(a) A pacemaker pulse picked up by any combination of leads, by the
lead selector switch and applied to the CMR's fuse, is amplified differentially
(approximately five times) at pin 14 of U3 together with the regular ECG signal. The much
shorter pacemaker pulse, through differentiation, is presented at pin 2 of U4. It is inverted
in phase, amplified 50 times, and clipped before it proceeds to the voltage comparator
stages of U5.
(b) Either positive or negative polarity pulses, whichever occurs first,
triggers a monostable oscillator, U6, which delivers a 4.5msec long pulse at its Q output
(pin 10). This throws the analog gate of U8 into full conduction exciting the LED of opto-
coupler K8 which transfers the signal to the grounded section.
(c) To avoid overload and long recovery times in the Hi gain section of
the preamplifier, this pulse has to be cut out from the normal signal flow. The other three
sections of U8 are driven by the complementary output of U6 (pin 11) and will open during
the pulse. This prevents overload. The signal then progresses to the voltage to frequency
(VTF) converter U7.
(d) At the grounded side of K8, the pacemaker signal triggers U116 in a
monostable configuration. The output signal is inverted into the last gain stage prior to the
v/mv output and A/D converter so as to be recorded in any chart recorder mode and on
the CRT as a negative going marker.
c. Functions of the Circuits in the Ground Amplifier Section.
(a) The pcb accepts two signals emanating from separate preamplifiers
and with distinctive frequency characteristics. A pacemaker signal entering the patient
cable pickup can be recognized and displayed as a marker pulse on the CRT and on the
chart recorder in real time or delayed mode.
(b) The signal presented at the chart recorder can be of diagnostic
quality real time or delayed, monitoring quality, with a total delay of five seconds. This
permits the recording of what happened during the five seconds before any event, such as
defibrillator firing or heart rate alarm, to be displayed when the chart recorder is in AUTO
mode. The display quality of the CRT is always of monitor quality (0.5Hz-40Hz).
(c) A calibrated 1mv pulse can be injected into the signal line for
comparison purposes. A three digit heart rate meter combined with an independent
selectable Hi and Lo rate alarm completes the main features of the system. A
synchronized pulse selected at the defibrillator assembly is displayed as a marker on the
CRT and delayed output to the chart recorder in cardioversion mode.