A note to the amazing maker community trying to put together a DIY Mechanical Ventilator.

Ventilators, test kits and protective gear are the need of the hour. Right now, observing a lot of interest in the maker community and non-medical industry to come out with a Mechanical Ventilator in the midst of this COVID 19 pandemic.

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13 May, 2020

A note to the amazing maker community trying to put together a DIY Mechanical Ventilator.

Ventilators, test kits and protective gear are the need of the hour. Right now, observing a lot of interest in the maker community and non-medical industry to come out with a Mechanical Ventilator in the midst of this COVID 19 pandemic. In this write-up, I would like to emphasize on the things to be considered in your efforts, from a technological and clinical standpoint for the project to have a real impact on saving lives.

It is important to note that mechanical ventilation does not heal the patient. Rather, it allows the patient a chance to be stable while the medications and treatments help them to recover. Generally, manual ventilation (using an Ambu bag/Bag Valve and Mask) is done as a last resort and is done until the patient can be provided with actual ventilator support. In low resource settings and when the patient has to be shifted to a higher-level healthcare centre, it is found to be used for the prolonged duration due to lack of an actual ventilator or a transport ventilator.

A Bag Valve Mask(BVM) as the name suggests is supposed to be used with a mask to resuscitate critically ill patients who cannot breathe on their own due to several underlying causes, in case of COVID 19, it has been found to be ARDS (Acute Respiratory Distress Syndrome).

How Manual Bagging is performed

As mentioned, in low resource settings during unavailability of a ventilator, it is a widely observed practice to manually compress a BVM while the patient is intubated (an endotracheal tube is inserted into the airway). This bag compression is done by trained nurses or paramedics. The parameters they look for while performing this are Breath Rate (compressions per minute), Inspiratory Expiratory Ratio (ratio of time of compression to retraction of the bag), and the Pressure delivered (from a small manometer connected in series to the airway circuit). All this is kept track of by the professional by keeping an eye on their wristwatch or a clock, and the parameters are decided by age, weight and health condition of the patient.

In essence, most of the hacker community is trying to automate this action of a trained professional. And this type of ventilation is called Continuous mandatory ventilation (CMV)

[Without intubation, a tightly sealed mask can also deliver this positive pressure, but this is usually associated with a condition of Aerophagia(air swallowing) and abdominal distension. Design of this mask to provide a tight seal is also a broad topic having to deal with varied sizes of mask for different age groups.]

Mechanism

Our lung makes use of negative pressure generated by the contraction of our diaphragm to suck in air. Any mechanical ventilator tries to do the reverse; it applies a positive pressure to inflate the lung. A conscious patient’s body would try to act against this action. One strategy used to prevent the patient from breathing against the ventilator is to paralyse the patient or heavily sedate him, but to keep a patient in paralysis we need good intensive care, respiratory therapists and ancillary support, this is a pretty big ask during an outbreak.

Generally, ventilators are set to operate by a patient trigger for pneumonia and ARDS ie, whenever the ventilator senses the patients’ effort to breath, it initiates a breath cycle. This requires highly sensitive pressure sensors and algorithms for triggering the positive pressure cycle.

Parameters to be controlled and displayed

The key parameters that a physician looks for are Respiratory Rate (RR) ie, the number of breaths delivered per minute, the Tidal Volume (TV- the volume of air pushed into the lung per breath) and the ratio of the length of time for Inhalations to Exhalations (I E Ratio)

There has to be an interface in the device to set these most critical values and display these in real-time.

Any mechanical ventilator should have the ability to support a range of 10 to 30 breaths per minute, rising in increments of two, with the settings adjustable by the physician. They should also be able to changeTidal volume is typically about six or seven millilitres per kilogram of body weight or about 500ml for someone weighing 80kg.

The average proportion of oxygen in the air is 21%. The percentage of oxygen inhaled is called FiO2. The ventilator should be able to offer 50% and 100% at the very least and ideally 30% to 100%, rising in increments of 10 percentage points.

Some of the other crucial patient vitals that needs to be measured via external sensors are:

  1. SpO2 -Oxygen Saturation (measured from the finger, gives valuable information about arterial oxygen content, tissue perfusion and heartbeat rate) This can be done via an inexpensive Pulse Oximeter.
  2. EtCO2-(End Tidal CO2) maximal concentration of carbon dioxide (CO2) at the end of an exhaled breath, which is expressed as a percentage of CO2 or mmHg. Measured using an infrared-based sensor, known as Capnography (this is quite expensive at the moment, any effort to simplify this will have a massive impact)

PEEP

PEEP (Positive End Expiratory Pressure) is extremely essential for ARDS patients. Imagine you inflate a balloon and release half of the air in it, the remaining air in it exerts a pressure above the atmosphere, that prevents the balloon from collapsing. Now, instead of ballon, think of the alveoli (air sacs) in the lung, PEEP is the pressure over atmospheric pressure that helps prevent the alveolus from collapsing.

The only mode of delivering PEEP through a BVM is using an off-the-shelf PEEP Valve. These valves work well for the initial half-hour or so, then there would be leaks in these valves and make them unreliable. Ensuring the reliability of these PEEP Valves or creating a pressure regulator valve that works in 10cmH2O to 50cmH2O range is necessary.

Lung Injury

Any sort of positive pressure ventilation on a patient induces a certain amount of injury to the lung. This is termed as VALI(Ventilator-associated Lung Injury). Underlying causes are classified as Volutrauma, Barotrauma etc. The high pressures pushing air into the lung and can overinflate those little balloons. They can pop. It can destroy the alveoli. Even when they survive COVID related ARDS, although some damage can heal, it can also do long-lasting damage to the lungs. They can get filled up with scar tissue. ARDS can lead to cognitive decline. Some people’s muscles waste away, and it takes them a long time to recover once they come off the ventilator.

Weaning

The term “weaning” is used to describe the gradual process of decreasing ventilator support, once the underlying cause of respiratory distress subsides. Weaning patients off ventilators usually require highly experienced respiratory therapists.

Expired air filter

The air patient breaths out are contaminated and need to be trapped or filtered before letting out in the ICU environment, a Viral Filter at the outlet may seem like the most logical option, but this adds to higher resistance in the airway which is not preferred.

To sum up…

A mechanical ventilator is not just about pumping air into the lung, although it had beginnings in that, over the years it has developed into one of the most sophisticated fields in biomedical engineering. It requires an experienced clinician, support staff, lot of accessories and a complex air delivery system to complete the equation. The maker community can be a huge support in delivering these accessories such as adaptors, splitters and sensors for airway circuits. At present these parts are mostly custom made for each brand of ventilators and there is no cross-compatibility among the manufacturers. This is driven by a huge market for these consumables which are onetime use and disposable.

The maker community at this time should pool their knowledge and resources, and collaborate with clinicians to reverse engineer these ventilator accessories and vital sensors and support them with the supplies as needed. This, in turn, can have a higher impact than providing them with experimental DIY ventilators which were developed in a hurry without significant time spent on its usability and clinical testing.

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13 May, 2020

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