Rapid Measurement of Total Body Water to Facilitate Clinical Decision Making in Hospitalized Elderly Patients

BODY composition measurements are important to assess nutritional depletion and lean muscle mass wasting in elderly individuals and acutely ill patients. Information on the total body water (TBW) can be used to estimate body composition and its compartments such as fat-free mass, fat mass, and body solids (1–4). Rapid knowledge of patient TBW can contribute to diagnosis, and management of fluid and electrolyte imbalance. TBW is composed of two compartments: intracellular water (ICW) and extracellular water (ECW). ICW consists of the fluid in the muscle and organ cells and ECW includes plasma, interstitial fluid, and connective tissue fluids. In healthy humans, for rapid approximate estimates of TBW, simple anthropometric measurements can be used with considerable accuracy (5–9). However, in acutely ill patients, estimated TBW does not reliably reflect fat-free mass due to disturbances of ICW caused by protein malnutrition, changes in TBW, and changes in the ratio of ICW and ECW due to injury and inflammation (10–14). ECW occupies approximately 25% of the total body mass; however, a 40% increase in ECW would result in only a 10% increase in total volume, which may not be detected (15). This is of considerable clinical importance because excessive fluid retention in the extracellular space is known to cause increased morbidity in acutely ill patients (15–17). Moreover, a shift in the ICW/ECW ratio could occur without a large change in total volume. To accurately measure TBW and the distribution of water between the ICW and ECW compartments, isotopic dilutions are used. Briefly, TBW is measured after oral administration of deuterated water into the blood stream and measuring the exchange of labeled hydrogen atoms with hydrogen atoms associated with carboxyl, hydroxyl, and amino acid groups. ECW is measured after oral administration of sodium bromide and measuring the bromide blood concentrations. Bromide space is determined by the area under the curve as compared with known bromide standards. The ICW is calculated as the difference between TBW and ECW. Although these methods are considered the “gold standard” methods to determine TBW, they are expensive, time consuming, and not readily available at the bedside. Urine osmolality is another important indicator of hydration status (18), but does not permit quantification of TBW. An alternative technique to predict TBW in acutely ill patients is bioelectrical impedance analysis (BIA). This simple technique introduces a single-frequency (50 kHz) signal through two outer pairs of electrocardiograph-type electrodes placed on the right wrist and right ankle. Two inner pairs of electrodes detect resistance to the introduced signal as a function of body conductance. The circuits are designed to measure two components of the alternating current signal: resistance and reactance. The resistance reflects extracellular space, and the reactance indicates cellular activity. These currents are conducted almost completely through the fluid compartment of the fat-free mass, which is an equivalent of TBW. The opposition to the flow of current, or resistance, is measured and used to predict TBW (3,4). BIA has been shown to be a simple, rapid, noninvasive, and reproducible technique to measure total body impedance in healthy adults (19–21). It has previously been used in assessing TBW in malnourished patients who underwent abdominal surgery (12,16,22). What is not clear is whether BIA is a useful and an accurate tool to predict TBW in acutely ill patients with nonuniform fluid distribution. In acutely ill patients, the amount of lean muscle mass is often overestimated by anthropometric equations when compared with actual values measured by the segmental compartments of TBW or ECW. Once validated, BIA could be used to determine fluid status, estimate fat and fat-free mass, assess nutritional states, and test hypotheses related to drug doses on the individualized level in acutely ill elderly patients. The goal of the study was to determine the agreement between TBW for commonly used equations for the prediction of TBW against the reference methods, deuterium oxide and sodium bromide dilution, in acutely ill hospitalized elderly persons.