An Overview of Hypovolemia

Symptoms, Causes, Diagnosis, and Treatment

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When the amount of fluid in the intravascular system is too low, it is known as having not enough volume, or hypovolemia. (In most cases, this refers to blood volume, but this can include lymphatic fluid as well.) This article will focus on hypovolemia as it pertains to the volume of blood relative to the available space inside the circulatory system.

Each person's need for fluid is a little different and depends on lean muscle mass, cardiovascular health, body fat, and various other things. There are clinical signs of hypovolemia, but it could be possible to lose up to 30 percent of total circulatory volume before any signs or symptoms of hypovolemia become apparent.

Background

The body is basically a bag (or several bags) of fluid. Each cell has 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 a condition known as shock. Hypovolemia and shock are closely related.

Symptoms

The symptoms of hypovolemia 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. Squeezing blood vessels makes 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 noticable 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 (see below) 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 patient could become unconscious.

Causes

Generally, 60 percent of body weight in men is made up of fluid while it is approximately 50 percent in women.

There are several ways to lose fluid volume. Sweating, excess urination, vomiting, or diarrhea can all cause rapid water loss. If the fluid is not adequately replaced through drinking water, a person can become dehydrated and eventually hypovolemic.

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

The location of bleeding can be internal (such as bleeding into the abdomen), gastrointestinal (bleeding into the stomach, esophagus, or bowel), or external. In cases of internal or gastrointestinal hemorrhage, sometimes the signs and symptoms of hypovolemia are the first indications of blood loss, rather than the observation of the bleeding itself.

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 patient with severe edema (swelling) in the extremities—such as a patient with congestive heart failure—can have hypovolemia.

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

If the amount of fluid in the body is unchanged, but the size of the cardiovascular system expands, the patient can experience relative hypovolemia. In this case, 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 the cause of a patient losing consciousness during syncope.

Diagnosis

There is no definitive blood test for hypovolemia. A clinical assessment is required to diagnose it. Vital signs including blood pressure, pulse 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), and respiratory rate all give clues about a patient's blood volume relative to his cardiovascular capacity.

When doing a thorough history and physical exam, the healthcare provider may ask the patient about fluid intake, history of vomiting or diarrhea, and urine output. The patient might also need to have her 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.

Treatment

Fluid intake is the treatment for hypovolemia. In the case of direct blood loss, a blood transfusion could be necessary for severe cases. Otherwise, an intravenous infusion may be required. The most important treatment is to correct the underlying cause of the hypovolemia.

A Word From Verywell

Hypovolemia can lead to shock and shock is very dangerous. If you haven't been getting enough fluids or you've been bleeding (even a simple nosebleed that won't stop) and you are feeling dizzy, weak, or nauseated, it's best to see your doctor or healthcare professional immediately. Early intervention is the best for diagnosis and treatment.

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