Where does the fluid go ?
Volume kinetics shows that increasing volumes of crystalloid fluid sequentially expands three body fluid compartments : the plasma, a fast-exchange interstitial volume, and a more remote slow-exchange interstitial volume ( “the third fluid space”).
The skin, intestinal wall, and lungs are known primary locations of excess amounts of crystalloid fluid in humans.
The compliance for volume expansion is twice as high in the skin compared to muscle. Therefore, most of the excess fluid ends up in the skin where it may impair wound healing and exacerbate local infection.
The lungs are another location where excess fluid is found.
The gastrointestinal wall is the third site for the accumulation of excess fluid.
Also …
Liberal administration of crystalloid fluid is often required to maintain adequate tissue perfusion when treating life-threatening conditions <=> either overhydration or underhydration can promote complications.
Apart from overhydration, accumulation of fluid in the slow-exchange space occurs in inflammatory conditions, whereby cytokines and vasoactive molecules create a suction pressure that withdraws fluid from the fast-exchange space. This suction decreases lymphatic flow, causing hypovolemia and hypoalbuminemia in addition to peripheral edema (preeclampsia, sepsis).
Severe disease involves redistribution of blood flows due to blunting of the adrenergic tonus and subsequent vasodilatation. This redistribution, in addition to possible disease-specific toxic effects, requires that the plasma volume (PV) be expanded with an electrolyte-based crystalloid (first choice) or colloid (secondary choice) fluid to maintain adequate circulation and preserve organ function.
Many ICU studies associate positive fluid balance with increased mortality.
Extreme fluid overload causes morphological organ damage and may even be fatal.
Despite the demonstration of infusion-related lesions in experimental animals, the occurrence of morphological lesions in overhydrated humans remains poorly documented.