This invention relates to ventilators of the kind for supplying breathing gas to a patient.

Resuscitators or ventilators (the two terms are used interchangeably in the description) are used to supply breathing gas to a patient who may not be able to breathe spontaneously. Portable resuscitatdrs may take the form of a resilient bag that is squeezed manually to supply a volume of air to the patient, the bag refilling with air when it is released so that a new volume of air can be supplied. Electrically-powered and electrically- controlled resuscitators are also available including various sensors and solenoid valves to control the supply of gas to the patient. Alternatively, the resuscitator may be a pneumatic mechanical arrangement including a timing valve and various other controls. The ventilator is connected to an oxygen cylinder, which both provides the breathing gas, or a part of this, and also provides the power to drive the components of the ventilator. These ventilators are arranged to supply gas in a cyclical manner to the patient at a selectively adjustable rate and volume compatible with normal breathing. Powered ventilators tend to be relatively complex, which makes them expensive, and they are not generally suitable for use by untrained people. It is desirable to provide ventilators in public places, workplaces and the like for emergency use by the general public but the cost and complexity of conventional ventilators precludes this.

Conventional electrically-powered, servo-controlled motors are generally too big, heavy, power hungry and slow to be used in ventilators. Also, some form of gears is needed to connect a servo motor to a piston/cylinder pump in order to transmit force. Gears introduce friction and other mechanical losses, and require lubrication and regular maintenance.

It is an object of the present invention to provide an alternative ventilator.

According to one aspect of the present invention there is provided a ventilator of the above-specified kind, characterised in that the ventilator includes a cylinder and a piston, that the piston is movable relative to the cylinder, that the piston and cylinder have mounted therewith respective cooperating magnets, at least one of which is electrically energisable, and that the magnets are arranged to develop a force to move the piston relative to the cylinder such as to pump gas into or out of the cylinder.

The cooperating magnets preferably include a plurality of electromagnetic coils spaced along a path of displacement, different ones of the coils being arranged to effect displacement at different regions along the path of displacement. The magnets preferably include a permanent magnet on one of the piston and cylinder and an electromagnetic coil on the other of the piston and cylinder. The permanent magnet may have a hollow, cylindrical shape. The ventilator preferably includes a plurality of coils mounted coaxially spaced from one another along the length of the cylinder. The cylinder preferably includes an inlet with a one-way valve arranged to allow gas to flow into the cylinder and an outlet with a one-way valve arranged to allow gas to flow out of the cylinder. The ventilator preferably includes a sensor responsive to the position of the piston relative to the cylinder and a control unit arranged to receive an output from the sensor and to provide a drive signal accordingly to drive the coils. The magnets may be arranged to develop a force to move the piston relative to the cylinder in both directions such as to pump gas into and out of the cylinder.

Alternatively, the magnets may be arranged to develop a force to move the piston relative to the cylinder in one direction such as to pump gas into or out of the cylinder, the ventilator including resilient means to move the piston relative to the cylinder in the opposite direction.

According to another aspect of the present invention there is provided a ventilation system including two ventilators according to the above one aspect of the present invention, characterised in that the outlets of the two ventilators are connected together, and that the ventilators are arranged to deliver different gases or gas mixtures.

A ventilator according to the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a partly cross-sectional side elevation view showing the ventilator schematically; Figure 2 is a schematic cross-sectional side elevation view of an alternative pumping arrangement for a ventilator; and

Figure 3 illustrates schematically an arrangement of a modified ventilator.

With reference first to Figure 1, there is shown a ventilator employing a voice-coil actuator 1 arranged to move a piston 2 relative to a cylinder 3 together providing a pumping unit 4. Voice-coil actuators (VCAs) have a coil of wire that drives a magnet according to the signal current flowing in the coil. VCAs are simple electromechanical devices that can move a mass along a line. The direction of movement can be changed by reversing the polarity of excitation. There are two main types of VCA, namely those with a moving coil and those with a moving magnet. The first type consists of the usually stationary field (permanent magnet) assembly and the moving coil assembly. In contrast, moving magnet VCAs have the coil attached to a stationary soft magnetic housing, which also serves as a conductor of the magnetic flux. The field assembly typically consists of an axially-magnetized, permanent cylindrical magnet and two soft magnetic pole pieces attached to both ends of the magnet. Applying a voltage across the terminals of the coil causes the VCAs' moving part, magnet or coil, to travel in a given direction. The generated force is proportional to the flux crossing the coil and the current that flows through this coil. VCAs are very useful in applications where more precise control is necessary, primarily because they are available with position feedback devices.

One problem with VCAs is that the stroke is usually very small, typically around 5 mm, which is insufficient to displace the required volume of gas in a ventilator with a piston of a reasonable cross-sectional area. In the present invention the VCA 1 is arranged to have a longer effective stroke length, sufficient to displace a useful volume of gas for ventilation purposes.

The outer cylinder 3 of the VCA 1 includes a gas inlet/outlet 11 at its upper end. The lower end 12 of the cylinder 3 is shown open but could be closed. Three circular electromagnetic coils 13 to 15 embrace the outside of the cylinder 3 coaxially at its upper end and are spaced from one another at fixed locations along its length. Inside the outer cylinder 3 the piston 2 has a central piston rod 16 attached at its upper end to a circular head 17 extending orthogonally of the rod. A cylindrical sleeve 18 depends from the lower side of the head 17 with an outer diameter equal to that of the head and slightly less than the internal diameter of the outer cylinder 3. The sleeve 18 supports an O-ring seal 20 that makes a sliding sealing fit with the inside of the outer cylinder 3. Alternatively, the seal between the piston 2 and the cylinder 3 could be provided by, for example, a flexible diaphragm seal. Inside the piston 2, the sleeve 18 supports a permanent magnet 21 of cylindrical shape fixed to the inside of the sleeve. The magnet 21 is axially magnetised and has two soft pole pieces of magnetic material attached to both ends (not shown). The VCA 1 is completed by a second series of three electromagnetic coils 23, 24 and 25, which are fixed relative to the outer cylinder 3 at a location below the lower end of the upper set of coils 13 to 15 and slidingly embrace the central piston rod 16 such that the piston rod can move up and down relative to these coils. The length of the permanent magnet 21 is such that, when the piston 2 is in the upper position shown in Figure 1, the magnet extends inside the upper set of coils 13 to 15 and also extends along the outside of the lower set of coils 23 to 25.

All six electromagnetic coils 13 to 15 and 23 to 25 are electrically connected by wires 30 to a control unit 31. The control unit 31 supplies drive voltages across the coils 13 to 15 and 23 to 25 such as to generate magnetic fields within the VCA to produce the desired movement of the piston 2 along a displacement path axially of the cylinder 3. In particular, it can be seen that both the lower set of coils 23 to 25 and the upper set of coils 13 to 15 can set up magnetic fields that interact with the permanent magnet 21 attached to the piston 2. The coils are arranged such that the lower set of coils 23 to 25 interacts with the magnet 21 to produce a displacement along a length towards the lower end of the piston's stroke or path of displacement and that the upper set of coils 13 to 15 interacts with the magnet to produce a displacement along a length towards the opposite, upper end of the piston's stroke. For example, the piston 2 might start in a position at the lowest extent of its stroke where the upper end of the piston is below the lower end of the upper set of coils 13 to 15. The lower coils 25, 24 and 23 would be energised one after the other to displace the piston 2 up the cylinder 3 to its halfway point. The upper set of coils 15, 14 and 13 would then be energised one after the other to move the piston 2 further up the cylinder 3 to complete its full stroke along the displacement path.

The pumping unit 4 also includes one or more sensors 40 positioned in or around the outer cylinder 3 to provide an output signal representative of the position of the piston 2 relative to the cylinder, which output is supplied to the control unit 31 for use in controlling the drive applied to the coils 13 to 15 and 23 to 25.

The coils could be oppositely energised to return the piston to its lowest position to start another cycle. Alternatively, the VC A could include some form of resilient means, such as a helical spring between the cylinder 3 and the piston 2 in order to displace the piston in one direction when power to the coils is removed.

In this way, the piston 2 is moved up and down the cylinder 3 to pump gas out of and into the cylinder via the inlet outlet 11.

The ventilator is arranged to pump fresh air or other breathing gas into the patient. This is achieved by a valve arrangement 50 coupled between breathing tubing 51 connected to the inlet/outlet 11 on the pump 4 and extending to a patient interface such as a breathing mask 52. The valve arrangement 50 has three limbs. One limb 53 connects with the inlet/outlet 11, a second limb 54 connects with the breathing tubing 51 and a third limb 55 opens to atmosphere. The second limb 54 includes a one-way valve 154 arranged to allow gas to flow from the ventilator to the breathing tube 51 (during an up stroke of the piston 2) but prevents flow in the opposite direction. The third limb 55 also includes a one-way valve 155, this being arranged to enable gas to be drawn through the valve into the ventilator (during a down stroke of the piston 2) but to prevent gas flow in the opposite direction. The patient interface 52 includes a patient valve (not shown) that allows exhaled gas to escape to atmosphere during the expiratory phase of the ventilator.

The VCA 1 used in the ventilator can have zero backlash, hysteresis and cogging. Because there is no contact between the coils 13 to 15 and 23 to 25 and the magnet 21 there is no wear and tear. Movement of the piston 2 can be very smooth at low speeds with a limitless resolution, depending on the feedback mechanism used.

These advantages enable the ventilator to have a compact size, light weight, and high force sensitivity. The VCA 1 improves the consistency and precision with which a valve can be actuated, and provides extremely fast control of valve opening and closing.

Instead of using fixed coils and a moving magnet it would be possible to use moving coils and a fixed magnet. Such an arrangement can have a very low coil mass leading to a very fast response and a high bandwidth.

Alternative arrangements for increasing the stroke length of a VCA 1 are possible. Instead of the concentric configuration used in the arrangement shown in Figure 1 it would be possible, for example, to arrange the two sets of coils in a line, one after the other, and locate the piston between the two coils as shown in Figure 2. This arrangement has a cylinder 200 with an inlet port 201 and 202 at opposite ends with respective one-way valves 203 and 204 that open to allow gas to flow into the cylinder but close to prevent gas flowing out of the cylinder through the ports. The cylinder 200 also has two outlet ports 205 and 206 located to the left and right respectively of the midpoint of the cylinder. The outlet ports 205 and 206 have respective one-way valves 207 and 208 that allow gas to flow out of the cylinder but close to prevent gas flowing into the cylinder. A piston 210 is slidably positioned in the cylinder 200 centrally along its length. The piston 210 supports a permanent magnet 211 that interacts with two series of electromagnetic coils 212 and 213 fixed to the outside of the cylinder 200 to the left and right respectively of the midpoint along the length of the cylinder. By appropriately energising the coils 212 the piston 210 can be moved along its displacement path to the right or left along the cylinder. When the piston 210 is moved to the right the valve 208 in the outlet port 206 opens and the valve 204 in the inlet port 202 closes so that gas in the cylinder on the right-hand side of the piston is pumped out to the patient. At the same time the valve 207 in the other outlet port 205 closes and the valve 203 in the inlet port 201 opens to allow air or other gas into the left-hand side of the cylinder 200 so that the piston 210 is not prevented from moving by a reduced pressure on its left-hand side. The air drawn into the left-hand side of the cylinder 200 is then pumped out to the patient when the polarity of the voltage applied to the coils 212 and 213 is changed to cause the piston 210 to move in the opposite direction, to the left. As it does this, the valve 207 in the outlet port 205 is opened and the valve 203 in the inlet port 201 is closed so that gas flows out through the left-hand outlet port 205. During this movement the reduced pressure to the right of the piston 210 causes the valve 208 in the outlet port to close and the valve 204 in the inlet port 202 to open,

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Ventilators according to the present invention can be used to provide standard ventilation to a patient and can also be used to deliver high frequency ventilation. The ventilator can be driven to produce different pressure or flow waveforms to ventilate patients with particular problems. Two or more ventilators could be connected together, as illustrated in the arrangement shown in Figure 3 where two similar ventilators 301 and 302 have their outlets connected together in parallel. In this arrangement the two ventilators could be used to deliver different gases or gas mixtures, such as, for example, air and pure oxygen or oxygen and an anaesthetic gas.