Bioimpedance: do we actually have to use gel?

Resumen

To the Editor: Medical techniques and expertise have evolved during the last decades. Nevertheless, various methods are still tiresome and time consuming. Our team specialises in obesity measurements, being one of the largest centres in Obesity in London, UK, and forming part of a worldwide plan which aims at diagnosing obese patients and thus help in preventing all co-morbidities that result from obesity and morbid obesity. We hereby would like to provide some preliminary results of a pilot study we performed in order to simplify obesity measurements.

Body composition measurements are useful in obesity to distinguish adiposity from fluid retention and/or muscularity (1). Various bioimpedance techniques have been described and used to evaluate body composition. Early bioimpedance methods used invasive type of electrodes, but have been replaced by plate electrodes. Gels were used to optimize electrical contact in much of the early literature validating bioimpedance methodology. Modern bioimpedance analysis (BIA) systems use simple touch-pad electrodes without the need for invasive techniques (2, 3). Although it is not considered the gold-standard method for determining body composition, bioimpedance generally compares well with techniques such as DEXA, isotope dilution and underwater weighing. It is cheaper and more readily applicable than such methods. For example Mally et al (4) reported their experience using a Tanita BC-418-MA compared to DEXA and found that the device performs well but they suggest some improvements. One issue not addressed by Mally et al was whether gel can enhance the electrical connection and remedy the deficiencies they identify. Valid BIA requires a good contact between the subject’s bare foot and the touch-pad electrode. Our Tanita machine’s manual suggests moistening the feet if the impedance measurement is problematic. Historically hydroscopic gel has been used to optimise electrical contact with skin. This recommendation may be based upon the finding of Nunez et al (5) who compared a BIA method with and without the use of gel. Those authors found small but significant differences in parameters when gel was used. If gel improves the electrical connectivity, that would be a powerful reason to use it routinely. Use of gel is however time consuming, unpopular with patients and renders the foot plate of the device slippery (which can be hazardous in the sort of morbidly obese or disabled patient we see). We therefore followed the approach of Nunez using an MC-180, a modern machine with performance comparable to that used by Mally et al. Our objective was to compare BIA estimates of body composition obtained with and without the use of gel.