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Theory - Complex Admittance

Scisense is the first company to manufacture a revolutionary PV loop system ADVantageTM which derives volume based on measuring complex admittance.

Traditional conductance technique

  • LV blood conductance is measured on a beat by beat basis using a tetrapolar catheter placed in the LV
  • Original theory proposed by Baan et al.1 which relates measured conductance to volume through a simple equation

Baan's Equation: V= 1/α(ρL2)(Gblood-G||)

where
ρ - blood resistivity
L - length between voltage sensing electrodes
α - constant dependent on the stroke volume (Baan assumed it to be 1)1
Gblood - conductance of blood
G|| - parallel conductance of muscle

Drawbacks of this technique

  • The relationship between blood conductance and volume is non linear2, due to non linear shape of stimulating electric field
  • Conductance measurement extends into blood pool AND into surrounding tissue (muscle)
    • This implies that the measurement will artificially increase the volume because the catheter will see further than only the blood pool
    • The correction for the parallel conductance G|| is a calculated constant in the above equation, but the parallel conductance is known to be non-constant3
  • The accepted methods for parallel conductance measurement are outdated
    • Hypertonic saline bolus injection is commonly used to measure G||
    • This measured value is a constant, and is not time dependent
    • Need a better technique to separate the blood and muscle components of the signal
  • Conductance approach is modeled after an inaccurate circuit model for blood and myocardium
    • Traditional approach models both blood (Gblood) and cardiac muscle (Gmuscle) as real or resistive components only, ignoring the imaginary or capacitive properties of cardiac muscle
    • Hence, separating blood and muscle is difficult and often done incorrectly in the traditional approach

DC Circuit

Admittance technique

  • Admittance technique measures both conductive and capacitive properties of blood and muscle
  • The basis of measuring admittance instead of conductance is that at frequency ranges of about 20 kHz, blood is purely resistive and has no measurable capacitance, but muscle has both capacitance and resistance properties
  • This allows separation of the admittance of the muscle from the admittance of blood, using electric field theory
  • New proposed circuit model which models the blood (G_blood) as resistive, the cardiac muscle as both resistive (G_muscle) and capacitive (ωC_muscle)

AC Circuit

Advantages of Admittance technique

  • Eliminates need for hypertonic saline injection to determine parallel conductance
  • Contribution of muscle to the measured admittance signal can be subtracted real time, regardless of catheter orientation, heart size and catheter positioning
  • Greatest advantage is the accurate removal of parts of the admittance signal which do not contribute to the blood volume; hence more accurate blood volumes are obtained

References

  1. J. Baan, E. T. van der Velde, H. G. de Bruin, G. J. Smeenk, J. Koops, A. D. van Dijk, D. Temmerman, J. Senden, and B. Buis, "Continuous measurement of left ventricular volume in animals and humans by conductance catheter." Circulation, vol. 70, no. 5, pp. 812-823, Nov 1984.
  2. C.-L. Wei, J. W. Valvano, M. D. Feldman, and J. A. Pearce, "Nonlinear conductance-volume relationship for murine conductance catheter measurement system." IEEE Trans Biomed Eng, vol. 52, no. 10, pp. 1654-1661, Oct 2005.
  3. C.-L. Wei, J. W. Valvano, M. D. Feldman, D. Altman, A. Kottam, K. Raghavan, D. J. Fernandez, M. Reyes, D. Escobedo, and J. A. Pearce, "Evidence of time-varying myocardial contribution by in vivo magnitude and phase measurement in mice." Conf Proc IEEE Eng Med Biol Soc, vol. 5, pp. 3674-3677, 2004. [Online]. Available: http://dx.doi.org/10.1109/IEMBS.2004.1404032