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.

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BIA

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

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BIA

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

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BIA

• The individuals TBW can be estimated from the
  impedance measurements because the electrolytes
  in the bodys water are excellent conductors of
  electrical current.

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BIA

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

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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.

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BIA

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

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BIA

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

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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.

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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.

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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.

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Assumptions

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

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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.

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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).

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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.

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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.

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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.

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BIA

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

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BIA

• Several limitations to this assumption are
  immediately apparent

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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.

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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.

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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.

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BIA

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

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BIA

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

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BIA

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

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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.

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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.

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Applicability

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

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Applicability

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

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Applicability

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

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Accuracy

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

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Accuracy

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

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BIA

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

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BIA

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

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BIA

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

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BIA

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

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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.

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BIA

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

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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.

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BIA

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

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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.

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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).

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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.

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BIA

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

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BIA

• Each approach has specific advantages and
  disadvantages.

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BIA

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

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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.

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BIA

• There are two major disadvantages to this method
• 1. the impedance measured reflects both the
  impedance of the body as well as that due to
  electrode polarization, which may be high at low
  frequency.

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BIA

• 2. Needle electrodes must be used to avoid the
  high impedance of the skin. These must be
  inserted subcutaneously in a standardized fashion
  and may result in minor pain and local tissue
  trauma that reduce both the acceptability and the
  accuracy of the impedance measurements.

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BIA

• The four-electrode technique overcomes the main
  disadvantages of the two-electrode approach.

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BIA

• In this method, the current is applied with one
  pair of electrodes located distally while a
  second pair located proximally measures the
  electrical potential across a segment of the
  conductor.

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BIA

• Because the electrodes that inject the current
  are separate from those that detect the
  potential, impedance due to electrode
  polarization can be eliminated.

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BIA

• The use of spot or band electrodes that are
  attached to the surface of the skin, rather than
  penetrating it, avoids problems associated with
  pain and tissue trauma.

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BIA

• The high cutaneous impedance is overcome by using
  current with higher amplitudes.

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BIA

• The two main disadvantages are considered to be
  the control of stray capacitance produced by the
  paired wires and electromagnetic leakage to
  external metallic objects when very high
  frequencies of current are used.

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Measurement Procedures

• It is very important to adhere to standardized
  measurement procedures when using BIA.

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Measurement Procedures

• BIA is measured in a standard fashion while the
  subject lies supine on a flat, nonconductive bed,
  cot, or couch.

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Measurement Procedures

• The arms are abducted slightly so that they do
  not touch the sides of the trunk.

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Measurement Procedures

• The legs are separated so that the ankles are at
  least 20 cm apart and, ideally, the thighs do not
  touch.

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Measurement Procedures

• Complete separation of the thighs may be
  difficult to attain in extremely obese subjects.

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Measurement Procedures

• The head should be flat against the bed or
  elevated minimally with a thin pillow.

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Measurement Procedures

• It is important to adhere to this standard body
  positioning, which has been used in virtually all
  calibration studies, because deviations produce
  large differences in the measured impedance.

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Measurement Procedures

• The subject may wear clothing, with the exception
  of shoes and socks, and must not wear metallic
  jewelry.

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Measurement Procedures

• Generally, care should be taken to ensure that
  the subject and the impedance analyzer are
  isolated from all metallic objects and other
  electronic devices by a distance of at least 50
  cm.

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Measurement Procedures

• For whole-body measurements of adults using the
  four-electrode technique, the electrodes are
  attached in the standard fashion to the dorsal
  surface of the hand and anterior surface of the
  ipsilateral foot.

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Measurement Procedures

• One voltage-sensing electrode is attached to the
  wrist midway between the styloid processes the
  other is attached to the ankle midway between the
  malleoli.

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Measurement Procedures

• The electrodes for introducing the current
  (source electrodes) are attached to the foot and
  hand at least 5 cm distal to the sensing
  electrodes generally on the third
  metatarso-phalangeal and third metacarpo-phalangea
  l joints, respectively.

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Measurement Procedures

• The skin should be lightly washed with alcohol
  before attaching the electrodes.

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Measurement Procedures

• The side of the body to which the electrodes are
  attached does not usually have a significant
  effect on impedance measurements in healthy
  subjects.

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Measurement Procedures

• Impedance measurements should be taken after a
  minimum 2 hour fast and at least 8 to 12 hours
  after any strenuous exercise, alcohol, or other
  factors that may alter hydration.

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Measurement Procedures

• It is recommended that the stage of the menstrual
  cycle be recorded in longitudinal studies of
  premenopausal females, although some
  investigators have not found effects of
  menstruation impedance measurements.

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Measurement Procedures

• Although few studies have demonstrated
  significant effects, it is recommended that the
  subject void the bladder completely immediately
  prior to measurement.

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Measurement Procedures

• To minimize changes in impedance due to
  gravity-induced fluid shifts in healthy subjects,
  it is recommended that impedance measurements be
  taken within 5 to 10 minutes after lying down.

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BIA

• BIA accuracy for prediction of percent BF is
  3.5-5

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BIA

• These figures should be regarded as ideal or
  minimal. Actual errors are likely to be greater
  but will be unknown unless a cross-validation
  study is performed.