Hyperhydration has a detrimental effect on mortality risk and morbidity, increases the risk of acute kidney failure, the need of renal replacement therapy (RRT), worsens recovery of renal functions and worsens lung injury (ALI), infectious complications, and causes prolonged artificial pulmonary ventilation (APV), the length of stay on Intensive care unit (ICU), and impairs wound healing.

Real-time assessment of fluid status and management of fluid administration in critically ill patients is challenging. Echocardiography can rapidly identify hemodynamic phenotypes, but it is rather intermittent than continuous methods and requires experienced and trained staff. Semi-invasive methods, based on stroke volume monitoring as the area under the arterial curve and variability of stroke volume variation (SVV) evaluate intravascular volume. However, these methods lack information about the interstitial fluid, part of extracellular water (ECW), or intracellular fluid water (ICW). This problem is partially solved by transpulmonary thermodilution with extravascular lung water (EVLW) measurement and lung ultrasound. Calculating the cumulative balance (CBF) is imprecise, especially in the area of fluid output for insensible losses or third-space fluid losses. Even more imprecise is the clinical assessment of peripheral edema and blood flow. And gold standard deuterium dilution methods for total body water (TBW) are not usable in daily practice in the ICU settings.

In addition to hyperhydration, the rapid loss of muscle tissue in critically ill patients has a negative impact on the course of the disease. Polyneuromyopathy affects up to 40 % of critically ill patients, patients in a severe catabolic state with an activated systemic inflammatory response (SIRS), with corticosteroid therapy, and immobilized on long-term artificial lung ventilation are at risk. Monitoring lean body mass, especially skeletal muscle mass (SMM), is still difficult. Anthropometric measurements and ultrasound measurements of the quadriceps muscles are not ideal because they are time-consuming and require well-trained staff. Some laboratory parameters such as albumin are likely to be influenced by inflammation (CRP), and hydration. Dual-energy X-ray absorptiometers (DEXA) using two different wavelengths of low-intensity X-rays give a relatively accurate picture of bone mass and soft tissues (fat-free mass, active mass, fat). However, repeated X-ray examination in immobilized critically ill patients is not the method of choice.

Bioelectrical impedance vector analysis (BIVA) is a simple, rapid, and noninvasive bedside technique, based on the principle that the flow of altering electrical current through a particular tissue differs depending on the content of water and electrolytes. It is thus able to measure body composition as skeletal muscle mass (SMM), and body cell mass (BCM), including total body water and extracellular water. And with the use of 50 frequencies of bioimpedance spectroscopy (BIS), it is possible to distinguish TBW, ECW, and from their different intracellular water, because only electric current with a frequency higher than 100 Hertz (Hz) passes through the cell membrane. However, the technique cannot distinguish between intravascular and interstitial volumes in the extracellular compartment. According to a number of studies, the results of bioimpedance parameters of body composition are comparable to DEXA. However, BIA overestimates the representation of muscle. An important parameter is the phase angle (PA), which detects a time delay of the passage of current through the cell membrane, i.e., a phase shift between the sinusoidal voltage and current waveforms. PA reflects BCM and serves as an important prognostic factor, with a normal value of 4-15°.

Of the laboratory markers, presepsin (PSEP) has prognostic significance. Presepsin, soluble Cluster of differentiation 14 (sCD14), is a glycoprotein expressed in the membranes of monocytes and macrophages in response to pathogen-associated molecular patterns (PAMPs: lipopolysaccharide, peptidoglycan) part of the bacterial wall or to other damage to cells - damage-associated molecular patterns (DAMPs). An interesting finding is its prognostic role, i.e. higher values in non-surviving patients, evaluated by a number of studies.