GB2451161A

GB2451161A - Cardiac pump

Info

Publication number: GB2451161A
Application number: GB0810644A
Authority: GB
Inventors: Kevin Fernquest; Graham Foster
Original assignee: Calon Cardio Technology Ltd
Current assignee: Calon Cardio Technology Ltd
Priority date: 2007-07-20
Filing date: 2008-06-11
Publication date: 2009-01-21
Anticipated expiration: 2028-06-11
Also published as: GB0714124D0; GB2451161B; GB0810644D0

Abstract

The pump is suitable for implantation into the human heart or vascular system, and is an axial flow rotary pump having an elongate tubular casing 1 with an electric stator 4 defining an inlet 2 for blood and an outlet 3 for blood longitudinally spaced from the inlet. Within the casing is a rotatable element 9 arranged to be driven within the casing. A substantially axial primary blood flow path 10 extends from the inlet to the outlet and a secondary blood flow path 17 flows in spacing between an outer surface of the rotatable element and an inner surface of the outer casing. The rotatable element has an electric rotor portion 12 arranged to be rotatably driven by the electric stator, and an impeller 11 axially spaced from the rotor portion. Part of the casing 6 and a corresponding part of the rotatable element 14 is flexible and in use inwardly compresses so that the outside diameters of the casing and the rotatable element can be constricted during implantation and then allowed to expand upon reaching its implantation destination.

Description

Cardiac pump The present invention concerns cardiac pumps suitable for implantation into the human heart or vascular system Heart Failure is a major global health problem resulting in many thousands of deaths each year. Until recently the only way to curatively treat advanced stage heart failure has been by heart transplant or the implantation of a totally mechanical heart.

Unfortunately donor hearts are only able to meet a tiny fraction of the demand and totally mechanical hearts have yet to gain widespread acceptance due to the technical difficulties involved with these devices.

Ventricle assist devices (VAD5) have been gaining increased acceptance over the last three decades primarily as a bridge to transplant devices. The devices are implanted long term and work alongside a diseased heart to boost its output and keep the patient alive and/or give a better quality of life whilst awaiting transplant.

The use of these devices has had an unexpected result in some patients: the reduction in strain on the heart over a period of time has led to significant spontaneous recovery of the left ventricle. This gives hope to many patients for whom a donor heart may not become available as it could be the case that the early implantation of a VAD may allow their condition to recover before the disease reaches the most advanced stages. It is also a far more preferable outcome to have one's own heart recover than undergo a transplant, even if donor hearts are available.

At present, the main reason preventing VADs from being fitted on a more routine basis is the highly invasive surgical procedure required to fit the devices. Typically a sternotomy, full heart lung bypass, and major procedures to the heart and thoracic aorta are required to fit a VAD. Presently the expense and risk of such an operation cannot be justified except in the case of those in the most advanced stages of heart failure. If the long term implantation of a VAD or an equivalent circulatory assist device (CAD) could be achieved with a less invasive surgical procedure, ideally eliminating the need for a sternotomy and heart lung bypass, then the use of CADs to treat heart failure in its earlier stages could become far more widespread and routine.

The way to minimise the invasiveness of the implantation procedure fora CAD is to make the device as small as possible so that it can be implanted using a keyhole' type procedure. There are currently available devices for short term use only (maximum two weeks) that are intended for use as a bridge to recovery.

These short term devices are implanted into the heart via the femoral artery using a simple catheter delivery procedure which results in a very low level of patient trauma. If a long term implantable CAD could be developed that is able to be inserted in a similar way to short term CADs, then the vision of routinely using CADs to treat earlier stage chronic heart failure (CHF) would be a significant step closer.

However, short term implantable pumps have mechanical bearings and seals which will wear out over a relatively short period of time. Also they are generally connected to a catheter that feeds a purge fluid to the seals and bearings to decrease wear and prevent heat build up. This connection to a catheter ties the patient to a base station which provides the purge fluid supply. These factors mean that the currently available blood pumps of a size small enough to be implanted via a peripheral artery are not suitable for long term implantation.

It is desirable to provide a completely blood immersed impeller, in whch a portion of the main flow is diverted so that a secondary flow can be created between all moving and stationary areas of the pump. In cardiac pumps, additional consideration needs to be given to the level of shear stress created in the fluid, or else haemolysis will occur. It is therefore required that surfaces that move relative to one another should be separated by a gap sufficient to avoid high shear stresses; as a result known secondary blood flow paths require significant extra space to be accommodated.

Furthermore, the addition of a secondary blood flow path will increase the clearance between the stator and rotor in the motor portion of the pump. This will increase the flux gap in the motor and a drop in efficiency will result. This drop in efficiency will have to be offset by larger motor components. This increase in the size of the motor components combined with the extra space required for the secondary blood flow paths has the overall result of significantly increasing the size of contactless pump compared to a non blood immersed design of a similar throughput.

This required increase in size of a long term implantable cardiac pump, compared to a short term pump, means that implantation of such pumps becomes increasingly difficult though a peripheral artery, especially in smaller patients. However, the considerable benefits of using a peripheral artery as a route to implantation in the heart means that the development of long term pumps that overcome the problems of increasing size is of particular interest.

We have therefore devised a cardiac pump in which the above problems are at least alleviated. The pump according to the invention is capable of being inwardly compressed during implantation (to alleviate the problem of the size constraint of the peripheral artery) and is subsequently able to re-expand outwardly upon reaching its implantation destination. This is possible because the destination for a heart assist pump is typically in either the left ventricle or in the aorta adjacent to the ventricle, both of which locations are many times larger than any peripheral artery, thereby allowing the possibility for considerable expansion of an implanted device when it reaches its implantation destination.

A miniaturised cardiac pump according to the invention (suitable for implantation into the human heart or vascular system) that can be compressed inwardly during implantation and then re-expanded outwardly upon reaching its destination is particularly beneficial and can offset the size disadvantages of a long term implantable pump compared to a pump which is only suitable for short term use.

According to the invention therefore, there is provided a pump suitable for the application just described, the pump being an axial flow rotary pump having: an elongate tubular casing defining an inlet for blood, an outlet for blood longitudinally spaced from the inlet, and a substantially axial primary blood flow path from the inlet to the outlet, the casing having an electric stator therein; an elongate tubular rotatable element arranged to rotate within the casing, the rotatable element having an electric rotor portion arranged to be rotatably driven by the electric stator and an impeller axially spaced from the rotor portion; wherein part of the casing between the inlet and the stator and/or between the stator and the outlet is inwardly compressible, and a corresponding part of the rotatable element between the inlet and the stator and/or between the stator and the outlet is also inwardly compressible, such that inward compression of said part of the casing and said corresponding part of the rotatable element permits the outside diameter of the casing and of the rotatable element to be constricted during implantation and then allowed to expand outwardly upon reaching the implantation destination of the pump.

The inward compressibility of the part of the casing and the part of the rotatable element is preferably radial.

A pump according to the invention is electric motor driven and as described includes an embedded brushless DC motor. The pump is of a contactless design employing hydrodynamic and/or magnetic forces to suspend the rotating part in the blood and is therefore near wearless in operation, making it suitable for long term use.

The pump comprises an outer casing that defines the inlet and outlet and also houses the motor stator components. Within the outer casing is an inner tubular rotating element that defines a primary blood flow path through its centre and comprises a motor rotor portion and an impellor portion. The motor rotor is positioned to be able to co-operate with the motor stator in the casing.

The pump according to the invention preferably includes a secondary blood flow path in spacing between an outer surface of the rotating element and an inner surface of the outer casing. This secondary flow path serves to suspend the rotor in blood and prevent the inner rotating element from contacting the outer casing. Centring forces are provided by hydrodynamic bearings and/or magnetic bearings.

At least part of the outer casing and of the inner rotatable element is flexible (either of a flexible material or of a flexible construction) so as to allow the relevant parts to be compressed inwardly to reduce the outside diameter of the pump (and at the same time constricting the primary blood flow path). In a preferred embodiment, such flexible parts of the outer casing and of the rotatable element are provided in axially spaced locations both upstream and downstream of the respective motor components as these latter components of the pump require the largest diametrical space.

As indicated, the flexible parts of the casing, and of the rotatable element, are preferably axially spaced so as to be positioned either side of the motor stator, so that the casing and the rotatable element can be compressed in combination.

The pump according to the invention preferably also includes an axial offset or spacing between the impellor section and the motor section of the pump, which allows the motor components to be compressed into the void left by the main blood flow path without the need to compress the impellor. This arrangement allows for greater overall compressibility, and avoids having to compress the impellor blades, the geometry of which is critical. However, embodiments are envisaged where the impellor is made from a compressible material so that this part of the pump could be compressed also. A compressible impellor would also allow for the axial offset between the impellor and rotor to be reduced or eliminated, which could enable the pump to be made shorter axially. Alternatively, a compressible impellor could enable it to have increased size and consequent improved pumping efficiency.

Furthermore, rigid ends of the casing and/or of the rotatable element are preferably provided to give a stable base for attachment of the flexible portions and also to facilitate attachment of other components to the pump, such as inlet and outlet cannulae.

The motor typically is of a brushless DC design employing electronic commutation for speed control. It should be noted however that motors of this type are well known and for this reason the conventional motor components shown in the accompanying representations are schematic representations only.

An exemplary embodiment of the invention will now be described in more detail, with reference to accompanying drawings, in which: Fig 1 is an axial sectional view of an exemplary cardiac pump according to the invention in its uncompressed configuration; Fig 2 is an axial sectional view of the pump illustrated in Figure 1 in its compressed configuration; Fig 3 is a radial sectional view of the pump in the uncompressed configuration shown in Figure 1; and Fig 4 is a radial sectional view of the pump in the compressed configuration shown in Figure 2.

Throughout the accompanying drawings and the following description, like parts are denoted by like reference numerals.

Referring to the drawings, the illustrated pump comprises a tubular outer casing 1 defining an inlet 2 at one end and an outlet 3 at the other. Integral to the outer casing is a motor stator 4 that houses the stator windings 5.

The outer casing includes a flexible portion 6 that surrounds the motor stator 4 and allows the stator windings 5 to move inward and closer together when the pump is compressed (see Figs 3 & 4). Longitudinally extending channels or voids 7 between adjacent stator windings 5 allow greater compressibility. Alternatively a foam type material (not shown) could be used for the flexible portion provided such material has high inherent compressibility and a non-porous surface.

Alternatively or additionally, flexible material may be provided between adjacent windings of the rotor portion Rigid end pieces 8 are provided at each end of the outer casing 1. These provide a stable point for attachment of the flexible portion 6 and also provide a rigid point for the mounting of other components that are likely to be attached to the pump, for example inlet or outlet cannulae (not shown). They will also help to prevent damage that could be caused by over compression of the pump.

Rotatably held within the outer casing 1 is a tubular rotatable element 9 that defines a primary blood flow path 10 through its central bore. The tubular rotatable element 9 comprises an impellor 11 and a motor rotor portion 12. The latter motor rotor portion 12 further comprises rotor permanent magnets 13 which are positioned to be able to co-operate with the motor stator 4.

The rotatable element 9 also includes a flexible portion 14 which is aligned axially to the flexible portion of the outer casing 6 so that both flexible portions can be compressed in combination. The flexible portion 14 of the rotatable element 9 surrounds the motor stator permanent magnets 13 and allows them to move inwardly and closer together (see Figs 3 & 4).

Longitudinally extending voids 15 between the permanent magnets 13 allow greater compressibility. In contrast to the voids in the outer casing 5, the voids in the rotor 15 should be enclosed in order to preserve a smooth surfaced cylindrical blood flow path. Voids that had a channel shape as in the outer casing 5 would give a good degree of compressibility, but would disadvantageously introduce an unwanted whirl or turbulent component to the flow. Rigid end pieces 16 on the inner rotatable element provide stable attachments for the flexible portion 14.

A clearance between the tubular rotatable element 9 and the outer casing 6 defines a secondary blood flow path 17 which allows the rotatable element 9 to be completely suspended in blood, and therefore allows the pump to be near wearless in operation.

The inlet 18 for the secondary blood flow path 17 is at the end of the rotatable element 9 which is downstream of the impellor 11 on the high pressure side of the pump. The outlet 19 is on the opposite side of the rotatable element 9 in the low pressure area of the pump upstream of the impellor 11. Thus the secondary blood flow path will naturally flow from a high to low pressure.

In a preferred embodiment, the secondary blood flow is augmented by helical ribs 20 that serve the dual function of both an additional pumping means for the secondary flow and radial hydrodynamic bearings. The additional pumping means is achieved by the helixes giving an Archimedes screw pumping effect and ensures that the flow rate in the secondary path 17 is sufficient to avoid risk of thrombus formation. The hydrodynamic bearing function is accomplished by the helical ribs 20 having a radial slipper pad bearing profile when viewed in section (see Fig 3). The large area covered by the helical ribs ensures that the centralising loads are well spread out over the entire rotatable element, which will reduce the shear stress at anyone point and therefore reduce the risk of haemolysis.

Additionally, the radial bearing effect of the helical ribs 20 prevents the overexpansion of the flexible portion 14 of the rotatable element 9 from centripetal forces generated by its rotation.

In order to resist the axial thrust load imparted onto the rotatable element 9 by the impellor an inclined slipper pad bearing 21 is provided. There is also provided a second opposing inclined slipper pad bearing 22. Even though this bearing is acting in the same direction of the axial thrust of the impeller 11, it will give improved overall system stability and provide resistance to shock loadings.

In general the whole of the pump is designed to provide a smooth, low shear stress blood passage in order to reduce the likelihood of haemolysis, without areas in which the flow could stagnate and give a risk of thrombosis.

Various modifications of the pump are envisaged. For example, a pump similar to the one described in which the direction of flow is reversed can be employed. Also there may be employed an additional flow stator adjacent to the impellor that can either pre-whirl or un-whirl the flow depending on the direction of flow. The addition of such a flow stator may also provide an additional means for retention of the inner rotating element and also provide a means for directing the inlet/outlet flow of the secondary blood flow path.

Claims

1 A pump suitable for implantation into the human heart or vascular system, said pump being an axial flow rotary pump having: an elongate tubular casing defining an inlet for blood, an outlet for blood longitudinally spaced from the inlet, and a substantially axial primary blood flow path from the inlet to the outlet, the casing having an electric stator therein; an elongate tubular rotatable element arranged to rotate within the casing, and defining a secondary blood flow path in spacing between an outer surface of the rotatable element and an inner surface of the outer casing, the rotatable element having an electric rotor portion arranged to be rotatably driven by the electric stator and an impeller axially spaced from the rotor portion; wherein part of the casing between the inlet and the stator and/or between the stator and the outlet is inwardly compressible, and a corresponding part of the rotatable element between the inlet and the stator and/or between the stator and the outlet is also inwardly compressible, such that inward compression of said part of the casing and said corresponding part of the rotatable element permits the outside diameter of the casing and of the rotatable element to be constricted during implantation and then allowed to expand outwardly upon reaching the implantation destination of the pump.

2 A pump according to claim 1, wherein the inward compressibility of the part of the casing and the part of the rotatable element is radial.

3. A pump according to claim 1 or 2, in which defines the primary blood flow path is through the centre of the inner tubular rotatable element, said rotatable element comprising a motor rotor portion and an impeller portion.

4 A pump according to any of claims 1 to 3, wherein at least one part of the outer casing and of the inner rotatable element is flexible.

A pump according to claim 4, wherein a plurality of said flexible parts are provided in axially spaced locations, at least one being upstream and at least one being downstream of the respective motor components.

6 A pump according to claim 5, wherein said locations are axially spaced so as to be positioned either side of the motor stator, so that the casing and the rotatable element can be compressed in combination.

7 A pump according to any of claims 1 to 6, wherein the stator comprises a plurality of windings with flexible material between adjacent windings to aid inward constriction of the stator.

8 A pump according to any of claims 1 to 6, wherein the rotor portion comprises a plurality of windings with flexible material between adjacent windings to aid inward constriction of the rotor portion.

9 A pump according to any of claims 1 to 8, which includes an axial offset or spacing between the impeller section and the motor section of the pump.

10 A pump according to any of claims 1 to 9, wherein at least one of the casing and the rotatable element has a rigid end.