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Deriving volume from a conductance catheter is based on very simple electrical principles. Conductance catheters are comprised of both excitation electrodes and recording electrodes. The excitation electrodes (most distal and proximal electrodes on the catheter) generate an electrical field inside the heart. This field is generated as a result of an alternating current being applied (at a constant magnitude) between these 2 outermost electrodes. The inner recording electrodes measure voltage change which is proportional to a change in resistance.
Conductance can be derived by considering Ohm’s Law:
V=IR
Where V= voltage, I = current and R = resistance.
Since we measure resistance, we can easily calculate conductance (G) given the following inverse relationship:
G=1/R
In 1981, Baan proposed a relationship between time-varying measurements of conductance (Gt) to time-varying changes in volume (Vt) as follows:
Vt = 1/α(ρL2)(Gt-Gρ)
This volume formula takes into account the distance between the recording electrodes (L) and blood resistivity (ρ). It also takes into account the importance of correcting for parallel conductance (Gρ) which generally results in an overestimation of volume as well as the field correction factor alpha (α) which often results in an underestimation of stroke volume.
Theory - Classical Conductance