Volume status effected by gravity and body fluid shifts: new protocol for bioimpedance measurements

The analysis of body composition gained much more interest with the use of non-invasive practical method of bioimpedance. This technique uses electrical properties of materials like body cells. Cell membrane has high impedance and depending on the frequency an alternate current may flow through the cell and quantification of extracellular water (ECW) and total body water (TBW).1 Body temperature, fluid and electrolyte balance and skin contact resistance could have effect on bioimpedance results.2 Orthostasis cause fluid re-distribution between intracellular and extracellular water (ECW). Bioimpedance analysis measured in supine position but shorter stay before measurement leads falsely increased ECW. Aim was to find out fluid shift equilibration time and clearly define how long to wait before bioimpedance measurements and reveal new protocols.

 Bioimpedance spectroscopy measured with the Body Composition Monitor (BCM) from Fresenius Medical Care, Deutschland GmbH. Four electrodes were placed on the ipsilateral hand and foot. The BCM analyses total body electrical impedance to an alternate current (0.2 mA) with fifty different frequencies (5–1000 kHz). First the ECW volumes and total body water are calculated via determining electrical resistances, then values of overhydration (OH), intracellular water, body mass index, lean tissue index (LTI), fat tissue index, body cell mass are provided by the BCM software.3 Because normal ICW or ECW can be determined for a given weight and body composition, fluid overload can be calculated from the difference between the normal ECW expected and measured ECW.

Thirty-three healthy individuals, 17 (51.5%) of male with the mean age of 23.3 ± 4.6 (18–39) years, weighting 69.9 ± 16.9 (48–110) kg and 170.5 ± 8.7 (150–189) cm in length were included. Mean albumin and sodium levels were 4.83 ± 0.35 g/dL and 140 ± 4 meq/L. Firstly measurement done in supine, than in standing positions up to reaching maximum OH value while standing still and later on measurements done during supine positions serially up to reaching minimum OH value. Mean OH change (OHdelta) calculated from maximum OH in standing subtracted by minimum OH in supine (OHmax-OHmin). Time OHdelta calculated from time elapsed to reach maximum OH in standing added to time elapsed to reach minimum OH in supine (Time OHmax + Time OHmin) positions. Overall 10 times measurements done for each individual. Time elapsed from 1st measurement to reaching OHmax during standing position for male and female were 37.2 ± 12.1 and 36.6 ± 16.9 minutes. Time elapsed from reached OHmax to OHmin during supine position for male and female were 44.6 ± 16.2 and 46.6 ± 27.2 minutes (p > 0.05). OHdelta was 1.17 ± 0.44 L (Figure 1). Intracellular water shifted in exactly opposite to OH (Figure 2) but there were no significant change in fat mass. Although there are positive but weak correlations present in OHdelta and BMI (r = 0.38, p = 0.030) also OHdelta and LTI (r = 0.41, p = 0.016), but there is no correlation found with age. Systolic and diastolic blood pressures were not changed significantly 127.1 ± 72.4 and 7.78 ± 5.66 to 126.2 ± 71.9 and 7.07 ± 8.49 in supine and standing positions (p > 0.05).

Figure 1. OH: Overhydration.

Figure 2. ICW: Intracellular water.

Bioimpedance analysis used for the measurement of fluid and nutritional parameters4 and could be used to show fluid shifts. Fenech et al. conducted a study including eight patient tests in supine and sitting positions and found that resistance of ECW decreased instantaneously by an approximately 2.3% when the patient sits up, due to interstitial and plasma fluid shift into the lower limbs which decreases leg resistance, the major contributor to total resistance. This drop in resistance attributed to the device Xitron 4200 impedance meter used for that study with averages 235 ml.5 In our study we demonstrated the decrease of ICW with the increase of OH in standing position. This difference may be attributable to fluid shift from ICW to ECW and interstitial space. Scharfetter et al. used a tilt table experiments and found that a steady state could not be reached within 30 min. During first phase the ECW decreased, in the tilt phase it increased, and in final phase it decreased again. The ICW changes measured in the opposite way of ECW. They concluded that the measured volume changes attributed to artifacts which were caused by extracellular fluid redistribution.6 In our study, we measured OH changes up to maximum value during standing which were 37.2 ± 12.1 and 36.6 ± 16.9 minutes, time elapsed from reached OHmax to OHmin during supine position were 44.6 ± 16.2 and 46.6 ± 27.2 minutes for male and female. Some fluid shifts toward the lower body during standing. Depending on the duration of standing when a healthy person stands, 10–15% (650–700 ml in a person) of blood is rapidly pooled in the legs. Lower body negative pressure of 40 mmHg causes similar fluid shifts as during standing fluid shifts toward the lower body during standing.7 In present study we found 16.7% changes. Lean tissue measured by Dual-energy X-ray absorptiometry found as a function of E/I measured by gold standard dilution tests.8 We found hydration changes correlated with LTI.

Previously Matthie reported that most of the shift was in first few minutes with more over time.1 In our study, OH change of >1 liters shown in relation to body position and the difference is positively correlated with BMI and LTI. Body position significantly affects the volume status due to gravity and redistribution of fluid between intra and extracellular compartments. Most accurate bioimpedance results need patients to be waited in supine position as long as possible before measurements. It may be logical to measure patients just after HD sessions or in the morning before getting up in hospitalized patients but most important was to make measurement at same time each time.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.