Mechanical Ventilation Basic Requirements

Community General Discussion
1

I watched this set of 5 videos about mechanical ventilation:

Mechanical Ventilation Explained Clearly - Ventilator Settings & Modes
https://www.youtube.com/watch?v=gk_Qf-JAL84

and found the following basic information about typical ventilators:

1. There are several modes of operation, here are a few common ones:
A. PS, or Pressure Support Mode: - The ventilator fills the lungs to a specified maximum pressure upon each breath, which can be triggered by the patient or the machine. The maximum pressure cannot be exceeded or severe damage will occur to the lungs.
B. AC, or Assist Control Mode: - The ventilator delivers a set volume of air (Tidal Volume or TV) on each breath. The breath can be periodic and initiated by the machine, or initiated by the patient - when the patient begins to breathe in a negative pressure is sensed and this triggers the machine to deliver the volume of air specified at a constant specified flow rate. If PEEP is utilized the pressure may not be negative, but a dip in pressure. Maximum pressure is controlled and depends on the compliance of the lungs which can easily be damaged by over-pressurization.
C. The machine can wait a certain time for the patient to initiate a breath and if it doesn't occur then it will initiate the breath.

Other very basic info from the video lectures I thought is initially important to a simple ventilator design:

1. Maximum pressure typically supplied to the lungs varies from 5" H2O to 20" H2O (0.2 - 0.75 psi). The averaged volume flow rate may be 20L per minute when averaged over a minute. Typically exhalation time is 3x inhalation time. The volume delivered (tidal volume, or TV) may be estimated as a volume per kg of body mass such as "7 mL/kg". For instance a 200lb person (91kg) might require 637ml tidal volume (0.637 liters).

2. Blood gas is monitored closely when on a ventilator, 4 results are reviewed at minimum: pH, pCO2, pO2, [HCO3-].

3. A typical ventilator basic setting has 4 parts: 1. Mode, 2. TV (Tidal Volume), 3. FIO2 (Fractional Inspiration of O2), 4. PEEP (Positive End Expiratory Pressure).

4. Compliance of the lungs varies over time depending on the lung condition and this needs to be sensed by the machine for correct operation. Compliance is the change in volume divided by the change in pressure to achieve that volume (dV/dP).

5. Acute Respiratory Distress Syndrome (ARDS) - the disease C19 patients die from, typically uses low tidal volumes (TV) ie. Small Breaths. ARDS makes the lungs very stiff, they have low compliance. This makes control by volume difficult, as the pressure will rise quickly during inspiration and may exceed the maximum because the lungs are stiff, this must be monitored accurately.

6. Dead space (air volume in the machine) is critical to machine operation and must be known.

7. If too much O2 is delivered, this can cause bronchitis and inflammation in the lungs. There is a limit, < ~50% FIO2 is desired.

8. Mechanical ventilation affects the body in many ways in particular higher pressures reduce blood return flow into the heart (venous return) and can lower blood pressure. The body is a very complex sensitive system.

9. Higher PEEP (Positive end expiratory pressure,this is the pressure maintained at the end of the breath btwn breaths), and can recruit more alveoli and push fluid out of the lungs but as mentioned higher pressures can lower blood pressure and cause lung damage among many other potential harmful effects to the body.

10. If the alveoli collapse and expand too much this causes inflammation and can lead to death. This is what ARDS is. ARDS has a death rate of 30-50%.

11. RSBI is the Rapid Shallow Breathing Index, it is defined as respiration rate [breaths per minute] / tidal volume [liters]. If RSBI < 105 when off the ventilator is a good sign someone is recovering.

I have no medical training or expertise. I am a mechanical engineer by degree and automotive product development engineer by occupation. I have spent time in hospitals as a patient recently in serious condition for abdominal surgery due to pancreatitis.

2

I am not knowledgeable with ventilator design or their operation, but there is a lot of information immediately available if you search. This article seemed to have useful information:

“Ventilator management strategies for adults with acute respiratory distress syndrome"

From which I found:
"For patients with ARDS, we and others recommend LTVV (LOW TIDAL VOLUME VENTILATION also known as lung protective ventilation; 4 to 8 mL/kg predicted body weight [PBW]) (table 1 and table 2). LTVV is typically performed using a volume-limited assist control mode, targets a plateau pressure (Pplat) ≤30 cm H2O, and applies positive end-expiratory pressure (PEEP) using a strategy outlined in the table (table 3). This approach is based upon several meta-analyses and randomized trials that report a mortality benefit from LTVV in patients with ARDS. It is thought that low tidal volumes (VT) mitigate alveolar overdistension induced by mechanical ventilation, which can cause additional lung injury and mortality in patients with ARDS. (See 'Efficacy and harm' below and "Ventilator-induced lung injury", section on 'Mechanisms'.). "

3

hey buddy, you're doing a great job in researching
I needed it
I want to ask you, are ambo bags enough in case of this virus
Like, If I automate its pumping and handling, is this something worth,
Or is it that we need those heavy ventilators, if so , why?

4

Thanks. I actually haven't looked into the details of the problem very deeply yet. I decided that the most important and useful thing for me to do was to get people together that were working on this, because I knew many people know much more than I ever will about the subject, and it was most important to get the inventors in contact with experts that can help them, as quickly as possible. So I focussed solely on that and haven't gotten into the details of the machine requirements yet. I see there are a lot of projects progressing rapidly, I will try to sort them by state of development next, although this has already been done by Public Inventions on their Github list: Ventilator Verification Project

I believe they might cross reference and consolidate my list with theirs. I would look into the projects that are furthest along, some are already getting FDA approval and looking for manufacturers. I think the demand may be so high, and in so many parts of the world, that many of these potential devices once developed may be viable as producible products immediately. I think time is of the essence here. It appears to me that the level of control needed for simple ventilators is well with the capabilities of Arduino microcontrollers.

5

Perigalacticon, this is VERY useful information, thank you!

I am close to done with the mechanical design, construction and programming of an automated ventilator that squeezes a BVM bag (Ambu-style bag). It's able to vary the breathing rate, tidal volume, inspiratory and expiratory times and the pauses (if any) between them, and monitor the patient's breathing pressure and changing its manipulation of the BVM bag accordingly. I will use the references you have given here to fine-tune the programming. Especially the info about hardening in the lungs and the effect it has on pressure, airflow, and Tidal Volume.

Started this project on March 31. I got a lot of information from a friend who is an Emergency Room doctor. I wish I'd had these references then!

Do you know if a ventilator like this (made from a BVM unit) suffers from the use of a hose to connect the bag to the mask? Normal manual BVM bags are connected directly to the mask, of course, which is practicable with a light, soft unit like the bag. But with an automated device like this ventilator, you can't have it resting on the patient's chest or pillow, a hose must be used, similar to a CPAP hose. The ventilator would sit on a table or shelf a few feet or more from the mask.

Might the compressibility of the extra air in the hose, disrupt measurements of pressure and flow? And possibly even degrade the airflow to the patient?

6

If you need detailed information regarding ventilator requirements, this has been the most thorough spec I have worked with.
“Ventilator specs Open Source"

This is the list of open ventilator projects being tested and scored by a large community of volunteers.
"Open Source Ventilator Submissions and Testing Results"