Intravenous fluid resuscitation is an important component of anaesthesia and critical care practice, and is required when a reduced blood volume results in hypovolaemic shock. If untreated, hypovolaemia will lead to a lack of perfusion and oxygenation of the tissues and eventually cause permanent vital organ damage and failure, leading to one of the major causes of death in trauma (Hicks, 2011).
Hypovolaemia can be referred to as absolute or relative. Absolute hypovolaemia refers to the total loss of volume from the extracellular space, whilst relative hypovolaemia refers to an inappropriate redistribution of body fluids or excessive dilation resulting in a reduction of the effective intravascular volume.
Hypovolaemia can be caused by trauma, both blunt and penetrating, or due to internal bleeding - for example, due to gastrointestinal bleeding disorders or aneurysms. Blunt trauma is categorised by there being no penetration of the skin, while penetrating trauma is categorised by a breach of the skin. Hypovolaemia can also be caused simply by not being able to eat or drink, or keep fluids in the body; therefore, patients sometimes require maintenance fluids.
The human body reacts to hypovolaemic shock by initiating four main systems:
- The cardiovascular system increases the heart rate and constricts the peripheral blood vessels, redistributing the blood away from the skin, muscles and gastrointestinal tract, and to the heart, brain and kidneys - the vital organs.
- The haematological system reacts by activating the coagulation cascade. Platelets are activated in order that they form clots at the bleeding area and fibrin is deposited at the clot. There is also a contraction of the bleeding vessels.
- The renal system stimulates renin secretion. Renin causes angiotensin II to be formed by the lungs and liver, and once released causes vasoconstriction of arteriolar smooth muscle and stimulation of the adrenal cortex, which in turn secretes aldosterone. Aldosterone causes the reabsorption of sodium, reducing loss of water.
- A decrease in blood pressure and sodium levels also stimulates the neuroendocrine system to release greater amounts of antidiuretic hormone (ADH). ADH increases the water permeability of the collecting ducts and the loops of Henle within the kidneys, thus allowing an increase in the reabsorption of water and salt.
The use of fluids
Part of the treatment for hypovolaemia is to re-introduce intravenous fluids to the patient. Fluids help to restore blood volume and increase blood pressure. The most commonly used fluids in the UK are crystalloids and colloids. Donor blood is still very scarce and expensive compared with crystalloids and colloids; therefore, these cheaper, more common plasma substitutes are usually used in the early stages of bleeding. The benefits of each type of fluid have been widely debated for many years and the discussion continues as to whether crystalloid or colloids are preferred for intravascular volume replacement, although colloids are often reserved for supplementation of the intravascular volume.