Multiple frequency impedance measurements in critically ill patients

The distribution of fluids in the human body is strictly governed by a system of homeostatic mechanisms. About 55% of the fluids is maintained within cellular membranes (intracellular water, ICW) whereas 45% forms the extracellular water compartment (ECW). The onset of disease often alters the otherwise constant ratio of ICW/ECW and may lead to a considerable ECW expansion during a period of critical illness. Recovery is associated with the mobilization and excretion of edema from the 'third space' resulting in normalization of the compartmental relationships. On the other hand, when sepsis persists compensatory mechanisms may fail which leads to a progressive 'extracellular state' (extracellular fluid expansion), haemodynamic instability and possibly death. Knowledge on the volume of ICW may be of considerable importance to the ICU physician. The ICW is considered an important part of the metabolically active portion of the lean body since it provides the environment for cellular metabolism. Feeding regimens aim at maintaining or improving the composition of the intracellular compartment. Haussinger has presented data that the induction of cellular swelling may drive anabolism (1). A measurement of ICW volume may therefore inform the physician about the 'metabolic condition' of a patient and may help to evaluate the efficacy of supportive nutritional strategies. The gold standard for the measurement of the quantity and distribution of body fluids is dilution of compartment specific tracers. Unfortunately, these methods are invasive and do not give immediate results that can be utilized at the bedside. Bioelectrical impedance analysis (BIA) is a noninvasive electrical method that has been introduced for com-positional purposes. This technique claims that the extent to which the human body opposes electrical current (impedance) depends on the quantity of body fluids. If electrical current of low intensity (that cannot be felt) is maintained constant across the body, the electrical resistance R (expressed in Ohm) reflects total body water TBW (2). The exact clinical importance of the second component of impedance, electrical reactance Xc (Ohm) is incompletely understood, but this electrical parameter may reflect the capacitance of cellular membranes and tissue interfaces. Xc and ECW volume appeared to be inversely related in a small group of critically ill patients (3). Most of the research has been performed in healthy subjects using a single frequency (50 kHz) alternating current (typically 800 ktA). The data obtained in patient populations are conflicting although the majority of studies suggests a potential value in using BIA in disease (4). Recently, a new generation of bioimpedance machines that use currents emitted at various frequencies (MFA, multiple frequency impedance analysis) has become available for research purposes (5). The body is 'swept' with currents whose frequencies range e.g. from 1 to 1000 kHz, and at each of these frequencies a measurement of R, X c and phase angle (tangent of Xc/R ) is performed. We have studied this experimental technique in a group of healthy volunteers and critically ill patients. A Xitron 4000B (Xitron Technologies, San Diego, CA, USA) was used that measures electrical parameters between 5 and 1000kHz. Dilutional studies using non-radioactive heavy water (Campro Scientific BV, Veenendaal, The Netherlands) and bromide (specific for TBW and ECW, respectively) were performed in all of these subjects. ECW was calculated according