Biomedical Engineering Reference
In-Depth Information
Figure 6.19
c
shows how the demodulated signal faithfully reproduces the dc o
ff
set and
low-frequency components of the ECG.
Since the sound card output is in the audible range, the modulated signal can be trans-
mitted to the demodulator via a voice radio or telephonic link for remote signal genera-
tion. To do so, however, the tone frequencies produced by the sound card for a full-scale
input must be limited to the bandpass of the communications channel. For a plain tele-
phone line, this range is 400 Hz to 3 kHz, while a commercial FM audio link is speci
ed
to cover the audio bandwidth 30 Hz to 15 kHz. Another interesting possibility is to use a
small 1 : 1 audio isolation transformer and a
fi
floating power supply to turn the demodula-
tor into an isolated output stage. Finally, it should be noted that the full bandwidth of a
single sound card channel can be shared by multiple software modulators occupying sep-
arate audio bands to convey various simultaneous low-frequency signals to an array of
PLL demodulators
fl
RESPONSIVE SIMULATORS
Signal generators are OK for testing medical instruments that only measure, process, ana-
lyze, or display physiological signals. However, many medical devices are used to deliver
a therapy that dynamically changes the physiological signals that are measured. In this
case, output-only signal generators are of only limited use. Take, for example, a DDD
pacemaker, described in Chapter 8. This pacemaker can pace both the right atrium and the
right ventricle separately at dynamically variable time delays to mimic the natural heart-
beat whenever one or both chambers fail to contract on their own. To do so, the pacemaker
can sense intrinsic electrical signals from both chambers. Whenever timely intrinsic activ-
ity is present in both atrium and ventricle, the device inhibits pacing. However, when ven-
tricular intrinsic activity does not follow the atrial activity in a timely manner, the device
triggers pacing on the ventricle in sequence after the atrium.
Testing a DDD pacemaker requires a simulator that is capable of emulating many of the
heart's electrophysiological properties. Many subtleties about the heart's conduction sys-
tem need to be designed into the simulator, including the way in which the atria and ven-
tricles become refractory for some time after being excited intrinsically or arti
cially. In
addition, a cardiac simulator suitable for interacting with a pacemaker should be able to
exhibit pacing thresholds similar to those of a typical heart, and the cardiac signals gener-
ated by the simulator's “chambers” must have morphologies, amplitudes, and timings sim-
ilar to real P- and R-waves detected with intracradiac electrodes.
We designed a responsive cardiac simulator as a test tool for three-chamber pacemak-
ers (pacemakers that cannot only stimulate the right atrium and right ventricle, but can also
synchronize the activity of the left ventricle to the pumping of the right heart). The circuit
and timing characteristics for this responsive simulator are shown in Figures 6.20 to 6.28.
The heart's electrical activity as seen by intracardiac electrodes is simulated by the
CENELEC signal
.
2
Three signal generators, one corresponding to the heart's right atrium,
one to the right ventricle, and the last to the left ventricle, provide programmable, 0- to 9-
mV CENELEC signal outputs. The following explanation refers to the signal generator for
the right atrium but is also applicable to the other two, since the right- and left-ventricle
signal generator circuits are similar. The CENELEC waveforms are stored as 12-bit values
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2
CENELEC
stands for “Comité Européen de Normalisation Electrotechnique” (European Committee for Elec-
trotechnical Standardization). The CENELEC signal is speci
ed in Figure FF.103 of the preliminary draft of the
EN-45502-2-1 standard:
Active Implantable Medical Devices—Part 2-1: Particular Requirements for Active
Implantable Medical Devices Intended to Treat Bradyarrhythmia (Cardiac Pacemakers)
, January 2001. This wave-
form is intended as a test signal used for the exact determination of sensitivity (sensing threshold) of pacemakers.
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