shock is referred as “compensated” and when it falls it is called “decompensated.” Some authors call compensated shock as hypovolemia and decompensated shock as hypovolemic shock, but both compensated and decompensated shock should be called hypovolemic shock, which means inadequate perfusion of the peripheral tissues, which already occurs in compensated shock with pale and cool skin and normal blood pressure.

Measurement

Invasively, hypovolemia can be detected by pulmonary arterial catheter.
Noninvasively, hypovolemia can be detected by pulse oximetry ¹⁰or diffuse optical spectrometry (DOS), which measure blood flow through the tissues ⁹. Using these methods, hypovolemia can be detected before hypovolemic shock develops.
Prolonged capillary refill time (pressing on the nail) and orthostatic hypotension are not reliable signs of mild to moderate hypovolemia ⁵.

References

Hypovolemia