Hypovolemia (Low Blood Volume)

What hypovolemia is

An average adult has about 5 liters of blood (roughly 70 ml per kg of body weight). Blood volume keeps blood pressure up and delivers oxygen to organs. When volume falls, the body compensates:

1. The heart beats faster.
2. Blood vessels in the skin, gut and kidneys tighten to redirect blood to the brain and heart, which is why the skin turns pale and cool and urine output drops.
3. Thirst increases and the kidneys hold on to salt and water.

These responses can keep blood pressure normal for a while, so blood pressure often drops late. When compensation fails, organs are starved of oxygen. This is hypovolemic shock.

Hypovolemia is related to, but not the same as, dehydration. Dehydration strictly means loss of water from the whole body, raising salt concentration. Hypovolemia means loss of volume from the bloodstream, which can happen with bleeding even when the body's water balance is otherwise normal. In everyday use the terms overlap.

Causes of hypovolemia

Blood loss (hemorrhagic hypovolemia)

• injuries, including fractures of the pelvis or thigh bone, which can hide large amounts of blood
• internal bleeding from stomach ulcers, ruptured aortic aneurysm, ruptured ectopic pregnancy or a ruptured spleen
• heavy bleeding after childbirth or from the uterus
• surgery

Loss of other body fluids

• Vomiting and diarrhea, the commonest cause worldwide and especially dangerous in babies and older adults. Gastroenteritis and cholera can cause losses of many liters a day.
• Burns: large burns leak plasma from damaged capillaries.
• Heavy sweating: endurance exercise and heat illness.
• Excess urine: diuretics, uncontrolled diabetes with very high blood sugar, diabetic ketoacidosis, diabetes insipidus and Addison's disease.
• Fluid shifting out of the bloodstream ("third spacing"): pancreatitis, bowel obstruction, severe infection and liver failure.

Not drinking enough

Older adults with reduced thirst, people with dementia, and anyone too unwell to drink can become volume-depleted without unusual losses.

Low blood volume symptoms

These ranges come from trauma classifications and are approximate. Children compensate well and then deteriorate suddenly. Older people and those on beta-blockers may not develop a fast pulse.

Common symptoms include:

• thirst and a dry mouth
• dizziness or lightheadedness, especially on standing
• a fast, weak pulse
• cold, pale, clammy skin and slow capillary refill (skin stays pale for more than 2 seconds after pressing a fingernail)
• dark urine or very little urine
• tiredness, weakness and confusion
• in babies: sunken eyes, a sunken soft spot (fontanelle), no tears and few wet diapers

How hypovolemia is diagnosed

Diagnosis starts at the bedside: pulse, blood pressure (lying and standing), breathing rate, skin temperature, mental state and urine output. Tests then look for the cause and the effects:

• Blood count: hemoglobin may be normal early in bleeding because the blood is lost whole; it falls as fluid is replaced
• Electrolytes and kidney function: sodium, potassium, urea and creatinine
• Lactate: a high level shows organs are short of oxygen
• Blood glucose and ketones
• Blood group and cross-match if transfusion may be needed
• Imaging: bedside ultrasound (FAST scan) for internal bleeding after trauma, CT scans, and endoscopy for gut bleeding

Treatment

Mild hypovolemia

Oral rehydration solution (ORS) is better than plain water or sugary drinks for diarrhea and vomiting because its balance of salt and sugar maximizes absorption. Give small, frequent sips. Babies should keep breastfeeding or take ORS.

Moderate to severe hypovolemia

• IV fluids: balanced salt solutions or saline.
• Blood products: for significant bleeding, red cells with plasma and platelets, often in a fixed ratio (massive transfusion protocol).
• Stop the loss: pressure on bleeding wounds, surgery, endoscopy, medicines for vomiting, or treatment of diabetes or Addison's disease.
• Oxygen and close monitoring of urine output.

Complications

• Hypovolemic shock and cardiac arrest
• acute kidney injury
• damage to the heart, brain, liver and gut from poor blood flow
• blood clotting problems after large blood loss

Outlook

Mild volume depletion recovers within hours of rehydration. Outcome after hypovolemic shock depends on how much fluid was lost, how quickly treatment began, and the person's age and health. Rapid control of bleeding and early fluid replacement greatly improve survival.

Frequently asked questions

What are the symptoms of hypovolemia?

Early symptoms are thirst, a dry mouth, dark urine and dizziness on standing. As volume falls further, the pulse becomes fast and weak, skin becomes cold, pale and clammy, urine output drops, and breathing speeds up. Confusion and fainting are late, dangerous signs.

What are the symptoms of low blood volume from bleeding?

They are the same as other causes of hypovolemia: rapid pulse, pale and clammy skin, dizziness, thirst, anxiety and reduced urine, followed by low blood pressure and confusion. Bleeding may be hidden inside the body, so these symptoms after an injury need emergency care even without visible blood.

What is the difference between hypovolemia and dehydration?

Dehydration is a loss of body water that concentrates the blood's salt. Hypovolemia is a loss of volume from the circulation, from bleeding or from losing salt and water together, such as with diarrhea. Both cause thirst and dizziness, and the terms are often used loosely as synonyms.

How is hypovolemia treated?

Mild cases are treated by drinking oral rehydration solution. Moderate or severe cases need IV fluids in the hospital, and blood transfusion if bleeding is the cause. The source of loss, such as bleeding, vomiting or high blood sugar, must also be controlled.

Can hypovolemia be fatal?

Yes, if it progresses to hypovolemic shock and is not treated quickly. Losing more than about 30 to 40% of blood volume is life-threatening. With early recognition and treatment, most people recover fully.