Interstitium and intravasal space are separated by a permeable capillary membrane and are interacting constantly. Composition of fluids within interstitium and intravasal space is similar with the exception of an higher protein content in plasma

This article aims at providing an overview on fluid therapy in cardiac anesthesia in order to support training of anesthetists. Intraoperative fluid therapy is an integral part of anesthesia management [1, 2]. In an attempt to prevent development of organ damage it is of major importance to ensure adequate fluid and volume supply [3]. Patients who have to undergo cardiac surgery present a major challenge to the anesthetist beyond the problem of fluid therapy: These challenges include specific features of the patients’ underlying cardiac disease, complexity of the surgical intervention and, particularly, pathophysiological impact of extracorporal circulation. During cardiac surgery the patients partly experience extreme conditions like cardiac arrest or deep hypothermia unlike in any other surgical subspecialty. In the immediate postoperative period, relative insufficiency of blood volume may often occur, in many occasions even absolute insufficiency of blood volume may also be observed. Especially intraoperative use of cardio-pulmonary bypass often induces capillary leackage which may lead to interstitial oedema during concomittant intravasal volume depletion [11]. However, intraand postoperative hemorrhage as well as administration of diuretics or vasoactive drugs may also cause inadequate blood volume. Fluid therapy should not only lead to stabilization of macro-circulation, but also of micro-circulation. Micro-circulation especially seems to be affected by different volume substitution fluids. Physiology and pathophysiology of fluid compartments should be accounted for when decision has to be made among different solutions [4]. The main part of body fluids (40% of TBW) can be found within the 75 trillions of body cells and is, in its entire volume, referred to as intracellular space (ICS). As composition of the individual cell fluids of the different body cells is uniform, the term of ‘total compartment’ can be applied. All kinds of fluids outside the cell are part of the so-called extracellular space (ECS), which amounts to approx. 20% of total body water in adults. ECS and ICS mainly differ from each other by the osmotic balance of extracellular sodium ions and intra-cellular potassium ions. Interstitial (interstitium, 16% of TBW) and intravascular fluid compartments (plasma volume: 4% of TBW, erythrocytes volume in adults 3,5% of TBW) are subsets of the ECS. Interstitium and intravasal space are separated by a permeable capillary membrane and are interacting constantly. Composition of fluids within interstitium and intravasal space is similar with the exception of an higher protein content in plasma water. This proportion of plasma proteins is responsible for the colloidosmotic pressure (COP) of approx. 26 28 mmHg which prevents intravascular fluid from draining to the interstitium (5 mmHg COP) [5].