Dual Field Excitation - Millar MPVS Ultra User Manual


A thick gauge wire (large diameter) will have a low resistance and a low voltage drop across
it (diastole). The wire contains more volume because it has a larger diameter.

A thin gauge wire (small diameter) will have a high resistance and a high voltage drop across
it (systole). The wire contains less volume because it has a smaller diameter.
Multi-Segment vs. Single-Segment
Millar's multi-segment P-V catheters measure volume using the conductance technique
described above. Unlike the single-segment catheter, a multi-segment catheter measures
conductance over several electrode segments. The system then sums individual segments'
signals to generate a total volume signal over the length of the catheter. In the MPVS
Ultra system this summation is performed in PVAN Ultra when the user records data for
individual segments. The composite volume channel is an analog sum of the voltages in
the active channels.
Baan's Equation
The basis for calculating volume from conductance is Baan's equation. The general
equation for volume measurement is given to be:
where alpha (α) is the volume calibration factor, rho (ρ) is the resistivity of the blood, L is
the segment length, G is the conductance and Gp is the parallel conductance of surrounding
structure.
In a multi-segment measurement, G is actually a summation of the segmental conductance
measurements.

Dual Field Excitation

The dual field method sends the primary excitation current through a selected primary
excitation electrode (E5 through E11) located at the proximal end of the measurement area
and through primary excitation electrode (E1) located at the distal end of the measurement
area to generate an electrical field within the heart. In addition to the primary excitation
current, a secondary excitation current of opposite polarity is sent through the next
electrode, located distal to the selected primary excitation electrode at the proximal end of
the measurement area and secondary excitation electrode (E2) at the distal end of the
measurement area. This secondary excitation current creates a second electrical field
which serves to expand the primary field to create a more uniform electrical field over the
length of the measurement area. The change in conductance as the blood pool changes is
then measured across each electrode segment and then summed for a total volume signal in
the same manner as the single field configuration.
Dual field catheters can be quickly identified by the group of three closely spaced
electrodes at either the proximal or distal end of the measurement area.
Field lines and equipotential planes
Ideally, when measuring the resistance of a uniform (homogeneous) material, the paths of
the electrons are straight from one end of the sample to the other. Equipotential planes,
which are perpendicular to the current flow, would be flat surfaces. The result would be a
current density that is uniform throughout the sample, which means that every portion of
the sample affects the measurement equally. This requires the current generated in the
M.I. P/N 004-2163 Rev. C
2
Volume = 1 ρ L (G – G
α bb
26
)
p