anthropometry

Values ​​of BIA (Bioimpedance) - How to interpret them

Bioimpedance or BIA

BIA (or rather BIA) is the acronym of the English term Body Impedence Assessment, which in Italian can be translated into bioimpedance geometry .

BIA is one of the fastest and most precise techniques for measuring and evaluating body composition (CC); its operation is indirect (like plicometry) and is based on the measurement of IMPEDANCE (Z) offered by the human body to the passage of an alternating electric current at a fixed frequency, by virtue of the fact that the organism's conduction capacity is directly proportional to the amount of water and electrolytes it contains (Total Body Water - TBW).

It should be emphasized that the Z-impedance:

  1. it increases with increasing fat mass and decreases with increasing lean mass
  2. decreases as the fat mass decreases (which logically has greater resistance - R)

Ultimately, BIA measures DIRECTLY and in a specific manner ONLY electrical quantities dependent on tissue hydration; on the other hand, in this way it is possible to identify variations in total body water (TBW) of clinical interest and, by means of appropriate regression equations, to predict the ratio between fat-free mass (FFM - containing high quantities of water and electrolytes, therefore more conductive) and fat mass (FM - containing little water and electrolytes and therefore with high resistance).

Types of BIA

As anticipated, BIA is an analytical technique of body composition (fat-free mass [FFM] vs fat mass [FM]) of INDIRECT type; it is highly specific sample, since the adiposity-density relationship depends on many variables such as: hydration and body density, muscularity, compressibility and thickness of fat, distribution of fat and amount of intra-abdominal fat.

The so-called CONVENTIONAL BIAs detect ONLY the impedance (module of the vector Z), while others defined VECTOTRIALS are able to measure DISTINCTLY also the two components that constitute the vector of impedance-Z (resistance [R] and reactance [Xc]).

Conventional BIA

Conventional BIA uses numerous software (using regression equations that include height, weight, age and sex) that transform the impedance measurement - Z into VOLUMES (intracellular and extracellular), masses (FM, FFM, cellular or BCM), basal metabolic rate (MB), and other quantities of body composition analysis.

It provides values ​​of: TBW (composed of intracellular, extracellular vascular and extracellular interstitial water), FFM (composed of lean mass and bone mineral mass) and FM (fat mass) in healthy adults; the conventional BIA has correlation coefficients always close to 1 (r> 0.95) and an estimation error that falls within the order of 3-4 kg.

NB. To obtain the TBW accurately it is also necessary to enter the height (h) and other specific variables.

BIA vector

The vector BIA (as in the reading of the electrocardiogram [ECG]) interprets the direct measurement of impedance by way of graphics providing a semi-quantitative evaluation of tissue hydration.

It uses alternating current (at constant intensity and frequency), injected onto the skin by electrode-patch, which crosses the electrolytic solutions of the extra-cellular liquid (LEC) and the intra-cellular fluid (LIC) of all tissues (excluding fat and bone ) generating an impedance vector-Z.

The cell membranes and tissue interfaces SFASE the conduction of current (PHASE ANGLE between voltage and current of the vector) generating the capacitive component of impedance-Z, or the reactance-Xc .

The vector BIA provides specific values ​​on conductive tissues, compartment expressed as resistance-R value by intra and extracellular electrolytic solutions, and as X-reactance value by the set of cell membranes of the compartment itself.

NB. The extension to other compartments is statistically usable in healthy subjects but absolutely NOT in the pathological.

Soft tissue analysis: direct measurement

Through the combination of the non-invasive BIA measurements with the specific formulas for instrument and population (assuming that the subject's hydration is at 73%, therefore normal), mass or volume data are obtained regarding: TBW, FFM and FM.

However, Akern Soft Tissue Analyzer (STA) is currently the only instrument that measures directly, and therefore independently of: weight, age or height of the subject, the fundamental compartments for assessing nutritional status: Cellular Mass (BCM) and the percentage Extracellular Water (ECW%).

NB. The BCM in kilograms and the percentage of ECW clearly reflect the state of hydration and nutrition of the subject.

In addition to BCM and ECW%, Akern STA also provides the values ​​of: Basal Metabolism (MB) in Kcal, Reactance (Xc), Resistance (R) and Phase Angle in degrees. From the BMC, with a formula, it is also possible to understand what should be the minimum quantity THEORETICAL that delimits the LOWER normality limit, that is:

BCMmin in kg = height in cm - 100 x 0.3 (males) {0.28 (females)}

As for the ECW% (which we recall being the percentage of Extracellular Water), the normality threshold is about 40%, while values> 45% indicate water retention and <38% reflect extracellular dehydration.

The phase angle decreases when the BCM (Cell Mass) is reduced and is very useful for the interpretation of the state of nutrition; WARNING! Its decrease may be caused by the increase in ECW due to disturbances in the water balance; the phase angle of a well-nourished and well-hydrated young goes from 6 to 8 degrees.

The values ​​of the BIA can be interpreted separately or through the reading of the NORMOGRAMMA which graphically shows the condition of a subject and allows a more rapid control of the results. The graph is composed of three zones, defined as confidence ellipses: normality (50%), tolerance (75%) abnormality (95%); this system shows with excellent sensitivity and specificity the real state of hydration in any clinical condition and regardless of body weight.

Use of BIA

The BIA, from its invention to the present day, has undergone numerous technical and application changes; to date, the most used is the polar tetra technique. Its operation is simple:

" four skin electrodes are applied, one pair on the back of the hand (the right is preferred), one of which is on the metacarpophalangeal joint of the third finger (injector electrode) and the other on the radio-ulnar joint (sensor electrode), and a pair on the back of the ipsilateral foot, one of which on the metatarsophalangeal joint of the third finger (injector), and the other on the ankle joint (sensor). The patient, supine, not in contact with metal elements, can keep uncovered only ipsilateral hand and foot, with lower limbs 45 ° apart and upper limbs abducted 30 ° to avoid skin contact with the trunk, the analyzer cables are connected with tweezers to the electrodes ".