We will look at two example applications and describe the test setup in each. The first is the Pacemaker and Implantable Cardioverter Defibrillator (ICD). The second is glucose monitoring devices
Testing Pacemaker and Defribrillator
A defibrillator is a device implanted into the body and under cardiac arrest conditions provides a shock to the heart to reestablish a beat. It also provides a pulse generation to pace the heart. A pacemaker provides primarily the later function.
Pacemakers and defibrillators consist of complex digital circuitry that must interface to its exterior environment through analog connections. A telemetry channel provides interfacing for device diagnostic purposes after implantation. Defibrillators must provide a kilovolt level shock to the heart without damaging the internal circuitry of the device. Because the device is implanted it must have a substantial battery life.
The test challenge relates to making precision measurements on complex digital circuitry that has been stimulating and interfaced through analog signals. While this is not a traditional mixed-signal test, signal analysis is a key element of testing the device. The second challenge is the wide range of signals to measure. The shock voltage reaches kilovolts and amperes, but must run on nanoamps and sense microvolts.
For pacemakers, the primary function is generating a pace pulse to ensure continuous heart beats. This pace pulse consists of a waveform signal, whose rise time, droop, and recharge pulse amplitude yields information about the condition of the digital circuitry. Figure 2 shows a graphical representation of the pulse signal to be measured.
Figure 2. Pace pulse from a pacemaker.
Test instrumentation for the pace pulse test consists of a signal source and a digitizer. The signal source provides the stimulus representing the heartbeat. A digitizer reads the response from the device representing the pacing pulse from the device to the heart.
Figure 3. Instrument setup for pacemaker test.
The sensing test uses the signal source to generate Haversine waveforms that simulate the electrical activity of a heartbeat. External shock tests the sensing circuitry after an external shock has been applied. This simulates the condition in which an external defibrillator has generated a shock to a patient with a pacemaker implanted. To perform this test, relays are used to switch high voltage capacitors onto the leads. With an external voltage applied, the device is tested to confirm its ability to detect heart rhythms.
Additional tests include the following:
- Battery Voltage
- Lead impedance
- Safety core
- Gross Fail Shock
- Timm – Leads
- Sense Amp Differential Gain
- Sense Amp Common Mode
- Temperature Measurement
- Battery usage
- Sensitivity Refractory Testing
LabVIEW provides an environment for developing individual tests for pacemakers and defibrillators. Here is an example application screenshot.
Figure 4. Example implementation of a Pacemaker test in a LabVIEW program.
A series of tests could be run in a sequence.
Figure 5. Teststand Sequence for Defibrillator testing
Glucose Monitoring Tester
Glucose monitoring is moving from the hospital bedside to at-home patient care. Handheld testers are coming onto the market to fulfill this demand. Testing a glucose monitor device is another example of NI tools performing medical device testing. The elements of a glucose monitor device include:
- Built-in microprocessor provides data collection, processing, user interface
- Patient interface for applying a fiber optic tip probe to a patient’s finger.
- Fiber probe tips are in standard fiber optic connector
- LCD Display and electronics enclosure
- Serial port for internal program updates
The National Instruments Mixed-Signal Platform tools test the internal electronics used for data collection and processing. NI data acquisition tools test the fiber optic interface to calibrate it for the measurement. For high-volume manufacturing, NI Vision tools can test the LCD display to ensure the proper functioning of the display and automate the testing of the correlation of the displayed information with the state of the instrument. NI motion tools can test the mechanical action of the fiber optic tip to ensure the precise positioning of the tip. Finally, NI LabVIEW can read the serial interface to compare external test results with internal test results for validation and verification purposes as the device must meet FDA requirements.
Innoventor Embedded Controls (IEC) worked with a start-up company that was developing a new glucose measurement technique. The technique is based on measuring the reduction in a florescence signal returned from a fiber sensor. The fiber sensor is etched to a sharp point and coated with a proprietary chemical mixture that produces the florescencing signal when stimulated with the proper wavelength.
IEC developed a meter to be used in the clinical trials of the new technique. The meter saves raw data internally to support more in-depth evaluations of probe responses. A simple LabVIEW application was created to simplify reading the data files, presenting the data of multiple probe samples and doing additional analysis.
Operating the glucose meter during development and production of clinical trial units required separating the performance of the meter from the variations of the fiber probe. Since the probe chemistry was still being modified and adjusted and the production of these probes was not consistent, a predictable and repeatable method of test was required. This was achieved by using an output of a PXI-MIO16E series board to create a programmable optical source.
The output of a National Instruments data acquisition board is used to drive a simple voltage-to-current drive circuit fashioned with an op amp to modulate the current of an LED. The LED is selected for the actual probe florescence wavelength peak. Light from the LED is coupled to a fiber with a simple lens thus completing the probe simulator. A LabVIEW program is created that generates a waveform of any desired probe model. This allows a flexibility to follow any probe changes be a simple adjustment of a model in the program.
Figure 6. LabVIEW example program for Glucose Monitor testing.