Corrigendum to ‘Towards personalized hydration assessment in patients, based on measurement of total body electrical resistance: Back to basics’ [Clin Nutr ESPEN 35C (2019) 116–122]

Impact of diffusion, ultrafiltration, and posture on total body electrical resistance in patients on hemodialysis

Monitoring of hydration in patients on hemodialysis (HD) by currently available bioelectrical impedance analysis (BIA) methods is hampered by limited accuracy. This may be caused by changes in total body electrical resistance (TBER) that are induced by processes other than ultrafiltration (UF). To identify these sources of error, we examined the impact of UF, diffusion, and postural change (PC), separately. Extracellular TBER (TBER_e) was measured by bioimpedance spectroscopy every 30 min in 23 patients on HD, for 2 h during diffusion-only (DO), followed by 2-h UF-only (UFO). The impact of PC from upright to semi-recumbent position was assessed by a 2-h TBER_e measurement on the day after HD. TBER_e increased by 23.5 ± 12.4 Ω (P < 0.001) during DO and by 40.0 ± 16.2 Ω (P < 0.001) during UFO. PC, evaluated on a separate day, was associated with an increase in TBER_e of 27.6 ± 26.0 Ω (P < 0.001). TBER_e changes during DO were mainly attributed to PC and to a lesser extent to electrolyte exchange. Extrapolation of the data to a conventional 4-h HD session indicates that about 32% of the total increase in TBER_e is not related to UF. In conclusion, a significant part of the increase in TBER during HD is not related to UF but can be attributed to other processes such as the effects of PC and diffusion-related electrolyte exchange. These factors have to be taken into account when TBER-guided UF is considered.NEW & NOTEWORTHY Current BIA methods have limited accuracy in patients on HD. This may be related to the incorrect assumption that all changes in total body electrical resistance (TBER) are caused by changes in body water volumes. The present study indicates that two-thirds of the change in TBER during a conventional 4-h HD session can be attributed to fluid extraction, and that the remaining part is caused by other processes such as postural change and electrolyte exchange. This may cause volume prediction errors when not recognized.

Prediction of total body electrical resistance normal values based on limb muscle thickness assessed by ultrasound

BACKGROUND

Recently, a new model has been proposed to assess hydration in patients by measurement of total body electrical resistance (TBER), with results expressed in ohm rather than in liter body water. According to this approach, hydration is considered to be normal if TBER is within the normal range. As TBER is inversely related to the size of the limb muscle compartment, this relationship can be used to calculate the patient-specific TBER normal value (TBER_norm). The present study investigates whether the prediction of TBER_norm can be improved by the use of ultrasound (US) instead of anthropometrically derived parameters of limb muscularity.

METHODS

In total, 129 healthy subjects (60 men and 69 women) ranging in age from 18 to 75 yr, and in BMI from 17.4 to 52.0 kg/m² were included in the study. Arm muscle cross-sectional area assessed by anthropometry (AMA_caliper) was compared with mean muscle thickness (MMT) of arm and leg assessed by B-mode US.

RESULTS

MMT correlated stronger with TBER than AMA, and reduced the standard error of the estimate (SEE) by 15% in men and by 26% in women. Muscularity was overestimated by AMA_caliper due to a systematic error directly proportional to subcutaneous fat layer thickness. The gender independent relation between MMT and TBER_norm is described by the equation: TBER_norm = 705-75.4⋅MMT (R² = 0.85, SEE = 22.3 Ω/m, P < 0.001).

CONCLUSIONS

US-based measurement of limb muscularity provides a more precise prediction of TBER_norm, in particular in obese subjects, and is recommended as the method of choice.