M.I. P/N 004-2163 Rev. C
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Resistivity – Dip Cuvette
The dip cuvette method used in the MPVS-300, MPVS-400, and MPCU-200 ARIA
pressure-volume measurement systems was used as a way of relating measurements to
volume in a more empirical manner. The dip cuvette contains wells into which blood is
injected. The catheter is then dipped into each well so that the system is reading from a
known volume in a material assumed to have the same resistivity as that in the subject’s
ventricle. Relating the resulting readings to the known volumes (typically through a linear
regression) allows the user to calibrate measured voltages to volumes. This method is less
direct than that employed by the MPVS
Ultra
system and is not practical for use with
multi-segment P-V catheters.
IECG vs. ECG
The MPVS
Ultra
can measure the ECG signal from the PV (catheter) input or the ECG
input. When the system measures ECG from the PV catheter, the signal is an internal
ECG, or IECG. Since the electrodes are in the ventricle, the source of the R wave, the R
wave will be much larger than the other aspects of the ECG signal. While this may not
provide some of the information for a detailed ECG analysis, the signal-to-noise ratio will
be reduced and the signal will be ideal for timing in PVAN
Ultra
calculations. The input
to the IECG signal is the electrodes that are presently in use for the primary excitation
field. This ensures that the spacing between the electrodes will be as wide as possible for a
maximal signal.
When the system measures the ECG through the ECG connector, the signal will resemble a
normal ECG signal.
Since the hardware only has one ECG amplifier circuit, the MPVS
Ultra
can only measure
one signal at a time.
Strain Gauge Pressure Sensors and Catheter Tip
Transducers
Strain Gauge Pressure Sensor
Millar Mikro-Tip pressure transducers use a microscopic pressure sensor embedded in the
tip of a catheter to measure pressure directly at the source. The design offers superior size,
accuracy, and frequency response. The pressure sensor contains a pair of strain-gauges on
a very thin diaphragm. These gauges are resistors whose resistance changes as they are
stretched. The gauges are stretched when pressure exerted on the diaphragm makes it
bend. These resistors are in turn wired into a Wheatstone bridge circuit. When the
resistors change resistance in response to pressure changes, the output voltage of the
Wheatstone bridge changes. The voltage output of the circuit is amplified by the MPVS
Ultra
, whose output is factory calibrated to a 1V output for an input pressure of 100mmHg.
Theor
y
of Operation
