A High Frequency Oscillatory Ventilator Design For Use in Pandemics

This is a continuation of some of the ideas I expressed in a previous post on HFOV design. You can see it here: http://panvent.blogspot.com/2009/08/crisis-is-near-now.html

Some of the news items I have been reading say that there will be a particular shortage of high frequency oscillatory ventilators. Many ICU units do not have any or may only have one. This type of ventilator is required to care for patients with the most damaged lungs. Here is a little more information on a design for an HFOV. This is very preliminary. It surely needs more work. Someone would have to to build and test a prototype to determine if it is feasable.


What is HFOV?

An HFOV (High Frequency Oscillatory Ventilator) is an advanced ventilator design that is sometimes used in ARDS (Acute Respiratory Distress Syndrome) patients when a conventional ventilator will no longer provide adequate ventilation. Using a HFOV is considered a “lung sparing” technique.

When using conventional ventilators, the ventilation levels can be increased by increasing the percentage of oxygen fed to the ventilator, increasing the stroke volume, or increasing the rate or frequency. Other measures to improve ventilation can be increased PEEP (Positive Expiratory End Pressure) levels, reversed I/E (Inspiration/Expiration) ratios, methods to increase the average airway pressure, or even PLV (Partial Liquid Ventilation) has been tried, where a perflourocarbon solution (perfluorooctyl bromide) is put into the lungs to reduce lung damage without reducing oxygen transfer. ECMO (Extra Corporeal Membrane Oxygenation) is another method that can prevent damage to lungs by the use of high pressures and oxygen levels in conventional ventilation.

The HFOV can maintain a fairly high mean airway pressure, resulting in better ventilation without causing as much lung damage. With the high rates required for HFOV, each breath is less than the dead space in the lungs. There are various mechanisms that explain how it works (http://priory.com/cmol/hfov.htm), but it does work quite effectively. The breathing mechanism is similar to a dog panting at a high rate.


How HFOV Works

To make a HFOV work, you need to have a system that maintains a set average airway pressure and then have another device that oscillates this column of air at a desired rate, amplitude and I/E ratio. That’s the fundamentals of what it does.

Now to optimize this design you need to have a gas management system that controls the oxygen level, temperature, humidity and inlet pressure and flows to the HFOV device. We do not have to worry about designing these parts. They are all standardized respiratory equipment that is also used on conventional ventilators.

Other design considerations are that the air flow goes through the tubing in such a way as to optimize gas exchange, and we will also need other alarms to warn us of low or high pressures, improper rates and loss of supply gas. We may also want additional alarm systems that warn us of equipment failure modes. The tubing should be relatively non-compliant and the system should have minimal dead space.











Fig 1. HFOV design from IEEE Transactions on Biomedical Engineering
this link.

This design would be based mostly on the diagram in Fig 1. The controls and operator interface could be modeled to be similar to the 3100B from Sensormedics. The 3100B is the most commonly used HFOV for adults. Many RTs are already trained in its operation. By making the controls and alarms similar to the 3100B, it could be more easily deployed in a pandemic situation.

This is how the device in Fig 1 works:

• A filtered, humidified air/oxygen mixture is fed into the feed tube near the ET (Endo Tracheal) tube. The flow rate is monitored and controlled by the mass flow meter
• It travels down the tube towards the oscillator unit and exits via the servo controlled restriction valve.
• The pressure sensor is that thing on the tube between the inlet and outlet ports. The electronics control system will receive this pressure signal and adjust the servo controlled restriction valve so that the average airway pressure is equal to the desired set point.
• The pneumotach is not really required for operation. They have to do measurements for their study. Vacuum is not really required either, as average airway pressures will always be positive.
• The oscillator is that plunger looking thing on the right hand side. It looks and works like a speaker in the 3100B but is really a special purpose built device. They call it the driver.
• The plunger moves in and out at the desired rate, wave shape and amplitude as determined by the driver circuitry and the operator settings.
• Now, you can see, that column of air is going to push and pull air in and out of the ET, which goes into the lungs. When the air comes out of the ET, the fresh bias flow gas will flush it away and out toward the servo controlled restriction valve. Fresh bias air is pushed into the lungs when the plunger moves toward the ET tube.
• The oscillations of the plunger will change the instantaneous pressure in the tube positive and negative with respect to the average pressure.
• The position feedback device improves the performance of the oscillator circuit and can also be used as a part of a safety system
  

Parts And Controls

I will group parts into 3 general categories.

• Oscillator driver and driver circuit.
• Sensor, actuator and associated circuitry.
• Control and display system.
  
  

Oscillator Driver and Driver Circuit

For the oscillator driver we would want to use a big, high power subwoofer type speaker. It should be tough and able to handle high duty cycles and long periods of operation. It should have a metal cone to make it inflexible. We might have to glue a metal plate to the cone to make it more rigid. In order to reduce the dead space, we could make a mold of the front of the cone surface in resin or silicone with an air access hole drilled in the center to mate with the speaker cone assembly.

I am not certain how well a speaker will work though. There must be technical reasons why the designers of the 3100 use that design. Speakers, even subwoofers have a certain compliance and harmonic resonance built into them that is at a higher frequency than the rates we would need to use. It may require a large speaker using only a small portion of it's maximum designed excursion in order to minimize the effects on the output airflow pattern caused by the damping effects of the speaker cone suspension.

I am not sure if a position sensor is absolutely required, but something can be attached to the back of the cone if it is. Cooling may also be required. We could use lots of air and fans, or perhaps an active system using peltier devices.

The drive circuit would be a high output audio amplifier. I think it is best to use one designed for automotive use. These are generally more rugged, modular and can easily run on a 12 volt battery for electrical backup purposes.


Sensor, Actuator and Associated Circuitry

The sensor and circuitry would be similar to the one my son Jeff used in his ventilator design (Norman). It would convert the pressure pulse to a digital value encoded and sent on an RS232 port. We may wish it use more than one pressure sensor in order to provide redundancy for safety reasons. The pressure controller and alarm board would be a servo controlled valve and driver circuitry that operates by RS232. It could also house the audio alarm. This alarm would also engage and cause the valve to open if communications were lost. These circuits would have to be hand built unless there is a commercially available alternative.

Control and Display System

The control and display system would be a computer. It would probably be a PC and probably a laptop. A laptop has its own integral battery backup system. A program such as Labview can be run to show a display that looks similar to the control interface from the 3100B.

The instantaneous pressure readings received from the sensors could be integrated over time for display. The minimum and maximum pressures would be the peak recurring pressure extremes integrated over a short time interval. The average pressure would be integrated over a longer time period.

Operating parameters could be entered by selecting the appropriate box on the screen and entering the parameter via the keyboard. Alarms could also be displayed and color coded.

The computer would also output an audio signal to the speaker amplifier. This wave shape is normally a square wave pulse produced by a pulse circuit with variable duty cycle, frequency and amplitude in the 3100B. In our machine, we would have full control of the waveform via software. We could add pre-emphasis and custom wave shaping to the output to compensate for physical design shortcomings in the speaker and driver circuit or shortcomings in the housing and tube.
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Here are some resources if you want to learn more about HFOV.

Guidlines for the use of HFOV
http://priory.com/cmol/hfov.htm

HFOV guidelines from Stanford Hospital and Clinics
http://scalpel.stanford.edu/ICU/HFOV%20Guidelines.pdf

The use of HFOV in surgical patients.
http://www.surgicalcriticalcare.net/Guidelines/High%20frequency%20ventilation.pdf

Slide show of HFOV in the adult patient.
http://intranet.unchealthcare.org/hospitaldepartments/respiratorycare/practice-education-and-research/inservices-presentations/HFOV%20in%20the%20Adult%20Patient.pdf?searchterm=suction

University of Virginia experience with HFOV.
http://www.healthsystem.virginia.edu/internet/respiratory/Research/Retired/HFOVposter.pdf

ARDS and HFOV from Express Healthcare.
http://www.expresshealthcare.in/criticare2009/criticarefrontiers200912.shtml

Ventilation article from Answers.com.
http://www.answers.com/topic/mechanical-ventilation

Wickipedia Article about HFOV (please improve this)
http://en.wikipedia.org/wiki/High_frequency_ventilation

Spec sheet for the 3100B
http://www.viasyshealthcare.com/prod_serv/downloads/062_3100B_Spec_Sheet.pdf

Picture of a 3100 HFOV
http://commons.wikimedia.org/wiki/File:HFOV_3100A.jpg

Competency exam for 3100B operators.
http://www.viasyshc.com/smc/Reference/Critical_Care/Exams/3100BExam.pdf

Video showing operation of the 3100B
http://www.youtube.com/watch?v=jLroOPoPlig

Video showing initial operator calibration of the tubing set for the 3100B
http://www.youtube.com/watch?v=O2TaDyzxQAY

The Crisis is Near Now

A High Frequency Oscillatory Ventilator Design

Some of the news items I have been reading say that there will be a particular shortage of high frequency oscillatory ventilators. Many ICU units do not have any or may only have one. This type of ventilator is required to care for patients with the most damaged lungs.

I found a design for such a device in the IEEE transactions on Biomedical Engineering publication. Here is the drawing.















The link to the PDF is located at this link.
The PDF essentially gives instructions on how to build this ventilator.

This device is much more complex than the basic ventilators I have built, but it seems that it would possible to build such a ventilator from common valves, sensors and control systems substituting for some of the components that are listed. The diaphragm actuator for example could be a large bass (subwoofer) type speaker.

It would be great if someone could build one of these unists and get back to me about how feasible it is and how well it works in testing.

Next week an idea for using ECMO.
Don't know what ECMO is?
Have a look.
http://panvent.blogspot.com/2009/09/using-dialysis-machine-to-do-ecmo.html

Now My Rant
Many of the recent reports that I have read indicate that there is a high likelihood that the current H1N1 pandemic will result in very busy ICU units and shortages of ventilators this fall in Canada. Doctors that work in ICU units that have had to treat H1N1 patients with Acute Respiratory Distress Syndrome (ARDS) say that the patients may have to be on a ventilator for a longer time period than they have seen with patients that got lung function complications in normal seasonal flu.

Here is a link to one such news item from CP.
Intensive care units likely to be main battlegrounds in the war against H1N1
There have been many stories like it originating from hospitals in Canada, the US, India, Australia, UK, Brazil, etc.

When I first proposed the Pandemic Ventilator Project two years ago as a means to alleviate some of the needs for ventilators in a pandemic, I expected it might be used for the H5N1 Avian flu. It seemed like a good insurance plan for a pandemic that, if we were lucky, might not even happen. Things have changed a lot since then. Now it is H1N1, people have actualy died from it, and this pandemic might return stronger this fall and winter, killing even more people. The risk of ventilator shortages has gone from a remote possibility to more likely. A lot has changed in the last six months.

Unfortunately, something that has not changed is the development status of the Pandemic Ventilator Project. As I have said several times before, the project has largely reached the limits of what I can do on my own to develop it. I need people with more ideas, more technical knowledge, more clinical expertise and better management and organizational skills than I possess on my own to carry the project further than I have. It would be a shame if we reach the stage where we really could use a ventilator from he pandemic ventilator project, but no one worked on it beforehand to get it ready in time.

Everything Old is New Again

Workshop Built Ventilators

I return to something I mentioned in an earlier post about homemade (wooden) iron lungs described in a 1950s Popular Mechanics article. the link under the picture will take you to a site that posts the original complete assembly plans.

Intended only for emergency use until a commercial respirator could be obtained, this “wooden lung” was designed by engineers and built by a volunteer group under the supervision of Dr. Gerald M. Cline, Dr. Homer O. Dolley, and Sister Celine of the medical staff of St. Joseph’s Hospital, Bloomington, Illinois. On completion, the unit was put into immediate use in emergency treatment of eight-year-old Rudy Landheer, a victim of polio in the epidemic of 1949. The original unit did emergency service for 12 hours until a conventional iron lung could be obtained.

Popular Mechanics Jan 1952 Emergency Wooden Iron Lung

I will also talk about some of the earliest positive pressure ventilators built in Great Britain and home made iron lungs used in the United States.

The British ventilators were built in Newcastle for anesthesia and also the polio epidemic in the late 1940s and 1950s. These ventilators were built by the inventive Newcastle professor of Anesthesia Edgar Pask, his chief technician Norman Burn and his staff of highly able technicians using war surplus parts.

This is the Newcastle ventilator. (Known as a respirator at the time.) Just as we named our ventilator “Vinnie”, they named the ventilator they designed and built. They named theirs “Mother”.

"Mother"

The electric motor was controlled by mercury switches. A spinning disc allowed speed variation, and a camshaft moved four small bellows up and down to provide the ventilation.

After the success of their first ventilator design they wanted to design another that could be easily and cheaply and quickly constructed by any able technician. They were building these ventilators to save lives in the polio epidemic. There simply were not enough “iron lungs” available in Britain to deal with this epidemic. There were about 1000, but many of these were already in use. There were 1000 cases of polio in 1946 in England and Wales, but this increased to 9000 by 1947. The use of a ventilator could reduce the death rate of those polio victims that were afflicted with respiratory paralysis from 90% to 30%.

One version of a cheap and easy to build ventilator was the Bang ventilator of 1953. This design was shared with others and similar units were built elsewhere.

The Newcastle "Bang" ventilator

The “Bang “ ventilator design worked well, but it required frequent adjustment and maintenance. The mercury switches it used were important for anesthesia, but were not required for polio patients. They could also be difficult to obtain.

They then built two prototype "polio ventilators" out of materials that were readily available in any workshop. The “blow” side of a domestic vacuum cleaner powered these ventilators. Up to 10 ventilators could be powered from one Hoover.

Two Polio Ventilators

These ventilators were made of plywood and perspex. The one on the left, used a magnet to operate its main valve. Inflation pressure was varied by adding weights onto the hinged flap on top much as we do with our design. There were no electronic controls and alarms. Cycling was controlled by regulating the leak out of the plastic bag under the flap. When the bag emptied, the magnet lowered and opened the main valve, allowing inspiration until the bag had filled enough to again lift the magnet from the valve, which would then shut until the bag was empty again

The device on the right used the "flick over" rocker from a light switch as a main switch which opened and closed a piece of tubing to allow gas flow. Inflation pressure was varied by altering tension in springs rather than by using weights, while controlled gas leak was still used to control cycling.

Both these devices split the inlet air, from the vacuum cleaner, to both the patient and the control bag. The expansion of the bag triggered the main valve that stopped the flow of air and allowed expiration.

The inspiratory and expiratory phases of the wooden polio ventilator

Above is a more functional diagram of one of the “Polio” ventilators. These devices seem incredibly crude by today’s standards. Even with the most basic of control systems and electrical valves, a far more sophisticated and reliable system can be constructed today. The people that designed and built these early ventilators were very inventive, brave and capable. They saw the need for ventilators during the polio epidemic and did whatever was required, in order to supply them. Although these units seem crude, they, and others like them saved many lives that would have been lost.

These early ventilators were followed by many more, for both adults and infants. "Home made" ventilators of all shapes and sizes were used for another thirty years at the Royal Victoria Infirmary.




In America, the polio epidemic also caused a large demand for ventilators. There was more demand than could be supplied by the existing stock of "iron lung" negative pressure ventilators.

An Iron Lung ward filled with polio patients. 1953

Rancho Los Amigos Hospital, Downey California

During the height of the polio epidemic years, the iron lung became such a crucial part of treatment that Philip Drinker and Edgar L. Roy offered instructions for building a makeshift emergency ventilator for cases of life-threatening paralysis. Although this emergency version of an iron lung was only intended for use until a board-approved version arrived, it was functional enough to save lives.

Image of an emergency ventilator, from the article by Drinker and Roy.

In contrast to the original, professionally crafted iron lung of 1929, Drinker and Roy employed common household and conveniently available hardware store materials in their emergency respirator. The materials included a car inner tube for a rubber collar, a common vacuum pump to provide pressure, a six-inch square piece of double-thick glass, a piece of sole leather to serve as the valve, a glass U-tube with colored water to show pressure, and several pieces of spruce wood. Drinker and Roy provided detailed construction plans and clearly indicated that it was an emergency respirator that could only accommodate small children and was not meant to replace the standard-size iron lung. Nonetheless, they fully supported its use in emergency situations

Assembly diagram

You can see these and more at :

The Brian Welsh Memorial Museum of Anaesthesia

Popular Mechanics Jan 1952 Emergency Wooden Iron Lung

University of Virginia Historical Collections at the Claude Moore Health Sciences Library


Here is a personal account of a homemade ventilator quickly built and used to save the life of a child

Assembly Instructions

Here are some details of how the Pandemic Ventilator is constructed.

It basically consists of the bellows unit, which is made of wood, valves and piping, a PLC controller, some wires and switches and a power supply unit.
The whole unit is mounted on a piece of ½ inch thick that is plywood 18 inches by 21 inches.


To get started you will need the valves. You may be able to find some used ones somewhere. New valves are expensive, over $100 each. The inlet valve can be ¼” normally open valve, the other two should be a minimum ½” diameter, one normally open (NO) and the other normally closed (NC). They must be of a direct acting solenoid type.

Direct acting solenoid types are required to operate with air. Pilot operated types will only operate with liquids. Pressure ratings do not matter for the 1/2'' valves, but the 1/4' valve should have at least a 50 psi rating. If you can only get either NO or NC 1/2" valves you can still make it work by adjusting the PLC output. The inlet 1/4' valve must be a NC valve or you will end up blowing up a lot of bags in the testing phase.

The valves are connected with pipe and mounted in such a way that the “T” to the Bellows lines up with the center of the bellows unit.
I used threaded pipe fittings. I used:

• Two ½” NPT “T”s,  One ¼” airline fitting (to connect to an air supply)
• Four ¼” pipe nipples (short pipe sections threaded at both ends)
• Three ¼” to ½” adapters
• Two ½” pipe nipples
• One ½” plug

• The bellows unit has a hinged section of ¼ inch plywood that is 10 ½ inches by 12 ½ inches.
• There is a 1 and a 1/2” by 1 and a 1/2” by 9” sensor pole attached beside the bellows that is used to position the magnetic switches. It should be angled at the bottom so that the top leans back about 1and ½ inches from the bottom.
• The bellows hinge is constructed of 4 pieces of 1 and a 1/2” by 7” 5/8” plywood pieces and one 1 and a 1/2” by 1 and a 1/2” by 17 inch piece of wood, two 3” hinges and a 2” by 12 and a ½” reinforcement.
• The bellows is made by screwing down the bottom 2 plywood pieces to the backing board.
• Arrange the other 2 plywood pieces directly over the first two, cover with the 17 inch piece of wood and and clamp together.
• Drill holes centered 2 and ¾” in from each end to accept ¼” carriage bolts inserted from the bottom.
• Unclamp and remove the top 17” piece of wood.
• Insert carriage bolts from the bottom to line up the middle layer plywood pieces and clamp again.
• Screw hinges to middle layer plywood. Ensure the screws do not protrude through the plywood to the bottom layer.
• Screw the other side of the hinges to the bellows lid between the lid and the reinforcing strip.
• The bag is clamped between the two plywood sections during operation using the nuts and washers on the carriage bolts.
• The magnet is attached to the end of the bellows near the sensor pole, and the sensors are attached to the sensor pole.

• To make the bag for the bellows, I used a large size Ziplock freezer bag.

• Cut off the ziplock part.
• Inert ½” plastic tubing into the center and use Tuck tape to seal and reinforce the edges.
• The tubing should stick out of the bag far enough to be able to be slipped over the end of the ¼” nipple section of piping.
• The taped seam of the bellows bag should be in on the bottom plywood section.
• Install the hinged cover and then the top 17” section.
• Clamp together with the 4” long ¼” carriage bolts, two nuts and 2 washers.

• The valves, sensors, switches and power supplies are wired to the PLC unit.
• Wiring diagram and program will be posted later.

Closeup of a Valve

Bags

Back of hinge and bellows feed

Bellows bolted together