Passport 2®/Passport 2 LT™ Service Manual
0070-10-0441
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Theory of Operation
Front End Module
2.2.10
Pacer Pulse/Electrosurgical Interference Detection
It is necessary to detect pacer pulses so that the rate meter can be made unresponsive to
them, and so that enhancement of the pacer pulse can be indicated on the monitor display.
Electrosurgical interference is similar in nature to pacer pulses, but while a pacer pulse
occurs at a repetition rate related to the normal range of heart rates, electrosurgical noise
has much higher repetition rates, due to rapid sparking at the active electrode. Therefore, a
single circuit can detect both types of signal, and categorize the detected event as a pacer or
noise according to the repetition rate. The detection circuit processes the ECG signal present
at the I, II, and III/V outputs.
The basis for detection of the pacer pulses is frequency - the frequency content of a pacer
being assumed to be higher than any normal physiologic signal. The most difficult pacer to
discriminate would therefore be one with the lowest slew rate, that is, the slowest rise time
and smallest amplitude. From the range of pacers defined in AAMI EC13-1992, this is a
pulse amplitude of 2 mV and a duration of 2 ms. The means of performing the frequency
discrimination is shown in the figure below.
In order to identify the pacer or noise spike, a window comparator, U214, is used to detect
when the output of U212 has a moderately high frequency content signal. U212 provides
rejection of the Respiration excitation carrier, but permits passage of pacer signals. Positive
and negative input pins of this comparator are biased near half of the +5 rail voltages. The
threshold of the window is approximately 0.4 volts wide. The comparator output network
consisting of C246 and R261, provides stretching for narrow input pulses of variable width
to output pulses having a wider width. Since the window comparator is symmetrical, the
system works equally well for pacer/noise pulses of either polarity. The outputs (open
collectors tied together) are used as the pacer flag connected to the microcontroller.
2.2.11
Respiration
Respiration is obtained through the ECG electrodes by impedance pneumography, in which
the AC impedance between a pair of the ECG electrodes is monitored. This impedance
varies with the chest motions associated with breathing. The respiration signal consists of a
small modulation on the order of 1 ohm, superimposed on a much larger baseline
impedance. The baseline impedance measured at the patient has a typical value of 500
ohms, and may reach 2000 ohms. However, when AAMI connector ECG cables are used,
the cable contains 1000 ohm resistors in series with each leadwire. Since the respiration is
measured through a pair of leadwires, 2000 ohms are added to the patient impedance.
When the connector board is equipped with additional defibrillation protection provisions
(series ECG resistors and high voltage respiration coupling capacitors), the front end board
can be used with resistor less ECG cables. Therefore, the total range of baseline impedance
ranges from somewhat less than 500 to 4000 ohms.
