Update: With much research done, we are NO longer investing our time in MANUFACTURING a low-cost ventilator. Feel free to change our minds with your opinions, and we mean no disrespect to the engineers still pursuing the manufacture of low cost ventilators. We just have our own concerns due to our findings which we will share with you in this post. Please read this first before stating your opinions:

In mid-March, my business partner and I were in a rush to procure materials for building a low cost ventilator before most shops closed due to lock-down period. In conjunction with building up the prototype, we had done intensive research on the mechanical design of ventilators.

Many makers and engineers want to help during the pandemic, by putting their time and energy in developing or building an open source low cost ventilator, so hats off to them – some designs have theoretically proven to be functional.

Many Open Source designs implement a BVM, which is a Bag Valve Mask. Usually a first responder will use this device on a patient that isn’t breathing, rather than performing mouth to mouth resuscitation. Using this device with the use of robotic arms to inflate and deflate the BVM at specific frequencies are the majority of designs open for anyone to download and build. This is a cheap and easy way to force air into the lungs automatically, and can be mass produced.

The problem is, ventilators are not just air pumps that force air into the lungs. The main problem makers and doctors are facing, is managing the side effects of mechanical ventilation, called Barotrauma. Baro meaning “pressure” and Trauma meaning “wound”.

Before I continue, here is some Biology 101 information on breathing:

Under normal conditions, two muscle groups act to control breathing, the intercostal muscles and diaphragm. When one breathes in, the diaphram as well the external intercostal muscles contract which increases the volume of the thoracic cavity. The increase in volume creates a corresponding decrease in pressure which makes air outside the body at atmospheric pressure to fill the lungs and equalise the pressure. The key thing to note here is that negative pressure drives inhalation. The lungs don’t inflate like a balloon. They expand and equalize with atmospheric pressure. On exhalation the process reverses with a small spike above atmospheric pressure to push the air out again.

Continuing, the BVM has to force air from outside the body and basically blow up the lungs like a balloon. The problem is, that if this is not tightly controlled, the air pressure from the BVM can work against the diaphragm and the intercostal muscles and then potentially increase the pressure in the alveoli in the lungs above its limited maximum pressure. alveoli are very sensitive pieces of tissue, over-expanding them with higher pressures than normal can lead to inflammation, or even rupture them. This is what Barotrauma is.

To make this worse, those suffering from ARDS (acute respiratory distress syndrome), like those affected by Covid-19, are more at risk of suffering from this side effect of mechanical ventilation, as the alveoli that are filled with fluid prevent air from entering them, causing the pressure to elevate even higher in the functioning alveoli. The last thing we want to do is damage the healthy tissue of a patient suffering from damaged lung tissue. That is the opposite of helping.

To avoid this doctors need to carefully choose their settings on a ventilator. The primary guidance for this is to limit the volume and pressure of air entering the lungs. So, any low cost ventilator will need a method to control these settings.

The earliest design that proposed using these BVM was from an MIT student project in 2010. This paper has been online that entire time and if people are truly copying it, they are leaving out some clever design ideas that make it more functional. Their design included a spirometer, which measures the air flow rate out of the BVM, by integrating this value they can calculate the volume of air delivered. This then feeds into a controller which can vary how tightly the BVM was squeezed to change the volume of air delivered. This gave the device a nice range of tidal volumes ranging from 200 to 750 millilitres.

This is a better design, and may be useful in a do or die situation. But, it is not perfect. The breaths per minute controller is simply set on a time based frequency, ranging from 5 to 30 breaths per minute. This is called a mandatory breath. It’s entirely determined by the machine. You will take a breath whether you like it or not. This would obviously be uncomfortable and requires the patient to be heavily sedated to the point of paralysis, but it can also exacerbate barotrauma if the patient’s diaphragm and intercostal muscles are resisting the inhalation.

High performance ventilators can work like this, but they typically don’t. Their breath sequences are normally triggered by the patient. They are still able to breathe. They just need help because they are exhausting themselves with the effort. In order to do this the machine needs some way of triggering the breath cycle and ending it too, based on observations of the patient. This can be done in a number of ways.

It can be pressure triggered, where a sensor detects a drop in airway pressure indicating the thoracic cavity is expanding. It can be flow triggered, where a sensor detects airflow into the lungs, or it can be triggered by a sensor detecting electrical activity of the diaphragm, indicating that the diaphragm is contracting to expand the thoracic cavity. This also requires very fast microprocessors to detect and react to the triggers.

At this time there are no low cost ventilators incorporating this vital component of ventilator design and is truly important to have.

A very difficult part of the ventilation process is weaning people off it again. A ventilator which requires someone to be sedated to the point of paralysis makes it very difficult to get them breathing naturally on their own again. There are a multitude of other design considerations to be made with ventilators.

For example, ventilators also have the function to provide PEEP (Positive End-Expiratory Pressure). Which give a constant air pressure exerted on your airway, making it ever so slightly harder to breathe out. PEEP is a constant positive pressure that prevents those alveoli collapsing at the end of each breath and also helps open up or recruit collapsed areas of the lung. In COVID-19 we are seeing patients requiring very high levels and tight control of PEEP to maintain their oxygen levels and protect the lungs and this is something that a basic bag-squeezing vent cannot really achieve.

From the mechanisms seen so far there may be risks of baro and volu-trauma. Covid-19 patients are on a ventilator for a few weeks at the moment. A basic bag-squeezer might be adequate for the first day or so when a patient is deeply sedated, but simply won’t work as you try to ease off the sedatives.

ADDITIONALLY, your upper airways warm and humidify air entering the lungs, but they are taken out of the equation by the endotracheal tube which goes directly into the lower airways. Without the warming and humidifying features of modern ventilators, lung tissue will get rapidly damaged.

So with this, there is lot more to ventilation than just pumping air into a patient. Tight regulation of pressure, volume, oxygen percentage control and humidification would all require more complicated mechanics than these simply BVM pumps. Designing a ventilator fit for purpose with cheap and easy to manufacture components is a difficult job,but with more research and development, with time, there will be someone who will create a solution.