Vanderbilt University
Department of Biomedical Engineering
BME 273
Improving Simulations in the
Post Anesthesia Care Unit
Written by: Alyssa Z. Cherif
Group 28, Department of Biomedical Engineering
Advisors:
Dr. Matt Weinger, Ray Booker, Bobby Gibbons, Dr. Paul H. King
Submitted: April 24, 2007
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ABSTRACT
The Post Anesthesia Care Unit Simulation Center was created to allow medical and nursing
students to practice a variety of procedures on a simulated patient, SimMan. To efficiently
simulate all possible scenarios that could arise in the PACU, it is necessary to provide a full
range of pulmonary function. At the start of this project, there were only two options for lung
size: fully functioning or completely collapsed. The overall goal for this project was to allow for
a range in the available volume in SimMan's lungs and to have it be externally controlled;
simulating an obstructive pulmonary disease. This goal was to be achieved in the most efficient
and inexpensive manner possible. A prototype was built by inserting a saline bag into one of
SimMan's lungs; this saline bag had a tube extend outside the body. The tube can be injected
with air using a syringe, allowing for total external manipulation of available lung volume. Data
was collected from the screen of the anesthesia monitor. To effectively simulate an obstructive
pulmonary disease, there must be a decrease in the amount of air expired by the lung per breath.
This was achieved, as shown by the steady decrease in the volume of air expired as the saline
bag volume was increased, and hence available lung volume decreased. Also, the pressure
needed to push air in and out of the lungs increased with the volume of air added to the saline
bag; the relationship is well-represented by a third order polynomial. Overall, this prototype was
extremely inexpensive to build and it successfully simulates an obstructive pulmonary disease.
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INTRODUCTION
After undergoing a surgery involving anesthesia, the patient is sent to the Post Anesthesia Care
Unit, or the PACU. Because general anesthesia affects the entire body, postoperative effects
tend to appear, the most common being pulmonary complications.
The PACU Simulation Center was created to enable medical and nursing students to
practice a variety of procedures on a simulated patient called SimMan. SimMan has several
working bodily systems, including fully functioning lungs. His vital statistics can be externally
controlled using a program called SimQuest. SimQuest can alter his statistics such as blood
pressure and heart rate, and then display them on a monitor inside the Simulation Center. At the
start of this project, there were only two options for lung size: fully functioning or completely
collapsed.
To efficiently simulate all possible scenarios that could arise in the PACU, it is necessary
to provide a full range of pulmonary function. The primary way in which pulmonary function is
assessed is by volume measurements (Sherwood 479). The most important of these is tidal
volume, which represents the amount of air entering and leaving the lungs in the span of one
breath. The expiratory reserve volume is the amount of air which can be expired after normal
exhalation, with maximum force of the expiratory muscles. The residual volume is the amount
of air left in the lung after exhalation, included after maximally forced exhalation. Functional
residual capacity is the amount of air in the lung after a normal, involuntary expiration; this is
calculated as the sum of expiratory reserve volume and residual volume. The vital capacity is
the largest possible volume change in a single breath in the lungs, after the patient maximally
inhales and then exhales. The total lung capacity is the largest possible amount of air that can be
held in the lungs and is calculated as a sum of residual volume and vital capacity. The final
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measurement is FEV1, forced expiratory volume in one second; this is calculated as the amount
of air exhaled in the first second of a vital capacity measurement (Sherwood 478).
There are two types of respiratory dysfunctions which can be diagnosed using these
volume measurements: obstructive and restrictive pulmonary diseases.
A patient with an
obstructive lung disease tends to experience problems with full expiration. Obstructive lung
disease patients exhibit standard values of total lung capacity, and increased levels of functional
residual capacity and residual volume; they also show a decrease in vital capacity and FEV 1.
These measurements show that although there is a normal amount of air going into the lung, the
patient presents with difficulty in reaching full expiration: there is always some extra air left
inside the lung (Sherwood 479).
Patients with restrictive lung diseases suffer from a loss of compliance, causing problems
filling their lungs. These patients show reduced total lung capacity and vital capacity, caused by
their inability to properly expand their lungs. The residual volume is typically normal, while the
FEV1 is elevated.
The fourth leading cause of death in the United States is chronic obstructive pulmonary
disease, or COPD1. The leading cause of COPD is smoking and it tends to affect women more
often than men1. COPD is an irreversible obstruction of air expired by the lungs. The damage
typically worsens with time, causing a continual decrease in air exhaled2. In certain cases,
spasms cause further blockage of the airways. The severity of COPD is determined by the
amount of both airway blockage and tissue damage.
COPD refers to one of two diseases, either chronic bronchitis or emphysema. Chronic
bronchitis results in inflamed bronchi and an overproduction of respiratory mucus, sputum. It is
1
2
http://www.lungusa.org/site/pp.asp?c=dvLUK9O0E&b=35020
http://www.medicinenet.com/chronic_obstructive_pulmonary_disease_copd/article.htm
4
the sputum that leads to the progression of the disease and the difficulty in reduction of
inflammation.
The second COPD, emphysema, is an enlargement of the alveoli.
This
enlargement damages the alveoli walls, causing tissue loss as well as a decrease in functional
breathing3.
The overall goal for this project was to allow for a range in available volume in
SimMan's lungs and to have it be externally controlled, thereby simulating an obstructive
pulmonary disease; this was to be achieved in the
most efficient and inexpensive manner possible. To
insure that the prototype built actually simulated an
obstructive pulmonary disease, data was collected
using the anesthesia machine monitor, as can be
seen in Figure 1. Monitor of the anesthesia
Figure 1. Monitor of the anesthesia machine.
machine..
The screen shows several volumes
including VE, VT and VTE. VE is the amount of air expired over a minute, and is measured in
liters per minute. VT is the tidal volume; in this case, it is the amount of air delivered to the
patient per breath. VTE is the amount of air actually expired in each breath; in an obstructive
pulmonary disease, this value should be less than the tidal volume. The final measurement
important to the data collection in this project was PMAX, the pressure needed to get air in and out
of the lungs; this value should increase in an effectively simulated pulmonary dysfunction.
METHODOLOGY
Several requirements were considered before embarking on this project. First, the prototype
must be able to withstand regular SimMan operations, including cardiopulmonary resuscitation
3
http://www.diseases-explained.com/COPD/home.html
5
(CPR). The prototype must also fit inside SimMan's chest cavity and cause as little disturbance
as possible. SimMan, like a regular human being, operates at a range of pulmonary parameters;
the prototype must work within these ranges. The lung must always be airtight, so the prototype
should not allow air leakage.
The prototype should be efficient, inexpensive and easily
repairable. Most importantly, the prototype should allow for external control of the lung volume.
When first exploring possible options, measurements were made to get base knowledge
of SimMan's lungs. Knowing the available lung volume is vital when manipulating the lungs.
To make the measurement, the lung was filled with water. Then, the water was poured into a
graduated cylinder. The final volume of water that fully expanded the lungs was found to be
1263ml. After this volume measurement, length was then considered. The bronchus was found
to be 11.4cm with an inner diameter of 15.9mm and an outer diameter of 19.1mm. These
measurements were all considered while selecting the best possible method.
As mentioned, the prototype must work at the full range of lung parameters. Each
parameter has a range specified by the person operating the anesthesia machine; the actual values
are displayed on the ventilator screen. When SimMan's values go outside the range, alarms alert
the user. Typically, SimMan's tidal volume is in the range of 400 – 600ml, although 600ml is a
bit high for normal operation. The ranges of VE and VTE are 3.8 – 5.8 L/min and 140 – 1000ml,
respectively. Finally, the Plimit is usually set at 40cm H2O, which can be adjusted higher or
lower, depending on a given situation.
The first idea considered was a valve, to be placed either in or around the bronchus of the
lung. This would easily allow variable amounts or air to enter the lung, and it would take up
minimal space within the chest cavity. To implement this method, a solenoid with variable
resistance would be attached to the inside of the lung bronchus; the resistance of the solenoid
6
could be externally manipulated. Unfortunately, this method would change the lung resistance
as opposed to its volume; solenoids are also generally expensive and fragile. This prototype
would most likely not withstand daily SimMan activities, namely CPR, so this idea was rejected.
The second idea was a weight of some sort to be placed atop the lung, inside the chest
cavity. The weight would be of variable mass; as the mass of the object changed, so would the
internal volume of the lung. This method would allow for a large range of lung volumes, but it
also brought up several concerns. First, if the weight was moved during operation, the lung
could be completely collapsed; it would not be able to be repaired until the end of the simulation.
The chances of the object moving would be high due to CPR and other chest compressions.
Second, while the object could be externally controlled, it would take up a great deal of space
within the chest cavity. Due to these possible complications, the weight idea was overruled.
The final idea was to insert a balloon of some sort inside the lung; this balloon would
have a tube to extend outside the lung and chest cavity to allow for external manipulation.
During balloon expansion, available lung volume would decrease. As long as a balloon with
dimensions smaller than those of the lung could be found, the standard parameters should be
achieved. Hence, this method was the optimal choice, and taken to the next level.
The first step to making the prototype was finding a balloon to fit comfortably inside the
lung. Saline bags come in a variety of sizes are quite inexpensive. A 1000ml saline bag was
chosen because its volume was closest to that of the lung. Due to this similarity, the prototype
would be able to simulate a deflated lung, a fully expanded lung and every volume in between.
Another convenience of saline bags is that they have tubing that fits directly into their own tubes.
After acquiring the saline bag and its tubing, it needed to be inserted into the lung. The
saline bag has a 12.1cm width, making it too wide to be fit through the bronchus.
A
7
bronchoscope was used to examine the interior of the lung; it was discovered that the bronchus
could not be removed. The final decision was to make a slit at the bottom of the lung, and slip
the saline bag in through this hole. The tube receiver of the saline bag was placed at the top of
the lung, at the rear. This positioning allows for the tubing that attaches to the bag to exit the
chest cavity via holes previously drilled in SimMan's back.
Once the saline bag was placed inside the lung, the next concern was resealing the lung
and ensuring that it stay airtight. At first, silicon glue was suggested because this type of glue
typically works to seal vinyl, the material of which the lung is made. After setting for 36 hours,
the two sides of the lung did not adhere. The next method tried was a heat gun. Heat was
applied to the bottom of the prototype. Due to prolonged exposure without any cooling, the lung
became extremely singed. This led to over-sealing and a great reduction in available lung
volume. The first prototype was discontinued and used later for extra material.
Upon insertion of the saline bag into the second prototype, the heat gun was used again.
This time, intermittent blasts of hot and cold air were administered to prevent over-sealing.
Although this method worked, it took at exceptional amount of time and it was far from flawless.
Hence, a new type of glue was sought after. This time, a flexible adhesive was used; this glue
works on vinyl, fabric and plastics.
This glue was completely effective and provided an
appropriate amount of flexibility.
The next step was to allow access to the saline bag from the exterior of the lung. The
saline bag's tubing measures a 6.35mm diameter. A hole was punctured in the rear of the lung
roughly of similar size. Due to the suppleness of the vinyl lung, it tore easily. To cover up any
exposed areas, additional pieces of vinyl from the primary prototype were glued around the
saline bag tubing, using the same flexible adhesive. To secure these additional pieces, rubber
8
washers were positioned on the tubing that exits the lung. The washers have a 6.35mm inner
diameter, so they fit perfectly.
After the second prototype was completed, it was taken to the simulation center for
testing. Once hooked up to SimMan, the anesthesia machine was turned on. At this point, the
bellows began pumping air at the specified VT. The bellows constantly returned to maximum
height, guaranteeing that the prototype was in fact airtight. The first trial was then run to begin
collecting data.
The prototype had been placed on the right side of the chest cavity; this caused the right
side to sit a bit higher. The saline tubing attached to the prototype was pulled through the hole in
SimMan's back. This tubing was rather bulky due to parts that are only needed for intended
saline bag use; so, these parts were removed. After removing a big of the bulkiness and
rearranging a few items, the chest cavity sat lower and new trials were begun.
RESULTS
During all trials, VT was set to 450ml. The frequency was set to 12 breaths per minute and the
inhale to exhale ratio (I:E) was set as 1:2. Finally, the Plimit was set at 40cm H2O and the PEEP
was off. At the start of the first trial, air was
syringe. After 180cm3 of air was added to the
saline bag, a linear trend was no longer observed;
Maximum Pressure
(cm H20)
added in 60cm3 increments, using a 60cm3
Trial 1, Pressure versus Air Added
40
35
30
25
20
15
10
5
0
0
3
so, air was added 10cm at a time. The saline bag
was inflated until it reached 840ml. The results
can be seen in Figure 2.
200
400
600
800
Volume of Air Added (cc)
Figure 2. Results from trial 1, pressure exerted
by the anesthesia machine versus volume of air
added to the saline bag.
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Figure 2 shows the relationship between pressure observed and the volume of the saline
bag. Again, the pressure is the amount needed in order to inflate and deflate the lungs. With a
third order polynomial trendline applied to this data, there is an R2 value of 0.9942. This is a
terrific R2 value, showing that this relationship is well-represented by a third order polynomial.
After the saline bag was filled to 840ml, the air was slowly pulled out of the bag, still
using the syringe. The results from this drainage
Trial 1, Pressure versus Air Removed
was much sharper than when the air was injected
into the saline bag.
At the start, the pressure
Maximum Pressure of
Lung (cm H20)
can be seen in Figure 3. The change in pressure
40
35
30
25
20
15
10
5
0
0
200
values dropped at 2cm H2O per 60cm3 air
removed. As the saline bag volume decreased,
400
600
800
Volume of Air Removed (cc)
Figure 3. Trial 1, relationship between
pressure exerted by the anesthesia machine
and air removed from the saline bag.
the drop became less sharp. An interesting point is that the final pressure value of the deflated
saline bag was lower than the pressure value initially observed at the start of the first trial.
The next trial was conducted after the readjustment of objects inside of SimMan. This
was the point when all extra bulkiness was removed from the tubing and the two sides of the
chest were made to be level. Figure 4 shows the data from the second trial. Another important
difference between this trial and the first is that
beginning.
When a third degree polynomial
trendline was applied to this data, the R2 value
Maximum Pressure of
Lung (cm H20)
the air was added in 10cm3 increments from the
Trial 2, Air Added vs. Pressure
35
30
25
20
15
10
5
0
0
100
200
300
400
500
Volume of Air Added (cc)
increased to 0.9963. This decrease in error is
most likely due to the steady addition of air. It is
Figure 4. Trial 2, relationship between
pressure exerted by the anesthesia machine
and air removed from the saline bag.
600
10
also most likely due to the lower pressure values,
Relationship Between Saline Bag Volume and
Lung Pressure
Maximum Pressure (cm
H20)
caused by a more comfortably arranged chest
cavity. A direct comparison between the two trials
can be seen in Figure 5.
33
Trial 1
28
Trial 2
23
18
0
200
400
600
800
Volume of Air Added (cc)
During the second trial, VTE was also
Figure 5. Comparison of trials 1 and 2.
monitored. As the volume of the saline bag and
pressure needed to inflate the lungs both increased,
Air Expired by Lung as Compared with Volume
Increase in Saline Bag
Volume Expired by
Lung (ml)
VTE decreased. The decline can be seen in Figure
6. With a third order polynomial trendline applied,
2
the R value is 0.9703; this shows that VTE is less
360
350
340
330
320
310
0
100
200
300
400
500
600
Volume of Air Added (cc)
proportional to a change in saline bag volume.
The VTE values observed while SimMan
Figure 6. Relationship between air expired by
the lung and volume of the saline bag.
was attached to the prototype lung were as much as 132ml less than the VT of 450ml. To test if
this was related to the prototype design, the VTE values were recorded again while both of the
normal lungs were attached to SimMan. The observed VTE values were in the range of 360 –
370ml. While these values are closer to 450ml, they are still considerably smaller. Interestingly
enough, when the normal lungs are in place, the PMAX is observed to fluctuate between 25 and
26cm H2O. So, while the VTE is slightly closer to the VT, the PMAX values are those observed
when the lung is nearly halfway filled by the saline bag.
This prototype is easily marketable as there are many simulation centers across the world.
As this design simulates an obstructive pulmonary disease, the prototype will help to expand the
breadth of situations attainable by SimMan.
The prototype was extremely inexpensive to
produce. Table 1 shows the cost summary of the entire project. Using one of SimMan's original
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lungs, a mere $14.25 was spent to
purchase the remaining materials.
This
prototype will not lead to an increase in
revenue, yet it will enhance the education
Material
1000ml saline bag with tubing4
1.5 oz. Vinyl Flexible Adhesive
10 rubber washers
Total
Cost
$10.00
$2.98
$1.27
$14.25
Table 1. Summary of cost of prototype production.
of both medical and nursing students.4
This prototype is quite easy to repair: glue is essentially the only necessary material.
Under proper conditions, the prototype should not fail and hence would have a life cycle
comparable to an unaltered SimMan lung. If this prototype were to be manufactured, the cost of
production would decrease dramatically per output lung. This decrease in cost would be due to
several improvements, to be discussed in the Future Directions section. Also, the prototype
could be distributed with standard SimMan parts, minimizing its distribution costs.
CONCLUSIONS
The results from the trials show that the project requirements are all satisfied. The prototype is
airtight, proven by the anesthesia machine's bellows. Also, SimMan is capable of working
within normal ranges while connected to the prototype lung. Chest compression occurred during
the trials, and the prototype did not fail. After readjustment, the prototype proved to fit in the
chest cavity without disturbing any other elements. The device was extremely inexpensive to
build and quite easily repairable.
Most importantly, the device effectively simulates an obstructive pulmonary disease. As
the available lung volume decreased, the pressure needed to inflate the lungs increased, as is the
case with an obstructive pulmonary disease. Also, the volume of air actually expired by the
lungs steadily decreased, a classic sign of progressive obstructive pulmonary disease.
4
http://shopping.netsuite.com/s.nl/c.624515/it.A/id.95/.f
12
As this project mainly pertains to a simulated patient, there are minimal ethical concerns.
Still, it can be argued that to be entirely ethical and effective, a simulated patient must be able to
simulate all possible patient conditions. Clearly, this is nearly impossible as the human body is
extremely complex, but this project does help to minimize any possible ethical problems.
This prototype has proven itself to be very safe. It has worked at a wide range of
parameters inside SimMan without bursting, tearing or causing any damage to surrounding areas.
Also, the prototype should not burst or cause any harm to those manipulating and working with
SimMan.
Overall, this project has effectively achieved all of its initial requirements. The prototype
responds to external control, among the most important goals of this project. In addition, it is a
rather simple concept, which will allow for ease in repair and well as reproduction.
FUTURE DIRECTIONS
There are several improvements that could be made to this prototype to maximize its efficiency.
The first is using a longer tube without bulky appendages to hook up to the saline bag. The
added length would permit the tubing to extend into the control room during simulations. Also,
the lack of extra pieces on the tubing would make the entire apparatus less bulky and allow it to
sit in the chest cavity without shifting any other objects.
A second improvement would be to find a bag similar in dimension and structure to the
saline bag, but without any contents. The main source of cost in the building of the prototype
came from the saline bag and its tubing. Hence, without the saline or the gadgets on the tubing,
the cost would be much less.
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The third improvement would involve the point of exit of the saline bag tubing from the
lung. This was the most problematic part of the entire building process. Due to the circular hole,
it was difficult to find a way to secure it whilst keeping it airtight. In the end, it was sealed with
glue and excess vinyl from the original prototype. One option would be to create a plastic
grommet with a 6.35mm inner diameter; at the time of this project, there was not one of this size
on the market. Another option would be to flip the saline bag inside the lung, so that it exited
where the original slit in the lung was made. This would help simplify the sealing process and
assist in troubleshooting to find any air leaks.
The final improvement would be to find an efficient way to use heat to seal the lung. The
vinyl will certainly seal with a heat application, but it will take a specific course of heating as
well as a great deal of time. Still, it will be the most efficient way to ensure the lung remains
airtight. It will also help to reduce the cost of production.
If the prototype is used in a variety of simulation centers, a future goal will be to integrate
it into the SimQuest program. The amount of air desired to be injected into the saline bag could
be entered directly into the computer and then directly affect the lung.
If all of the possible improvements are made, the prototype should become very simple to
replicate.
production.
This would help ensure its marketability and continue to decrease its cost of
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REFERENCES
American Lung Association. "Chronic Obstructive Pulmonary Disease (COPD) Fact Sheet."
Diseases. 18 Apr 2007. <http://www.lungusa.org/site/pp.asp?c=dvLUK9O0E&b=
35020>
Barash, Paul, Bruce Cullen, and Robert Stoelting. Clinical Anesthesia. 5th ed. New Haven:
Lippincott, Williams & Wilkins, 2005. 18 Apr 2007. <http://www.R2Library.com/marc_
frame.aspx?ResourceID=208>
Datex-Ohmeda. "Aestiva/5 MRI." Products and Services: Anesthesia. 15 Apr 2007.
<http://www.us.datex-ohmeda.com/products/anesth_aestiva5_MRI.htm>
Datex-Ohmeda. "SmartVent Ventilator." Products and Services: Anesthesia. 15 Apr 2007.
<http://www.us.datex-ohmeda.com/products/anesth_smartvent_1.htm>
James, Francis M, Griselda M. Cooper, and Peter Hutton. Fundamental Principles and Practice
of Anaesthesia. Birmingham: Informa Healthcare, 2002. 415. 18 Apr 2007.
<http://books.google.com>
LexiComp Inc. "COPD (Chronic Obstructive Pulmonary Disease)." DiseasesExplained. 18 Apr
2007. <http://www.diseases-explained.com/COPD/home.html>
MedicineNet. "Chronic Obstructive Pulmonary Disease (COPD)." Lungs. 18 Apr 2007.
<http://www.medicinenet.com/chronic_obstructive_pulmonary_disease_copd/
article.htm>
Piezosurgery. "Infusion Set for Solution." Surgical Supplies. 15 Apr 2007.
<http://shopping.netsuite.com/s.nl/c.624515/it.A/id.95/.f>
Piezosurgery. "Sterile Saline Bag – 1000ml." Surgical Supplies. 15 Apr 2007.
<http://shopping.netsuite.com/s.nl/c.624515/it.A/id.104/.f>
Sherwood, Lauralee. Human Physiology: from cells to systems. 5th ed. Belmont: Brooks/Cole,
2004.