Evaluation of the use of bioelectrical impedance analysis in assessing the hydration and fluid balance of infants with bronchiolitis requiring intensive care

Objective

To examine whether regular bioelectrical impedance(Z) measurements in babies with bronchiolitis detects daily changes in fluid status and if it may be used as a guide to hydration when compared to age matched well babies.

Methods

Bioelectrical impedance was measured using a Xitron 4000B machine, with electrodes placed 6 cm apart on the dorsal surface of the hand and foot. In babies with bronchiolitis measurements were made as soon as possible after admission and every 24 h until discharge. Data on fluid balance over the previous 24 h were recorded at each measurement period.

Results

There was a clear linear relationship between impedance (ohms) plotted against body mass index (r = 0.95, P < 0.0001) in the control babies. Using control data for BMI and Z, an equation was developed using linear regression to predict Z from BMI values. This equation was used (Z=19.04BMI + 114.6) to predict values for Z_expected in the babies with bronchiolitis on admission. These values were compared with those at admission (Z₁), where using paired t-test a significant difference existed (P < 0.0008) and at discharge (Z₂) where there was no significant difference. There was no significant relationship between changes in body impendance and changes in fluid balance (both positive or negative) although impedance changed appropriately in association with fluid boluses or with diuretics.

Conclusion

Regular impedance measurements give a guide to the state of hydration of babies requiring intensive care and help determine whether the baby is adequately hydrated or not. There is a poor relationship between measured changes in fluid balance and changes in impedance and it may not be used to calculate absolute values of fluid required.