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In medicine, volume status refers to the volume of blood in a patient's circulatory system. This is related to the patient's state of hydration, but is not identical to it. For instance, volume depletion can exist in an adequately hydrated person if there is loss of water into interstitial tissue (e.g. due to hyponatremia or liver failure).
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Clinical assessment
Volume depletion
Signs of volume depletion (low blood volume) include, in order of severity:
- a fast pulse
- infrequent and low volume urination
- dry mucous membranes (e.g. a dry tongue)
- poor capillary refill (e.g. when the patient's fingertip is pressed, the skin turns white, but upon release, the skin does not return to pink as fast as it should)
- decreased skin turgor (e.g. the skin remains "tented" when it is pinched)
- a weak pulse
- orthostatic hypotension (dizziness upon standing up from a seated or reclining position, due to a drop in cerebral blood pressure)
- cool extremities (e.g. cool fingers)
Volume overload
Signs of volume overload (high blood volume) include:
- an elevated Jugular venous pressure (JVP)
Volume Measurement
Blood volume may be measured with the BVA-100 blood volume analyzer from Daxor Corporation.
Pathophysiology
Volume depletion
The most common cause of volume depletion is diarrhea or vomiting. The other causes are usually divided into renal and extrarenal causes. Renal causes include overuse of diuretics, or trauma or disease of the kidney. Extrarenal causes include bleeding, burns, and any causes of edema (e.g. congestive heart failure, liver failure, etc.).
Volume depletion is divided into three types based on the blood sodium level:
- Isonatremic (normal blood sodium levels) Example: a child with diarrhea, because both water and sodium are lost in diarrhea.
- Hyponatremic (abnormally low blood sodium levels). Example: a child with diarrhea who has been given tap water to replete diarrheal losses. Water is replenished, but sodium is not, so water flows out of the vasculature into the interstitial tissue.
- Hypernatremic (abnormally high blood sodium levels). Example: a child with diarrhea who has been given salty soup to drink, or insufficiently diluted infant formula. Here sodium has been replenished, but not enough water has been provided with it.
Volume overload
Volume overload can occur during surgery, if water rather than isotonic saline is used to wash the incision. It can also occur if there is inadequate urination, e.g. with certain kidney diseases.
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