Flow Restrictors

Flow restrictors can be used to control the pressure and the tidal volume delivered to each patient.

Insert an adjustable flow restrictor at the inspiratory limb of each patient, between the one-way valve and the individual pressure and tidal volume monitoring devices.

How it works

An adjustable device that restricts the flow will allow the caretaker to control the pressure and tidal volume for each patient individually. This hinges on the fact that there is only a fixed, short time, given by the time needed for the ventilator to reach peak inspiratory pressure (PIP),  wherein there is a flow passing through the restriction. Depending on the applied restriction, the pressure behind the flow restrictor will thus be lower than what you would get without restriction, simply because less volume has been able to flow past it in the given time. Each patient should have an individual pressure and tidal volume meter such that the changes in tidal volume and pressure resulting from the flow adjustments can be visualized.

Advice and comments from our contributors

To be used in combination with an individual tidal volume meter or a pressure transducer, preferably both.

Important: at least one of the circuits should have a low resistance. The reason behind this is that at least one circuit with low resistance, meaning a circuit where the ventilator’s internal pressure measurement at the inspiratory and expiratory end is almost the same, is needed for the ventilator to work properly. The more the flow is restricted, the more resistance is added to a circuit. The maximal amount of resistance tolerated is dependent on the specific ventilator and other components in the circuit, and is also influenced by both the flow restrictor diameter and the amount of closure. The first option is to make this the circuit for the patient with the lowest compliance, such that the flow restrictor on this circuit can be left open. The second option is to introduce a third, ‘short’ circuit, to ensure there always is an uninterupted flow. 

We know that in terms or regulating individual volumes and pressure, an actual pressure regulator would make for a better solution, but these are not as readily available as the proposed flow restrictors.

Important properties

  • safe materials: all materials should be safe for breathing purposes and for use with high oxygen concentrations
  • possible to disinfect: should at least have the possibility to be disinfected with ethanol. 3D printed components should be printed with professional equipment to reduce the chances of bacterial hotbeds forming in filaments and pores.
  • maximal achievable pressure drop: should be at least 20cmH20
  • pressure drop when fully open: should be as low as possible, although a small added resistance can be allowed in favor of better sensitivity. This is the case for valves with an inner diameter smaller than the ventilation tubing.
  • sensitivity: amount the applied pressure/tidal volume changes when making a change to the flow restrictor. Can be expressed in change in tidal volume per percentage of applied restriction. E.g., 10ml / % closure of the valve. This sensitivity is not constant for most flow restrictors; an example of a drastic change in restrictor sensitivity is, e.g., a restrictor where no tidal volume changes happen from when it’s fully open to when it’s moderately closed (very low sensitivity), and where you can only affect the tidal volume  when the flow restrictor is almost completely closed.
  • linearity: the sensitivity is thus not a fixed constant. Some flow restrictors can, e.g.,  become far more sensitive the more they are closed. If the flow restrictor yields the same sensitivity over a wide range of adjustments, it can be called linear. Linear flow restrictors will be easier to operate.
  • weight: if the valve is too heavy, it will weigh down the circuit or increase chance of disconnection; heavy valves need to be supported.

Current Status

We’re working on a unified test protocol. Our contributors Tormod Martinson and Christian Tronstad, from Oslo University Hospital, Norway, have started comparing different flow restrictors. In the graph below you can see the measured tidal volume in function of the applied restriction, for the different flow restrictors they tested so far. While the diaphragm valve and their 3D printed valve version are non-linear and only become sensitive after around 60-70% of valve closure, the in-line PEEP and (in-line) APL valve are linear over a very wide range of adjustments to the valves. The APL valve is not a good option though, since it is way too sensitive. But the in-line PEEP valve will allow for fine adjustments of the delivered tidal volume. See our in-line PEEP valve page>>  for ideas on how to convert a standard, adjustable PEEP valve to an adjustable in-line valve.

List of flow restrictors

We’re currently still working on a unified test protocol to compare all the different proposed flow restrictors. The information given below is thus so far collected ad hoc.

In-Line, Adjustable PEEP valve

Brass diaphragm valve DN20 G3/4 (3/4″)

Brass valve DN20 G3/4 (3/4″) with visual indicator of applied adjustment

3/4″ Thermoplastic Ball Valve

1/2″ Thermoplastic Ball Valve

3D printed flow restrictor, team Tobin Greensweig

3D printed flow restrictor, team No2Covid

3D printed flow restrictor, team Stephen Gordon

3D printed flow restrictor, team Jamie Claye

3D printed flow restrictor, team PANDA

3D printed flow restrictor, Flynn restrictor Valve

In-Line Adjustable PEEP Valve

Advantages

  • Linear over a wide domain of adjustments, thus safer and more practical to use.
  • Good sensitivity.

Drawbacks

  • Almost no commercial products available, so one needs to convert a standard, adjustable PEEP valve to an in-line valve; alternatively the whole in-line, adjustable valve can be printed.
  • Given the above, this might not be the most reliable option.

Materials

Depends on method of production.

First test

Tested by Tormod Martinsen and Christian Tronstad, University hospital Oslo, Norway. See results descibed above. They used an Ambu valve (1.5-20cmH20) in their in-line PEEP construction.

Independent test

Tested by Steven Roy, MD, Calgary, Canada.

Additional Resources

Please see our in-line adjustable PEEP valves page >>

Brass diaphragm valve

DN20 G3/4 (3/4″)

Advantages

  • Easy to obtain from local plumbing/HVAC supplier.
  • No mechanical parts in the flow (since it is a diaphragm valve).
  • Does not increase resistance in circuit, even when open, due to large diameter (unless inspiratory times are extremely short).

Drawbacks

  • Non-linear, and can only influence the flow/volume in a relatively small range of valve settings (e.g., only restricts the flow in the range of 70% to 90% valve closure).
  • Relatively high sensistivity (small adjustments to valve yield large pressure drops) in that range of valve settings.
  • Heavy, so increased chance of disconnection unless supported.
  • No visual indication of applied adjustment.

Materials

Depends on brand

First test

Tested by Dr. Mergeay, Dr. Stiers and Dr. Janssen, hospital Geel, Belgium. Test results and protocol available soon.

Independent test

No information about independent test yet.

Send us your test report.

Additional Resources

Instruction video by Dr. Mergeay, hospital Geel, Belgium:

Brass valve

DN20 G3/4 (3/4″)

with visual indicator of applied adjustment

Advantages

  • Visual indicator of applied adjustment.
  • Does not increase resistance in circuit even when open (due to large diameter).

Drawbacks

  • No complete information on linearity/sensitivity yet.
  • Heavy, so increased chance of disconnection unless supported.
  • Not that easy to obtain.

Materials

IMI hydronic version: AMETAL®, EPDM O-ring, PTFE, stainless steel, Polyamide and TPE (all materials deemed safe by R&D department)

First test

Tested by Dr. Mergeay, Dr. Stiers and Dr. Janssen, hospital Geel, Belgium. Test results and protocol available soon.

Independent test

No information about independent test yet.

Send us your test report.

3/4″ Thermoplastic Ball Valve

Advantages

  • Quite easy to obtain.
  • Lightweight.
  • Does not increase resistance in circuit even when open (due to large diameter).

Drawbacks

  • No complete information on linearity/sensitivity yet.
  • But will be more sensitive (thus worse in terms of sensitivity) than its 1/2″ counterpart.

Materials

Homewerks Worldwide version: CPVC

First test

Tested by Dr. Bunting, MD.

Independent test

No information about independent test yet.

Send us your test report.

Additional Resources

1/2″ Thermoplastic Ball Valve

Advantages

  • Quite easy to obtain.
  • Lightweight.
  • Better sensitivity than the 3/4″ valve version.

Drawbacks

  • No complete information on linearity/sensitivity yet.
  • Might increase resistance in circuit even when open (due to small diameter).

Materials

Homewerks Worldwide version: CPVC

First test

Test results and protocol available soon.

Independent test

No information about independent test yet.

Send us your test report.

Additional Resources

3D printed flow restrictor, team Tobin Greensweig

Advantages

  • Open source.
  • Lightweight.

Drawbacks

  • No complete information on linearity/sensitivity yet.
  • Threads are only tight for a relatively short time, after that you need to add an o-ring.

Materials

Should be printed with Med610, Dental SG/LT, or Teeth A2

First test

Tested by Dr. Greensweig, MD.

Independent test

No information about independent test yet.

Send us your test report.

Additional Resources

Print files and instructions >>

Demonstration video by Tobin Greensweig:

3D printed flow restrictor, team No2Covid

Advantages

  • Lightweight.
  • Visual indication of applied adjustement.
  • Will not increase resistance in circuit when fully open.
  • Clamps the tubes from the outside, so no contact between flow restrictor and gases in the circuit.

Drawbacks

  • Not open source.
  • No complete information on linearity/sensitivity yet.
  • Clamp might damage tubing over time.

Materials

To be determined.

First test

Test results and protocol available soon.

Independent test

No information about independent test yet.

Send us your test report.

Additional Resources

3D printed flow restrictor, team Stephen Gordon

Advantages

  • Open source.

Drawbacks

  • No complete information on linearity/sensitivity yet.

Materials

To be determined.

First test

Test results and protocol available soon.

Independent test

No information about independent test yet.

Send us your test report.

Additional Resources

3D printed flow restrictor, team Jamie Claye

Advantages

  • Open source.

Drawbacks

  • No complete information on linearity/sensitivity yet.

Materials

To be determined.

First test

Test results and protocol available soon.

Independent test

No information about independent test yet.

Send us your test report.

Additional Resources

3D printed flow restrictor, team PANDA

Advantages

  • Open source.

Drawbacks

  • No complete information on linearity/sensitivity yet.

Materials

To be determined.

First test

Testing starting soon.

Independent test

No information about independent test yet.

Send us your test report.

Additional Resources

Link to files will be posted here soon.

3D printed flow restrictor, Flynn restrictor valve

Advantages

To be determined.

Drawbacks

  • No complete information on linearity/sensitivity yet.

Materials

To be determined.

First test

By team Bath, UK. Link to pdf of test results>>

Independent test

No information about independent test yet.

Send us your test report.

Additional Resources

The International Workgroup on Differential Multiventilation

Disclaimer

None of the contributors, or any other person connected with this international working group, can be held responsible for the information provided on these webpages. The content provided on this website might be incomplete or incorrect. The use of the described methods or any other reliance on the information provided by us is solely on your own risk.