excitation signal. Admittance technology uses this phase shift to determine the instantaneous muscle conductance and remove it from the total measured conductance. Removal of Gm The total Admittance (Y), of the blood filled ventricle is given by Y = Gb + Gm + iωCm where: Gb is the measured conductance of the blood (the desired signal) Gm is the measured conductance of the cardiac muscle (unwanted signal) Cm is the measured capacitance of the cardiac muscle ω is the angular frequency of the excitation signal The signals Gm and Cm are both properties of cardiac muscle and vary in a fixed ratio. Thus the ratio of Gm to Cm is equal to the ratio of muscle conductivity (σ) to muscle permittivity (ε). The ratio σ/ε is the constant of proportionality. Although both σ and ε are functions of the health of the heart tissue, they are relatively constant for short periods of time. Using this proportionality, one can rewrite the equation for Gm as Gm = (σ/ε)Cm Note that the imaginary component of Y depends only on the amount of muscle in the field of the catheter. This makes it easy to isolate by measuring the phase shift, φ, of the measured signal: cos(φ) = (Gb + Gm)/Y sin(φ) = ωCm/Y Hence, Cm = Y.sin(φ)/ω also, Gm = (σ/ε)Cm Thus, blood conductance is determined as Gb = Y.cos(φ) - Gm Wei's equations can be applied to this calculated blood conductance Gb to obtain blood volume. Unlike Baan's equation, Wei's equation takes into account the non-linear nature of the electrical field and the dynamic nature of the cardiac cycle to give a more accurate representation of the blood volume. Experimental results Admittance technique involves the measurement of both phase angle and total conductance in the ventricle. Thus, it is possible to observe how the parallel conductance (muscle conductance) varies throughout the cardiac cycle. A plot showing both the blood and muscle contribution are shown in the figure. References Further reading Cardiology