DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

RESPONSIVE SIMULATORS 277Figure 6.19c shows how the demodulated signal faithfully reproduces the dc offset andlow-frequency components of the ECG.Since the sound card output is in the audible range, the modulated signal can be transmittedto the demodulator via a voice radio or telephonic link for remote signal generation.To do so, however, the tone frequencies produced by the sound card for a full-scaleinput must be limited to the bandpass of the communications channel. For a plain telephoneline, this range is 400 Hz to 3 kHz, while a commercial FM audio link is specifiedto cover the audio bandwidth 30 Hz to 15 kHz. Another interesting possibility is to use asmall 1 : 1 audio isolation transformer and a floating power supply to turn the demodulatorinto an isolated output stage. Finally, it should be noted that the full bandwidth of asingle sound card channel can be shared by multiple software modulators occupying separateaudio bands to convey various simultaneous low-frequency signals to an array ofPLL demodulatorsRESPONSIVE SIMULATORSSignal generators are OK for testing medical instruments that only measure, process, analyze,or display physiological signals. However, many medical devices are used to delivera therapy that dynamically changes the physiological signals that are measured. In thiscase, output-only signal generators are of only limited use. Take, for example, a DDDpacemaker, described in Chapter 8. This pacemaker can pace both the right atrium and theright ventricle separately at dynamically variable time delays to mimic the natural heartbeatwhenever one or both chambers fail to contract on their own. To do so, the pacemakercan sense intrinsic electrical signals from both chambers. Whenever timely intrinsic activityis present in both atrium and ventricle, the device inhibits pacing. However, when ventricularintrinsic activity does not follow the atrial activity in a timely manner, the devicetriggers pacing on the ventricle in sequence after the atrium.Testing a DDD pacemaker requires a simulator that is capable of emulating many of theheart’s electrophysiological properties. Many subtleties about the heart’s conduction systemneed to be designed into the simulator, including the way in which the atria and ventriclesbecome refractory for some time after being excited intrinsically or artificially. Inaddition, a cardiac simulator suitable for interacting with a pacemaker should be able toexhibit pacing thresholds similar to those of a typical heart, and the cardiac signals generatedby the simulator’s “chambers” must have morphologies, amplitudes, and timings similarto real P- and R-waves detected with intracradiac electrodes.We designed a responsive cardiac simulator as a test tool for three-chamber pacemakers(pacemakers that cannot only stimulate the right atrium and right ventricle, but can alsosynchronize the activity of the left ventricle to the pumping of the right heart). The circuitand 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 theCENELEC 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 forthe right atrium but is also applicable to the other two, since the right- and left-ventriclesignal generator circuits are similar. The CENELEC waveforms are stored as 12-bit values2CENELEC stands for “Comité Européen de Normalisation Electrotechnique” (European Committee for ElectrotechnicalStandardization). The CENELEC signal is specified in Figure FF.103 of the preliminary draft of theEN-45502-2-1 standard: Active Implantable Medical Devices—Part 2-1: Particular Requirements for ActiveImplantable Medical Devices Intended to Treat Bradyarrhythmia (Cardiac Pacemakers), January 2001. This waveformis intended as a test signal used for the exact determination of sensitivity (sensing threshold) of pacemakers.