Field Ventilator logo

An emergency field ventilator designed for 12 people

Provides basic simultaneous respiratory support for 1 to 12 people depending on respiratory mode (CPAP, BiPAP and manually determined I:E)

Patents Pending

Note: Please contact Tom Greany for information regarding the Prototype and the Rev 2 Field Ventilator design.

This project was started in Louisville, Colorado on March 16, 2020. Sixteen days later, on April 1, 2020, a prototype was successfully lab tested in San Francisco, California. (Read the back story below.)

This website provides information about the design, development, testing and deployment of a basic respiratory support ventilator designed during the 2020 COVID-19 pandemic in anticipation of a critical mechanical ventilator shortage worldwide.This design conforms with the new (enacted 26 March 2020) FDA Enforcement Policy for Ventilators and Accessories and Other Respiratory Devices During the Coronavirus Disease 2019 (COVID-19).

Enough information is given on this website to allow persons with appropriate mechanical and electrical expertise to duplicate the prototype field ventilator for emergency use in areas that may not have adequate ventilator facilities to meet demand. Though functional, the prototype was intended only as a proof of concept, and significant revisions were made to allow oxygen to be safely incorporated into the design; as well as to improve the unit's anticipated service life and flexibility in providing respiratory support for a broad range of patient conditions.

The revised Field Ventilator can also be built according to the specifications set forth on this website. However, its power module and programmable logic controller are more sophisticated than that of the prototype. All liability for construction and deployment of the field ventilator is assumed by the user of this site. See full “Legal Disclaimer” at the bottom of every page of this site.

The prototype and revised field ventilator units described on this website were designed as multi-patient units, providing manually adjustable tidal volume control on each patient port to suit individual patient needs. It is critically important to realize that these units have not been tested on humans. The REV2 model programmatically disallows negative I:E ratios to help prevent overinflation of lungs; and both models include pressure relief on each patient port. However, misapplication and unsupervised use of any ventilator can lead to patient injury and death.

Many areas in the world have no ventilators and surgeries are routinely performed while the patient is being manually respirated with a bag valve mask. Parts of the developing world may suffer significant loss of life during the COVID-19 pandemic. These field ventilators may provide a solution for those applications, but require further testing and clinical validation.

This ventilator was developed, and is intended for the sole purpose of providing emergency respiratory support for patients when there is no alternative solution in response to the COVID-19 pandemic of 2020; to prevent emergency medicine personnel from having to deny any patient access to a ventilator (or removing patients from a ventilator) based on social value or any other reason. Any other use of the device by any party for any reason is not covered by laws or suspension of laws and statutes developed in response to COVID-19 development.

Field ventilator basics (based on the prototype)

This project began on March 16, 2020, when Dr. William Whetstone contacted Dr. Thomas Greany, stating that conditions looked grim regarding current forecasts of COVID-19 cases in the San Francisco Bay area. In addition, that a nationwide shortage of ventilators was anticipated. Whetstone asked Greany whether he thought a field ventilator could be designed and built quickly to provide basic respiratory support for multiple patients to prevent a situation of having to deny any person rescue ventilation in the event of a shortage of FDA approved hospital ventilators. Dr. Whetstone is a CU-trained engineer, currently a Clinical Professor of Emergency Medicine at UCSF; and an emergency physician at San Francisco General Hospital. He completed his medical degree at the University of Colorado Health Sciences Center and his Emergency Medicine residency at the University of Michigan. Dr. Greany is a CU-trained engineer/dentist and Assistant Professor at CU's Anschutz Medical Campus. He has practiced as an engineer and dentist most of his career.

Fifteen days after the initial conversation took place, Greany delivered a prototype unit (1:1 I/E respiration mode only) to Whetstone in San Francisco. On April 1, the unit successfully demonstrated simultaneous, asynchronous 1:1 I/E air flow to two manifolds of four patient ventilator ports each (eight total). Dr. Whetstone recommended an additional 1:2 I/E ventilation mode be added if possible. Greany modified the design the same day to include this new mode, introducing the need for a second digital relay timer and solenoid valve (this design was later simplified through the use of a more sophisticated programmable logic controller- see the Rev 2 notes). The 1:2 I/E model allowed for addition of a third manifold of four patients (twelve total).

The following day, in response to new data received from UCSF regarding evolving COVID-19 protocols, a method of rapidly converting from 1:2 I/E to CPAP mode was configured. CPAP mode does not require valves or logical timers, and may be useful for patients in avoiding non-invasive ventilation. Finally, a method of integrating 100% oxygen from stationary hospital-supplied, or portable tanks was configured, which can improve outcomes for patients whose inflamed lungs cannot efficiently accomplish respiratory function. Low flow (~2.5 Liters per minute) CPAP can be delivered to eight patients simultaneously via the prototype unit with a quick tubing re-route through normally open solenoid valves. If done, oxygen cleaned solenoid valves should be substituted for those used in the prototype. Depending on the availability of 100% oxygen under field conditions, higher flow can be delivered to additional patients. For CPAP mode on the prototype, valve timing is disabled.

This site documents the design of Dr. Greany's resulting three-mode field ventilator Prototype; as well as the subsequent Rev 2 modifications which added respiratory modes; enhanced safety; improved reliability; and added flexibility for extended continuous duty service life. Rev 2 modifications were made in collaboration with an outstanding RK Mission Critical team in Aurora, Colorado; and six Rev 2 Field Ventilators were built in their facilities.

Either revision of the ventilator can supply simultaneous rescue ventilation for up to 12 patients at variable flow rates to accommodate individual tidal volume needs, although Rev 2 would certainly recommended for its improved safety; respiratory mode flexibility; and anticipated service life. The site also provides plans for building, operating, and troubleshooting the prototype units. Manual operation of flow distribution in the event of solenoid failure as presented for the prototype has been eliminated in Rev 2. This is because the Rev 2 solenoids are anticipated to have a much greater service life than those used in the prototype. Photos and videos documenting the Rev2 build are supplied; however the Rev2 power module and programming will require approval by and interaction with the authors of this site.

Please note: this emergency field ventilator is not intended to replace clinically validated FDA-approved traditional hospital ventilators, which provide additional flexibility, monitoring and user interface improvements. Nor should this site be construed as an authorization or recommendation to use the field ventilator device in clinical applications. The decision to do so rests entirely with physicians supervising the care of the patients.

Dr. Greany, Dr. Whetstone and the RK Mission Critical team acknowledge that design refinements are certainly possible beyond Rev 2. Individual component and system longevity cannot be estimated or extrapolated from short term testing; nor can clinical performance of the device or patient outcomes be predicted. But with little time to prepare for the possibility of a nationwide shortage of ventilators, this device may provide emergency ventilation to patients who may otherwise receive nothing. The primary purpose of this website is to enable others to innovate and improve based on this multi-patient design with the hope of helping to save lives. It is further hoped that the design may provide a basis for patient ventilation at a reasonable cost in developing countries, where ventilators may not be available. Such derivative works will be based on extensive feedback and testing from the medical community.

The rapid design, build, and deployment of this project could not have been accomplished without the technical input and support of multiple ER physicians and respiratory therapists in Colorado and California. Ken Lambrecht (another CU-trained engineer) also helped Greany brainstorm, assisted in the documentation of this project, and created this website. Thanks to Haidar Malhas, a hydraulics engineer living in Ghana for input on pressure relief. The leadership, planning, supply chain, technical and build teams at RK Mission Critical have been extraordinary in their individual roles.

Our hope is that the critical national shortage of rescue ventilators identified during the COVID-19 pandemic, which exposes a strategic vulnerability; as well as the realization that developing nations have no ventilators at all will find a potential solution in this Field Ventilator device. Through testing and refinement, we hope that this device will provide a real solution that can be readily produced at reasonable cost.

Finally, a special thanks to all of the ER physicians, nurses, respiratory therapists and other health staff who are putting themselves in harm's way every day to provide critical care to those suffering from the COVID-19 virus.

Thank you to everyone involved in helping out during this crisis!


March 16-22, 2020

Researched existing solutions for patient ventilation, both in-hospital and in rescue situations. Developed an idea to support multiple patients from a single air source (pump), with solenoid valves and digital control timers employed to direct air to multiple groups of patients according to a prescribed respiratory rhythm. Air pumps can simply produce forward air. The flow coming from them should not be reduced or restricted (i.e. "deadheaded") in order to avoid overheating and reducing the life of the pump. Compared with an ambu bag style ventilator, which provides forward air only when the bag is squeezed, pumps produce air continuously, thereby offering potentially increased efficiency. While one group of patients is exhaling, flow from the pump can be re-directed to another group of patients who are inhaling.

For purposes of a prototype, large commercial aquarium pumps were selected as the primary air mover. These devices provide oxygenation to large, exotic fish aquariums and are designed to operate continuously. Reliability and failure modes were evaluated. The need for enclosing and cooling the pumps to prevent overheating was established. A basic design was drafted and parts for a prototype were procured. Since this is to be a field unit, maximum power demand of 400 watts was established (able to be powered by a light pickup truck's auxiliary AC power supply). The unit should be housed in a durable enclosure (utility toolbox system by Milwaukee Tool Division was ultimately selected and permission obtained via phone from Bryan Sanchez to use branded boxes without removing the Milwaukee branding. Bryan is their Director of Business Development). Also notified Group President, Milwaukee Tool Division, Steven P. Richman of the project. He was supportive. The unit should be buildable as quickly as possible by a team with average mechanical/electrical abilities using as many off-the-shelf parts as possible. Most of the parts can be obtained at a hardware store, with exception of the programmable logic controllers and oxygen-cleaned solenoid valves.

March 22-25, 2020

Procured parts for the prototype. Began building. Created annotated photos. Posted all plans, engineering calculations, photos to a shared Google Drive (which has since been deactivated in favor of this webite). Shared the design with CU Anschutz friends and colleagues, who put it out to their networks.

March 26, 2020

Update 1- Heard from engineer, Haidar Malhas in Ghana last night — he had the following suggestion after reviewing the design and finding it essentially satisfactory. We consider this a critical update, although the anticipated port pressure from the system is low:

— SPLICE IN A PRESSURE RELIEF VALVE JUST BEFORE EACH PATIENT VENT PORT. I AM USING 1/2″ Tees inline with the PVC gate valves— bush the 90 degree port down to 1/4″ NPT and using a 1/4″ NPT 0-20 psi (0-1400 cmH2O) safety relief— It will allow you to set the valve down to a safe range in case there might be pressure fluctuations in the system. Will share photos later.

A secondary suggestion to filter the air on the output side of the pump instead of the intake side is not going to be implemented at this time since I am trying to filter out whatever COVID-19 virus particles we can (it's a 5 micron filter). I know we can't filter out aerosolized particles, but it is filtering moisture. I'm using a much larger filter than needed to avoid choking the pump on the intake.

I added the pressure relief ports and programmed the digital relay timer. I decided to include two identical timers for redundancy. If one fails, then the other one simply needs to be turned on. I added a folder (Video) and put a 5 minute clip on how to program the timer since it doesn't come with any instructions. I'll do my best to clarify that as I have time.

March 27, 2020

First test of the unit today— Digital relay timer controlling valves and redirecting flow as designed. There is now an “inhale” manifold and and “exhale” manifold with 4 vent ports on each manifold. Will flow/pressure test these later. I added the pressure relief ports on all patient lines.

Yesterday I was working on built-in redundancy. Here is were we are with that:

1. In case the primary pump fails, a second pump is provide. Simply plug it in and switch the air supply line from primary to secondary. While the pump is running, connect the intake filter to the new pump (simply move the intake tubing to the new pump from the old).

2. I added a second digital relay timer programmed identically to the first. Simply turn on its power supply and hit the start button. It will control the switching solenoids the same way the first one did. If the doctors have re-programmed the “inhale” and “exhale” times, they will have to do that for this timer as well.

3. I added a 5 minute video on how to program the timers (see previous day update).

4. Added tubing adapters to the end of the manifold opposite the solenoid valves. If those were to fail, a person could manually open and close flow into the manifolds (one for inhale, one for exhale) while the solenoids are being changed. Include two solenoids (one normally open, one normally closed) for each unit, so they can be swapped out while the person is being manually vented.

Created 8 videos of the testing from today. Promising results. Still awaiting a second cooling fan and the pressure relief valves. Planning to leave for San Fran on Sunday or Monday to deliver the unit to UCSF.

March 28, 2020

Six port testing was done. Unit can supply two manifolds with six ports on each at 2.5LPM; however you'll need to add an additional 4-port manifold to each side, connect it to the first, and turn off two of the ports. The marginal cost to do this is very low per patient added however!! Delighted that you could provide respiratory support for 12 patients on this one pump (supplying six at a time, out of phase). Added a couple more videos.

March 29, 2020

Worked to get word out. Spoke with Milwaukee Tool senior leadership about use of their boxes; interacted with senior leadership at CU about best steps to move forward. Planned to deliver the first unit to Dr. Whetstone in San Francisco on Wednesday 4/1.

March 30, 2020

Drove to Utah. While driving, thought of way to add a ventilation mode that would provide Inhale:Exhale of <1 (in other words Inhale time less than exhale time. This is doable. It will require using the second timer in the circuit and not as a backup. Will sketch this tonight and post. Unclear if this mode will be needed. Will ask Whetstone when I get to San Fran.

March 31, 2020

Posted the sketches I made last night for the I < E timer modes. It is definitely possible but you will lose your fail safe ability to manually operate valves (because it requires 3 patient vent manifolds). You will need to set the second timer such that time on and time off is evenly divisible into those of the first timer. I demonstrated this with a 1:2 Inhale:Exhale logic diagram— it requires the other two patient manifolds to operate in a binary 1:1, 1:3 mode (which alternates)— so the net breaths per minute to the patient and net volume per breath are the same. Take a look at the photos in the 31MAR2020 folder of the “Build…” folder (under “Images”). That should make it clear.

Reorganized all the build photos to make more logical. Heading to San Fran from Utah.

April 1, 2020

Exuberant day in San Francisco. Met with Dr. Whetstone (from UCSF) to review assembly, operation of the Field Ventilator unit. Discussed respiratory modes 1:1 IE ratio, 1:2 IE ratio (which he preferred and requested be implemented). Made the necessary modifications to the unit in order to achieve 1:2 Inhale/Exhale ratio. This meant adding a second digital relay timer to the unit; re-programming the first timer; adding two solenoids, and an additional patient vent manifold. I will post all videos and photos from this when I'm back in Denver. But the net of this is that we tested the modified unit and found it can push air to 12 patients on a 1:2 I/E ratio for 4; and a 1:1/1:3 I/E ratio for the other 8. It is important to note that both groups of patients receives 20 Breaths per minute and has a positive net I/E ratio of 1 second of inhalation to 2 seconds of exhalation. At 2.5 LPM of flow (without added oxygen), this provides 125 mL of air per breath.

Note: an improved programmable logic controller was later substituted for two digital relay timers that eliminated the need for 1:1/1:3 E/E ratio.

A few new images of the final device are included in the Build Folder. The videos we took today in general are large and I may need to break them into sections when I get home before sharing them to the Drive.

April 2, 2020

This morning we received from the UCSF team another query as to whether the unit could be used as an early intervention device in “CPAP mode”— providing continuous positive airway pressure to patients who are not yet in a situation requiring invasive respiratory support (i.e. intubation). This is how the unit started. Essentially using it that way means bypassing the timers and solenoid valves, connecting the air tubing directly to the patient vent manifolds from the pump and adjusting flow rate accordingly. As tested, the unit can provide continuous flow of 6 liters per minute to at least 8 ventilator ports. The simplest way to achieve this is to use Patient Vent Manifolds with normally open solenoid valves (which I've designated Manifold 1 and Manifold 3 in the Electronic Schematics); remove air supply to the normally closed solenoid at the “Bridge” and the normally closed solenoid valve at the Patient Manifold 2. Reconnect the tubing supply that had been going to Patient Manifold 2 up to the Air supply Tee from the Pump. Turn both digital relay timers off (lower right button on each). You are now providing air to the two manifolds with the gold normally open solenoid valves. Attach your CPAP vent hoses to those manifolds and you will be flowing continuous airway pressure to those (eight) patient ports.

Added the electrical schematic for converting the Field Ventilator Unit to a 1:2 Inhale/Exhale ratio.

April 3-4, 2020

Traveled from San Fran back to Colorado. Prototype #1 left with Dr. Whetstone in San Francisco Worked with CU Chancellor's office to plan construction of one or more units for Colorado. Upon return, discussed (with Dr. Whetstone) addition of High Flow Oxygen at variable flow rates (FiO2) to the ventilator. Completed the design for this upgrade, added the schematic and a couple additional parts to the Shared Folder. Plan is to put up the Build site at tonight after review by Dr. Whetstone. Oxygen cleaned solenoid valves will need to be substituted for those used in the prototype.

April 5, 2020

Worked on FiO2 calculations and integration ports. Posted these modifications to the working website.

April 6, 2020

Ken Lambrecht got the site up and SSL working. We will be shutting down the shared drive so we only have to update in one place. Spoke with people who may be able to help source parts and build units; spoke with several control system and flow experts who had good inputs. We will continue to update here as design changes are integrated. Remember, the purpose of this ventilator is as a field unit for crisis intervention mode when the shortage of vents exceeds the supply– as an option for anyone who would not otherwise be prioritized for respiratory support when indicated. Thanks everyone!

Rev 2

April 8, 2020

Today we began building the second unit, aided by an outstanding team of engineers, flow mechanics specialists and supply chain professionals at RK Mission Critical in Aurora, Colorado. Special thanks to President and CEO of RK Mission Critical, John Marinnucci for his leadership, flexibility, people and resources to make this happen. The “Can Do!” spirit of this team is amazing. RC Mission Critical is providing the resources to build five units for the Colorado healthcare community and the people it serves. As with other areas, Colorado may be facing a critical shortage of ventilators in the coming days. Thanks as well to Plant Manager, Scott Dickerson; Pre-Construction Manager Phillip Powers; and Floor Manager, Sam Soares. Thanks to SVP and Chief Customer Officer, RK Corporate, Marc Paolicelli for putting together this team and making it happen. Finally, special thanks to CU Anschutz Medical Center Chancellor Don Elliman; and CU AMC Excecutive Vice Chancellor For Biotechnology, Steve VanNurden for catalyzing, supporting, and participating in this project.

Today Dwayne Kessinger (RK Mission Critical Engineering Manager) and I critically evaluated the potential failure modes of the ventilator as designed and prototyped. We identified better solenoid valves (Spartan Scientific later supplied a 20 Million Cycle valve built specifically for these field ventilator units)  vs. the 1 Million cycle valves used for the prototype); and better digital relay timers with 100% solid state electronics to replace the existing DRT's, which have mechanical relays and may also have limited life. We will provide specs on the new valves and controllers shortly. The valves are made by Spartan Scientific. Use of a new solid state programmable logic controller offers the possibility of simplifying the design. We will have a spec ASAP.

April 9, 2020

Today we revised the prototype to improve anticipated service life in the digital valve timers, solenoid valves and switching relays. Thanks to Dwayne Kessinger for spec-ing a drop-in power module, complete with new programmable logic controller (PLC); 24V DC Power Rail, and solid state relays for each patient ventilator manifold. Thanks to Jim Koller at Spartan Scientific (Canfield) for spec'g new 20 million cycle solenoid valves. These will be custom O2 cleaned valves. Jim and company spec'd entirely new valves and promised them in under a week. Thanks as well to Jesse Davern and Zdenka Mueller at RK for their flexibility andextraordinary patience with design modifications; as well as their incredible supply chain connections/efficiency.

(Contact Tom Greany for the Updated Power Module, Solenoid Valve Part number and Programmable Logic Controller part numbers/costs/suppliers.)

April 10-14, 2020

We've been busy building the Rev 2 vents. Working on training the RK team; building manifolds, enclosure modifications; vibration and sound damping; enclosure cooling; trying to find parts; substituting parts on the fly; training the RK Mission Critical assembly teams and Floor Manager (shout out to Sam, Danny, Leon and Cesar. You guys learn quickly and are making things happen. Thanks to Veronica and the custodial staff for clearing and cleaning the assembly area.

Between yesterday and today, also introduced a room air bypass feature to the vent. This will avoid dead heading the pump and will expand the range of FiO2 the unit can deliver. Not deadheading the pump allows the unit to run CPAP on one manifold; BiPAP on the second manifold; and 1:2 or 1:1.6 on the third manifold, where four intubated patients can potentially be supported (both requested I:E ratios by the ER physicians and respiratory therapists). The 1:2 I:E mode provides pure square wave inspiration/exhalation; whereas the 1:1.6 mode provides some clipping to the wave forms. These will be posted as soon as available. It is also important to note that running simultaneous CPAP, BiPAP and asymmetrical I:E ratios does produce fluctuations in CPAP flows, though air is always moving. 1:1.8 would also be possible, with peak flows lying between the 1:1.6 and 1:2  we have analyzed. There would also be clipping to the inhale waveforms as with 1:1.6 I:E mode.

April 15-17, 2020

Acquired parts to complete six Rev2 field ventilator units. Passed the one month anniversary date of the project on April 16. Continued to model system performance, inhale/exhale wave forms when each manifold is running different respiratory modes (e.g. CPAP, BIPAP and 1:X Inhale:Exhale). Developed concept of peak shifting (synchronizing the inhale of manually entered inhale:exhale and BIPAP modes to minimize manifold flow fluctuations). RK Electronics assembly personnel Ricardo, Matt and Mike joined Dwayne in assembling and testing the new power and control modules. They did a great job and even provided some video footage to help others with the build. We are withholding instructional video for Rev2 units at this time. Completed unit number 2 on 17APR2020. It will be delivered to the Pulmonology Research Lab on the CU Anschutz Medical Campus. Posted more build photos and videos for Rev2 in the Rev2 Build section of this site. Planning to complete assembly and bench testing of all six units on 20APR2020 and begin deploying the units.

April 18-24, 2020

Received a few components that were needed to complete the build of Rev2 units 2 through 7. Delivered unit 2 to CU Anschutz Medical Center Pulmonology Research Laboratory (Dr. Jeff Sippel, Associate Professor of Clinical Practice, Medicine-Pulmonary Sciences & Critical Care) for evaluation and testing. Units 3-7 should be completed this week. Unit 3 will go to UCSF for testing. UC Health Colorado Springs will receive a unit for testing by Kevin McQueen, MHA, RRT, RRT-ACCS, CPPS, CM, Director, Southern Colorado Region, Respiratory Therapy/Pulmonary Diagnostics, UC Health Memorial Hospital; and Rob Scott, BSRT, RRT-ACCS, CPFT, Clinical Manager, Respiratory Therapy, UC Health Memorial Hospital, Colorado Springs.  Although the role of ventilators in helping to manage patients with COVID-19 has continued to evolve very quickly, the Field Ventilator's viability in multiple respiratory modes as an emergency solution; and as a possible solution with appropriate modifications for developing nations with no ventilators appears sound at this point.

April 1, 2020

Tom and All,

Thank you for the incredible work on this project. Your in-person successful demonstration of the Emergency field ventilator leads me to recommend it as a temporary backup device when I no longer have access to a ventilator. Obviously the patients would have to be sedated and paralyzed which is a typical post-intubation state while in the Emergency Department. Given the current grave situation in our country and in many of our largest cities I will be pursuing further testing and research at our hospitals under the current FDA guidelines. Although it would be a public health victory to avoid using this field ventilator, we have to prepare for the possibility of our hospital and EMS systems being completely overwhelmed with critical patients. Your design and testing have led me to believe that I could use this unit to save lives in that setting.


William D. Whetstone M.D.
Clinical Professor of Emergency Medicine
Department of Emergency Medicine
University of California San Francisco &
San Francisco General Hospital TRACK-SCI
505 Parnassus Ave L-126
San Francisco CA 94143-0209

Prototype: Overview

Prototype: Build

Prototype: Testing

Prototype: Operating

Rev 2: Build

Rev 2: Testing

Other videos

The content on this website is being released in this manner to maximize the potential public benefit during this urgent need for measures to respond to the COVID-19 crisis, including promoting potential ventilator manufacturing methods.

The content has not been reviewed or approved by the U.S. Food and Drug Administration (FDA). Interested readers are encouraged to contact the FDA and review available FDA materials, including their guidance on ventilators as well as the Department of Health and Human Services (DHHS) declaration of liability immunity for medical countermeasures against COVID-19.

PLEASE NOTE: The content has not been peer reviewed. The Authors make no representations or warranties of any kind (express or implied) relating to accuracy, safety, usefulness, usability, marketability, performance, or otherwise of the content released here. The Authors disclaim all express and implied warranties of merchantability and fitness of the content for a particular purpose, and disclaims all express and implied warranties regarding non-infringement of any patent, copyright, trademark, or other rights of third parties in the content or use of the content, or in the making, using, or selling products or services by any person or entity.

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The Authors are proud of their affiliation with the University of Colorado as alumni and, in the case of Dr. Thomas Greany, as a professor at the Anschutz Medical Campus. The Authors express their appreciation to the University of Colorado for its support of this project.