Third spacing of fluids

# Overview
In human physiology, extracellular fluids are distributed between the interstitial compartment (i.e. tissue) and intravascular compartment (i.e. plasma) in an approximately 75%-25% ratio. Third spacing is the physiological concept that body fluids may collect in a "third" body compartment that isn't normally perfused with fluids. Third-space fluid shift is the mobilization of body fluid to a non-contributory space rendering it unavailable to the circulatory system. The third space where bodily fluid resides is the interstitial space, or the space filled with interstitial fluid between the cells within the tissues. Extensive tissue swelling occurs when the third space fills with excess fluid, known as edema. An example of severe third spacing is ascites, as seen in severe liver failure as a result of a low albumin level in the circulating blood.
Third space is also a term used to refer to, for example, the bowel with an ileus, and the collection of fluid therein, usually post-operatively. The interstitial space could be considered to be a sub-section of the extracellular compartment. The distinction one could make is that fluid in the interstitium is still readily available to either the intravascular or the intracellular compartments, in response to ionic changes, whereas that which is pooled in the bowel (third space) is not so readily available. It is a recurrent clinical phenomenon requiring swift identification to minimize deleterious effects. Third Spacing takes fluid away from the normal fluid compartments and causes the patient to have hypovolemia.

# Causes
For example, with severe burns, fluids may pool in the burn site and cause depletion of the fluids in the first and second compartments. With pancreatitis, fluids may "leak out" into the peritoneal cavity, also causing depletion of the first and second compartments.  Sepsis syndrome may also cause third spacing. Other causes include viral and bacterial infections and major surgery. The underlying causes however are mainly trauma and inflammation.
- Decreased protein levels
- Heart Failure
- Increased capillary permeability
- Liver failure
- Lymphatic obstruction
- Major Surgery
- Pancreatitis
- Sepsis syndrome
- Severe Burns
- Trauma
- Viral and Bacterial infections

# Diagnosis

## History and Symptoms
- Weakness
- Weight Loss
- Decreased urinary output

## Physical Examination
The patient will manifest symptoms of volume depletion and will be hypovolemic and dehydrated due to a shift of fluid from the intravascular compartment to the third space. Other symptoms include:

### Vital Signs
- Decreased body temperature
- Tachycardia
- Weak pulse
- Low blood pressure
- Increased respiration

# Treatment

## Medical Therapy
Clinically, the extent of "third spacing" is usually unknown, and therefore it serves more as a theoretical concept for problem-solving rather than a concrete value that requires repletion. In general third spacing is dealt with treating the underlying disturbance or illness rather than giving more fluids to correct the electrolyte and protein imbalances, since that will only help relieve the symptoms in the short term. While the patient may be edematous and is total body fluid overloaded, it should be realized that the patient is paradoxically often "dehydrated" or under filled in their intravascular space. For this reason, aggressive diuresis should be avoided to avoid pre renal azotemia. Diuresis should be performed at a rate that is commensurate with the mobilization of fluid from the third space into the intravascular space.

# Related Chapters
- Ascites
- Pleural effusion
- Edema
- Interstitial fluid
- Anasarca