Mashavu Pulse Rate Project

[Laura Arnold]

Contact Information

University and Department: Penn State, Department of Bioengineering

Team Members: Mike Perone, Kevin Hom, Gabriela Hernandez, Nick Bizzaro, Vy Nguyen, Lauren Anderson

Project Information

Describe the challenge that your project is trying to solve:

The goal of our team is to successfully construct an accurate pulse rate monitor that will be used in Kenya as part of the Mashavu project to provide Kenyans with a basic level of information regarding their health.

There is a great need for fast and efficient basic health care in Eastern Africa due to the high cost and time consumed in walking miles to the nearest clinic.  People in this region make very little money, so the customer need for a cheap screening of vital statistics would drastically help the local health care infrastructure. 

The goal of the Mashavu Project is to create user-friendly kiosks where biomedical devices may interface with a computer to inform the patient whether or not it is necessary to travel to a hospital.

Customer Needs:

Low Cost

Simple Hardware, substituted by software.

Can be used multiple times

Easy to use, easy to repair, easy to sanitize.

Non Invasive measurement.

Adjustable for use in adults and children.

Durable

Parts have to be readily available in Kenya.

5 volts is the maximum input voltage.

The kiosk staff will need to be trained to use the device and to fix it.

Make the measurements without requiring the patient to disrobe.

Implementation in other East African countries.

Common diseases that affect the pulse rate:

Dengue fever a disease caused by mosquito bites, may cause internal hemorrhage and needs immediate treatment in which several non steroidal analgesics, such as aspirin need to be avoided.  Dengue fever results in a weak rapid pulse.

Hookworm is a parasitic worm that causes damage to the intestines, as well as anemia.  Hookworm symptoms include heart palpitations coupled with a barely perceptible pulse.

Yellow fever is a viral disease transmitted by mosquitoes. It leads to 200,000 illnesses and 30,000 deaths every year in unvaccinated populations.  Bradychardia is a symptom of yellow fever.

Typhoid fever is caused by the salmonella bacteria and may lead to encephalitis and intestinal perforation and hemorrhaging.  In its first stage a symptom of Typhoid fever is bradycardia.

Describe how you addressed the challenge through your project.

There are a number of design criteria and specifications that are important for the pulse rate monitor to be appropriately implemented in Kenya, which are outlined below:

      Accuracy:

The pulse rate monitor must be accurate so as to cut down on the number of false positive and false negative diagnoses.  This means that whether or not there is a genuine problem with the subject’s heart rate the device will provide a reading that could help accurately describe the condition of the subject.  This requires the sensor to be sensitive enough to pick up each individual pulse.  The device is expected to have an error <5% compared to measurements from working devices and methods.  Our monitor is designed to measure heart rates ranging from 50 - 220 bpm.

Minimal Cost:

If the cost to build or repair the pulse rate monitor is too high, then the device will not be appropriate for use in the Mashavu project.  Fortunately, the total cost for the materials in our project is less than ten dollars.  Using only a sensor, a few resistors, a wrist strap, a plastic link, and some adhesives the design is very simple.  The pulse rate monitor should cost no more than $10 to build.

      Ease of use:

The device needs to be easy enough for a non-health professional to be able to use with minimal training.  This means the operation of the device must be very simple.  The Velcro strap is adjustable so that it can easily be used on both small children and large adults.  Also, an easy to use LabVIEW program that is designed to automatically calculate the pulse rate of the subject is necessary so that no calculations are necessary to get the desired data.  Using the force sensing resistor and LabVIEW there is no need for calibration of the device.

      Durability/Ease of repair:

Kenya is an area with few resources and educated professionals who have the ability to make repairs to the device if necessary.  This makes it important that the device is fairly resistant to damage, easy to repair and cleanable in the event that it gets dirty.  Our design incorporates a strip of plastic that is adhesive on one side and Velcro on the other.  The adhesive side is attached to the force sensing resistor and the Velcro side attaches to the wrist strap so that replacing the strap or the sensor is an easy task if necessary.  Also, the device is lightweight so that if it gets dropped it will not impact the ground with any significant force, preventing damage to the sensor, which is housed in a protective shield  The number of components in the device design is limited to 4, excluding the DAQ device.

Availability of materials:

The materials used to construct the wrist strap and the housing for the sensor should be available in the USA and Kenya so that any parts can be replaced if necessary.  Every part of our device can be found at Walmart with the exception of the sensor.

      Cultural issues:

Amongst our possible designs, most of them involved using a strain sensor on various pulse points on the body. Unfortunately, not all of them were appropriate for use in Kenya.  Our group decided on the wrist strap design so that it was not necessary to remove any clothes (there is no privacy at the kiosks) or put the sensor in an uncomfortable or dangerous place, like the neck

Our design for the pulse rate monitor is based around a force sensing resistor (FSR).  This sensor changes in resistance given an input voltage proportional to the pressure on the sensor when a force is applied against the active area.  The FSR is placed on the wrist so that it is in close contact with the radial artery, which provides pulsating force that can be measured.  When the heart beats the radial artery will pulse and cause a deflection in the sensor that will send a signal.

Fig. 1 Close up Picture of the sensor.

The radial artery force due to pulses applies a stimulus to the FSR.  This FSR is available from Sparkfun.com part number SEN-09673.  To detect the pulses, the FSR is connected to a voltage divider circuit as seen in Fig. 2 that includes a 5V input from the DAQ device and a 2 kOhm resistor.  The voltage divider equation results in an output voltage to the DAQ device from the equation Vout = 5V(FSR/(FSR+2kOhms)).  The circuit is connected to the DAQ device to an analog input (AI) with respect to ground (AI GND).

Fig. 2.  Voltage divider circuit applied in the design

The voltage difference signal will pass through the DAQ device and into the computer via USB where it will be read by the LabVIEW program.  LabVIEW will take the signal and use it to computer pulse rate from it two different ways.  It will determine the period of the signal and use that to calculate a heart rate.  At the same time the program will also be measuring the number of peaks above a certain threshold over a period of time.  The peak counter can then take its measurement and convert it to a pulse rate as well.  These results will be read out on the screen.

The construction of the actual device is quite simple.  The sensor is attached to a small piece of tape that is adhesive on one side and Velcro on the other.  The Velcro side is then attached to the inside of the strap.  The other side of the strap is composed of the polyester loops commonly seen in Velcro.  The strap can be wrapped around the wrist (and the sensor pressed against the skin) by threading the end of the strap through the hole at the top and tightened before being fastened down by the Velcro.  Since it is adjustable the strap can be used on both children and adults.

Fig. 3. Sketch of the final design.

Finite Element Analysis Modeling using COMSOL

Our design for measuring pulse rate utilizes the Force Sensing Resistor [FSR] (FSR-400 from Images SI, Inc.), which we have chosen to model using COMSOL.  We intended to simulate the voltage of the film over a given external pulsating signal at the center of the circular active area in order to simulate the pulsating effect of the radial artery (located at the wrist) with our sensor. We also intended to determine the output voltage of a FSR versus a piezoelectric film sensor (PZ-02) in order to decide which sensor to use. Ultimately, we proceeded with the FSR sensor. Our decision to use the FSR over the piezoelectric film is explained below.

Using experimental data from past research on radial artery pulse rate measuring allowed a more precise way of simulating the effect of the pulse going through the radial artery. We assumed in our model that the FSR was firmly adhered to the skin so that the model only simulates the presumed radial pulse measured at the skin.  We used benchmark results from the Phoenix Ambulatory Blood Pressure Group that did similar measurements to obtain an approximate force. We noticed that the benchmark results essentially contained sine wave pulses, which we brought into our model.   

Additionally, the exact material properties of each element in the film were also utilized in order to have a more precise solution.  We used a force of 20g (5g by radial artery, 15g of force created by the strap) working at a frequency of 1.3 Hz, which corresponds to an average pulse rate of 78 beats/min (bpm) for a person. Once the model was solved, we intended to view the voltage graph at the center point of the active area for 15 seconds via post-processing to see how the FSR changes in voltages with time and if it would match our benchmark results; i.e. pulses (multiply number of large peaks measured this way by 4 to get calculated beats per min – should be ~80 from model benchmark). Therefore, if our COMSOL model matched our model benchmark results, we can use it in turn as a benchmark for our experiment.

The FSR is shown below as it appears after simulation in COMSOL:

Fig 1: FSR COMSOL model upon simulation

Upon simulating the FSR model, there were some surprising results, shown in the figure below.

Fig 2: FSR Voltages at the center point of active area as simulated by COMSOL

As shown above, our FSR results show an output of ~0.2 Volts which is significant enough to be useful for our design. However, we don't want to proceed without knowing how much voltage the piezoelectric sensor (PZ-02 from Images SI, Inc.) can output given the same pulsating input signal. So, another model was created to simulate the piezoelectric film with approximately the same amount of pulsating force.


When the FSR is compared to the piezoelectric model results (shown below), there is a vast difference in measured voltages. In comparing the first principal strains between the free end and where the leads are, there shows a small but significant difference in strains. These strains can be translated into electric potential by the piezoelectric piece between 15-20 mV per microstrain (1e-6). Based on the differences in peaks (they are simulating R peaks at ~80 bpm), it is approximately equal to 3.8e-7 which at 15 mV/ustrain, equals 5.7 mV at the peaks. This, in effect, is significant enough to be detected by LABVIEW where we can proceed with a piezoelectric film sensor design, but the voltages are far smaller. Seeing how the voltages are relatively small in comparison to the input voltages, there may be a need to filter noise from external sources in LABVIEW.

Fig. 3: Piezoelectric film COMSOL model

            Fig. 4: First principal strains at x=0.038 m, y=0.00016 m   

                 




















         Fig. 5: First principal strains at x=0.010 m, y=0.00016 m

Data Acquisition and Analysis using LabVIEW

The purpose of the pulse rate device is to measure the number of pulses over a given period of time.  The FSR detects the changes in the force applied caused by a pulse; this changes the voltage drop across the variable resistor.  The DAQ device measures the voltage change and imports the data to LabVIEW for analysis.

LabVIEW Goals:

1. Count pulse and out put a rate in beats per minute
   
2. Filter ambient noise (i.e. 60 Hz power interference)

Program Block Diagram:

1. "DAQ Assistant" - Obtains voltage change with respect to ground
   • 120k samples at 4 kHz (samples are taken over 30 seconds)
   • Data taken in RSE mode (with respect to ground)
2. "Filter Express VI" - Smooths curve to isolate pulse peaks
   • Set to operate as a "smoothing" filter through a moving average technique
   • Half-width = 1600 samples
3. "Butterworth Filter" - Attenuates ambient noise
   
   • Set as a low-pass filter (break frequency of 5 Hz)
   • Sampling rate = 4 kHz
   • 5th order filter to maximize effectiveness
4. "Peak Detector VI" - Set to counts peaks from data arrays
5. Peaks are doubled and outputted to a numerical indicator as pulse rate in beats per minute
6. A time indicator counts down 30 seconds for aesthetic purposes

Figure 1 - LabVIEW program block diagram

Program Front Panel:

1. "Waveform chart" - Provides visual interest of peaks detected
2. "Pulse rate numerical indicator" - Outputs measured pulse rate (bpm)
3. "Time count-down indicator" - Counts down 30s
4. "Go button" - Initiates program when the customer is ready
5. "Stop button" - Immediately terminates program if needed.

Figure 2 - LabVIEW program front panel user interface

Example of the Expected Pulse Rate Waveform:

Figure 3- Resting pulse (top); exercise pulse (bottom)

The final design of the pulse rate monitor is quite simple, using only a few inexpensive pieces.  
Attached to an adjustable Velcro strap is a force sensing resistor that is wrapped around    the wrist to measure pulses.  When the strap is placed around the wrist so that the sensor is near the radial artery it picks up the small changes in pressure that correspond to the pulse rate of the patient.  This signal is transmitted through a low pass filter meant to remove noise, through the DAQ device and into labVIEW where pulse rate will be calculate in two ways.

Determining the period of each peak and converting it to frequency will provide the number of beats per second

which can be calculated as a heart rate.  Another way to measure pulse rate is to look at the number of peaks that occur above a certain threshold over the course of a minute.  The sum of these peaks is the patient’s heart rate.

Our Kenyan customers are in need of a simple way to measure their pulse rate since they cannot afford to use nearly any modern medical technology.  Having a simple method of measuring pulse will aid in the diagnosis of many diseases that are common in Kenya in addition to providing standard health information to the doctors and customers.

Fig. 1 Pulse Rate Monitor being tested on a patient