Programmable Biofeedback Chest Exerciser
BME 273, Spring 2007
Group 23
Group Members: Eileen Bock, Lauren Cassell, Margaret Gipson, Laurie McAlexander
Advisors: Douglas Sawyer, M.D, Ph.D.; John Newman, M.D; Paul King, Ph.D.
Abstract
The purpose of the project is to design, build, and implement a programmable
biofeedback chest exerciser for patients with heart and lung disease. Deep breathing has
been found to be an effective form of exercise for heart failure patients. A biofeedback
chest exerciser will monitor depth and rate of breathing and communicate with the
subject so that he or she follows a physician’s breathing exercise protocol. The change in
a subject’s thoracic circumference as he or she breathes in and out is measured, and a
variable resistor is incorporated into a strap to be worn around the subject’s chest. A two
stage inverting amplifier circuit amplifies the voltage change that occurs as a result of the
change in resistance. This output voltage is connected to LabVIEW which detects when
depth and rate thresholds are not met in accordance with a physician’s prescribed
protocol. A linear relationship between output voltage and tidal volume was established
by performing experiments on members of the group to verify the validity of the circuit
design as a method of measuring breathing rate. Once the linearity was verified, the
optimal starting length of the bend sensor was found since sensitivity changes with
different lengths. A deep breath was then determined by performing tidal volume
experiments to obtain an accurate data manipulation and results. This device effectively
measures the breathing depth and rate of a subject while keeping cost low and is much
less conspicuous than other devices on the market. In the future, an improved
biofeedback system will be incorporated into a wireless data acquisition device to notify
the patient when an adequate breathing rate or depth is not accomplished. IRB protocol
will also be submitted for clinical testing of the device.
Introduction
Chronic heart failure affects 5 million Americans, with 550,000 new cases every
year. This disease carries a 50% mortality rate within 4 years of diagnosis and affects
many Americans' quality of life. This disease has a number of causes, including heart
attack, coronary artery disease, valve disease, chronic hypertension, cardiomyopathy, and
hormonal disorders such as thyroid disease. Coronary artery disease is the most common
cause of heart failure. These problems either originate in the heart or place excessive
demands on the heart, causing chronic heart failure (Klabunde).
During heart failure, a number of adverse effects occur. Vascular function
changes occur to try to maintain cardiac output, such as raising arterial blood pressure
and blood volume, but these compensation mechanisms can actually make heart function
worse. A patient with heart failure has a decreased cardiac output, increased end-diastolic
pressure, impaired ability to fill the heart with blood, and a reduced ejection fraction.
These changes affect the vascular system, resulting in increased vascular resistance, a
decrease in arterial pressure, and impaired organ perfusion. Because the body cannot get
the nutrients it needs from the blood, symptoms such as shortness of breath, fatigue, chest
pain, and an irregular heartbeat are common in patients with chronic heart failure
(Klabunde).
There are many options for the treatment of heart failure. Drug therapy is a
common method of treating heart disease and many different drug therapies are used.
Diuretics work by decreasing blood volume in the body by increasing the amount of fluid
that is excreted from the body. Decreasing the amount of blood decreases the pressure
placed on the heart. Vasodilators and nitrates dilate blood vessels, allowing more blood to
flow to both the heart and other places in the body for more effective pumping and better
nutrient delivery to tissues. Surgery is another option. A left ventricular assistive device
can be implanted, which acts as a pump for the left ventricle to aid in blood circulation.
Heart transplants may be necessary at advanced stages of the disease, and pacemakers
can also be implanted to restore regular heartbeat (Klabunde).
Exercise has also been found to be an effective treatment of the disease, and
unlike other methods of treatment, this can actually help the body gain strength and
reverse some of the damage heart disease causes. Exercise improves vascular function by
improving heart contractility, reducing adrenaline levels and the "fight or flight"
response, improves muscle function, thereby increasing ability to deliver nutrients to
tissues, and reduces the risk of an irregular heartbeat. Unfortunately, many patients with
heart failure already experience shortness of breath and fatigue, so exercising in a
traditional sense is difficult. Deep breathing, however, has been thought to be an effective
method of exercise for these patients because it exercises the muscles of the chest wall
and specifically strengthens these muscles, which can help strengthen the heart muscle
and reverse some of the adverse effects of heart disease. The device created aims to
measure the breathing depth and rate of a patient who has been prescribed a deep
breathing regimen by a physician as a treatment for heart failure and the device will alert
the patient when a desired breathing rate or breathing depth is not attained.
Currently, there are several devices available that perform similar functions. The
incentive spirometer is a common device available in hospitals and clinics. In order to use
this device, a patient breathes into the mouthpiece, which moves a piston in the device to
record the volume of air the patient expels. This accomplishes the task of measuring
depth of breath, but gives no indication of breathing rate, and does not give the user any
information about the breath depth that is adequate to accomplish chest exercise.
Additionally, the patient must be stationary to use this device, and it has a large,
conspicuous shape ("How to Use an Incentive Spirometer").
The Pneumotrace measures respiration changes through a series of belts around
the patient and a strain gauge transducer. The multiple belts on this device, though, may
make it inconvenient to use. The patient must also be lying down to use the device
(REST1 Impedance Pneumograph).
The RESPeRATE device is mainly used as a method to decrease blood pressure
in a patient. The device consists of a belt, headphones, and a handheld electronic
component. After measuring the patients breathing rate, the device tells the user the depth
and rate of breathing through the headphones in an attempt to lower the breathing rate
and increase the depth of each breath. This device may be inconvenient because the
headphones are not conducive to use during daily activities that require a patient to be
alert of the sounds of his or her environment (Buyamag). Finally, the VivoMetrics
LifeShirt is a garment that is worn continuously. It resembles a vest and constantly
records respiration data. This device is large, conspicuous, and also cannot be
incorporated into daily life very easily (VivoMetrics).
The goal of this project is to create a chest exerciser that will record the rate and
depth of breathing and alert the patient if a deep breath has not been taken in a
predetermined time period, which will be prescribed by a patient's physician. Patients
will be more likely to wear this device and use it effectively because of its small size and
ability to privately alert the patient when the appropriate breathing protocol is not
performed. Using this device should increase the strength of the chest wall muscles and,
ultimately, improve heart function.
Methodology
The first step in designing the programmable chest exerciser was to determine a
method for detecting and measuring respiration in a subject. There are several ways to
measure the respiration of a subject, including sensing intranasal pressure changes,
intranasal air flow, intrapulmonary pressure changes and flow, and chest circumference
changes. Preliminary experiments were conducted with several different members of the
design team to determine the relationship between chest circumference expansion and
tidal volume during respiration using a spirometer. It was concluded that chest
circumference and tidal volume are linearly related.
Some of the most pertinent goals of the device are accurate breathing
measurement, inconspicuousness, patient
comfort, and patient compliance. The
discovery of a linear relationship between
inspiratory volume and chest circumference
Figure 1. Original circuit design. This
design was discarded due to lack of
sensitivity.
expansion and the need to accomplish the
aforementioned goals led the design team to
conclude that detecting changes in chest circumference would be the best way to monitor
a patient's breathing. The design team decided to convert the changing chest
circumference to a varying output voltage using a stretch sensor as a variable resistor in a
two-stage inverting operational amplifier circuit. Originally, the design team had planned
to use a circuit containing a bend sensor and a differential amplifier. However, when this
circuit design was tested, it was not sensitive enough to detect the small changes in output
voltage from the circuit. Additionally, the single stage inverting design did not provide
enough gain to accurately record the output signal. It was decided that the bend sensor
was too large and would not allow the device to be worn under the clothing, so a stretch
sensor was used in its place. Figure 1 shows the original circuit design, in which the
variable "Rs" is the variable resistor.
The experiments relating tidal volume to chest expansion led the design team to
conclude that a two-inch stretch sensor with a high degree of elasticity would be needed
for the device. The team also decided on a low-power operational amplifier for the
circuit that would require only six volts of input power. A six volt positive-groundnegative battery pack consisting of four three-volt batteries was constructed and the
designed circuit was built using resistors that resulted in a gain of 20.
The software that would receive the changing voltage as input is able to display
voltages in a negative 10 volt to positive 10 volt range in increments of 10-5 volts. With
this high degree of sensitivity, the circuit could be designed with a either high or low
gain. The voltage changes that resulted from the stretching of the variable resistor were
too small to be measured without multiplying them by some moderate gain. Building a
circuit with higher gain, however, may have increased the noise in the circuit and would
have made the circuit design more complex than necessary. A two-stage circuit design
was used to increase the gain of the circuit while reducing the error that would result
from using non-precision resistors.
In order to achieve a large amount of gain in the circuit with a single operational
amplifier, resistors with large impedance would have to be used. Individual gains of
stages in series are multiplied to give the overall gain of the circuit, so using a two-stage
circuit permits the use of smaller resistors to achieve the desired high gain. The final
circuit design is shown in Figure 2.
Both the
battery pack and the
wire-wrapped circuit
were attached to an
inelastic band that
included a small
Figure 2. The circuit design used in the final prototype. The two-stage gain
allows for greater sensitivity and a gain around 20.
elastic portion the
length of the variable resistor, over which the stretch sensor was secured. While the
patients that will wear this device will vary in size significantly, the prototype was
designed to be worn by one of the members of the design group.
Experiments were conducted that investigate the relationship between tidal
volume and output voltage. A subject was outfitted with the device which was connected
to the appropriate software. The subject was asked to take breaths that varied in depth.
The tidal volume of each breath was measured using an incentive spirometer and the peak
voltage reached for each breath was recorded. The data was graphed and the expected
result of a linear relationship between tidal volume and output voltage was obtained. The
maximum tidal volume that the subject could exhale was about 3.5 liters, which
corresponded to an output voltage of about 0.632 volts.
After discussing the concept of deep breathing exercise with Dr. Sawyer, it was
concluded on his advice that the threshold for a deep breath be set at 75% of the
maximum tidal volume exhaled. In order to determine the value for this, each group
member recorded maximum tidal volume ten times during physical activity, and the
mean for each group member and standard deviation between maximum tidal volumes
was taken to determine the deep breath threshold. The deep breath voltage threshold for
these tests was 0.629 volts.
The output voltage of the circuit was connected to LabVIEW software. The signal
was filtered with a low-pass third order Butterworth filter with a cutoff frequency of 2
Hz; normal breathing rate is about 0.2 Hz. The signal was displayed on a waveform graph
and displayed the output voltage of the circuit. The output voltage was then compared to
a preset voltage (0.629V) that corresponded to the deep breath threshold. When this deep
breath threshold was reached, a Boolean operator LED lighted up, indicating that a deep
breath had been taken. The signal was then converted from dynamic data and sent to a
peak detector. The number of breaths taken was determined by the number of peaks that
reach another preset threshold. This threshold for a normal breath was also measured
experimentally using a similar method to the one described above for determining a deep
breath threshold. These normal breaths were summed to display the total number of
breaths taken. The elapsed time was recorded and displayed on the virtual interface.
Dividing the total number of breaths taken by the elapsed time provided the user with a
dynamic evaluation of the subject's breathing rate. The visual alarm could be further
modified so that if the threshold is not reached in a given amount of time, another alarm
could notify the subject that the physician's protocol is not being followed and the
subject's inspiratory muscles are not being properly exercised. The VI for the LabVIEW
program is shown below.
Figure 3. The above figure shows the block diagram for the virtual instrument created in LabVIEW
8.0 to capture, display, and manipulate the breathing data acquired from the subject. Delivered data
includes a graphical output of the patient's breathing, the voltage, the time elapsed, the total number
of breaths taken, the subject's breathing rate, the total number of deep breaths taken, and the number
of deep breaths taken per minute.
A protocol for testing the device on human subjects was drafted and was submitted to the
Institutional Review Board in December 2006. The protocol was returned with a request
for revisions and has been resubmitted to the IRB. The submitted protocol consisted of
two testing stages. First, the device is to be tested on ten subjects for one hour. The
subjects will be surveyed at the beginning and end of the hour-long session about their
mobility and comfort while wearing the device. They will then be asked to perform
several common daily tasks, such as walking and conversing. The subjects’ surveys will
be used to design improvements for the device. The second stage of testing involves ten
new subjects wearing the device for a three-hour period. Again, the subjects will be
surveyed at the beginning and end of the three-hour period about their mobility and
comfort. If further improvements to the design are suggested, those improvements will
be considered and may be incorporated.
An example of a physician's protocol was also created with the advice of Dr.
Sawyer. Since the aim of this device is the ability for it to be inconspicuous and therefore
allow the patient to wear the device throughout the day, the number of deep breaths
required in a given period of time will be less than would be required for a protocol that
lasts 30 minutes or an hour, as other protocols do. For this reason, the required number of
deep breaths per hour will be 12, and the patient will wear the device 4-5 hours per day.
However, if the physician so chooses, the parameters on the device can be changed to
allow the physician to change this protocol based on individual patient needs.
In order to make the device portable, the design team plans to incorporate the
output of the system into a PDA to enable a patient wearing the device to have constant
feedback from the system. A Compaq Brand IPAQ was originally obtained from Dr. Paul
King and was configured. A meeting was arranged with Dr. Stacy Klein to discuss
software options on the PDA and after researching Vernier and PASCO software at her
suggestion, it was determined that these two software brands are only compatible with
Palm Brand PDAs. Dr. Klein also put the design team in contact with Bill Rodriguez at
the University School of Nashville, who had a joint grant with Dr. Klein previously for
the purchase of Palm Brand PDAs for scientific applications. They had previously used
these PDAs as an output display of magnetic field using voltage sensors, an application
which is similar to that of the design team. PDA equipment was obtained from Mr.
Rodriguez, including a Palm IIIc, a MELD interface, and a voltage sensor. The Palm IIIc
has been set up on a laptop and compatible software is being researched, since the
software used by Dr. Klein and Mr. Rodriguez has been misplaced.
Results
The relationship between chest expansion and tidal volume was determined to be
linear, and the amount of chest
3.5
expansion was found to be no greater
y = 1.1539x + 0.3692
R2 = 0.8503
3
Exhaled Air (L)
2.5
than two centimeters. As shown in
2
1.5
1
Figure 4a and 4b, tidal volume
0.5
0
0
0.5
1
1.5
2
increased linearly with chest
Chest Expansion (cm)
A
expansion in two different test
Exhaled Air (L)
4
3.5
y = 0.8493x + 0.6736
2
R = 0.842
3
2.5
2
1.5
subjects. Figure 4a shows tidal
volume as a function of chest
1
0.5
0
expansion for one group member,
0
0.5
1
1.5
2
2.5
3
3.5
Chest Expansion
4
and Figure 4b shows expansion for
B
another group member; the R^2
Figure 4. The graphs above show tidal volume vs.
chest expansion for two group members, indicating
that this relationship is linear.
value for the first member was
0.8503 and the R^2 value for the
second member was 0.842. The differences in amount of chest expansion for each group
member are likely due to differences in original chest circumference and lung volume,
which varies from person to person.
The completed device and screen shots in LabVIEW of its functionality are
shown below:
Figures 5 and 6. Figure 5, on the left, shows a deep breath being taken and the light on the right of
the interface lighting as a result of reaching this threshold. Figure 6, on the right, shows multiple
breaths being taken and recorded in LabVIEW. These are counted by a peak detector.
Once the circuit was
completed, it was found that
5.5 cm is the ideal starting
length of the stretch sensor.
Starting lengths of 5.5 cm,
6.0 cm, and 6.5 cm were
Figure 7. The device on a group member. On the left is the
circuit, with wires to power and ground, and in the middle
over the elastic part of the strap, is the stretch sensor.
tested to determine which
starting point would give the
most linear relationship between output voltage and tidal volume.
3.5
3
3
2.5
2.5
2
2
1.5
1.5
1
y = 362.46x - 225.4
R2 = 0.8216
1
y = 244.58x - 151.95
R2 = 0.5709
0.5
0.5
0
0.624
0.625
0.626
0.627
0.628
0.629
0.63
0.631
0
0.624
0.625
0.626
3.5
0.628
0.629
0.63
0.631
R^2 Values
0.9
3
0.627
0.8
2.5
0.7
2
0.6
0.5
1.5
0.4
1
0.3
y = 313.02x - 195.12
R2 = 0.5254
0.5
0
0.625
0.626
0.627
0.628
0.629
0.63
0.2
0.1
0.631
0
5.5 cm
6.0 cm
6.5 cm
Figures 7,8,9,10. Figure 7 (top left) shows tidal volume vs. output voltage of the circuit at an
initial stretch sensor length of 5.5 cm. Figure 8 (top right) shows this relationship for 6.0 cm
initial length, and Figure 9 shows an initial starting length of 6.5 cm. Figure 10 (bottom right)
shows the R^2 values for each starting length, and indicates that the 5.5 cm initial length yields
the most linear relationship between tidal volume and output voltage.
As shown in the Figures above, a starting length of 5.5 cm is optimal for obtaining a
linear relationship between output voltage of the circuit and tidal volume. The bar graph
shows that an R^2 value of 0.8256 is returned; the correlation coefficient decreases with
longer starting lengths of the stretch sensor.
Finally, exercise tests were performed to determine where the threshold for a deep
breath should be set, and this value was determined to be about 2.5 volts for the members
of the design team. This was determined by the group members expelling the maximal
tidal volume possible during physical activity. For group members, the deep breath
threshold should be set at 2.586 +/- 0.17 L. The data from this experiment is shown
below.
Subject 1
3.45
3.17
3.34
3.4
3.31
3.38
3.59
3.35
3.29
3.2
3.348
Subject
2
3.17
3.14
3.18
3.21
3.16
3.26
3.14
3.15
3.14
3.24
3.179
Subject
3
3.34
3.55
3.49
3.75
3.55
3.58
3.87
3.55
3.6
3.61
3.589
Subject
4
3.48
3.6
3.55
3.7
3.75
3.7
3.72
3.76
3.75
3.75
3.676
deep
breath
2.511
2.38425
2.69175
2.757
deep
breath
Avg
st dev
2.586
0.170051
average
Figure 11. Values of maximum tidal volume for each
group member, and the corresponding value for deep
breath threshold (75% of maximum tidal volume).
Conclusion
The designed prototype for the programmable biofeedback chest exerciser is a
successful breathing rate monitor. A linear relationship of tidal volume versus output
voltage was established for a particular subject. The relationship was determined
experimentally by measuring the exhaled air volume with a spirometer and recording the
resulting output voltage. Establishing the linear relationship for a given subject proves
the validity of using a stretch sensor and the proposed circuit design for breathing rate
and depth analysis. Without a linear relationship, the ability of the monitor would have
been questionable in establishing the difference between a normal breath and a deep
breath, as well as separate breaths. A peak detector appropriate for the subject must be
implemented to allow for a biofeedback system. Without linearity, the peak detector
could have inaccurate counts thus providing error in the patient's compliance check.
After linearity was established, the optimal starting length for the stretch sensor
was determined experimentally. The sensitivity of the sensor changes with different
lengths. In order to determine the appropriate length for our subject, a series of tidal
volume experiments was carried out according to the same protocol as that performed to
establish the linearity of output voltage and tidal volume. A linear regression analysis
was preformed on each set of data to determine which length produced the highest degree
of linearity for the tidal volume versus output voltage relationship. A length of 5.5
centimeters was determined to be the optimal length for our subject, which was just the
exact point of slight stretch. The results with this amount of stretch in the sensor allow
the biofeedback system to have a higher degree of accuracy. Allowing the stretch sensor
to start from a variable length changes the output voltage sensitivity range. Establishing
an appropriate range for a patient based on chest expansion of breath depth allows the
physician to adjust the starting length of the stretch sensor accordingly to obtain
maximum sensitivity of feedback data.
For a biofeedback system to be properly implemented, the threshold of a deep
breath must be determined. In order to be a chest exerciser, the device must measure
when a deep breath is taken and the threshold was determined to be 2.58 +/- 0.17 L. This
measurement did not vary much among the members of the group; however, this device
will be used for a wide variety of subjects in differing states of health and as a result this
threshold will vary from patient to patient and will have to be individually set by a
physician.
A goal of the project was to build a device that was not only portable,
comfortable, and user-friendly, but also inexpensive. To construct the prototype, the
design team spent $15 on a bend sensor and $13.70 on eight operational amplifiers and
twenty 3 volt batteries. The strap used was made out of already acquired material and
elastic. The battery casing was also made out of existing materials in the lab. The PDA
(about $200), connection cords, software (LABPro: $220), wire, resistors (about 2 cents
each), wire wrapping tools, breadboard, and soldering tools were all borrowed from the
lab or Dr. Galloway. The cost of these extra items would dramatically increase the cost of
the device initially, but would be a one time investment. Once the sunk cost is met, the
cost per device would be inexpensive compared to existing products.
Recommendations
The original device prototypes were built with components borrowed from the
Biomedical Instrumentation lab, since the team was unsure of the resistor ratings that
would be ideal for the final prototype. The resistors used in the prototype are not
precision resistors, meaning that the resistance values can be at least 10% different from
the resistor ratings. Precision resistors, however, ensure that resistor values are within 5%
or 10% of the resistor rating, creating much more certainty in the exact gain to the output
of the circuit. Output accuracy and reliability would be increased by using precision
resistors but this would also increase the cost of the device. In the future, the use of
precision resistors for the device would be preferred.
Additionally, the chest strap was constructed from materials purchased at a craft
store. The inelastic part of the band is similar to straps used on book bags and the elastic
portion was inserted by cutting the inelastic band and inserting a piece of elastic with a
length similar to the length of the stretch sensor. These parts of the bands were
connected by sewing the pieces together by hand. In the future, a band made out of a
more comfortable material will be used. A better method to attach the stretch sensor, such
as metal hooks, and a different closure device could also be incorporated to both make
the device more comfortable and to reduce the possibility of minor pinching injuries with
the clasp of the current model.
To make the device portable and ideal for everyday use, the output of the circuit
can be connected to a PDA that will display and record device output. With the
appropriate software, this part of the device could also contain the alarm system for
patients that do not follow their doctor's prescribed breathing regimen. The first step in
doing this is to connect voltage sensors from the output of the circuit to the PDA using a
MELD interface. Vernier LabPro software will display the output voltage of the circuit
in a manner similar to LabVIEW. The PDA will also be able to store data available to a
patient and his or her physician for analysis. The design group has experienced problems
with device failure after prolonged use, perhaps due to the overheating of the device
when the circuit is enclosed in a case. A more breathable case or a fan could be
incorporated into the device to avoid this problem.
Due to the nature of the device, there are limited ethical issues to be considered.
Few dangers arise from the use of the device other than a pinching risk due to the clasp
used and the unlikely event of electrical failure. The circuitry of the device presents
certain electrical hazards that a subject should be duly informed of because they could be
hazardous to his or her health; these include the possibility of shock if the interior of the
device becomes exposed or if the patient accidentally comes in contact with water while
wearing the device. The main ethical concerns arise from ensuring that informed consent
is obtained from each patient before use of the device and confirming that the device
meets its intended purpose of increasing negative inspiratory force, therefore improving
chest muscle strength.
Finally, additional protocol for clinical testing will be submitted to the
Institutional Review Board (IRB). The trial will consist of three groups of subjects in
double-blind studies with the device. One group will not wear the device; another group
will wear a device that has not been programmed and does not provide patient feedback;
and a third group will wear a fully functioning device. The inspiratory force of these
subjects will be measured before and after the trial using an incentive spirometer to
determine the ability of the device and the protocol to strengthen the inspiratory muscles
of the subject.
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