Haemodilution – Blood Dilution Explained

What is Haemodilution?

The term haemodilution (also spelled hemodilution) is derived from the Greek word “haima” (blood) and the Latin “dilutio” (dilution). It describes a condition in which blood volume increases without a proportional increase in the cellular and dissolved components of the blood. As a result, the concentration of red blood cells (erythrocytes), haemoglobin, platelets (thrombocytes), and coagulation factors decreases.

Haemodilution can be a natural physiological process – such as during pregnancy – or it can be deliberately induced for medical purposes, or occur as an unintended complication.

Types of Haemodilution

Normovolaemic Haemodilution

In normovolaemic haemodilution, the total blood volume remains constant while the cellular fraction decreases. This is typically achieved by administering intravenous fluids such as crystalloid or colloid solutions. A clinically important variant is acute normovolaemic haemodilution (ANH), in which blood is withdrawn from a patient before surgery and replaced with intravenous fluids. The collected blood is then retransfused during or after the procedure to reduce the need for allogeneic blood transfusions.

Hypervolaemic Haemodilution

In this form, total blood volume increases as more fluid is administered than the body can process. This may occur during aggressive fluid resuscitation or in conditions such as heart failure or renal failure.

Physiological Haemodilution

During pregnancy, natural haemodilution occurs because plasma volume expands more rapidly than red blood cell mass. This leads to a relative decrease in haematocrit and is commonly referred to as physiological anaemia of pregnancy. In most cases, this does not require treatment.

Causes and Triggering Factors

• Generous intravenous fluid administration (infusion therapy)
• Pregnancy (physiological haemodilution)
• Heart failure with fluid retention and oedema
• Renal disease with impaired fluid excretion
• Liver cirrhosis
• Deliberate medical induction (e.g., acute normovolaemic haemodilution before surgery)
• Cardiopulmonary bypass (extracorporeal circulation)

Clinical Effects and Symptoms

The effects of haemodilution depend on the degree of blood dilution. Possible consequences include:

• Anaemia: Reduced haemoglobin levels impair oxygen transport, potentially causing fatigue, dizziness, and shortness of breath.
• Coagulation disorders: Dilution of clotting factors and platelets increases the risk of bleeding.
• Electrolyte changes: Concentrations of sodium, potassium, and other electrolytes may decrease.
• Reduced blood viscosity: Thinner blood can improve microcirculatory flow but also reduces oxygen-carrying capacity.

Diagnosis

Haemodilution is identified through laboratory blood tests:

• Haematocrit: Measures the percentage of red blood cells in total blood volume; reduced in haemodilution.
• Haemoglobin: The oxygen-carrying protein in red blood cells; decreased in haemodilution.
• Full blood count: Assesses erythrocytes, leucocytes, and thrombocytes.
• Coagulation parameters: Prothrombin time (INR), aPTT, and fibrinogen levels to assess bleeding risk.
• Electrolytes and albumin: To detect concentration changes in the plasma.

Therapeutic Use of Haemodilution

In clinical practice, haemodilution is deliberately applied to achieve specific therapeutic goals:

Acute Normovolaemic Haemodilution (ANH) in Surgery

Before major surgical procedures, blood is collected from the patient and replaced with intravenous fluids. The collected blood is then returned to the patient during or after the operation. This reduces the loss of red blood cells during surgery and can help minimise the need for donor blood transfusions.

Haemodilution in Circulatory Disorders

In certain circulatory conditions, such as ischaemic stroke or peripheral arterial disease, haemodilution has been used to improve blood flow properties. However, the clinical efficacy of this approach remains controversial, and it is less commonly applied today.

Cardiopulmonary Bypass

During cardiac surgery using a heart-lung machine, blood is diluted by the priming volume of the bypass circuit. Targeted haemodilution management is therefore a standard component of cardiac surgical protocols.

Risks and Complications

Uncontrolled or excessive haemodilution can have serious consequences:

• Critical drop in haemoglobin leading to inadequate oxygen supply to organs
• Increased bleeding risk due to dilution of coagulation factors (dilutional coagulopathy)
• Oedema formation caused by a decrease in colloid osmotic pressure
• Cardiac strain due to increased blood volume (in hypervolaemic haemodilution)

References

1. Kozek-Langenecker, S. et al. - Management of severe perioperative bleeding: Guidelines from the European Society of Anaesthesiology. European Journal of Anaesthesiology, 2017.
2. German Medical Association (Bundesarztekammer) - Cross-sectional Guidelines for Therapy with Blood Components and Plasma Derivatives, 5th edition, 2020.
3. Pschyrembel Clinical Dictionary, 268th edition, De Gruyter, 2020.