Illinois State University Bioelectrical Impedance Analysis
(BIA) Chapter 6
Slide 2
Illinois State University BIA n In the last decade, the use of
bioelectric impedance and conductivity methods for prediction of
body composition has grown rapidly.
Slide 3
Illinois State University BIA n BIA is a rapid, noninvasive,
and relatively inexpensive method for evaluating body composition
in field and clinical settings.
Slide 4
Illinois State University BIA n BIA is now regarded as either a
substitute or supplement to conventional anthropometry in field
studies.
Slide 5
Illinois State University BIA n 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.
Slide 6
Illinois State University BIA n The individuals TBW can be
estimated from the impedance measurements because the electrolytes
in the bodys water are excellent conductors of electrical
current.
Slide 7
Illinois State University BIA n When the volume of TBW is
large, the current flows more easily through the body with less
resistance.
Slide 8
Illinois State University BIA n 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.
Slide 9
Illinois State University BIA n Because the water content of
the FFB is relatively large (~73% water), FFM can be predicted from
TBW estimates.
Slide 10
Illinois State University BIA n Individuals with large FFM and
TBW have less resistance to current flowing through their bodies
than do those with a smaller FFM.
Slide 11
Illinois State University Assumptions n 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.
Slide 12
Illinois State University Assumptions n 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.
Slide 13
Illinois State University Assumptions n 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.
Slide 14
Illinois State University Assumptions n The volume of these
tissues can be deduced from measurement of their combined
resistance.
Slide 15
Illinois State University 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.
Slide 16
Illinois State University 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).
Slide 17
Illinois State University 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.
Slide 18
Illinois State University 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.
Slide 19
Illinois State University 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.
Slide 20
Illinois State University BIA Thus, S 2 /R or S 2 /Z is the
basic variable used in BIA equations for predicting total body
water or fat-free mass.
Slide 21
Illinois State University BIA Several limitations to this
assumption are immediately apparent:
Slide 22
Illinois State University 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.
Slide 23
Illinois State University 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.
Slide 24
Illinois State University 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.
Slide 25
Illinois State University BIA Differences in the structure as
well as the relative proportions of the trunk versus the limbs also
affect the conduction of the current.
Slide 26
Illinois State University BIA The assumption that whole-body
resistance is linearly related to the conductive volume and its
electrolyte concentration may not be strictly true.
Slide 27
Illinois State University BIA Some have reported that BIA
predicts fat- free mass less well at the extremes of body
fatness.
Slide 28
Illinois State University 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.
Slide 29
Illinois State University 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.
Slide 30
Illinois State University BIA The changes in impedance due to
changes in fluid or hydration status are more complex than they
appear using the single frequency approach.
Slide 31
Illinois State University Applicability The BIA method of
estimating body composition is best suited to epidemiological
studies.
Slide 32
Illinois State University Applicability It can improve
population estimates of obesity and can be used to supplement other
field methods in assessing levels of protein-energy
malnutrition.
Slide 33
Illinois State University Applicability BIA can be used also in
clinical settings to quantify body composition.
Slide 34
Illinois State University Accuracy The accuracy of the
estimated variables is complicated by factors that may produce
shifts in body fluids or electrolytes.
Slide 35
Illinois State University Accuracy The ability of BIA to detect
small changes in body composition has practical limitations as
well.
Slide 36
Illinois State University BIA The BIA method is applicable
technically to all subjects regardless of age, sex, ethnicity, or
health status.
Slide 37
Illinois State University BIA In the tetrapolar technique, the
paired source and receiving electrodes must be separated by at
least 5 cm to avoid interaction.
Slide 38
Illinois State University BIA In adults, another possible
physical limitation to accurate impedance measurements could be
extreme obesity.
Slide 39
Illinois State University BIA The main limitation to the
general applicability of BIA is the availability of appropriately
calibrated, cross-validated predictive equations.
Slide 40
Illinois State University 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.
Slide 41
Illinois State University BIA Factors that affect the
distribution of fluids and electrolyte concentrations between
intra- and extracellular compartments can be expected to affect
resistance.
Slide 42
Illinois State University 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.
Slide 43
Illinois State University BIA Pregnancy and menstruation may
affect fluid balance and the accuracy of BIA predictions of body
composition also.
Slide 44
Illinois State University 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.
Slide 45
Illinois State University 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).
Slide 46
Illinois State University 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.
Slide 47
Illinois State University BIA Two very different approaches
have been used most frequently: two-electrode and four-electrode
techniques.
Slide 48
Illinois State University BIA Each approach has specific
advantages and disadvantages.
Slide 49
Illinois State University BIA In the two-electrode bridge
technique, the electrodes that sense the voltage drop are the same
as those that introduce the current.
Slide 50
Illinois State University 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.
Slide 51
Illinois State University 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.
Slide 52
Illinois State University 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.
Slide 53
Illinois State University BIA The four-electrode technique
overcomes the main disadvantages of the two- electrode
approach.
Slide 54
Illinois State University 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.
Slide 55
Illinois State University BIA Because the electrodes that
inject the current are separate from those that detect the
potential, impedance due to electrode polarization can be
eliminated.
Slide 56
Illinois State University 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.
Slide 57
Illinois State University BIA The high cutaneous impedance is
overcome by using current with higher amplitudes.
Slide 58
Illinois State University 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.
Slide 59
Illinois State University Measurement Procedures: It is very
important to adhere to standardized measurement procedures when
using BIA.
Slide 60
Illinois State University Measurement Procedures: BIA is
measured in a standard fashion while the subject lies supine on a
flat, nonconductive bed, cot, or couch.
Slide 61
Illinois State University Measurement Procedures: The arms are
abducted slightly so that they do not touch the sides of the
trunk.
Slide 62
Illinois State University Measurement Procedures: The legs are
separated so that the ankles are at least 20 cm apart and, ideally,
the thighs do not touch.
Slide 63
Illinois State University Measurement Procedures: Complete
separation of the thighs may be difficult to attain in extremely
obese subjects.
Slide 64
Illinois State University Measurement Procedures: The head
should be flat against the bed or elevated minimally with a thin
pillow.
Slide 65
Illinois State University 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.
Slide 66
Illinois State University Measurement Procedures: The subject
may wear clothing, with the exception of shoes and socks, and must
not wear metallic jewelry.
Slide 67
Illinois State University 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.
Slide 68
Illinois State University 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.
Slide 69
Illinois State University 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.
Slide 70
Illinois State University 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-phalangeal joints, respectively.
Slide 71
Illinois State University Measurement Procedures: The skin
should be lightly washed with alcohol before attaching the
electrodes.
Slide 72
Illinois State University 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.
Slide 73
Illinois State University 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.
Slide 74
Illinois State University 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.
Slide 75
Illinois State University Measurement Procedures: Although few
studies have demonstrated significant effects, it is recommended
that the subject void the bladder completely immediately prior to
measurement.
Slide 76
Illinois State University 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.
Slide 77
Illinois State University BIA BIA accuracy for prediction of
percent BF is 3.5-5%
Slide 78
Illinois State University 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.
