Hypovolemia


Hypovolemia, also known as volume depletion or volume contraction, is a state of decreased intravascular volume. This may be due to either a loss of both salt and water or a decrease in blood volume. Hypovolemia refers to the loss of extracellular fluid and should not be confused with dehydration. Dehydration refers to excessive total body water loss that results in cellular hypertonicity.
Hypovolemia is caused by a variety of events, but these can be simplified into two categories: those that are associated with kidney function and those that are not. The signs and symptoms of hypovolemia worsen as the amount of fluid lost increases. Immediately or shortly after mild fluid loss, one may experience headache, fatigue, weakness, dizziness or thirst. Untreated hypovolemia or excessive and rapid losses of volume may lead to hypovolemic shock. Signs and symptoms of hypovolemic shock include increased heart rate, low blood pressure, pale or cold skin, and altered mental status. When these signs are seen, immediate action should be taken to restore the lost volume.

Signs and symptoms

Signs and symptoms of hypovolemia progress with increased loss of fluid volume.
Early symptoms of hypovolemia include headache, fatigue, weakness, thirst, and dizziness.
The more severe signs and symptoms are often associated with hypovolemic shock. These include oliguria, cyanosis, abdominal and chest pain, hypotension, tachycardia, cold hands and feet, and progressively altering mental status.

Causes

The causes of hypovolemia can be characterized into two categories:

Kidney

The signs and symptoms of hypovolemia are primarily due to the consequences of decreased circulating volume and a subsequent reduction in the amount of blood reaching the tissues of the body. In order to properly perform their functions, tissues require the oxygen transported in the blood. A decrease in circulating volume can lead to a decrease in blood perfusion to the brain, resulting in headache and dizziness. Altered mental status progresses as hypovolemia continues.
Baroreceptors in the body sense the reduction of circulating fluid and send signals to the brain to increase sympathetic response. This sympathetic response is to release epinephrine and norepinephrine, which results in peripheral vasoconstriction in order to conserve the circulating fluids for organs vital to survival. Peripheral vasoconstriction accounts for the cold extremities, increased heart rate, increased cardiac output. Eventually, there will be less perfusion to the kidneys, resulting in decreased urine output.

Diagnosis

Hypovolemia can be recognized by a fast heart rate, low blood pressure, and the absence of perfusion as assessed by skin signs 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.
In children, compensation can result in an artificially high blood pressure despite hypovolemia. Children typically are able to compensate for a longer period than adults, but deteriorate rapidly and severely once they are unable to compensate. Consequently, any possibility of internal bleeding in children should be treated aggressively.
Signs of external bleeding should be assessed, noting that individuals can bleed internally without external blood loss or otherwise apparent signs.
There should be considered possible mechanisms of injury that may have caused internal bleeding, such as ruptured or bruised internal organs. If trained to do so and if the situation permits, there should be conducted a secondary survey and checked the chest and abdomen for pain, deformity, guarding, discoloration or swelling. Bleeding into the abdominal cavity can cause the classical bruising patterns of Grey Turner's sign or Cullen's sign.

Investigation

In a hospital, physicians respond to a case of hypovolemic shock by conducting these investigations:
Untreated hypovolemia can lead to shock. Most sources state that there are 4 stages of hypovolemia and subsequent shock; however, a number of other systems exist with as many as 6 stages.
The 4 stages are sometimes known as the "Tennis" staging of hypovolemic shock, as the stages of blood loss mimic the scores in a game of tennis: 15, 15–30, 30–40 and 40. It is basically the same as used in classifying bleeding by blood loss.
The signs and symptoms of the major stages of hypovolemic shock include:
Stage 1Stage 2Stage 3Stage 4
Blood lossUp to 15% 15–30% 30–40% Over 40%
Blood pressureNormal Increased diastolic BPSystolic BP < 100Systolic BP < 70
Heart rateNormalSlight tachycardia Tachycardia Extreme tachycardia with weak pulse
Respiratory rateNormalIncreased Tachypneic Extreme tachypnea
Mental statusNormalSlight anxiety, restlessAltered, confusedDecreased LOC, lethargy, coma
SkinPalePale, cool, clammyIncreased diaphoresisExtreme diaphoresis; mottling possible
Capillary refillNormalDelayedDelayedAbsent
Urine outputNormal20–30 mL/h20 mL/hNegligible

Treatment

Field care

The most important step in treatment of hypovolemic shock is to identify and control the source of bleeding.
Medical personnel should immediately supply emergency oxygen to increase efficiency of the patient's remaining blood supply. This intervention can be life-saving.
The use of intravenous fluids may help compensate for lost fluid volume, but IV fluids cannot carry oxygen the way blood does—however, researchers are developing blood substitutes that can. Infusing colloid or crystalloid IV fluids also dilutes clotting factors in the blood, increasing the risk of bleeding. Current best practice allow permissive hypotension in patients suffering from hypovolemic shock, both avoid overly diluting clotting factors and avoid artificially raising blood pressure to a point where it "blows off" clots that have formed.

Hospital treatment

is beneficial in hypovolemia of stage 2, and is necessary in stage 3 and 4. See also the discussion of shock and the importance of treating reversible shock while it can still be countered.
The following interventions are carried out:
Vasopressors should generally be avoided, as they may result in further tissue ischemia and don't correct the primary problem. Fluids are the preferred choice of therapy.

History

In cases where loss of blood volume is clearly attributable to bleeding, most medical practitioners prefer the term exsanguination for its greater specificity and descriptiveness, with the effect that the latter term is now more common in the relevant context.