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Written by Dr.Mary

What is Hypovolemia?

Hypovolemia is a modification that occurs inside the human body, characterized by a reduction of the blood volume (more specifically, blood plasma). This change is also known as oligemia or hypovolemic shock, being encountered in a wide range of medical emergencies. Hypovolemia should not be confused with a person becoming dehydrated – in the first case, sodium is reduced, while in dehydration one loses water excessively. In the medical field, the hypovolemic shock is considered a medical emergency – the decrease in the blood volume affects the heart, the overall circulation and it can lead to multiple organ failure.

Signs and Symptoms

Doctors and other specialists have come up with a classification of hypovolemia and, as you will see, depending on the amount of blood plasma lost, the signs and symptoms will differ.

In the first stage of hypovolemia, the patient loses up to 15% of the overall blood volume, which means approximately 750 ml. At this point, the loss of blood plasma is still compensated by the body, which means that the blood pressure remains stable. The patients present a respiratory rate within normal limits (12-20 breaths/minute). However, one can notice that the skin has become pale and there are some patients who might exhibit anxiety-like status. In the majority of patients, the mental status remains unaffected at this point. Both the capillary refill and the urine output are within normal levels.

The second stage of hypovolemia is characterized by the loss of 15-30% of the total blood volume, which means between 750 and 1500 ml. The heart muscle is affected by the reduced amount of blood plasma, resulting in tachycardia (>100bpm) and increased respiratory rate (over 20 breaths/minute). While the systolic BP remains stable, one can notice an increase in diastolic blood pressure. The pulse becomes narrow and skin becomes excessively pale – as the blood is redirected towards major organs (heart, lungs, brain), the skin will also turn clammy and cold. The patient can present symptoms of anxiety, feeling restless. The capillary refill is delayed and the urine output is reduced to 20-30 ml/h.

In the third stage of hypovolemia, the blood loss is more considerate, reaching a percentage of 30-40 (as quantity – 1500-2000 ml). In patients who are in the third stage, the systolic BP is very low – it can even go under 100 mm Hg. By now, the patient exhibits increased heart rhythm (tachycardia – >120bpm) and a higher respiratory rate (>30 breaths/minute). The mental status is no longer intact, the patient presents marked confusion, feeling anxious and agitated. The skin is cold and pale, with profuse sweating. The capillary refill is delayed and the urine output is reduced to 20 ml/h.

Within the fourth stage of hypovolemia, the blood volume loss is over 40%, which means a decrease over 2000 ml. The pulse is barely felt and the patient exhibits market tachycardia (>140bpm). The respiratory rate is increased as well, while the systolic pressure is as reduced as 70 mmHg or even less. The mental status is severely affected, the patient can lose consciousness, entering into a state of severe lethargy or coma. An intense paleness characterizes the skin, which is extremely sweaty and cold. The capillary refill is no longer possible, the urine output is barely noticeable and the condition is life-threatening.

General symptoms for hypovolemia include:

  • Anxiety
  • Confusion
  • Agitation
  • The lips and nails become blue due to the blood being re-directed to major organs
  • The urine output is reduced or barely noticeable
  • The skin is cold, clammy and the patient sweats profusely
  • The respiratory rate is increased
  • The patient might experience vertigo or pain in the chest (possible sign of heart attack)
  • Loss of consciousness – coma
  • Reduced blood pressure
  • Increased heart rate – tachycardia
  • Barely noticeable pulse

Hypovolemia can also be caused by internal bleeding and one must learn how to recognize the signs of such problems. These are:

  • Severe pain in the abdomen
  • Blood in the stool or in the urine
  • Severe bleeding from the vagina (not caused by the menstrual cycle)
  • Coughing or vomiting blood
  • Pain in the chest (possible heart attack)
  • Swelling in different parts of the body

Types of Hypovolemia

There are two main types of hypovolemia, meaning relative and absolute. It is said that the hypovolemia is relative when the vascular tree increases in size, while the blood volume is not sufficient to handle the new changes. On the other hand, absolute hypovolemia can be encountered in several situations. For example, absolute hypovolemia often appears after severe dehydration but also in those who have suffered from hemorrhages, severe diarrhea or excessive loss of liquids (through urine or sweat).



The human body is designed to perfection, with complex mechanisms for compensation in case of damage. In the situation that a person suffers from hypovolemia, there will be an individual response from each of the following systems: neuroendocrine, hematologic, cardiovascular and renal.

The hematologic system responds to the acute loss of blood volume by activating the coagulation process, causing the blood vessels to constrict. The cardiovascular system will try to compensate by making the heart pump more rapidly and constricting the blood vessels that are located in the periphery. As a secondary mechanism of compensation, it will distribute the blood to the major organs (brain, lungs, heart), taking it away from the skin, gastrointestinal tract and muscles (hence the pale skin).

The renal system has a complex response to the hypovolemia – it starts by increasing the secretion of serotonin. This allows for angiotensinogen to be transformed into angiotensinogen 1 – this will then be transformed in the lungs and the liver in angiotensinogen 2. The purpose of all these responses is to counteract the effects of the hypovolemic shock. Angiotensinogen 2 acts by constricting the smooth muscles and also by stimulating the secretion of aldosterone. This has the purpose of reabsorbing sodium and water, thus maintaining the homeostasis of the body. Last, but not least, the neuroendocrine system responds to the hypovolemic shock by increasing the production of ADH (antidiuretic hormone), with the purpose of reabsorbing sodium and water.

Hypovolemia Treatment

These are the most common methods of treatment recommended for hypovolemia:

  • Blood donation – it is important that the donor is healthy and that he/she does not suffer from anemia
  • The patient is advised to lie down and rest until the homeostasis is restored
  • The loss of blood plasma can be compensated by the administration of oral fluids – these have to include electrolytes and sugar, replenishing the body with what it has lost
  • If the hypovolemia is caused by an external wound with hemorrhage, it is important to apply pressure to the wound – this is a medical emergency and the patient should be taken to the hospital before it is too late
  • Emergency oxygen is applied as a first aid measure, helping the remaining blood volume to maintain the homeostasis of the body
  • Intravenous fluids represent a temporary solution for hypovolemia but it is important to understand that these cannot carry oxygen in the same way that blood does
  • Antidotes are administered in patients who suffer from hypovolemia induced by medication
  • If the hypovolemia is caused by trauma, surgical intervention will be necessary, at the same time with a blood transfusion.

Hypovolemia vs Dehydration

As it was mentioned at the start of the article, it is very important not to confuse hypovolemia with dehydration. Often times, we hear even medical professionals who are using these terms loosely, as they consider them to be synonyms. When one says dehydration, it means that a person practically loses water and nothing else. The state of dehydration can be confirmed by performing blood tests – these will reveal a normal level of sodium. Hypovolemia, on the other hand, refers to the blood volume being decreased – this means that, besides water, the patient also loses sodium and other electrolytes that are vital for life. While the state of dehydration can be compensated by the administration of water, the treatment of hypovolemia is more complex, as you have seen above.

Hypovolemia lab values

These are the changes you will encounter in the lab tests for hypovolemia:

  • Sodium excretion increased
  • Specific gravity increased
  • BUN (blood urea nitrogen) increased – demonstrates kidney function
  • Hct (hematocrit) increased
  • Osmolality
    • ICF volume increased
    • ECF volume decreased
  • Potassium decreased.


First and foremost, you should know that, untreated, this condition is life-threatening. The higher the blood volume loss, the higher the risk of damage to major organs or complete failure – most often affected: kidneys, heart or brain. The loss of blood volume can also lead to major circulatory problems, with gangrene affecting the limbs. As the heart pumps more and more, trying to compensate for the blood loss, there is an increased risk of heart attack.

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