How To Make Your Own Accurate Test Lungs for Testing Emergency Ventilators

Alex’s test lung set up
(A Commercial test lung with a resistance tube and barbed bleed adapter.)

How Doctors Measure Pressure

For respiration, only pressure differences matter. Breathing is driven not by atmospheric pressure, but by difference in pressure in the airway compared to atmospheric pressure. Although engineers normally use the SI unit of pressure, which is the Pascal, the medical profession usually measures pressure in terms of the height of a column of water it will raise — -hence, centimeters of water, denoted as cmH2O. 1 cmH2O is approximately equal to 98 Pa. It takes 70 cm H2O to make one psi. Breathing pressure differentials almost never go above one psi!

A smaller port in the airway makes taking a pressure measurement easier.

Compliance

Typical healthy lungs have a compliance around 20–50 milliliters/cmH2O. In other words, pushing in 20–50 ml air would raise pressure by 1 cmH2O.

Different lungs or diseases have different compliance curves

Resistance

Typical healthy lungs also have a resistance around 3–7 cmH2O/(liters/second). Thus, pushing air in at a constant flow rate of 1 liter/second (60 liters per minute) would raise pressure by 3–7 cmH2O (in addition to the change due to compliance).

Commercial Test Lungs

Instrumented commercial test lungs are available, but may be quite expensive, and may not be easy to obtain in the current crisis. Some examples of instrumented test lungs are the Michigan Instruments Model 1600 Test Lung; the IngMar ASL 5000 Breathing Simulator; and the IMT SmartLung.

Constructing Your Own Test Lungs

Several ways of constructing test lungs with known resistance and compliance are described in the ISO 10651–5 standard for ventilators. It turns out that some of those don’t require any expensive or difficult to obtain materials or equipment. They work as well as the commercial test lungs, and are suitable for certifying designs.

Constructing Compliance from a Glass Vessel

Compliance is measured in milliliters/cmH2O. This may be concisely written as just a C-value without the units, and those units are assumed. The typical adult range is around C20 (+-10), mainly depending on body size and physical condition. Compliance may decrease greatly with conditions that cause the lung tissue to stiffen (eg ARDS).

(Inexpensive 41.6 liter or 11 gallon test lung, measured compliance C28)
(Classic adult test lung from from Hill 1965; the 50 liter or 13 gallon glass demijohn inside a wicker basket on the right is the test lung, the device on the left is a pump)
Typical brewing carboys
(Vessels of different sizes: 3 to 14 gallons)

Safety Note

These suggested vessels are not pressure rated and may fracture at high pressures. The pressure differences used in normal breathing almost never exceed 1 psi, or 70 cmH2O. Compared to bicycle tires or air-powered tools, this is a tiny pressure. There is never a reason to test higher pressures, which may be dangerous.

Constructing Compliance with Plastic Sheets and Bags

A 2–3 liter inflatable bag (anesthesia bag or similar) sandwiched between two thin flexible sheets of plastic. The sheets of plastic should be connected together on two opposite edges, forming flat springs. The resistance to air flowing in should come (as much as possible) from the bending of the plastic sheets, not from stretching the bag.

(IMT SmartLung — commercial version of a bag between flat plastic springs; adjustable length of spring changes C value. Price: $650)

Constructing Resistance

Because resistance is a function of flow, it is measured in pressure divided by flow, or cmH2O/(liter/second). This may be abbreviated with the letter R and a number without stating the units. The typical adult range is around R5 (+-2). Resistance may increase significantly with any conditions that produce impaired airflow, such as asthma.

(A commercial adapter offering three resistances by having thin plates with 3 different hole sizes.)

Other approaches

When it comes to simulating lungs, the perhaps obvious DIY approach is to use some kind of expandable or inflatable bag, balloon or similar. This is then loaded in some way e.g. with elastic bands, weights, or by being put under water to make it harder to push air in. The problem with these approaches is that the pressure vs volume curves they have is completely different from the one that the standard test lungs (and real lungs) have. Most elastic bags are not nearly stiff enough; for example anaesthesia bags are much stiffer than balloons, but are still not stiff enough until over-inflated to 2x-5x their rated volume; and additionally, the stiffness changes a great deal once they stretch. Weights, water and similar static loads add a minimum pressure which is required to push any air in; they offset the pressure vs volume curve, rather than changing the slope. Springs, elastic bands and so forth do change the slope, but not in a consistent way — for most of these the slope gets much steeper as the volume gets higher. Trying to measure the R and C values, especially C, is meaningless because the shape of the curve is wrong. This may lead to very misleading conclusions when testing a ventilator prototype.

Conclusion

It is much more important to have a test lung which behaves in a realistic way, and for which the R and C values are known and within the expected range for real lungs, than it is to have specific R and C values or to use a commercial test lung. Even if a commercial test lung is not available, it is strongly recommended to use the types of construction described here — even if the specific sizes or volumes are slightly different — for all but the simplest tests of ventilator function.

Public Invention

Public Invention is a US 501(c)3 public charity. Your donation supports are attempt to foster tested, reliable, and monitored open-source pandemic ventilators.

References

International Standards Organisation. ISO 10651–5–2006 Lung ventilators for medical use — Particular requirements for basic safety and essential performance — Part 5: Gas-powered emergency resuscitators.

Materials and equipment needed

To build and test these test lungs, some equipment is essential. The following is a list of suggested gear.

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Robert L. Read

Robert L. Read

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Public Inventor. Founder of Public Invention. Co-founder of @18F. Presidential Innovation Fellow. Agilist. PhD Comp. Sci. Amateur mathematician.