Theory Of Operation; The Conductance Method - Millar MPVS Ultra User Manual

IV. Theory of Operation

The Conductance Method

Single Segment Measurement
Researchers are able to estimate left-ventricular volume changes via the conductance
method using the tiny electrodes built into the catheter tip.
A small constant-amplitude AC current is sent out through the distal electrode (E1) to the
proximal electrode (E4). The current flow creates an electric field within the ventricle.
The voltage potentials (differences) between the two inner sensing electrodes (E2 and E3)
are measured as the heart expands and contracts and the volume of the left ventricular
blood pool changes. The operational theory of the impedance/conductance method for
estimating left ventricular volume changes can be simplified in terms of Ohm's Law.
During systole, the heart pumps blood out of the left ventricle and the resistance of the
blood pool increases; therefore, the voltage potential between the inner two electrodes (E2
and E3) also increases.
During diastole, the left ventricle is filling with blood and the resistance of the blood pool
decreases; therefore, the voltage potential between the inner two electrodes (E2 and E3)
also decreases.
A good analogy is to think of the blood-filled left ventricle as a wire that changes size
during the cardiac cycle.
M.I. P/N 004-2163 Rev. C
Ohm's Law
V = IR
G = 1 , therefore R = 1
R
V = I , therefore G = I
G
Volume G = I
____________________
V = voltage potential between E2 and E3
I = constant current
C = conductance (the inverse of resistance)
R = resistance of the left ventricular blood pool
Systole = R ↑ V ↑ G ↓ and Volume ↓
Diastole = R ↓ V ↓ G ↑ and Volume ↑
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x
G
x
V
x
V