Rotary blood pumps are known which are inserted into the heart for supporting the pumping capacity of the natural heart. Insertion is performed intravascularly, namely through the blood vessel system of the patient. It is thus important that, upon insertion, the maximum diameter of the blood pump is small, preferably not exceeding 4 mm (12 French). Further, the blood pump should be flexible for the purpose of conforming to the bends of the vessel course.
The aforementioned rotary blood pumps that are currently in use are axially delivering blood pumps, i.e. they have an axial inlet and an axial outlet spaced therefrom, with an axially delivering impeller in between. The maximum pumped blood flow rate is thus limited by two factors, the cross sectional diameter through which the impeller delivers flow and the rotational speed at which the impeller is driven. Further increase of the diameter is limited by the blood vessel dimensions. Further increase of the rotational speed is difficult because of blood damage.
Therefore, foldable intravascular blood pumps have been proposed in which the impeller itself and the housing in which the impeller rotates can be deployed from a folded configuration into an unfolded, operational configuration (U.S. Pat. No. 4,753,221; U.S. Pat. No. 4,919,647; U.S. Pat. No. 5,749,855; U.S. Pat. No. 6,533,716; U.S. Pat. No. 7,841,976). If the insertion of foldable intravascular blood pumps is to be carried out through a catheter, the maximum diameter of the expandable blood pump should not exceed 4 mm (12 French), whereas, when deployed, the cross sectional diameter may be 9 mm (27 French).
A severe problem with foldable axial blood pumps is the required dimensional accuracy. The rotor must conform, within very close tolerances, to the inner shape of the housing for the purpose of attaining a flow rate of at least 2 l/min (liter per minute) at physiological pressure conditions and without excessively destroying blood. These requirements are difficult to fulfil with foldable blood pumps because of the flexibility of the foldable housing. More specifically, during use and in particular due to the bends in the vessel course, there is always the danger that the housing does not perfectly assume the predefined shape. Instead, unexpected forces acting locally on the flexible housing may cause the impeller to contact the inner wall of the housing and this may quickly lead to the destruction of the entire device or to unacceptable levels of blood damage and loss of parts in the patient's body.
There have also been other proposals for foldable blood pumps, such as in U.S. Pat. No. 5,827,171, which, instead of an impeller, employs concentrically arranged balloons. The balloons are inflated and evacuated to repeatedly collapse and expand an innermost balloon which is actually pumping the blood. US 2008/0103591 A1 proposes a foldable intravascularly insertable blood pump in which the impeller is a radially delivering impeller rather than an axially delivering impeller. Not only does the radially delivering impeller require a relatively low speed of approximately 5,000 to 15,000 rpm for delivering a typical amount of blood of approximately 2 to 5 l/min but, in addition, such a centrifugal pump does not require close radial tolerances between the impeller and the housing. This is due to the fact that the blood is radially expelled from the impeller blades against the circumferential wall of the housing and the wall redirects the blood flow from radial to axial. Consequently, there may be a large radial gap between the impeller blades and the circumferential housing wall. However, it is described that a gap between the front wall of the housing and the impeller blades and, if a back wall of the housing is present, between the back wall of the housing and the impeller blades is to be kept small at approximately 0.1 mm in order to prevent an undesired return flow of blood. This is described in relation to various embodiments, including one embodiment where the blades are held between two plates, the plates being attached to and radially extending from the drive shaft in spaced-apart relationship, and another embodiment where the blades are held between spokes rather than plates, the spokes being attached to and radially extending from the drive shaft.
Thus, while close tolerances between the impeller and the housing are not required in a radial direction, they are still required in the axial direction, thereby causing a risk of failure if the housing deforms towards the impeller under external loads or internal stresses.
It is therefore an object of the present invention to further reduce the risk of failure of radially delivering intravascular blood pumps, in particular foldable blood pumps as known from US 2008/0103591 A1.
According to the invention, the clearance between the front wall of the housing and the impeller is made sufficiently large to account for possible deflections of the housing wall, the minimum distance being at least 0.2 mm, preferably at least 0.3 mm, more preferably at least 0.4 mm, even more preferably at least 0.5 mm, yet more preferably at least 0.6 mm and most preferably 1 mm or more. It has been found that the throughput through the radially delivering impeller is sufficient to compensate any radial back flow axially before and/or behind the impeller blades by accordingly increasing the rotational speed of the drive shaft. This is no problem particularly where the axial length of the impeller blades is substantially greater than the clearance, e.g. 4 mm or more, preferably at least 5 mm, more preferably at least 6 mm and most preferably 7 mm or more. Power for driving the drive shaft is sufficiently available because the drive shaft is driven from outside the patient.
The aspect ratio (Lrotor/Drotor) of the axial length (Lrotor) and average rotational diameter (Drotor) of the impeller blades is preferably equal to or larger than 0.3 and, thus, substantially differs from typical centrifugal pumps whose aspect ratio would range below 0.2.
Along the length of the blades, the housing preferably tapers from a larger diameter at the back of the housing to a smaller diameter at the front of the housing. This taper plays a significant role in separating the inflow pressure from the outflow pressure and, therefore, reduces blood recirculation from the outflow towards the inflow. Considering the large clearance between the rotor and the housing walls, the taper is preferably larger than smaller, e.g. 5 degrees or more, preferably at least 7.5 degrees, more preferably at least 10 degrees and most preferably 12 degrees or more.
The invention is particularly useful with radially delivering impellers where the space between adjacent blades is open towards the front wall of the housing. This results in the pressure created between adjacent blades by the rotation of the impeller also being present next to the front wall, the front wall of the housing separating the impeller from the pump's low pressure side. Since the pressure in the blood flow between adjacent blades increases in a radially outward direction when the impeller rotates, the same pressure increase from radially inward to radially outward is present next to the front wall. Thus, when the space between adjacent blades is open towards the front wall, there is no negative radial pressure difference causing any significant radial back flow, contrary to what is suggested in US 2008/0103591 A1.
Therefore, a structure in which the blades are held between spokes (rather than plates) axially extending from the drive shaft is a particularly preferred embodiment according to the present invention. Alternatively, the blades can be arranged between a front plate and, possibly, a back plate radially extending from the drive shaft, provided that at least the front plate has through openings or perforations for pressure equalization towards the front wall of the housing. If the housing further has a back wall, such through openings or perforations are also provided in the back plate to provide for pressure equalization between the back plate of the impeller and the back wall of the housing.
The embodiment with the blades arranged between radially extending spokes is preferred over the embodiment with the blades held between perforated front and back plates, because a spoke-configuration can be folded about and/or along the drive shaft more easily. For similar reasons, the blades are preferably of planar configuration and may extend in a plane substantially parallel to the axis of rotation.
In essence, the impeller blades in the blood pump of the present invention swirls the blood around the drive shaft inside the housing. Due to the blood flow inlet being arranged radially inwardly (in the following also referred to as the “inlet diameter” of the impeller) relative to the radially outermost ends of the impeller blades (in the following also referred to as the “outlet diameter” of the impeller), i.e. preferably next to the drive shaft, the centrifugal forces acting on the swirling blood force the blood to flow radially outwards, thereby drawing in further blood through the inlet. The through flow is independent of the size of the gap between the impeller blades and the front wall and back wall of the housing. The structure of the blades is not critical either, provided that the swirling motion of the blood is sufficiently high to create the necessary centrifugal forces. The centrifugal forces generating the forward flow are directly related to the inlet versus outlet diameters. The larger the difference between the inlet diameter of the inlet orifices and the outlet diameter of the impeller blades is, the higher is the pressure gradient driving the forward flow.
While the invention is particularly useful with foldable intravascular blood pumps, it can likewise be realized in non-foldable blood pump types and would provide the same advantage that tolerances between the impeller housing and the impeller blades are not critical.
Where the housing has a back wall, the distance between the blades and the back wall of the housing is preferably in the same range as the distance between the blades and the front wall of the housing. However, where the blades are attached to the drive shaft via a back plate rather than spokes, the back plate of the impeller may replace the back wall of the housing and the outlet for the blood to exit the housing may be defined by a gap between the back plate of the impeller and the circumferential wall of the housing.
The redirection of flow of blood through the housing can be facilitated if the outer diameter of both the housing and the blades increases in an axial direction from the inlet side to the outlet side, which implies that the blood flow inlet and outlet are axially and radially spaced apart.
The housing itself is preferably made from a non-compliant polymer, preferably polyurethane, which is sufficiently flexible to be foldable about the drive shaft but retains a predetermined shape in its unfolded condition even at high internal pressures. It is particularly preferred to do without any frame structure, i.e. the housing is preferably formed substantially only by the non-compliant polymer film.