The Hydration Status and the Kidneys
Fluid overload and oedema are signs of excess extracellular fluid volume. The underlying pathomechanism is, in most cases, an increase of extracellular sodium which binds water and subsequently expands the extracellular space. Although sodium and water retention is a physiologic response to disease, trauma and stress, a significant proportion of fluid overload in the critically ill patient occurs as a result of liberal infusion of fluids containing high sodium amounts. Therefore, oedema is not only an indicator of disease severity but also reflects iatrogenic sodium and fluid overload. In critically ill patients, oedema is either already present at hospital admission (e.g. due to a cardiac, hepatic, renal or inflammatory condition) or develops/aggravates during the disease course. Depending on the type of critical illness, the speed of fluid accumulation and the presence of organ dysfunction, fluid overload can occur after a variable time and amount of fluid infused. Oedema is most frequently encountered in patients with capillary leak (e.g. sepsis, major surgery) or acute on chronic organ (e.g. heart or liver) dysfunction. Organs mainly compromised by fluid overload are the heart and lungs, the gastrointestinal tract, the kidneys and wound healing.