What is Hypovolemia?

Hypovolemia occurs when the amount of fluid, or volume, in the body's circulatory system is too low. In most cases, this refers to blood volume but this can include lymphatic fluid, too.

Causes of hypovolemia range from excessive bleeding caused by trauma, as with an auto accident, to medical conditions like severe dehydration. Symptoms depend on the cause, but typically include low blood pressure, a rapid heart rate, and increasing mental confusion.

This article will focus on blood-related hypovolemia within the circulatory system. It discusses the symptoms and causes of hypovolemia, as well as how hypovolemia is treated.

Hypovolemia in the Body

Hypovolemia happens when the body's own "containers" for blood and fluid fail to achieve their normal level of function. Cells have an outer membrane filled with fluid, inside of which are all the structures necessary for cell function. The cells make up tissues, many of which are organized into various structures that either channel or contain fluid.

All of this fluid is water-based and must have enough water to balance out all the salts and particulates in it. Water and salt are moved from cell to cell, as well as into and out of the bloodstream as the body needs to balance fluids.

When the body is adequately hydrated and there is enough relative fluid volume to fill the circulatory space available, the systems typically function properly. However, when the circulatory space is too large relative to the fluid that's available, it's known as hypovolemia.

The lack of volume affects the ability of the body to adequately perfuse (fill) the tissues with blood, oxygen, and nutrients. Inadequate perfusion is known as shock. Hypovolemia and shock are closely related.

Hypovolemia Symptoms

Hypovolemia symptoms and the symptoms of shock are very similar. As blood volume decreases, the body begins to compensate for the lack of volume by constricting blood vessels.

The body squeezes blood vessels to make the available space inside the cardiovascular system smaller, which means the relative volume of blood is adequate to create pressure and perfuse the tissues. This shunts blood away from the most distal parts of the body (which is usually the skin) and results in loss of color and less noticeable warmth (cool, pale skin).

The heart rate increases to circulate available blood more quickly and to increase the blood pressure enough to offset the loss of volume (and pressure) in the vascular space. At this point, there is often very little change in measurable blood pressure.

If the cause of the hypovolemia is not corrected and the body continues to lose fluid volume, the body responds by:

• Sweating (stress response to the loss of perfusion)
• Lightheadedness (as loss of perfusion affects the brain)
• Confusion
• Fatigue
• Decreased blood pressure

If hypovolemia remains untreated and the cause is not corrected, the person could become unconscious.

Hypovolemia Causes

Hypovolemia can have several causes. Sweating, excess urination, vomiting, or diarrhea can all cause rapid water loss. If the fluid is not adequately replaced, you can become dehydrated and eventually hypovolemic.

Bleeding, however, is the most common cause of hypovolemia. In fact, direct blood loss can result in hypovolemia very quickly.

Shifting fluid out of the bloodstream can also cause hypovolemia. Severe dehydration (loss of water) can lead to hypovolemia as the tissues pull water out of the bloodstream to balance the loss. Even a person with severe edema (swelling) in the extremities—such as someone with congestive heart failure—can have hypovolemia.

Even though they might have too much fluid in the body (resulting in swelling), they might not have enough in the cardiovascular system. This would result in hypovolemia.

If the amount of fluid is unchanged but the size of the cardiovascular system expands, you can experience relative hypovolemia. There is no loss or shift of fluid, but the sudden increase in space in the blood vessels leads to the same loss of pressure and perfusion as hypovolemia. This is why people lose consciousness during syncope (fainting).

Diagnosis

There is no definitive blood test for hypovolemia. A clinical assessment is required to diagnose it. Vital signs are evaluated, including:

• Blood pressure
• Heart rate
• Respiratory rate
• Capillary refill time (how long it takes for the color to return to your fingernails after you squeeze them—the faster it returns, the better)

These all give clues about someone's blood volume relative to their cardiovascular capacity.

When doing a thorough history and physical exam, your healthcare provider may ask about fluid intake, history of vomiting or diarrhea, and urine output.

You may also need to have your blood pressure and pulse taken while lying down, sitting up, and standing. Changes in the vital signs between these positions could indicate the presence of hypovolemia.

Treating Hypovolemia

Fluid intake is the treatment for hypovolemia. The type of fluids used for hypovolemia will depend on the individual case and the cause of the condition, and why an intravenous infusion may be required.

In the case of direct blood loss, a blood transfusion could be necessary for severe cases. The most important treatment is to correct the underlying cause of the hypovolemia.

A Word From Verywell

Hypovolemia can lead to potentially life-threatening shock. If you haven't been getting enough fluids or you've been bleeding, and you also feel dizzy, weak, or nauseated, see your healthcare provider immediately. An early diagnosis and treatment may lead to better outcomes.