Hypovolemia

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In physiology and medicine, hypovolemia (also hypovolaemia) is a state of decreased blood volume ; more specifically, decrease in volume of blood plasma.^[1][2] It is thus the intravascular component of volume contraction (or loss of blood volume due to things such as hemorrhaging or dehydration), but, as it also is the most essential one, hypovolemia and volume contraction are sometimes used synonymously.

It differs from dehydration, which is defined as excessive loss of body water.^[3] Furthermore, hypovolemia defines water deficiency only in terms of volume rather than specifically water.

Causes

Common causes of hypovolemia can be dehydration, bleeding, vomiting^[4], severe burns and drugs such as diuretics or vasodilators typically used to treat hypertensive individuals. Rarely, it may occur as a result of a blood donation^[5], sweating^[4], and alcohol consumption^[4]. It is also common during surgery due to the use of anaesthetics, nil-by-mouth, and in-operation bleeding.

Bodily response

To respond to hypovolemia is a task for the body fluid balance systems as well as osmotic balance systems.

Following an acute response, this function is accomplished by two sets of receptors; one in the kidneys and the other in the heart.

Acute response

The first response to hypovolemia is an inversed baroreflex, where a lack of activation of baroreceptors results in elevation of total peripheral resistance and cardiac output via increased contractility of the heart, heart rate, and arterial vasoconstriction,^[6]which tends to increase blood pressure.

There is also autoreperfusion, in which decreased blood pressure results in decreased filtration of fluid out of capillaries, in effect causing a volume shift from interstitial fluid to blood plasma.

Kidney

The kidneys have a specialized set of cells called granular cells that enable the recognition of changes in blood flow to the kidneys.^[4] Naturally, these cells detect the presence of hypovolemia and react accordingly to the loss of blood volume. These cells secrete a hormone called renin when there is a decrease in the flow of blood to the kidneys.^[4] Renin flows into the blood and there, initiates the conversion of a protein called angiotensinogen to angiotensin.^[4] In order to exert its effects on the body, angiotensin I must be converted by enzymes into its active form, angiotensin II. Physiologically, angiotensin II stimulates the release of hormones by the posterior pituitary gland (ADH, also known as vasopressin) and the adrenal cortex (aldosterone). Aldosterone causes the kidneys to reabsorb sodium, leading to the reabsorption of water. ADH (vasopressin) also causes the kidneys to reabsorb water. Angiotensin II increases blood pressure by contracting arterial muscles.

Heart

Further reading:Atrial natriuretic peptide

The next set of receptors responsible for detecting volumetric insufficiency are located in the heart atria. Commonly referred to as stretch receptors, these atrial baroreceptors detect the amount of blood that is being pumped back into the heart from the veins.^[4] The body constantly returns blood to the heart through veins. Therefore, when the volume of blood being transported back to the heart is decreased, these receptors detect the change in the amount of blood thereby reducing the release of atrial natriuretic peptide.

Thirst

Both the activation of the renin angiotensin system and the decrease in atrial natriuretic peptide, along with their other functions, contribute to elicit thirst, by affecting the subfornical organ.^[7]

Other responses

Furthermore, as intravascular fluid decreases, blood pressure is reduced and some compensation occurs as fluid from other cellular compartments moves into the vasculature. Fluid is passively transferred from all of the fluid compartments in the body, including intracellular, interstitial and other extravascular compartments.^[4]

Diagnosis

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Clinical symptoms may not be present until 10–20% of total whole-blood volume is lost.

Hypovolemia can be recognized by elevated pulse, diminished blood pressure, and the absence of perfusion as assessed by skin signs (skin turning pale) and/or capillary refill on forehead, lips and nail beds. The patient may feel dizzy, faint, nauseated, or very thirsty. These signs are also characteristic of most types of shock.

Note that in children, compensation can result in an artificially high blood pressure despite hypovolemia. This is another reason (aside from initial lower blood volume) that even the possibility of internal bleeding in children should almost always be treated aggressively.

Also look for obvious signs of external bleeding while remembering that people can bleed to death internally without any external blood loss.

Also consider possible mechanisms of injury (especially the steering wheel and/or use/non-use of seat belt in motor vehicle accidents) that may have caused internal bleeding such as ruptured or bruised internal organs. If trained to do so and the situation permits, conduct a secondary survey and check the chest and abdominal cavities for pain, deformity, guarding or swelling. (Injuries to the pelvis and bleeding into the thigh from the femoral artery can also be life-threatening.)

Stages of Hypovolemic Shock

Stage 1

• Up to 15% blood volume loss (750mls)
• Compensated by constriction of vascular bed
• Blood pressure maintained
• Normal respiratory rate
• Pallor of the skin
• Slight anxiety

Stage 2

• 15–30% blood volume loss (750–1500 ml)
• Cardiac output cannot be maintained by arterial constriction
• Tachycardia >100bpm
• Increased respiratory rate
• Blood pressure maintained
• Increased diastolic pressure
• Narrow pulse pressure
• Sweating from sympathetic stimulation
• Mildly anxious/Restless

Stage 3

• 30–40% blood volume loss (1500–2000 ml)
• Systolic BP falls to 100mmHg or less
• Classic signs of hypovolemic shock
• Marked tachycardia >120 bpm
• Marked tachypnea >30 bpm
• Decreased systolic pressure
• Alteration in mental status (Anxiety, Agitation)
• Sweating with cool, pale skin

Stage 4

• Loss greater than 40% (>2000mls)
• Extreme tachycardia with weak pulse
• Pronounced tachypnea
• Significantly decreased systolic blood pressure of 70 mmHg or less
• Decreased level of consciousness
• Skin is sweaty, cool, and extremely pale (moribund)

Treatment

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Minor hypovolemia from a known cause that has been completely controlled (such as a blood donation from a healthy patient who is not anemic) may be countered with initial rest for up to half an hour. Oral fluids that include moderate sugars and electrolytes are needed to replenish depleted sodium ions. Furthermore the advice for the donor is to eat good solid meals with proteins for the next few days. Typically, this would involve a fluid volume of less than one liter, although this is highly dependent on body weight. Larger people can tolerate slightly more blood loss than smaller people.

More serious hypovolemia should be assessed by a physician. When in doubt, treat hypovolemia aggressively.

First aid

External bleeding should be controlled by direct pressure. If direct pressure fails, other techniques such as elevation and pressure points should be considered. The tourniquet should be used in the case of hemorrhages that can not be controlled by any other means. The use of a tourniquet can kill all the tissue below its application upon a limb, making amputation necessary. If a first-aider recognizes internal bleeding, the life-saving measure to take is to immediately call for emergency assistance.

Field care

Emergency oxygen should be immediately employed to increase the efficiency of the patient's remaining blood supply. This intervention can be life-saving.

The use of intravenous fluids (IVs) may help compensate for lost fluid volume, but IV fluids cannot carry oxygen in the way that blood can. See also emergency medical services for a discussion of techniques used in IV fluid management of hypovolemia.

Hospital treatment

If the hypovolemia was caused by medication, the administration of antidotes may be appropriate but should be carefully monitored to avoid shock or the emergence of other pre-existing conditions.

Blood transfusions coupled with surgical repair are the definitive treatment for hypovolemia caused by trauma. See also the discussion of shock and the importance of treating reversible shock while it can still be countered.

History

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Hypovolemia has historically been termed desanguination (from Latin sanguis, blood), meaning a massive loss of blood. The term was widely used by the Hippocrates in traditional medicine practiced in the Greco-Roman civilization and in Europe during the Middle Ages. The word was possibly used to describe the lack of personality (by death or by weakness) that often occurred once a person suffered hemorrhage or massive blood loss.

References

1. ^ MedicineNet > Definition of Hypovolemia Retrieved on July 2, 2009
2. ^ TheFreeDictionary.com --> hypovolemia Citing Saunders Comprehensive Veterinary Dictionary, 3 ed. Retrieved on July 2, 2009
3. ^ MedicineNet > Definition of Dehydration Retrieved on July 2, 2009
4. ^ ^{a b c d e f g h} Carlson, N. R. (2005). Foundations of Physiological Psychology: Custom edition for SUNY Buffalo. Boston, MA: Pearson Custom Publishing.
5. ^ Danic B, Gouézec H, Bigant E, Thomas T (June 2005). "[Incidents of blood donation]" (in French). Transfus Clin Biol 12 (2): 153–9. doi:10.1016/j.tracli.2005.04.003. PMID 15894504. 
6. ^ Banic A, Sigurdsson GH, Wheatley AM (1993). "Influence of age on the cardiovascular response during graded haemorrhage in anaesthetized rats". Res Exp Med (Berl) 193 (5): 315–21. doi:10.1007/BF02576239. PMID 8278677. 
7. ^ M.J. McKinley and A.K. Johnson (2004). "The Physiological Regulation of Thirst and Fluid Intake". News in Physiological Sciences 19 (1): 1–6. doi:10.1152/nips.01470.2003. PMID 14739394. http://physiologyonline.physiology.org/cgi/content/full/19/1/1. Retrieved 2006-06-02. 

See also

• Volume status
• Hypervolemia
• Exsanguination

External links

• CRISP Thesaurus 00004050
• DDB 29217
• http://www.daxor.com - Daxor designed and developed the BVA-100 Blood Volume Analyzer. It is the first instrument approved by the FDA to provide rapid direct measurement of a patient's true blood volume.