The heart of the Nutritionist's activity , as we have repeatedly reiterated during our courses, should be a complete evaluation of the nutritional status , which takes into account anthropometric and blood chemistry parameters and the results of the execution of test batteries. Only after having collected this data it is possible to formulate specific nutritional indications for the patient. Whether it is a question of grammed nutritional plans , nutritional education paths or indications for the use of supplements, in the most appropriate ways and doses, a correct overall nutritional framework cannot be ignored. In some cases it is naturally essential to take into account pathologies previously diagnosed by the doctor and to orient the nutritional intervention in the most appropriate way.
BIVA is one of the anthropometric analyses and is a simple, economical and non-invasive test used to estimate body mass and fluids through measurements of bioelectrical impedance, a vector-type physical quantity which can be broken down into its resistance (R ) and reactance ( Xc ) components . These quantities represent the opposition of liquids and cellular membranes to the passage of a low intensity alternating electric current . Today, many instruments are commercially available that allow an estimate of body composition, but it is better to use scientifically validated equipment, which involves performing the test in a supine position with the application of electrodes and which always return the values of resistance and reactance. We remind you that in clinical studies, impedance scales are never used .
Operating principles of vector bioimpedance analysis (BIVA)
Vector bioimpedance analysis is based on the principle that biological tissues oppose a certain resistance to the passage of an electric current. When a low-frequency alternating current passes through the body, it encounters various resistances due to body composition, mainly water and adipose tissue. The fundamental quantities that are obtained from performing a BIVA are:
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Resistance ( R ): measures the resistance that tissues offer to an electric current. It is mainly influenced by the amount of fluid in the body. Lean tissues, rich in water, have a lower resistance than adipose tissues. It is therefore a quantity inversely proportional to the amount of water and electrolytes contained in an organism.
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Reactance ( Xc ): represents the ability of cell membranes to accumulate and release electrical charges and is also known as the capacitive component of impedance . It is indicative of cell mass and the integrity of cell membranes.
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Phase Angle (PA): Phase Angle (PA ) is a measurement derived from bioimpedance analysis that provides information on the quality of cells and the distribution of body fluids. It is calculated using the resistance (R) and reactance ( Xc ) values measured during bioimpedance analysis .
Phase angle formula
The phase angle is calculated as the arctangent of the ratio of reactance to resistance:
PA = arct an ( Xc / R )
Where:
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PA is the phase angle, measured in degrees (°).
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Xc is the reactance, expressed in ohms (Ω).
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R is the resistance, expressed in ohms (Ω).
Interpretation of the phase angle
The phase angle reflects the dielectric properties of cell membranes and the amount of intracellular and extracellular fluids:
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High Phase Angle: Indicates good integrity and function of cell membranes, as well as optimal distribution of body fluids. Generally, a higher phase angle is associated with better cellular health and greater cell mass. Caution: A high phase angle could also indicate possible dehydration.
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Low Phase Angle: Suggests compromised integrity of cell membranes and may indicate increased extracellular fluid relative to intracellular fluid. Low phase angle is often associated with pathological conditions or functional disorders , including inflammation, malnutrition, or edema.
Phase angle calculation
For example, if during a BIVA measurement we obtain the following values:
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Resistance (R) = 500 Ω
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Reactance ( Xc ) = 50 Ω
The phase angle is calculated as follows:
PA = arctan (50/500)
We therefore find that:
PA = arctan (0.1) ≈ 5.71°
Importance of phase angle in defining nutritional status in pathological conditions
Phase angle is a useful indicator in clinical settings because it provides rapid and non-invasive information about the patient's body composition and nutritional status. It is used in several medical areas, including:
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Oncology: M onitoring nutritional status and response to treatment in cancer patients. Scientific literature states that a phase angle less than 3 ° has a negative prognostic value for the cancer patient: in this case the patient should be immediately referred to the doctor. In general, whenever a strong deterioration in the nutritional status and/or prolonged fasting is detected, it is advisable to refer the patient to the doctor, who will evaluate whether to intervene pharmacologically and/or through artificial nutrition methods.
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Nephrology: assessment of water and nutritional status in patients with renal failure.
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Cardiology: monitoring of fluid retention and cardiac function in patients with heart failure.
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Geriatrics: assessment of nutritional status and cellular health in the elderly.
In summary, the phase angle is a fundamental measurement in bioimpedance analysis to evaluate cellular health and the distribution of body fluids. Its ease of calculation and clinical relevance make it a valuable tool in the practice of the profession of nutritionist. In the most advanced software today, the sPA (standardized phase angle) is also available , which allows to interpret the value of a patient's PA in relation to his/her sex and age group: this statistical correction allows to make reflections on comparable data within heterogeneous populations and is therefore very useful in research.
Starting from the measurement of resistance (R) and reactance ( Xc ), equations typically contained in software allow to evaluate different parameters, including total body water (TBW), intracellular water (ICW) and extracellular water (ECW). In particular, ECW is a crucial parameter to identify conditions of fluid overload, such as edema. ICW, hopefully, never varies: if it did, cellular physiology would be compromised.
Measurement procedure
The standard procedure for carrying out BIVA includes the following steps:
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Patient preparation: The patient should be in a supine position, with the upper limbs abducted 30° from the midline of the body and the lower limbs 45° between them . It is useful if the patient has not eaten, drunk or performed intense physical activity in the hours preceding the measurement.
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Electrode placement: Four electrodes are placed on the patient's skin: two (injector and sensor) on the back of the right hand. (third metacarpophalangeal and radio - ulnar joints) and two (injector and sensor) on the back of the foot (third metatarsophalangeal and tibio - tarsal joints ) . The electrodes must adhere well to the skin to ensure good conduction of the current, therefore it is essential to pay attention to the possible presence of cosmetic products that could alter the detection . It is also important to store the electrodes correctly during the hottest periods.
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Measurement: A low-intensity alternating current (usually 50 kHz) is passed through the body. The bioimpedance meter measures the resistance and reactance of the body to the passage of the current.
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Data Analysis: The collected data is analyzed using specific software that processes resistance and reactance values to determine body composition parameters.
Biavector Chart
The B iavector graph , or bioelectrical impedance vector, is a visual representation of resistance (R) and reactance ( Xc ) measurements in a Cartesian coordinate system. This graph allows interpretation of the patient's body composition and fluid status.
How to read thebivector graph
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X-axis: r represents the resistance (R) normalized to the subject's height (R/H, in ohms/m).
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Y-axis: r represents the reactance ( Xc ) normalized with respect to the subject's height ( Xc /H, in ohms/m).
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Measurement Point: Each point on the graph represents an individual measurement of normalized R and Xc . The location of the point indicates the subject's body composition.
Interpretation of theBiavector Graph
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Points within the tolerance ellipses: Tolerance ellipses represent the statistical distribution of impedance values in a reference population. There are several ellipses, generally at 50%, 75%, and 95% confidence levels, that indicate what fraction of the population falls within them.
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Points outside the ellipses: These indicate extreme values, suggesting abnormal body compositions or pathological states. For example, a point outside the 95% ellipse could suggest a significant clinical condition: in the event of abnormalities of this magnitude, following a correctly performed measurement, the patient should be referred to the doctor.
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Distribution of points:
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Points shifted to the bottom left : People with medium-low reactances and resistances that fall into this quadrant are often overweight or obese.
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Dots shifted down and to the right: result from lower reactance and higher resistance, suggesting lower muscularity /cellular integrity and possible cachexia .
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Points shifted upwards and to the left: associated with greater reactance and less resistance , they suggest a greater quantity of cellular mass and good hydration: these often concern athletes or people in good health.
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Point move up and to the right: with medium-high reactances and resistances, it is the typical placement of thin/malnourished patients who tend to dehydration : this is the case, for example, of patients suffering from anorexia nervosa.
Biavector Graph Applications
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Assessment of nutritional status : the graph can be used to identify states of malnutrition or obesity, comparing individual points with the reference population.
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Therapy monitoring: In collaboration with your doctor , it can help monitor changes in body composition during medical treatments, such as dialysis or artificial nutrition.
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Detection hydration status : can be used to identify states of dehydration or edema, observing the position of the points in relation to the tolerance ellipses.
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Training monitoring: It can be used, in collaboration with a medical team and physiotherapist, to evaluate recovery from an injury or can predict fatigue or muscle injury, not yet felt by the patient.
Interpretation of body fluids
BIVA allows to accurately evaluate the distribution of body fluids, distinguishing between intracellular water and extracellular water. This is particularly useful to identify states of dehydration or fluid overload.
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Total body water (TBW): It is the sum of intracellular water and extracellular water. It represents approximately 60% of the body weight in a healthy adult.
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Intracellular water (ICW): It is the water contained within the cells. It represents approximately 60-70% of the TBW.
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Extracellular water (ECW): This is the water outside the cells, including interstitial fluids, blood plasma, and lymph. It represents approximately 30-40% of TBW.
Identification of edema by BIVA
Edema is a condition characterized by excessive accumulation of fluid in the interstitial tissues. It can occur in different parts of the body, including the lower extremities, hands, face, and abdomen. BIVA is an effective tool for identifying and quantifying edema, due to its ability to measure extracellular water.
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Increased extracellular water (ECW): A significant increase in ECW over normal values is indicative of fluid accumulation between the tissues . This can be detected as a decrease in the overall resistance (R) of the body.
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ECW/TBW ratio: An increased ECW/TBW ratio may indicate a relative increase in extracellular water, suggesting the presence of edema.
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Reduced Phase Angle (PA): A low phase angle may indicate compromised integrity of cell membranes, often associated with edematous conditions.
Causes of edema
Edema can be caused by a variety of medical conditions and situations. Some of the more common causes are listed below:
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Heart failure: The heart's inability to pump blood effectively can cause fluid accumulation in the tissues, especially in the lower extremities (e.g., perimalleolar and peritibial edema ) .
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Kidney failure: If kidney function is impaired, the kidneys cannot adequately remove fluids and salts from the body, leading to fluid accumulation.
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Liver cirrhosis : reduced production of albumin, the protein that regulates the osmotic pressure necessary for the correct distribution of body fluids, due to impaired liver function can cause an accumulation of fluids in the abdomen (ascites) and in the lower limbs.
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Nephrotic syndrome: A kidney condition characterized by excessive loss of protein in the urine, which reduces oncotic pressure and causes edema.
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Lymphedema: A blockage or impairment of the lymphatic system can prevent adequate drainage of fluids, causing localized swelling, often in the limbs.
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Pregnancy: Hormonal changes and pressure from the growing uterus on blood vessels can cause fluid retention and swelling, especially in the legs.
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Medications: Some medications, such as calcium channel blockers, nonsteroidal anti-inflammatory drugs (NSAIDs), and corticosteroids, can cause fluid retention and edema.
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Infections and inflammation: Localized infections and inflammation can cause increased capillary permeability, leading to fluid accumulation in the tissues.
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Venous problems: Conditions that impede venous return, such as varicose veins or deep vein thrombosis, can cause fluid to build up in the legs.
If an overload of extracellular fluids is detected, the nutritionist may recommend the patient:
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To hydrate properly, to promote the flow and replacement of body fluids .
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Generally improve the quality of nutrition, to reduce the risk of fat accumulations that can promote an aesthetic problem known as gynoid lipodystrophy , commonly called cellulite. Attention: gynoid lipodystrophy , which is not a pathology, and water retention are different phenomena and not necessarily associated. Patients often confuse the two conditions, it is up to the nutritionist to orient them correctly.
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To evaluate the use of bromelain -based products , for which some studies have suggested a possible effect on inflammatory and/or edematous conditions. Caution: bromelain should not be recommended to patients undergoing antibiotic therapy, due to a possible interaction, nor to subjects undergoing anticoagulant or antiplatelet therapy, for the same reason.
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To direct the healthy patient to professionals able to correctly perform lymphatic drainage massages, preferably conducted manually and not through pressotherapy technology.
Conclusions
Bioelectrical impedance analysis (BIVA) is a valuable tool for assessing body composition and tissue water status. The ability to accurately measure intracellular and extracellular water makes BIVA particularly useful for identifying fluid overload conditions, such as edema. With a thorough understanding of how BIVA works and accurate data interpretation, this technique can be used to accurately diagnose and monitor patients' nutritional and hydration status over time.
Edema, being a symptom of many medical conditions, may also require a medical evaluation to determine the underlying cause. BIVA, with its ability to provide detailed data on the distribution of body fluids, represents an important complement to other diagnostic tools, helping physicians to effectively plan and monitor treatment strategies.
Clinical implementation of BIVA and continued education of healthcare professionals on this technique can significantly improve the management of patients with fluid retention disorders, contributing to improved quality of life and clinical outcomes.
