Use of Bioelectrical Impedance: General Principles and Overview
3.4.5 Whole-Body Versus Segmental Measurements
3.4.5.3 Segmental Electrode Arrangement
A more sophisticated approach to eliminating the disparity between body segments is to model the human body as fi ve cylinders, i.e. two arms, two legs and a trunk with varying cross-sectional areas.
Total conductive volume ( V tot ) can then be expressed as
= + +
2 2 2
leg 3
arm trunk
tot arm leg trunk
arm leg trunk
2 l 2 l l [m ].
V r R r R r R (3.18)
where r arm , r leg , r trunk , l arm , l leg , l trunk , R arm , R leg and R trunk refer to resistivity ( W ·m), length (m) and resistance ( W ) of arm, leg and trunk segments, respectively. Certainly, further refi nement of this approach is possible. For instance, the limbs could be further differentiated in their lower and upper
71 3 Use of Bioelectrical Impedance: General Principles and Overview
parts, respectively. Some of such developments are therefore further discussed in subsection 4.5.3.5.
There are several ways to relate segmental data to total conductive volume. Basically, resistivity of the three sections can be assumed to be either constant or different. Hence, in structural bioimped- ance modeling some models either rely on a single resistivity for all body segments or assume resis- tivity to be segment-specifi c. Similarly, in empirically derived segmental bioimpedance models the sum of the (weighed) resistance indices can be regressed against a dependent variable that is used as a reference. Alternatively, multiple regression analysis can also be used to predict a dependent vari- able from the three respective body segments.
The Role of Uniform Current Distribution for Segmental Measurements
The potential of segmental measurements was fi rst mentioned by Settle and his coworkers in 1980 and it was almost a decade later that Chumlea et al. ( 1988 ) fi rst published a segmental approach for determining fat mass and fat-free mass. For this purpose, current source and voltage-sensing elec- trodes were placed at the shoulder and wrist, ankle and hip, and hip and sterna notch for determining R arm , R leg and R trunk , respectively. Theoretically, the sum of ipsilateral R arm , R leg and R trunk should be equal to R wb measured between the ankle and wrist. Data by Chumlea et al. ( 1988 ) however exceeded R wb by roughly 16%. Such a discrepancy was too large to be purely attributed to experimental error.
Organ et al. ( 1994 ) showed that this phenomenon could be explained by the fact that data from dis- similar measurement confi gurations were compared. They further explained that the sum of R arm , R leg and R trunk would only be equal to R wb when the same current source and magnitude are achieved. The placement of the current source electrodes next to the voltage-sensing electrodes caused a larger potential difference of each segment, respectively. Similarly, as already suggested for the proximal electrode placement, current injection should therefore be maintained via the ankle and wrist.
Nonetheless, such segmental measurements still suffer from two major drawbacks. Firstly, subject comfort is reduced since at least partial undressing is necessary for correct electrode placement.
More importantly, however, reliability is put at risk since at least for less experienced personnel the correct location of specifi c landmarks at the shoulder, hip and chest is likely to be more prone to errors than the positioning of electrodes at the wrist and ankle.
Effective and Reliable Electrode Placement for Segmental Measurements
In order to avoid diffi culties underlying segmental measurements requiring electrode placements at the limbs and trunk, fi rst Patterson et al. ( 1988 ) and Patterson ( 1989 ) , and later Organ et al. ( 1994 ) , as well as Cornish et al. ( 1999, 2000 ) presented a segmental measurement technique which only requires the application of two additional electrodes at the equivalent positions of the hand and foot at the contralateral side of the body. This technique will now be explained in detail.
Assuming that the current between the ipsilateral wrist and ankle fl ows in a longitudinal direction, the equipotential lines should be perpendicular to the current distribution lines in narrow sections. 1 For this reason, it should be possible to obtain the same voltage signal at any specifi c anterior, pos- terior and lateral level. That means that there is a virtual connection between the left wrist and the right shoulder, allowing the determination of the segmental resistance of the right arm by measuring the voltage between the right and left wrist (Fig. 3.15 ).
1 Equipotential lines indicate points with identical potentials. If an electrical charge is moved along an equipotential line, it experiences no electric force and hence no work is performed. Since there is no force driving the charges, there is no current fl ow between two points with the same potential.
72 A. Stahn et al.
In accordance, R leg (right side) can be measured by sensing the voltage potential between the right and left ankle. R trunk then only requires the determination of the voltage between the left wrist and left foot. Alternatively, the suggestion of Cornish et al. ( 2000 ) was to sense voltage between the contral- ateral wrist and ankle to fi rst obtain the sum of R trunk and R arm ( R trunk and R leg ) and then compute R trunk by subtracting R arm (i.e. R leg ) from the sum of R trunk and R arm (i.e. R trunk and R leg ). The ideal assumption of a current fl owing only in a longitudinal direction is certainly not entirely valid. In regions where the electric fi eld is not aligned in parallel with the longitudinal body axis, the equipotential lines are not linear and complex to determine. This diffi culty increases in regions where the current has to pass regions with marked changes in cross-sectional area and where the poorly conducting tissues such as bone (e.g., shoulder and hip joints) complicate the development of a homogeneous electrical fi eld. Despite these constraints, Cornish et al. ( 1999 ) empirically showed that the distal extremity of both the upper and lower limb is at the same equipotential as all parts of the upper and lower limb respectively. The variation in Z relative to the standard electrode site along the proposed equipoten- tial lines was less than 1% and the standard deviation of relative Z did not exceed 2%. It should be noted that this deviation comprises both a technical measurement error as well as biological varia- tions. Due to orthostatic effects substantial fl uid shifts occur while lying supine, which might have confounded constant measurement conditions during that study. Thus, the observed discrepancies of 2% might simply be caused by a technical and biological variation.
Arm Trunk Leg
Fig. 3.15 Segmental bioimpedance measurements for right arm, trunk and right leg. This is an electrode confi guration for segmental measurements that is based on the conventional whole-body measurement with two electrodes attached to the right hand and foot, respectively. In addition, only two extra detector electrodes are needed. These electrodes are placed at the equivalent positions (with regard to the detector electrodes of the right side of the body) of the left side of the body to determine segmental resistance of the arms, legs, and trunk, respectively. This measurement confi gura- tion neither requires the current injection electrodes to be moved nor to locate additional electrode sites other than those for the whole-body approach. To obtain separate resistance measurements of the right arm, leg and trunk the crocodile clips for measuring the potential difference are connected to the electrodes of the left and right wrists and ankles as indicated
73 3 Use of Bioelectrical Impedance: General Principles and Overview
Potential and Limitations of Segmental Measurements
The largest uncertainty underpinning the segmental approach can be traced to the volume estimation of the trunk. Given the anatomical complexity of the trunk, the assumption of isotropy is far from being fulfi lled. Furthermore, given the disparity between segmental resistances and masses, small errors in R trunk can lead to substantial artifacts in volume estimations of the trunk. In addition, the full potential of segmental measurements can only be achieved when the corresponding segmental lengths are also determined. This introduces additional measurement error, and if the electrode setup as suggested by Organ et al. ( 1994 ) is employed, the reference points for segmental lengths need to be determined without knowledge of the true conductive length.
Nonetheless, the segmental approach seems to be promising. Similar to the proximal electrode arrangement, it seems to be less prone to errors related to fl uid shifts within body segments. For instance, Zhu et al. ( 1998a ) demonstrated that in contrast to the whole-body approach the seg- mental technique is independent of the body position. In a study by Thomas et al. ( 2003 ) 13 impedance measurements were performed over a 60-min period while lying supine. While the whole-body impedance index linearly and signifi cantly decreased by 6.6% after termination of testing, no signifi cant changes were observed for the segmental approach. Some authors also suggested that the poorer performance of BIA in obese subjects might be related to the single- cylinder assumption underpinning the whole-body approach (Swan and McConnell 1999 ) . Furthermore, a segmental approach would also be useful in the monitoring of body fat distribu- tion and for classifying different types of obesity (gynoid vs. android obesity). In summary, a number of studies monitoring different fl uid compartments during surgery and hemodialysis found the segmental technique to be a promising extension of the whole-body approach (Patterson et al. 1988 ; Patterson 1989 ; Bracco et al. 1996, 1998, 2000 ; Tatara and Tsuzaki 1998 ; Zhu et al.
1998b, 2000 ; Biggs et al. 2001 ) .
Commercially Available Segmental Bioimpedance Systems
It is worth noting that the segmental approach also led to the commercial availability of a number of instruments, which combine a scale with a bioimpedance monitor. These instruments use tactile electrodes at the feet and allow bioimpedance measurements of the lower extremities. Similarly, some manufacturers also introduced hand-held devices to measure the bioimpedance of the upper extremities. As long as these two types of devices are not combined, the techniques are generally prone to errors associated with body fl uid and tissue distribution since they both assume the upper or lower body to be representative of the total body. For instance, leg-to-leg instruments will hardly detect any changes in upper-body composition or refl ect fat mass in android obesity, whereas hand- to-hand devices might not accurately measure fat in subjects with gynoid obesity. Thus, unless such approaches consider differences in fat mass distribution or limit the analysis of the target quantity to the segments measured, these methods should be carefully applied to assess individual body compo- sition levels. A refi nement of the technique that overcomes these drawbacks is a segmental eight- tactile electrode system, which is based on the segmental principle suggested by Organ et al. ( 1994 ) , allowing the separate determination of the trunk and limbs, respectively. Recent validation studies have confi rmed the validity of the approach, at least in healthy populations (Jaffrin and Morel 2009 ) . Given its speed and ease of use it might promote the acceptability of BIA in the clinical, research and particularly epidemiological settings.
74 A. Stahn et al.
Directions for Future Research
Finally, ongoing research is trying to improve the segmental approach. For instance, Van Kreel et al.
( 1998 ) postulated that modeling the body segments as a combination of truncated cones and cylin- ders would more strictly refl ect the geometrical properties of the human body. Though promising, this approach failed to improve the prediction of total body water as compared to whole-body mea- surements. A fi eld of emerging interest is to restrict body composition analysis by bioimpedance measurements to specifi c body segments instead of using various segmental measurements to model whole-body composition. Limiting analyses to the extremities apparently avoids diffi culties associ- ated with assuming isotropy in the trunk. Furthermore, if measurements are reduced to small limb segments the assumption of representing the conducting volumes as cylinders will apparently gain signifi cance. A further step is to perform local bioimpedance measurements to estimate skeletal muscle cross-sectional area. This type of measurement confi guration has also been employed to provide diagnostic information on neuromuscular diseases (Tarulli et al. 2005 ) , on induced muscle damage after prolonged physical exercise (Elleby et al. 1990 ) as well as on subcutaneous fat layers (Elia and Ward 1999 ; Scharfetter et al. 2001 ) . Another approach combining the determination of individual measurements to cross-sectional areas and full segmental limb measurements is based on multiple measurements along the limbs (Fig. 3.16 ).
The basic idea of this approach is obtain a resistance profi le of the limb, and approximate these data by a fi tting algorithm so that it is possible to derive the resistance at any possible site along the limb (Fig. 3.17 ).
Fig. 3.16 Electrode confi guration for the determination of limb resistance profi les. This fi gure displays the electrode confi gurations for the determination of limb resistance profi les of the arm ( a ) and leg ( b ), respectively
75 3 Use of Bioelectrical Impedance: General Principles and Overview
Accordingly, assuming that the injected current is fl owing in longitudinal direction only and that the resistivity of the tissue of interest is much smaller than that of other tissues, an estimate of the cross-sectional area ( A ) at a specifi c site ( x ) can be obtained from:
( )
[cm ].2A x /
dR dx
= r (3.19)
where r ( W ·cm) is resistivity, and R ( W ) is the resistance of any specifi c site. Finally, the total limb volume can be computed as the integral over x of the estimated cross-sectional area. An example for an individual subject where skeletal muscle volume was determined for the leg and arm using this approach is given in Fig. 3.18 .
The agreement between MRI-measured skeletal muscle mass and BIA is excellent in healthy populations. Furthermore, at least for the leg this approach is clearly superior to conventional seg- mental BIA for estimating skeletal muscle mass. The total error can be reduced by as much as 50%
when compared to conventional segmental measurements. Furthermore, irrespective of the predic- tion accuracy the approach is promising as it does not imply the need of sample-specifi c regression equations. Yet, further validation of the technique is required to assess its accuracy in a variety of populations and to quantify the variability of specifi c resistivity and its impact on the estimation of skeletal muscle volume. Though the approach could also become more effi cient by employing mea- surements at fewer electrode sites, it should be noted that measurement and its analysis are more sophisticated and time-consuming than conventional segmental BIA. Thus, in spite of the fact that the approach might serve several promising clinical applications and provide useful research oppor- tunities, the trade-off between the gain in accuracy and the setback in effi ciency should be carefully considered prior to the implementation of the technique.