||This article may be too technical for most readers to understand. (June 2009)|
|Classification and external resources|
|ICD-10||E86, R57.1, T81.1|
In physiology and medicine, hypovolemia (also hypovolaemia, oligemia or shock) is a state of decreased blood volume; more specifically, decrease in volume of blood plasma. 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.
Common causes of hypovolemia are
- Loss of blood (external or internal bleeding or blood donation)
- Loss of plasma (severe burns and lesions discharging fluid)
- Loss of body sodium and consequent intravascular water; e.g. diarrhea or vomiting
- Vasodilation (involving widening of blood vessels) such as trauma leading to dysfunction of nerve activity on blood vessels and inhibition of the vasomotor center in the brain or drugs such as vasodilators typically used to treat hypertensive individuals.
Excessive sweating is not a cause of hypovolemia, because the body eliminates significantly more water than sodium.
Clinical symptoms may not be present until 10–20% of total whole-blood volume is lost.
Hypovolemia can be recognized by tachycardia, 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. Children will typically compensate (maintain blood pressure despite loss of blood volume) for a longer period than adults, but will deteriorate rapidly and severely once they do begin to decompensate. 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. ("Blood on the floor, plus 4 more" = intrathoracic, intraperitoneal, retroperitoneal, pelvis/thigh)
Also consider possible mechanisms of injury 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 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.
Stages of hypovolemic shock
The 4 stages are sometimes known as the "Tennis" staging of hypovolemic shock as the stages of blood loss (under 15% of volume, 15-30% of volume, 30-40% of volume and above 40% of volume) 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.
- Up to 15% blood volume loss (750 mL)
- Compensated by constriction of vascular bed
- Blood pressure maintained
- Normal respiratory rate (12-20 breaths per minute)
- Pallor of the skin (paleness)
- Normal mental status to slight anxiety
- Normal capillary refill (less than 2 seconds)
- Normal urine output
- 15–30% blood volume loss (750–1500 mL)
- Blood pressure cannot be maintained by arterial constriction
- Tachycardia >100bpm
- Increased respiratory rate (more than 20 respirations per minute)
- Systolic blood pressure maintained
- Increased diastolic blood pressure
- Narrow pulse pressure (gap between the systolic and diastolic pressure)
- Pale, cold, and clammy skin as blood flow is directed away to major organs such as the heart, lungs, and brain
- Mildly anxious/Restless
- Delayed capillary refill
- Urine output of 20-30 milliliters/hour
- 30–40% blood volume loss (1500–2000 mL)
- Systolic BP falls to 100mmHg or less
- Marked tachycardia (increased heart rate) >120 bpm
- Marked tachypnea (increased rate of respiration) >30 respirations per minute
- Alteration in mental status (confusion, anxiety, agitation)
- Sweating with cool, pale skin
- Delayed capillary refill
- Urine output of approximately 20 milliliters/hour
- Loss greater than 40% (>2000 mL)
- Extreme tachycardia (>140) with weak pulse
- Pronounced tachypnea
- Significantly decreased systolic blood pressure of 70 mmHg or less
- Decreased level of consciousness, lethargy, coma
- Skin is sweaty, cool, and extremely pale (moribund)
- Absent capillary refill
- Negligible urine output
- Survival is extremely unlikely
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, however blood substitutes are being developed which can. Infusion of colloid or crystalloid IV fluids will also dilute clotting factors within the blood, increasing the risk of bleeding. It is current best practice to allow permissive hypotension in patients suffering from hypovolemic shock, both to ensure clotting factors are not overly diluted but also to stop blood pressure being artificially raised to a point where it "blows off" clots that have formed.
Fluid replacement 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.
For a patient presenting with hypovolemic shock in hospital the following investigations would be carried out:
- Blood tests: U+Es/Chem7, FBC, Glucose, Cross-match
- Central venous catheter or blood pressure
- Arterial line or Arterial Blood Gases
- Urine output measurements (via urinary catheter)
- Blood pressure
- SpO2 Oxygen saturations
The following interventions would be carried out:
- IV access
- Oxygen as required
- Surgical repair at sites of hemorrhage
- Inotrope therapy (Dopamine, Noradrenaline) which increase the contractility of the heart muscle
- Fresh frozen plasma or whole blood
In cases in which loss of blood volume is clearly attributable to bleeding (as opposed to, e.g., dehydration), most medical practitioners of today 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.
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