Bioelectrical Impedance Analysis (BIA)

1. Bioelectrical Impedance Analysis (BIA) Chapter 6

2. BIA • In the last decade, the use of bioelectric impedance and conductivity methods for prediction of body composition has grown rapidly.

3. BIA • BIA is a rapid, noninvasive, and relatively inexpensive method for evaluating body composition in field and clinical settings.

4. BIA • BIA is now regarded as either a substitute or supplement to conventional anthropometry in field studies.

5. BIA • With this method, low-level electrical current is passed through the client’s body, and the impedance (Z), or opposition to the flow of the current, is measured with a BIA analyzer.

6. BIA • The individual’s TBW can be estimated from the impedance measurements because the electrolytes in the body’s water are excellent conductors of electrical current.

7. BIA • When the volume of TBW is large, the current flows more easily through the body with less resistance.

8. BIA • The resistance to current flow is greater in individuals with large amounts of body fat, given that adipose tissue is a poor conductor of electrical current due to its relatively small water content.

9. BIA • Because the water content of the FFB is relatively large (~73% water), FFM can be predicted from TBW estimates.

10. BIA • Individuals with large FFM and TBW have less resistance to current flowing through their bodies than do those with a smaller FFM.

11. Assumptions • The use of BIA to estimate body composition is based on the different conductive and dielectric properties of various biological tissues at various frequencies of current.

12. Assumptions • Tissues that contain a lot of water and electrolytes such as cerebrospinal fluid, blood, or muscle are highly conductive whereas fat, bone, and air-filled spaces such as lung are highly resistive or dielectric tissues.

13. Assumptions • An applied electric current always follows the path of least resistance, and in the human body this will include extracellular fluid, blood, muscle, and other conductive tissues that comprise the majority of fat-free mass.

14. Assumptions • The volume of these tissues can be deduced from measurement of their combined resistance.

15. Assumptions Assumptions are made that do not apply perfectly to the human body as a conductor, and it is important to understand these limitations when using BIA to estimate body composition.

16. BIA Impedance (Z) is the frequency-dependent opposition of a conductor to the flow of alternating electric current and is composed of two components, resistance (R) and reactance (Xc).

17. BIA Resistance is the pure opposition of the conductor to the flow of the current. Reactance is the storage of an electrical charge by a condenser for a brief moment in time.

18. BIA Electrical conduction in biological tissues is mainly ionic: that is, electric charges are transferred between ionized salts, bases, and acids dissolved in the body fluids.

19. BIA The conventional approach is to measure “whole body” resistance or impedance between the wrist and the ipsilateral ankle and to use stature (S) as an index of the length of the conductor.

20. BIA Thus, S2/R or S2/Z is the basic variable used in BIA equations for predicting total body water or fat-free mass.

21. BIA Several limitations to this assumption are immediately apparent:

22. BIA Geometrically, the body is not a cylinder with uniform cross-sectional area, but is better represented as five cylinders (two arms, two legs, and a trunk) connected in series that have large differences in their cross-sectional areas.

23. BIA When such a set of conductors is connected in series, the conductor with the smallest cross-sectional area (i.e., the arm) will determine most of the resistance of the series.

24. BIA • Thus, whereas an arm is about 4% and a leg about 17% of body weight, they account for about 47% and 50%, respectively, of whole-body resistance: conversely, the trunk comprises about 46% of body weight but may have little if any influence on whole body resistance when measured conventionally from the right ankle to the right wrist.

25. BIA Differences in the structure as well as the relative proportions of the trunk versus the limbs also affect the conduction of the current.

26. BIA The assumption that whole-body resistance is linearly related to the conductive volume and its electrolyte concentration may not be strictly true.

27. BIA Some have reported that BIA predicts fat-free mass less well at the extremes of body fatness.

28. BIA The prediction of total body water or fat-free mass using the conventional “whole body” BIA approach is dependent to a large extent on their strong associations with the mass and bioelectric characteristics of the appendicular skeletal muscle.

29. BIA Individuals who deviate markedly from the norm for the size of the trunk in proportion to the limbs are more likely to have erroneous estimates.

30. BIA The changes in impedance due to changes in fluid or hydration status are more complex than they appear using the single frequency approach.

31. Applicability The BIA method of estimating body composition is best suited to epidemiological studies.

32. Applicability It can improve population estimates of obesity and can be used to supplement other field methods in assessing levels of protein-energy malnutrition.

33. Applicability BIA can be used also in clinical settings to quantify body composition.

34. Accuracy The accuracy of the estimated variables is complicated by factors that may produce shifts in body fluids or electrolytes.

35. Accuracy The ability of BIA to detect small changes in body composition has practical limitations as well.

36. BIA The BIA method is applicable technically to all subjects regardless of age, sex, ethnicity, or health status.

37. BIA In the tetrapolar technique, the paired source and receiving electrodes must be separated by at least 5 cm to avoid interaction.

38. BIA In adults, another possible physical limitation to accurate impedance measurements could be extreme obesity.

39. BIA The main limitation to the general applicability of BIA is the availability of appropriately calibrated, cross-validated predictive equations.

40. BIA It is most important to make a careful selection of equations that were developed from a sample that is similar in age, sex, ethnicity, and health status to the subjects under study.

41. BIA Factors that affect the distribution of fluids and electrolyte concentrations between intra- and extracellular compartments can be expected to affect resistance.

42. BIA Factors that have acute, temporary effects on fluid and electrolyte equilibrium in healthy subjects, such as exercise, need to be controlled or significant errors may result.

43. BIA Pregnancy and menstruation may affect fluid balance and the accuracy of BIA predictions of body composition also.

44. BIA The prediction of fat-free mass using BIA is subject to additional complications that may limit applicability over and above those noted so far.

45. BIA The most important is considered to be variation in the concentration of water within the FFM, or the ratio of the total body water to FFM (TBW/FFM).

46. Equipment: All BIA devices consist essentially of: 1. an alternating electrical current source. 2. cables and electrodes for introducing the current into the body and for sensing the voltage drop due to impedance. 3. a system for measuring impedance.

47. BIA Two very different approaches have been used most frequently: two-electrode and four-electrode techniques.

48. BIA Each approach has specific advantages and disadvantages.

49. BIA In the two-electrode bridge technique, the electrodes that sense the voltage drop are the same as those that introduce the current.

50. BIA The main advantages of this approach are that highly accurate measures can be obtained with a very low amplitude current and that electromagnetic leakage toward nearby metallic objects is minimal.