Proximal and distal measurements of extracellular and total body water by multi-frequency bio-impedance analysis in surgical patients.

Measurements of extracellular water (ECW) and total body water (TBW) can be useful when assessing the nutritional status of surgical patients. The simple and non-invasive nature of bio-impedance techniques may allow repeat measurements to assess clinically significant changes in ECW and TBW. We have evaluated bio-impedance spectroscopy (BIS) and multi-frequency bio-impedance analysis (MFBIA) using radioisotope dilution as the independent method.

Comparison of bio-impedance spectroscopy and multi-frequency bio-impedance analysis for the assessment of extracellular and total body water in surgical patients.

1. Measurements of extracellular and total body water provide useful information on the nutritional status of surgical patients and may be estimated from whole-body bio-impedance measurements at different frequencies. 2. Resistance and reactance were measured at 50 frequencies from 5kHz to 1MHz in 29 surgical patients (17 males, 12 females) with a wide range of extracellular to total body water ratios. 3. A fit to the spectrum of reactance versus resistance data gave predicted resistances at frequencies zero and infinity. Values of extracellular and total body water determined by this bio-impedance spectroscopy technique were regressed against values obtained from radioisotope dilution. The standard errors of the estimate were 1.8931 and 3.2591 respectively. 4. Resistance indices (height2/resistance) at selected frequencies gave the highest correlations with extracellular and total body water at 5kHz and 200kHz respectively, and prediction equations derived from multiple stepwise regressions also showed these to be the optimum frequencies. The standard errors of the estimate for this multi-frequency bio-impedance analysis method were 1.9371 and 2.6061 for extracellular and total body water respectively. 5. To assess the ability of the two methods to measure changes in extracellular and total body water, reproducibility was assessed from repeat measurements 10 min apart in a subgroup of 15 patients. Bio-impedance spectroscopy gave mean coefficients of variation for extracellular and total body water of 0.9% and 3.0% respectively. For multi-frequency bio-impedance analysis the corresponding coefficients of variation were 0.9% and 0.6%. 6. It is concluded that a simple impedance analyser operating at only two frequencies compares favourably with the more complex spectroscopy technique for the determination of extracellular and total body water in surgical patients.

Body mass index as an estimate of body fat.

Body mass index (BMI) was compared with percentage body fat (%Fat) measured by dual energy X-ray absorptiometry (DXA) in 233 adolescent schoolgirl volunteers and 179 adult female patients. Repeat measurements were made on 67 of the adolescents and 51 of the adults. The correlations between BMI and %Fat were established from the 300 adolescent measurements and the 230 adult measurements. Although highly significant relationships were found between BMI and %Fat, only 58% of the variance in %Fat in adolescents and 66% in adults could be predicted by BMI. At the 95% confidence levels, a BMI of 20 kg m-2 can correspond to a range of 18-33% body fat in adolescents and 13-32% in adults. Without any change in BMI, an adolescent's percentage fat can change by as much as -3% to +7%. For an individual adult the same BMI can correspond to changes in fat of +/-5%. Since the strength of prediction of percentage body fat from BMI is poor, caution should be exercised in its use for eating disorders research.

Improved prediction of bone mineral content and density.

To establish improved predictive values for normal bone mineral content (BMC) and density (BMD) in adolescent girls a community based study of schoolgirl volunteers was carried out by dual energy x ray absorptimetry. Measurements were performed on 216 subjects aged 11.0 to 17.9 years; measurements were repeated one year later on 84 of the girls, providing a total of 300 studies. For total body BMC the standard error of the prediction was improved from 290.9 g to 134.1 g when weight, height, and shoulder width were added to the normal variable of age. For spine BMD the standard error of the prediction was improved from 0.105 g/cm2 to 0.066 g/cm2 when height, weight, and shoulder width were added to the normal variable of age. Significant improvements were also obtained for total BMD and spine BMC. Despite the normal practice of predicting bone density from age alone this was not selected as the first variable in a multiple stepwise regression for either total body or spine. The prediction of BMC and BMD can be significantly improved by the inclusion of simple body habitus parameters in the prediction equations. As the mean (SE) z score derived from the manufacturer's normal data was - 0.36 (0.02) these American data are probably not appropriate for use in adolescents in the UK.

Evaluation of multi-frequency bio-impedance analysis for the assessment of extracellular and total body water in surgical patients.

1. Multi-frequency bio-impedance analysis has been used to estimate extracellular and total body water in a heterogeneous group of 43 surgical patients (23 males, 20 females). 2. Radioisotope-dilution methods were used for the measurement of extracellular and total body water. 3. Resistance and reactance were measured between wrist and ankle at frequencies from 5 kHz to 1 MHz. 4. Extracellular and total body water were estimated by multiple stepwise regression using the radioisotope values as the dependent variables. The parameters included in the regression were: resistance and reactance at each frequency, body habitus parameters, plasma albumin and plasma sodium. 5. The standard errors of the estimates between the measured and estimated values were 1.73 litres (coefficient of variation 9.6%) and 2.17 litres (coefficient of variation 6.0%) for extracellular and total body water, respectively. 6. These errors represent a useful improvement relative to those obtained from anthropometric estimates. However, the improvements relative to the use of a single frequency (50 kHz) are not clinically significant.

Can bioelectrical impedance improve the prediction of body fat in patients with eating disorders?

Prediction equations for percentage body fat have been derived for a group of 93 female patients being treated for eating disorders, using fat measured by dual-energy X-ray absorptiometry (DXA) as the reference method. Separate bioelectrical impedance analysis (BIA) prediction equations were derived for patients with a body mass index above and below 16 kg/m2. This provided an improvement compared with the equations supplied by the manufacturer. The standard error of the estimate (SEE) for the entire group was 3.2% (r = 0.922). However, an alternative prediction equation based on anthropometry alone gave an SEE of 4.2% (r = 0.861). The ability of BIA and anthropometry alone to measure changes in percentage fat was assessed in a separate group of 24 females during treatment for eating disorders. The SEEs using the BIA and anthropometry prediction equations were 2.6% (r = 0.931) and 2.7% (r = 0.901) respectively. We conclude that in patients with eating disorders BIA does not provide any significant improvement over simple anthropometry prediction equations for the assessment of percentage fat or its changes. The apparently reasonable agreement between fat predicted by BIA and that measured by DXA is largely due to the inclusion of body habitus parameters in the BIA prediction equations.

Parametric imaging of cerebral vascular reserves. 1. Theory, validation and normal values.

We describe the theoretical basis and practical details of a method for creating parametric images of the ratio of regional cerebral blood flow (rCBF) to volume (rCBV) from planar gamma-camera images following intravenous injection of any intravascular tracer and after correction for dispersion of the bolus before it arrives at the head. This ratio, which is the most sensitive index of the extent to which perfusion is being maintained by vasodilatation in patients with cerebro-vascular disease, indicates whether the limits of compensation have been, or are in danger of being, exceeded and identifies any ischaemic penumbra. Evidence is provided of the validity of the correction and the accuracy of calculation of the ratio. The range of normal values established is not significantly different from that previously reported using positron emission tomography. This technique has proved sufficiently sensitive to be clinically useful in patients with abnormalities of cerebral perfusion.

Distribution of pemphigus and pemphigoid antigens, laminin, and type IV collagen in corneal epithelium.

Dog corneas were studied by indirect immunofluorescence with antibodies specific for pemphigus and pemphigoid antigens, laminin, and type IV collagen. The distribution of these antigens was similar to that observed in other squamous epithelia and skin. This study provides further evidence for close immunochemical similarities between these tissues.