Retrofitting an emergency ventilator for COVID-19
NORTHWESTERN UNIVERSITY VENTILATOR PRESSURE++ PROJECT!
SUMMARY: We have adapted a commercial emergency ventilator (Vortran's Automatic Resuscitator, called VAR or GO2Vent) to raise its inspiratory and expiratory positive pressures (PIP and PEEP) so that the device is better suited for COVID-19 patients. On hospital ventilators COVID patients are often treated with relatively high PIP and PEEP pressures. Hospital ventilators have adjustments to reach these higher pressures, but emergency ventilators do not.
Our retrofit raises both PIP and PEEP pressures by the same increment, to an additional pressure of 5-15 (cm-water), or more.
We also built a meter-high manometer that is viewable from a distance, for visually monitoring a patient's respiratory pressure cycle. The manometer has an alarm that can be set to warn of over-pressure. We are working on a respiration monitor to alarm if respiration slows or ceases for any reason.
Existing product, the gas-powered GO2Vent
Vortran's Automatic Resuscitator GO2vent, pictured at left, is a gas-powered ventilator intended for short-term use in an emergency situation. Xerox has announced they are building over 100,000 of them in partnership with Vortran (April 2020).
The GO2Vent is more automated and controllable than a BVM ("Ambu bag") but still requires continuous supervision.
GO2Vent appears under several names, including VAR, VAR-Plus, and (in India) SureVent. The UIUC RapidVent is very similar. Go2Vent user guide
GO2Vent requires no electrical power. Its cycle is driven by the supply of high pressure oxygen that is also breathed by the patient. Three control knobs must be set carefully: (1) an external valve that controls the flow rate of high pressure oxygen or air, (2) a built-in leak valve that controls exhalation rate, (3) a pressure valve that controls peak pressure during inhalation, called PIP.
GO2Vent provides an inhalation positive pressure (PIP) that is adjustable, in the 10-45 cm-water range. The exhalation positive pressure (PEEP) is strictly proportional to PIP, and is set by device design to the 2 to 9 cm-water range. This proportionality is not adjustable (except via the retrofit described here).
These PIP and PEEP pressures are sub-optimally low for COVID-19 patients. In particular, COVID-19 patients are often found to require PEEP values of 15 to 20 cm-water, or even higher.
Hospital ventilators vs. emergency ventilators
Hospital ventilators have many adjustments of a patient's breathing parameters that can be set. Hospital ventilators also make measurements that can be crucial diagnostically. And they include alarms on several conditions, that can be bracketed so that medical intervention is prompt when something goes wrong. These are sophisticated machines.
In an emergency there is a desire to push into service much simpler devices, some of them designed for only very brief use such as BVMs (Ambu bags) used by paramedics. However to treat a badly damaged or incapacitated lung requires skilled interpretation and titrated control over many parameters, over a period of days or weeks. This video explains the need for fine control, and the problems of crude ventilators: A Guide To Designing Low-Cost Ventilators for COVID-19
There are some simpler devices which might nevertheless be of value. One is the GO2Vent, which cycles lung pressures appropriately and automatically.
To make it a more clinically acceptable and valuable product, we retrofitted the GO2Vent to increase its pressure range. We also have added devices to it that monitor and display patient respiratory parameters, and that can also cause an alarm at a settable level.
Our "pressure++" theory of operation
Our approach is to enclose all the parts of the Go2Vent that make reference to atmospheric pressure, inside an airtight enclosure. The pressure in that enclosure is held at an elevated level, 5-15 cm-water above atmospheric pressure, or more. A manually adjustable pressure-relief valve is used to set the elevated pressure.
The GO2Vent continues to cycle as it normally does; no modifications to its internal mechanism are needed. All of its operating pressures are incremented by the "apparent" atmospheric pressure that it is exposed to, which is slightly higher than actual atmospheric pressure.
Both the PIP and PEEP pressures are raised the same amount, by our method.
At left is the control adjustment end of the Go2Vent. This part of the device includes 5-6 connections to outside air pressure, including several small but important holes on the body of the device.
This entire half of the Go2Vent is enclosed in the hermetic box shown above, so that all references to outside air pressure are elevated by the same amount.
Also shown on the left is an adjustable pressure relief valve ( called a PEEP valve by the manufacturer, Ambu) that we used to maintain a constant positive pressure in the hermetic enclosure.
The enclosure is one intended to be an electrical junction box in a "washdown" environment. It is very well sealed. The cover is removed easily with a few turns of four captive bolts in the corners.
Our design requires three holes to be cut in the box: one for the GO2Vent, one for the PEEP valve, and one for a hose barb that connects via tubing to the GO2Vent's one way valve.
With our design, the GO2Vent's PIP adjustment knob (red T-shaped knob in figure above) and rate adjustment knob (red ribbed knob above) are enclosed in the box, making them inaccessible to manual adjustment. The box, however, is easily opened for adjustment. It is also possible to pass knob extensions through the wall of the enclosure, via air-tight rotary seals (e.g., lip seals or o-rings). We have not done this, opting for simplicity.
The cover of the hermetic enclosure is transparent and you can see two of the controls of the Go2Vent through the cover. The object in the foreground is the pressure relief valve ("PEEP valve") which controls the elevated pressure in the hermetic box.
Video of operation including manometer
This video shows a few cycles of operation of the GO2Vent, operating at raised PIP and PEEP pressures. (or, same video on youtube)
Our lung model is a hot water bottle with a weight on it. Operation of the GO2Vent is affected by lung capacity and stiffness, so our lung model is far from general.
The device on the left side is a water-filled pressure manometer, for visualizing the pressure cycle.
VID_20200427_165515.mp4
Manometer
Above, the bottom of the manometer showing filling port (left), a copper tubing component that is part of an over-pressure alarm mechanism, and a fluidic damper so that the manometer responds smoothly to pressure changes, without bouncing.
Full System
At right, the manometer, the GO2Vent retrofit, and our model lung. The brass component affixed to the box is a needle valve that controls the flow rate of supply oxygen. Its setting is important because if the flow rate is too high or too low the GO2Vent will cease cycling.
