Electrical impedance myography at frequencies up to 2 MHz

Extension of the frequency range of electrical impedance myography (EIM) to 2 MHz discloses a major rise in the reactance of muscle above 3-500 kHz, together with a slow decrease in the resistance consonant with the Kramers-Kronig relations. This 'upturn' phenomenon is found when the distant current electrode configuration of EIM is employed, but not when the current electrodes are placed close to the voltage measuring area. In that case the impedance qualitatively mimics the commonly used 3-element resistor-capacitor model. The possibility that the upturn is an artifact of the measuring system rather than a true property of the tissue is examined in detail. In particular, experiments are reported which argue against the transmission line mechanism proposed to explain similar increases in reactance in some high frequency whole-body BIA studies. Also, scaling of X versus R plots for muscle segments of different lengths strongly suggests that the upturn is as much a property of the underlying tissue as is the low frequency maximum in reactance.