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

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.

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.

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).

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.