2 B. Body Fluid Compartments and Investigational ... - Data-input.de

More documents

Recommendations

Info

a. Anthropometry The thickness of skin folds of defined parts of the body is measured with the help of a callipers (mostly on the biceps and triceps, as well as sub-scapular and supra-iliac). Conclusions can be made about body fat from the thickness of the skin folds. b. Infra-red interactance This method uses harmless infra-red light in order to measure the percentage of total body fat on the biceps of the dominant arm with the help of a near infra-red probe. Fig. B.3 Infra-red interactance measurement (Futrex) c. Non-phase sensitive BIA measurement This method measures the total resistance of the body (impedance Z). By calculating body water, the compartments for body fat and lean body mass can be obtained. This does not give a determination of the phase angle and, as such, a subdivision of impedance into water and cellular resistance, so that no judgement can be made about the body cell mass BCM or the extra-cellular mass ECM with the nonphase sensitive BIA. The compartments measured using the last three methods are known as body fat (BF) and as lean body mass (LBM), in comparison to the fat-free mass (FFM) which is obtained in the case of densitometry. The difference is that densitometry measures pure triglycerides, whereas body fat also contains 15-18% water and approx. 2% protein. 3. Three-compartment measurement The 3-compartment model arises by subdividing the lean body mass into the body cell mass BCM and the extra-cellular mass ECM. The BCM is the sum of all aerobic oxidizing, metabolically active cells, in other words the muscles, internal organs and the CNS. The interstitial and trans-cellular spaces are part of the extra-cellular mass ECM, as well as the skeleton and connective tissue. Large displacements between the BCM and ECM can arise without changes to the bodyweight and lean body mass. Measurements in the 3-compartment model are possible with the following simple methods: a. Phase sensitive BIA measurement The phase sensitive technique of measuring allows the impedance Z to be differentiated into its two components resistance R (water resistance) and reactance Xc (cell resistance), so that a differentiation can be made between body cell mass and extracellular mass. Fig. B.4 Phase sensitive mono-frequency device COMPARTMENTS 3
4 b. Phase sensitive multi-frequency measurement A further step towards making a more exact diagnosis of body composition and nutritional status can be achieved using phase sensitive multi-frequency analysis. By using an additional determination of resistance at low frequency (1 or 5 kHz), it is possible to achieve a subdivision of the total body water TBW into intra-cellular and extra-cellular water. Deviations in mass of BCM and ECM can thus be estimated differentially with regard to anabolic/catabolic processes or shifts in water. Phase sensitive multifrequency analysis is indicated in particular in patients with disturbances in water balance, e.g. in heart and kidney failure, oedema, patients undergoing treatment with parenteral nutritional therapy and intensive care patients. Fig. B.5 Phase sensitive multi-frequency device 4. Other investigational methods for determining body composition a. Total body potassium Fat is potassium-free and bones contain only traces of potassium. For this reason, practically the entire content of potassium in the body (98%) is to be found in the cells of the BCM. The natural radioactive isotope K40 is present wherever potassium is found at a level of 0.012%. By measuring the isotope K40 it is possible to draw conclusions about total body potassium and thus the BCM. The measurement of K40 is done in a whole body counting chamber. b. DEXA (Dual Energy X-ray Absorptiometry) DEXA is carried out using a special x-ray apparatus at a very low radiation dosage. Two beams of photon Fig. B.7 DEXA apparatus (Hologic Inc., USA) radiation with different energy levels are used, which gives the method its name. DEXA can be used to differentiate between bone mass, fat mass and fat-free mass. c. Dilution methods The dilution techniques are used mostly to determine total body water TBW. Deuterated water is used as a non-radioactive tracer; it can be given orally or parenterally. About 2 hours after the dose, the concentration of the tracer is determined in the urine and blood using gas chromatography or mass spectrometry, allowing a conclusion to be drawn about the total body water. The extra-cellular fluid can be determined selectively using bromide or sulphate. d. IVNAA (In-vivo Neutron Activation Analysis) This is probably the most expensive of all methods for determining the body composition; but it also provides the most differentiated assessment. A beam of neutrons aimed at the body induces the emission of gamma radiation with a characteristic spectrum. In this way, single elements can be determined, such as total body potassium or calcium. The corresponding compartment can be calculated from the proportion of these elements. Further imaging processes for determining the composition of the body are mentioned below: Nuclear magnetic resonance technique (NMR) Computer tomography (CT) Magnet resonance tomography (MRT) Photon absorption measurement Fig. B.6 Potassium chamber of the University of Texas
Page 1: 2 B. Body Fluid Compartments and In

2 B. Body Fluid Compartments and Investigational ... - Data-input.de

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?