Literatura académica sobre el tema "Body impedance"

The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Title from PDF of title page (University of Missouri--Columbia, viewed on September 16, 2009). Thesis advisor: Dr. Steve Ball. Includes bibliographical references.

Whole-body bioelectrical impedance analysis (BIA) was evaluated for its reliability and accuracy in estimating body composition in children and youth. The established electrical principle for estimating volume in a geometrical system from conductor-length('2) divided by impedance is the basis for the use of this method in humans. The hypothesis that body-height('2) divided by the resistance component of impedance (resistance index) can be used to estimate total body water (TBW), fat free body (FFB), and percent fat (%FAT) was tested. Validation studies in adults indicate BIA is a reliable and fairly accurate method of estimating TBW, FFB, and %FAT but no testing has been completed on children. The subjects were 103 nonobese and obese anglo males and females from 10 to 14 years old. Within-day reliability of resistance and reactance was assessed by analysis of variance with built-in comparisons. Between-day reliability for all measurements, made four to five weeks apart, was evaluated by test-retest correlation coefficients and paired t-tests. The criterion variables were FFB and %FAT estimated using equations developed for children and youth based on: (1) skinfolds, (2) body density, (3) TBW, (4) density and TBW, (5) density, TBW, and bone mineral content. Regression and multiple regression analyses were used to select the most accurate method of measuring FFB and %FAT and to determine the relationship among criterion variables and the following independent variables: resistance index alone and combined with sex, fatness category, sex x fatness, age, sexual maturation status, weight, anthropometric variables, and reactance. From this study the following conclusions were made: (1) BIA measurements were reliable, (2) resistance index had a linear relationship with FFB estimated from several criterion variables, (3) weight, sex, fatness category, sex x fatness, age, and sexual maturation status were significant variables for predicting criterion variables used in combination with resistance index but were not significant when anthropometric variables were included in the analysis, (4) prediction accuracy for FFB and %FAT from resistance index was fair (SEE 2.58 kg and 4.21%) and from resistance index plus anthropometric variables and reactance was good (SEE 1.88 kg and 3.26%) and similar to that from the best anthropometric variables alone (SEE 2.11 kg and 3.19%).

Bioelectrical impedance analysis, (BIA), is a method of body composition analysis first investigated in 1962 which has recently received much attention by a number of research groups. The reasons for this recent interest are its advantages, (viz: inexpensive, non-invasive and portable) and also the increasing interest in the diagnostic value of body composition analysis. The concept utilised by BIA to predict body water volumes is the proportional relationship for a simple cylindrical conductor, (volume oc length2/resistance), which allows the volume to be predicted from the measured resistance and length. Most of the research to date has measured the body's resistance to the passage of a 50· kHz AC current to predict total body water, (TBW). Several research groups have investigated the application of AC currents at lower frequencies, (eg 5 kHz), to predict extracellular water, (ECW). However all research to date using BIA to predict body water volumes has used the impedance measured at a discrete frequency or frequencies. This thesis investigates the variation of impedance and phase of biological systems over a range of frequencies and describes the development of a swept frequency bioimpedance meter which measures impedance and phase at 496 frequencies ranging from 4 kHz to 1 MHz. The impedance of any biological system varies with the frequency of the applied current. The graph of reactance vs resistance yields a circular arc with the resistance decreasing with increasing frequency and reactance increasing from zero to a maximum then decreasing to zero. Computer programs were written to analyse the measured impedance spectrum and determine the impedance, Zc, at the characteristic frequency, (the frequency at which the reactance is a maximum). The fitted locus of the measured data was extrapolated to determine the resistance, Ro, at zero frequency; a value that cannot be measured directly using surface electrodes. The explanation of the theoretical basis for selecting these impedance values (Zc and Ro), to predict TBW and ECW is presented. Studies were conducted on a group of normal healthy animals, (n=42), in which TBW and ECW were determined by the gold standard of isotope dilution. The prediction quotients L2/Zc and L2/Ro, (L=length), yielded standard errors of 4.2% and 3.2% respectively, and were found to be significantly better than previously reported, empirically determined prediction quotients derived from measurements at a single frequency. The prediction equations established in this group of normal healthy animals were applied to a group of animals with abnormally low fluid levels, (n=20), and also to a group with an abnormal balance of extra-cellular to intracellular fluids, (n=20). In both cases the equations using L2/Zc and L2/Ro accurately and precisely predicted TBW and ECW. This demonstrated that the technique developed using multiple frequency bioelectrical impedance analysis, (MFBIA), can accurately predict both TBW and ECW in both normal and abnormal animals, (with standard errors of the estimate of 6% and 3% for TBW and ECW respectively). Isotope dilution techniques were used to determine TBW and ECW in a group of 60 healthy human subjects, (male. and female, aged between 18 and 45). Whole body impedance measurements were recorded on each subject using the MFBIA technique and the correlations between body water volumes, (TBW and ECW), and heighe/impedance, (for all measured frequencies), were compared. The prediction quotients H2/Zc and H2/Ro, (H=height), again yielded the highest correlation with TBW and ECW respectively with corresponding standard errors of 5.2% and 10%. The values of the correlation coefficients obtained in this study were very similar to those recently reported by others. It was also observed that in healthy human subjects the impedance measured at virtually any frequency yielded correlations not significantly different from those obtained from the MFBIA quotients. This phenomenon has been reported by other research groups and emphasises the need to validate the technique by investigating its application in one or more groups with abnormalities in fluid levels. The clinical application of MFBIA was trialled and its capability of detecting lymphoedema, (an excess of extracellular fluid), was investigated. The MFBIA technique was demonstrated to be significantly more sensitive, (P<.05), in detecting lymphoedema than the current technique of circumferential measurements. MFBIA was also shown to provide valuable information describing the changes in the quantity of muscle mass of the patient during the course of the treatment. The determination of body composition, (viz TBW and ECW), by MFBIA has been shown to be a significant improvement on previous bioelectrical impedance techniques. The merit of the MFBIA technique is evidenced in its accurate, precise and valid application in animal groups with a wide variation in body fluid volumes and balances. The multiple frequency bioelectrical impedance analysis technique developed in this study provides accurate and precise estimates of body composition, (viz TBW and ECW), regardless of the individual's state of health.