Update on the Shared Manifold Ventilator
The Shared Manifold Ventilator is an inexpensive ventilator that could serve 100 or more patients at once. It could be used in a typical hospital situation but is designed for shared ventilation wards that are being set up for COVID-19 patients.
Our design is based on readily available industrial components, although I have also made an invention to make one of the key components less costly, peristaltic solenoid valves that control oxygen, air, and exhalation for each particular patient.
The simplest version of the Shared Manifold Ventilator has only one air supply manifold, one oxygen supply manifold, and one pressurized exhaust manifold to which the patient may optionally be attached. (It is also possible to exhaust directly into the ward through a viral filter rather than linking the patient up to the shared exhalation manifold). The exhaust manifold is optional for each patient but will provide positive expiratory pressure for those who need it.
The key feature of the proposed Shared Manifold Ventilator is that there are three separate manifolds that link to each patient. Each patient can have their oxygen and air levels adjusted individually by electronic controls linked to solenoid valves between the patient and the manifolds. This is significantly different from the types of shared manifold ventilators which have been proposed by numerous people and companies, for example, the Combi-Ventilate which is supported by Enterprise Ireland.
Both money and complexity are saved by providing supply and exhaust manifolds which are at an appropriate pressure for the whole hospital ward.
By making the manifold pipes four times as large as the patient supply pipes, it becomes possible to have a nearly constant pressure everywhere on the manifold, even with 100 patients on the manifold. (This is an arbitrary example; the diameter of the manifold pipes depends on how many patients are to be served by the manifold, and also the total length of the manifold.)
Rather than controlling pressures for each individual patient, cost is reduced and also complexity is reduced by providing a shared manifold for each of the input gases, air and oxygen, and for the (optional for each patient) pressurized exhaust manifold.
All the patients on the shared manifold can have an individually optimized breathing gas composition in terms of the oxygen content of the air, but all of them have the same inhalation pressure in the simplest implementation of the Shared Manifold Ventilator.
The inhalation pressure can also be individually controlled for each patient by adding a bit more equipment at each patient’s bedside. In any case though, the air and oxygen manifold pressures would be low enough so that they would not damage the lungs of a patient in case of a failure of the pressure control mechanism at the bedside.
In prior attempts to use a single ventilator to ventilate multiple patients, there is only one mechanism controlling the pressure and oxygen concentration in the entire shared inhalation manifold, so every patient must have the same breathing cycle and breathing gas mixture. This is medically problematic as pointed out by this white paper which shows the flaws in the idea of shared ventilators with identical oxygen concentration and breathing cycle for several patients. The Shared Manifold Ventilator gets around this by mixing air and oxygen for each individual patient, and also controlling the breathing cycle for each individual patient through the use of solenoid valves.
Figure 1 shows a prior art version of a manifold ventilator in which the flow of a breathing gas mixture is simultaneously varied to all the patients on the manifold. In this mode, all the patients would experience the same pressure cycle for inhalation and exhalation, and all the patients must have the same oxygen concentration.
Figure 1 corresponds to the type of Shared Manifold Ventilator that has been tried by numerous hospitals since the COVID-19 outbreak. It suffers from a lack of individual control for each patient.
Figure 2 shows an improved shared manifold ventilator design which enables individualized control of the breathing mixture and the breathing cycle for each patient.
The figure above shows a 20 patient version of the Shared Manifold Ventilator. Each patient is connected to the air and oxygen manifold through solenoid valves which are between the manifold and the patient. These solenoid valves allow each patient to have a customized breathing cycle and oxygen concentration. Filters and one-way check valves prevent cross-contamination between the patient and the manifolds.
There are two breathing gas supply manifolds in Figure 2, an oxygen manifold and an air manifold. Both of these manifolds are controlled at a realistic inhalation pressure, for example, 30 centimeters of water head pressure. By holding the manifolds at a modest pressure there is no danger to a patient of having their lungs stretched too much, which is a particular risk for COVID-19 infections.
Using a common inhalation pressure across the ward simplifies the design significantly without compromising the ability to individualize breathing cycles and oxygen concentration for each patient. The air and oxygen manifolds should be at nearly the same pressure so that oxygen and air flow are determined by how long the solenoid valves are open.
A respiratory therapist could modify the rate of flow into a patient’s lungs by adjusting a manual flow restriction valve at the bedside (not shown in Figure 2). This variable flow restriction can also be incorporated into the solenoid valves controlling the oxygen and air intake, by making these valves proportional flow control valves as opposed to simple on-off valves.
It is useful to have a pressure accumulator on the air and oxygen manifolds to maintain a near-constant pressure as shown in Figure 2. These pressure accumulators can be elastomeric balloons similar to an exercise ball. This minimizes pressure fluctuations that would otherwise occur when individual patient solenoid valves open and close. Smaller balloons can also be deployed at each patient’s bedside, on the patient side of the solenoid valves (not shown in Figure 2).
There are several options to dispose the exhalation gas. One conventional approach is to have small holes drilled in the mask or the supply line (to the mask or the endotracheal tube). Exhaust through such holes is how many masks handle exhalation. Viral particles cannot easily be captured using this method of exhalation.
Positive expiratory pressure (PEEP) is desirable to prevent lung collapse, and also makes it easier to apply a HEPA filter to the exhaust gas to prevent virus particle escape. Exhalation pressure in the exhaust manifold as shown in Figure 2, would typically be on the order of 2–4cm of water pressure (~1.002 to 1.004 atmospheres).
In acute respiratory distress, which occurs in the most severe cases of the COVID-19 infections, the alveoli thicken and fill with fluid, which interrupts the transpiration of oxygen. This is why oxygen is so important, but for many COVID-19 patients requiring respiratory support, it is desirable to have a patient triggered inhalation cycle. This can be accommodated by adding a rest time at the end of the exhalation cycle where all the solenoid valves are closed. During this rest time, a pressure sensor would detect the patient’s inhalation which would trigger the opening of the oxygen valve at the beginning of the inhalation cycle (not shown in Figure 2).
Some patients may also need help with exhalation, in some cases due to prior conditions such as COPD. COVID-19 may also affect lung elasticity. This could be addressed via prior art methods such as the pneumobelt or Rethink Respironics’ patent-pending Conformal Vest Ventilator.
The time that the oxygen supply valve is open to each patient controls the amount of oxygen delivered per breathing cycle. This valve open time will clearly depend on the patient’s lung capacity as well as the patient’s need for oxygen.
Following the oxygen valve opening and closing, the air supply valve opens and closes. Inspiratory pressure below the manifold pressure could also be implemented by rapid opening and closing of valves coupled with a pressure sensor to feedback pressure data to the control system. This feature is not illustrated in Figure 2. This would be further facilitated by an elastomer balloon pressure accumulator at each patient’s bedside.
Pure oxygen increases the possibility of combustion of polymers that will be used in the piping and the pressure accumulator tank. However, this is not a significant problem in the current design because the oxygen pressure will barely exceed atmospheric pressure, unlike the oxygen manifolds used in typical hospital rooms.
PVC pipe and various types of elastomer pressure accumulators can be used safely with atmospheric pressure oxygen. Using these materials for the manifolds would make them very inexpensive.
The shared manifolds can be laid out in different ways. It would be desirable for there to be built-in redundancy. If the respiratory ward is organized with supply manifolds all along the walls and crossing above the door so that it is one big loop of manifold, there can be more than one source for air or oxygen.
If there are valves in the main loop supply manifolds, it would be possible to section each manifold so that there can be two different pressure zones in the same ward, with valves separating these zones. In an emergency, those valves would be opened and the entire ward would be supported by only one pressure source. That way if one of the sources failed, pressure could still be maintained by opening the valves to the next neighboring pressurized zone.
Having two pressure zones on the respiratory ward would allow, for example, 1.015 atmospheres for some patients, and 1.025 atmospheres for others.
The largest part of the mass of this device is plastic pipe and fittings that are widely available standardized parts. Solenoid valves and quick-connects are also readily available. The pressure supply for air would be based on a centrifugal blower such as those used in vacuum cleaners or leaf blowers.
We’d like to thank Sukrit Bala for illustrating our figures. He is available for similar project work via inquiry at asukritbala@gmail.com
