DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

RESPONSIVE SIMULATORS 283IC40B scales the summed signal and adds an offset of 2.5 V dc. IC40B’s output is thusV out 2.5 V Z2The change in impedance, which is seen between IC40D, pin 12, and ground, is given by100Z out 20 (5 V 1 XY/10Z )The function of a pacemaker is to stimulate the heart when the heart’s intrinsic pacemakeror conduction mechanisms fail to generate an action potential. This is where theresponsive behavior of the simulator comes into play. Pacing pulse detectors are used tosense pacing pulses generated by a pacemaker connected to the simulator. This cardiacsimulator responds to these stimuli by simulating the excitation of the heart chamberthrough which the pacing pulse was received. The atrial pacing detector circuit shown inFigure 6.25 is made up of IC7A, an inverting amplifier with a gain of 2, a comparator,IC8, and an inverter, IC9A. An external atrial pacing signal delivered by a pacemakeracross R73 and R74 is input to IC7A, which inverts and amplifies the pacing signal by afactor of 2. IC8 compares the voltage on its noninverting terminal, which is set to approximately1.0 V by trimmer R43. If the inverted pacing signal detected on pin 3 of IC8 isless than the voltage on pin 2, the output of the comparator is set high (5 V). This digitalhigh is inverted by IC9A to a low and input to the microcontroller’s RB6 input. Whenthe inverted pacing signal is greater than 1.0 V, the output of IC36 is low, which isinverted by IC35E to a high. This low-to-high transition generates a “change on port B”interrupt, which causes the microcontroller to execute the interrupt service routine code.R40 and C25 ensure that the pacing signal has sufficient amplitude and duration to tripthe comparator. This guarantees that narrow transients do not pass through and trigger themicrocontroller. The ventricular pacing detector circuit works in the same way as theatrial pacing detector described above, with the exception that the input pacing signal isinput across R75 and R76.The responsive cardiac simulator was designed to exercise three-chamber pacemakersand other devices intended to treat congestive heart failure. These sometimes deliverbipolar current pulses to the left ventricle, and it is important to monitor the approximatevalue of the positive and negative current pulses. The bar graph LED display of Figure6.26 shows the amplitude of the left-ventricle pacing pulse in 2-mA steps. D1 and D2steer each phase of the bipolar current pulse to the correct display circuitry. The currentfor the positive phase of the left-ventricular pacing pulse flows into the junction ofR52–R60. A minimum of 2 mA is required to turn on optocoupler IC19 and the first segmentof the bar graph display.For discussion purposes let’s suppose that the pacing pulse delivered to the left ventricleconsists of a 4-mA current pulse followed by a 4-mA pulse. Then 2 mA wouldflow through R61 and turn on the optocoupler and the first section of the bar graph display.The voltage across R61 will forward bias the base–emitter junction of the PNP transistorin IC19 (across pins 5 and 6). Once the PNP transistor is forward biased, the inputcurrent can also flow through the base–emitter junction of the NPN transistor. This currentflow switches on the NPN transistor in the IC and allows the additional 2 mA (what’sleft of our 4 mA) to flow through R52, the collector–emitter junction of the NPN transistorand the second LED of the bar graph display. Current will flow through the secondLED of the bar graph display and through R53 and should be enough to forward bias thenext PNP transistor (IC12). But there would not be enough remaining current flow to turnon the LED in the third section of the bar graph display. Each successive LED of the bar