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

Using a Dialysis Machine to do ECMO

(There is more on ECMO on my Oct 1, 2009 Posting)

http://panvent.blogspot.com/2009/10/ecmo-dialysis-h1n1-and-macgyver.html

Many of the recent case reports indicate that sophisticated machines are required to treat the patients infected by the current novel H1N1 strain of influenza. Basic ventilators such as the existing Pandemic Ventilator Project designs may not be adequate for these H1N1 patients that develop ARDS. Pandemic Ventilator Project type units, however could possibly be utilized on other existing patients to free up more sophisticated equipment for patients requiring advanced therapies. I have also found a design for high frequency oscillatory ventilator that I posted (here).

Another technology that almost certainly will be in shortage during the pandemic is access to ECMO (Extra Corporeal Membrane Oxygenation) machines. ECMO machines oxygenate the blood directly using a gas permeable membrane. These machines can keep people with severely damaged lungs alive long enough for their bodies to repair their damaged lung tissues. There is very little of this equipment around. Many centers do not have any ECMO machines, or have only one.

Consider this:
An ECMO machine pumps blood from the patient, adds an anticoagulant, runs it past a gas exchange membrane to remove CO2 and add O2, regulates the blood temperature with a heat exchanger, removes air bubbles via drip chambers, checks incoming and return pressures, and has safety systems to ensure air is not infused, or pressure limits are not exceeded.

A dialysis machine pumps blood from the patient, adds an anticoagulant, runs it past a dialyzing membrane to stabilize electrolytes and remove toxins and fluid, regulates the blood temperature by controlling dialysate temperature, removes air bubbles via drip chambers, checks incoming and return pressures, and has safety systems to ensure air is not infused, or that pressure limits are not exceeded.

Hemodialysis System

They are pretty similar eh?

Note that terminology for blood access is opposite in ECMO vs hemodialysis.

• In ECMO, the port where the blood is drawn into the pump is termed the Venous line and the port where the blood is returned to the body is termed the Arterial line.
• In Hemodialysis, the port where the blood is drawn into the pump is termed the Arterial line and the port where the blood is returned to the body is termed the Venous line.
• In CRRT, (a form of hemodialysis) the port where the blood is drawn into the pump is termed the access line, and the port where the blood is returned to the body is termed the Return line.

The Hemodialysis picture is from METU BIOMAT, and the ECMO picture is from Medscape. (Note there is an error in the Medscape ECMO drawing, both pressure ports are named "Post-Membrane Pressure Monitor". The lower one should be named "Pre-Membrane Pressure Monitor) Note also that fluids and heparin are normally infused post pump in hemodialysis, as this method is usually considered a safer method. Air removal, and monitoring safety systems are also not in the ECMO picture. Both VV-ECMO and Hemodialysis can use a Jugular Venous Dual Lumen Catheter for access.

VA-ECMO vs. VV-ECMO
There are two types of ECMO. VA-ECMO or Venous-Arterial ECMO, has a more complicated method of attaching to the patients circulation system. VA-ECMO operation is similar to the use of a heart-lung bypass machine in that it replaces the function of both the heart and lungs of a patient. VV-ECMO or Venous-Venous ECMO, has a less complicated method of blood system access. It is done using high flow central line catheters similar to the ones used for dialysis. It replaces only the lung function of the patient.

Some patients with H1N1 are getting lung damage and progressing to ARDS. They may require ECMO because their lungs are so damaged that they can no longer provide enough gas exchange to maintain other body functions. A ventilator may not be adequate in these situations. The heart is not usually compromised. These patients could benefit from VV-ECMO if a machine was available. As stated earlier, available ECMO machines would probably be in very short supply during the pandemic.

You can see that the equipment for ECMO is very similar to the equipment required to perform dialysis. In fact CRRT or SCUF are sometimes done in order to control electrolyte and fluid volume levels by adding a dialyser to an ECMO machine without needing any additional equipment.

It seems to me that one could do VV-ECMO treatments using a dialysis machine with a diffusion membrane oxygenator attached in line on the blood tubing set. Some extra gas and oxygen regulators and controls may also be required. If a standard hemodialysis machine is used, it can be run at a low dialysis flow rate (available on machines such as the Fresenius 2008K) to run in a SLED (Sustained Low Efficiency Dialysis) mode continuously. I would like to hear comments from people that have worked with ECMO equipment to hear if they think this is at all feasible.

This Just in (Sept 15, 2009)

Article in New York Times about ECMO use in H1N1 pandemic and potential shortage of ECMO machines.
http://www.nytimes.com/2009/09/16/health/research/16flu.html

Lancet article about the efficacy of ECMO for severe influenza treatment.
http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(09)61069-2/fulltext

Bloomberg article on using ECMO for near death swine flu cases.
http://www.bloomberg.com/apps/news?pid=20601080&sid=a3B182GF_auk

Belfast Telegraph article about ECMO
http://www.belfasttelegraph.co.uk/news/health/article14493762.ece;jsessionid=80D2A25F7E4033BF410D32971134D6DA?postingType=posting&mode=thanks&postingId=14493924

Update, Sept 18 2009

I have been thinking about this doing ECMO using a dialysis machine for a few days now. So far I have not had any comments either for or against on this blog.

I have done some further research into the equipment required for ECMO and some of the problems with ECMO therapies. It appears that maintaining systemic coagulation using heparin is sometimes a problem. Patients may not properly respond to the heparin therapy, they may have allergies, or there may be bleeding problems associated with systemic coagulation. These are problems that are also very common in hemodialysis and CRRT therapies. One solution to this problem is to use regional citrate anticoagulation. Citrate is infused into the blood circuit at the blood access port to initiate anticoagulation and calcium is infused at the blood return port to cancel the effect of the infused citrate.

This can be more complex than straightforward heparin infusion because the infusion of these chemicals also alters the calcium, pH, fluid volume and sodium levels of the patient. In CRRT and SLED therapies these parameters are monitored and controlled by adjusting the sodium and bicarbonate levels of the dialyzing and infusion fluids. Patient fluid volumes are also easily controlled by the dialysis machine.

Regional citrate anticoagulation has been shown to significantly extend the filter (dialyser) life compared to heparin coagulation by reducing clotting. It is sometimes used when the patient has HIT (Heparin Induced Thrombocytopenia). Regional citrate anticoagulation can also reduce other complications that would occur when using systemic anticoagulation protocols.

During a pandemic, it may be difficult to obtain enough membrane oxygenators to do ECMO. It is reasonable to assume that regional citrate anticoagulation could also extent the serviceable life of the membrane oxygenator by reducing clotting in the device. It will be important to make the best use of whatever supplies one has on hand. If it is indeed possible to use a dialysis machine to do ECMO, and also employ regional citrate anticoagulation with it, this could be a good way to save more lives with the possibly limited supplies available.

Here is a link to a PubMed abstract of an ASAIO journal article about using regional citrate anticoagulation with ECMO.
http://www.ncbi.nlm.nih.gov/pubmed/16883129?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum

Some More Info if this Intrigues You...

JAMA article shows that most patients with severe H1N1 that are treated with ECMO survive http://jama.ama-assn.org/cgi/content/full/2009.1535

Some general Info on ECMO systems and complications
http://www.anzcp.org/CCP/Clinical%20applications/ecmo.htm

Here is a link to a Patent for an ECMO system
http://www.google.com/patents/about?id=QoIcAAAAEBAJ&dq=ECMO

CDC info on the use of ECMO and CRRT on novel A H1N1 patients.
http://www.cdc.gov/mmwr/preview/mmwrhtml/mm58d0710a1.htm



Response to the first comment by Anonymous (see below)

Thank you for your comments. I have been waiting to get some feedback on this issue. Just to clarify, ELSO is Extracorporeal Life Support Organization centered at the University of Michigan.

Now you have question about the origin, purpose and legitimacy of the Pandemic Ventilator Project. It was started on Feb 22, 2007 to promote alternative methods of supplying additional ventilators during a pandemic. In order to reduce the death toll of people either ill from a pandemic or those who would be denied life support so that the ventilator they are using could be used to save a pandemic victim (due to triage protocols). Now when you question legitimacy, I am not quite sure what you are after. I am not trying to defraud or manipulate anyone, and my motives for the project are entirely humanitarian. It is not a commercial venture; in fact I have spent a fair bit of my own time and money on it. All of my work and postings are available for you to view and see for yourself. Now if by legitimacy, you mean authority, I really have none. The opinions I express are my own. It is up to the reader to determine if my arguments are rational and my sources of information are valid.

Now when you warn against an untrained person just setting up ECMO on a dialysis machine when no prior testing or feasibility studies have been done you are absolutely correct. When I proposed this idea, it was for people that are qualified to do ECMO treatments to try to find innovative alternative ways to provide this potentially life saving treatment even if there were a shortage of existing ECMO equipment during a pandemic. I was hoping that knowledgeable people could look at the idea and see if they could make it work safely rather than dismiss it out of hand. Perhaps a someone could find a solution to this problem with the pumps that you mentioned.

Now when you assuredly state that there will be NO shortage of ECMO systems in the US, I do not think you can say that for sure. When we have Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota (CIDRAP) http://www.cidrap.umn.edu/ worried about a shortage of ECMO machines http://legal-ledger.com/item.cfm?recID=12283 , http://www.startribune.com/lifestyle/health/59253022.html?elr=KArksD:aDyaEP:kD:aUbP:P:Q_V_MPQLa7PYDUiD3aPc:_Yyc:aUHDYaGEP7eyckcUr, and with Dr Dr. Giles Peek of Glenfield Hospital in Leicester, England talking about how few the number of ECMO machines are available in Britain. http://latimesblogs.latimes.com/booster_shots/2009/09/bypassing-lungs-helps-swine-flu-pneumonia-victims.html The World Health Organization is also warning developed countries "to anticipate this increased demand on intensive care units, which could be overwhelmed by a sudden surge in the number of severe cases." http://news.eirna.com/209051/h1n109-who-issues-warning-on-second-wave-of-pandemic

There is agood chance that the current H1N1 pandemic will remain mild and within the ability of our current infrastructure and surge capacity to manage, But I do not believe anyone can definitely say that this will be the case.

What you say about legal liabilities is unfortunately sadly true. The heroic measures undertaken by individuals during the polio epidemic to build their own ventilators to save the lives of children could never happen in today’s legal liability climate. The only hope for that is if legislatures provide legal liability exemptions to the individuals that decide who gets which machine and treatment in a pandemic. Under today’s legal climate it is more prudent for a physician let his patient die by denying access to a potentially life saving treatment than to risk a lawsuit by using an uncertified device.

I must say in defence of any nephrology professionals that read this, hemodialysis is also a type of life supporting treatment that is done extracorporeally. Most of the complications that can occur in VV-ECMO can also occur in hemodialysis. Hemodialysis is routinely done in a safe mode by trained individuals. There were over 300,000 patients safely dialyzed for more than 150 million hours of treatment in more than 4000 centers in the US last year alone.

Clarence Graansma

How Many Ventilators Does New York Really Have?

A couple of weeks ago, I posted a piece on how hard it is to get good numbers on the quantity of ventilators available. I checked through some documents from the New York State Workgroup on Ventilator Allocation in an Influenza Pandemic. Last year they published some preliminary information and asked for public input on setting up guidelines for ventilator use in a pandemic or similar emergency. I sent in some information to them about the Pandemic Ventilator Project and some of my ideas about how to expand the availability of ventilators and clinical capacity in a crisis, but they never asked me for more details. They recently published some guidelines based on that exercise in Disaster Medicine and Public Health Preparedness. I will have to see about getting a copy of that article and reviewing it.

Anyway, I found two documents by the same workgroup stating the number of ventilators in New York. Unfortunately they quote two different numbers. One is 60% higher than the other. Here are the details:

First we have:
Allocation of Ventilators in an Influenza Pandemic: Planning Document
NYS Workgroup on Ventilator Allocation in an Influenza Pandemic
NYS DOH/ NYS Task Force on Life & the Law
Feb 13, 2007 (listed access date)
Available here:

On page 9, it states:

• the population of New York State is approximately 19 million,
• there are currently 3,981 adult and pediatric ICU beds staffed,
• 15% of the admitted patients with pandemic influenza will require intensive care,
• 7.5% of the admitted patients with pandemic influenza will require ventilators,
• there are currently 6,100 ventilators in acute care settings in New York State,
• at any given time, 85% of the ventilators in acute care settings are in use, and
  
• 70% of deaths related to pandemic influenza are projected to occur in a hospital.

And then we have:
New York State Workgroup on Ventilator Allocation in an Influenza Pandemic
New York State Department of Health/ New York State Task Force on Life & the Law
March 15, 2007
Available here:


On page 1, it states:

a) Community Demographics
New York State has an estimated population of 19,254,630, which represent 6.5% of the total United States population. Approximately 13% of New Yorkers are age 65 or older; an estimated 18%of the state population over the age of 5 is disabled.

b) State & Local Public Health Infrastructure
NYSDOH is empowered to issue voluntary, non-binding guidelines for health care workers and facilities; NYSDOH is also empowered to issue binding regulations for hospitals that would app to standards of care during a pandemic.

c) Health Care Delivery System
There are more than 650 nursing homes in New York State housing 120,000 beds. Of the 240 hospitals in the state, 44 are classified as trauma centers, and 13 are classified as critical access hospitals (CAH) in rural areas. There are 3,981 adult and pediatric staffed intensive care unit beds throughout the state. There are currently 3,861 mechanical ventilators in acute care settings in New York State; at any given time, 85% of these ventilators are in use.


So here we have 2 documents. Both are produced by the same workgroup on ventilator allocation. Both of these documents list Gus Birkhead and Tia Powell as contributors. One of the documents says that New York State has 6,100 ventilators in acute care settings, and the other document says that they have 3,861 ventilators in acute care settings. Both of them say that they have 3,981 ICU beds.

It is hard to know what numbers to believe. As I said before, how can you know how many ventilators you have to stockpile if you are not even sure how many you have now? How can you know how far you can extend your resources and clinical skills capacity if you are not even sure how many ventilators those workers are supporting now? A definitive census is needed with plans that list actual (validated) numbers of ventilators that exist, how many will be added for surge capacity and how far it is possible to stretch clinical support capacity.

Maybe in their latest article, Powell and Birkhead can tell us which numbers are the real ones.