Insert an adjustable flow restrictor at the inspiratory limb of each patient, between the one-way valve and the individual pressure and tidal volume monitoring devices.
How it works
An adjustable device that restricts the flow will allow the caretaker to control the pressure and tidal volume for each patient individually. This hinges on the fact that there is only a fixed, short time, given by the time needed for the ventilator to reach peak inspiratory pressure (PIP), wherein there is a flow passing through the restriction. Depending on the applied restriction, the pressure behind the flow restrictor will thus be lower than what you would get without restriction, simply because less volume has been able to flow past it in the given time. Each patient should have an individual pressure and tidal volume meter such that the changes in tidal volume and pressure resulting from the flow adjustments can be visualized.
Advice and comments from our contributors
To be used in combination with an individual tidal volume meter or a pressure transducer, preferably both.
Important: at least one of the circuits should have a low resistance. The reason behind this is that at least one circuit with low resistance, meaning a circuit where the ventilator’s internal pressure measurement at the inspiratory and expiratory end is almost the same, is needed for the ventilator to work properly. The more the flow is restricted, the more resistance is added to a circuit. The maximal amount of resistance tolerated is dependent on the specific ventilator and other components in the circuit, and is also influenced by both the flow restrictor diameter and the amount of closure. The first option is to make this the circuit for the patient with the lowest compliance, such that the flow restrictor on this circuit can be left open. The second option is to introduce a third, ‘short’ circuit, to ensure there always is an uninterupted flow.
We know that in terms or regulating individual volumes and pressure, an actual pressure regulator would make for a better solution, but these are not as readily available as the proposed flow restrictors.
Important properties
- safe materials: all materials should be safe for breathing purposes and for use with high oxygen concentrations
- possible to disinfect: should at least have the possibility to be disinfected with ethanol. 3D printed components should be printed with professional equipment to reduce the chances of bacterial hotbeds forming in filaments and pores.
- maximal achievable pressure drop: should be at least 20cmH20
- pressure drop when fully open: should be as low as possible, although a small added resistance can be allowed in favor of better sensitivity. This is the case for valves with an inner diameter smaller than the ventilation tubing.
- sensitivity: amount the applied pressure/tidal volume changes when making a change to the flow restrictor. Can be expressed in change in tidal volume per percentage of applied restriction. E.g., 10ml / % closure of the valve. This sensitivity is not constant for most flow restrictors; an example of a drastic change in restrictor sensitivity is, e.g., a restrictor where no tidal volume changes happen from when it’s fully open to when it’s moderately closed (very low sensitivity), and where you can only affect the tidal volume when the flow restrictor is almost completely closed.
- linearity: the sensitivity is thus not a fixed constant. Some flow restrictors can, e.g., become far more sensitive the more they are closed. If the flow restrictor yields the same sensitivity over a wide range of adjustments, it can be called linear. Linear flow restrictors will be easier to operate.
- weight: if the valve is too heavy, it will weigh down the circuit or increase chance of disconnection; heavy valves need to be supported.
