A practical guide to bioelectrical impedance analysis using the example of chronic obstructive pulmonary disease

BIA is a method for estimating body composition. The principle of BIA is to determine the electric impedance of an electric current passing through the body [15]. The electrical impedance (Z) consists of two components, resistance (R) and reactance (Xc). Reactance is a measure of BCM and resistance a measure of total body water [15, 22]. From the determined impedance a number of BIA parameters can be estimated [20]:

The strengths and limitations of different BIA methods (e.g. single frequency, multi-frequency, segmental BIA) have been extensively reviewed [15, 16, 18, 23‐25].

Our experience is based on single frequency BIA (50 kHz); the software package we use (NUTRIPLUS from Data Input GmbH) includes BIVA and adapted reference values.

Detailed instructions for performing BIA measurements can be found elsewhere [17, 18]. To give a brief description here, single frequency BIA usually involves the placing of two distal current or signal-introducing electrodes on the dorsal surfaces of the hand and foot close to the metacarpal-phalangeal and metatarsal-phalangeal joints, respectively. The two voltage sensing electrodes are applied at the pisiform prominence of the wrist and between the medial and lateral maleoli of the ankle. The impedance analyzer delivers a constant alternating current at a fixed 50-kHz frequency via the distal electrodes and detects the drop in voltage via the proximal electrodes. The measured resistance and reactance are displayed by the analyzer [18].

Factors impacting BIA results [16, 18, 20, 23, 25]:

BIA parameters are largely dependent on the patient's hydration status. BIA enables the above mentioned parameters to be determined in subjects without significant fluid and electrolyte abnormalities [15].

Patient: female, 61.1 kg, BMI 22.7, 64 years old, mild COPD (GOLD stage I) [9].

Interpretation: With a BMI of 22.7 this patient is of normal weight. The measurement point in the BIVA nomogram (Figure 2) lies within the 50^th tolerance ellipse and thus indicates normal findings. The TBW, ECM, body fat, BCM, BCM % and phase angle values listed in table 1 are within the normal range.

Conclusion: All values in the table are within the normal range and the measurement point in the BIVA nomogram lies within the 50^th tolerance ellipse.

Patient: female, 90 kg, BMI 31.5, 73 years old, severe COPD (GOLD stage III)

Interpretation: With a BMI of 31.5 this patient is obese. The measurement point in the BIVA nomogram (Figure 3) in this patient is well below the line of normal BCM values (long axis) and above the line of normal TBW values (short axis) between the 75^th and the 95^th tolerance ellipse. The position of the measurement point in the lower right quadrant points to malnutrition.

The BIA parameter values listed in table 2 can be interpreted as follows: The fat mass lies above the normal range in line with the increased BMI. BCM lies within the normal range. At first sight this does not fit in with the finding of the BIVA nomogram, which indicates malnutrition. The fact that the calculated BCM is within the range of normal values here may be explained as follows: It needs to be considered that BCM is dependent on the patient's fluid status (TBW). This means that a BCM within the normal range does not necessarily mean a normal nutritional status but may also be due to increased TBW. The following two BIA parameters are helpful in drawing a distinction: BCM % and phase angle, which are usually altered along the same lines as BCM. In this case TBW is increased and the BCM % and phase angle are below the range of normal values. This indicates that BCM is actually reduced. BCM therefore only appears to lie within the range of normal values because of the increased TBW. In contrast to this somewhat complex interpretation of the calculated BIA values, the suspected diagnosis of malnutrition can be established at a glance by BIVA. In addition, it is confirmed that the calculated BCM is too high because of the increased TBW (position of the measurement point in the BIVA nomogram above the line of normal TBW values).

Conclusion: Despite the presence of obesity the patient is exhibiting malnutrition. The position of the measurement point in the BIVA nomogram in the right lower quadrant between the 75^th and the 95^th tolerance ellipse provides an indication for the suspected diagnosis of malnutrition.

Patient: male, 45 kg, BMI 16.7, 62 years old, severe COPD (GOLD stage III)

Interpretation: With a BMI of 16.7 the patient is underweight. The measurement point in the BIVA nomogram (Figure 4) in this patient is far below the line of normal BCM values (long axis) and well above the line of normal TBW values (short axis), far outside the 95^th tolerance ellipse. The position of the measurement point in the lower right quadrant points to malnutrition in the form of cachexia.

The BIA parameter values listed in table 3 can be interpreted as follows: The fat mass lies below the normal range in line with the reduced BMI. The calculated values for BCM und TBW are reduced. It needs to be considered as regards the reduced BCM value that BCM is dependent on the patient's fluid status (TBW). This means that a reduced BCM does not necessarily point to malnutrition but may also be due to a low TBW. The BIA parameters BCM % and phase angle are again helpful for drawing a distinction, as they are usually altered along the same lines as BCM. The values for BCM % and phase angle are reduced, which indicates an actually reduced BCM or malnutrition. In this example also BIVA provides a more efficient assessment of the nutritional status than the calculated BIA parameters.

Conclusion: All the values listed in the table are below the normal range and the measurement point in the BIVA nomogram is outside the 95^th tolerance ellipse in the lower right quadrant. This indicates severe malnutrition in the form of cachexia. The assessment of the BIVA nomogram is sufficient for the suspected diagnosis of cachexia.

Patient: female, 105.6 kg, BMI 38.8, 78 years old, COPD GOLD stage II

Interpretation: With a BMI of 38.8 the patient is overweight. The measurement point in the BIVA nomogram (Figure 5) in this patient is above the line of normal BCM values (long axis) and well below the line of normal TBW values (short axis) on the 95^th tolerance ellipse. The position of the measurement point in the lower left quadrant points to water retention in the form of oedema.

The BIA parameter values listed in table 4 can be interpreted as follows: Body fat mass lies above the normal range in line with the increased BMI. The determined TBW is increased and the calculated BCM lies in the upper range of normal. These findings are consistent with the position of the measurement point above the line of normal BCM values and below the line of normal TBW values in the lower left quadrant. With the derived normal BIA value for BCM it needs once again to be taken into account here that BCM is dependent on the patient's fluid status (TBW). This means that a BCM within the normal range does not necessarily indicate an actually normal BCM or normal nutritional status but may also appear normal due to an increased TBW. The BIA parameters BCM % and phase angle, which are usually altered along the same lines as BCM, in this case lie in the lower range of normal, so that it may be assumed that the BCM is actually in the normal range. In addition to the increased TBW, ECM is also markedly increased, indicating oedema. The suspicion of oedema is established at a glance with BIVA. BIVA confirms simply and rapidly the calculated BIA values BCM and TBW. The suspicion of oedema was confirmed on physical examination of the legs.

Conclusion: The values listed in the table for TBW and ECM are outside the normal range and the measurement point in the BIVA nomogram is on the 95^th tolerance ellipse in the lower left quadrant, indicating oedema. The determined BCM is in the upper range of normal and the measurement point in the BIVA nomogram is above the line of normal BCM values. The position of the measurement point in the nomogram provides an indication for the suspected diagnosis of oedema.

For the general differential diagnosis of underweight we present a female patient with anorexia: female, 34.1 kg, BMI 18.4, 41 years old, anorexia (unintentional loss of appetite).

Interpretation: With a BMI of 18.4 the patient is underweight. The measurement point in the BIVA nomogram (Figure 6) lies almost on the line of normal BCM values (long axis) and far above the line of normal TBW values (short axis) outside the 95^th tolerance ellipse. The position of the measurement point in the upper right quadrant points to the presence of anorexia.

The BIA parameter values listed in table 5 can be interpreted as follows: Body fat mass is reduced in line with the low BMI. TBW is markedly reduced and BCM also is decreased. With the reduced BCM it needs to be kept in mind here that BCM is dependent on the patient's fluid status (TBW). This means that a lower BCM may also appear reduced due to a lower TBW. The BIA parameters BCM % and phase angle which are helpful in drawing a distinction and which are usually altered in the same way as BCM, in this case lie in the upper third of the normal range. This indicates that BCM is normal and that the calculated value was too low only because of the low TBW. BIVA confirms the suspicion raised by the BIA values that the calculated BCM was too low because of the reduced TBW. Again, the suspected diagnosis of anorexia can be established more efficiently and more reliably by BIVA.

Conclusion: The patient exhibits a markedly reduced BMI, decreased body water and a normal BCM in the form of anorexia. The position of the measurement point in the nomogram in the upper right quadrant outside the 95^th tolerance ellipse provides an indication for the suspected diagnosis of anorexia.