GB2544989A

GB2544989A - Insert devices for pressure compensation

Info

Publication number: GB2544989A
Application number: GB1521249.1A
Authority: GB
Inventors: Weinberger Paul; Scott Gutsell Graham; Elghamaz Ahmed
Original assignee: DIASOLVE Ltd
Current assignee: DIASOLVE Ltd
Priority date: 2015-12-02
Filing date: 2015-12-02
Publication date: 2017-06-07
Also published as: GB201521249D0

Abstract

A device for use with a fluid delivery apparatus, and method of use thereof. The fluid delivery apparatus comprises a fluid delivery conduit 20, a fluid flow path 30 extending at least in part through the conduit and having a distal end for connection with a system for conveying/containing fluid, a proximal pressure sensor 23 for measuring a fluid pressure at a proximal end of the flow path, and compensation means 25 for determining a fluid pressure in the system based on the proximal pressure and a pressure drop along the flow path. The device comprises an elongate insert 100 for insertion into the conduit, having sensing means (8, 18, 19, Fig. 4) for determining the pressure drop along the fluid flow path. The device and the fluid delivery apparatus may be used in cardiac catheterisation procedures, such as FFR procedures. The device may be a pressure wire, pressure catheter or guide wire, and the conduit may be a guide catheter.

Description

Insert Devices for Pressure Compensation

Field of Invention

This invention relates to insert devices for deployment within fluid delivery conduits, pressure measurement devices and methods of measuring pressure. In particular, but not exclusively, the invention relates to insert devices for use in pressure measurement compensation in coronary catheterisation apparatus.

Background to the Invention

This disclosure relates particularly, but not exclusively, to devices such as pressure wires/catheters and/or guide wires deployed in fluid delivery conduits contained within apparatuses having various, different purposes, including the supply of a fluid to, and/or extraction of samples of fluid from, a system. Typically, the fluid(s) to be delivered and the fluid(s) contained or conveyed by this system are in liquid form. The invention is particularly applicable to coronary catheterisation apparatus which may be used, for example, in Fractional Flow Reserve (FFR) analysis of a coronary artery stenosis. If the apparatus is to be used as a coronary catheterisation apparatus, then the system is the patient, the fluid delivery conduit is a catheter, and the fluid in the patient is blood.

The coronary catheterisation procedure can involve first, inserting a guide wire into the patient’s peripheral arteries, for example (via an incision in the wrist or groin), followed by passing a guide catheter over the wire. This can ease the initial placement of the catheter, allowing it to be guided through the patient’s arterial network to the coronary arteries of interest.

The guide wire may subsequently be replaced by a either: a pressure wire, which is a similarly elongate, flexible wire, but that contains a miniature pressure sensor located at, or near, its distal tip; or a pressure catheter, which may be a rapid exchange microcatheter with a miniature pressure sensor located at, or near, its distal tip as described for example in US 2010/0241008. This subsequently introduced device can be advanced through the structures of the heart to measure the actual local blood pressure at different locations, for example downstream of an arterial stenosis. This local pressure can be compared to an aortic blood pressure measurement obtained using a proximal pressure sensor connected in a flow line at a point proximal of the guide catheter. This proximal sensor is supported, for example on a drip stand; and to avoid systematic errors arising from a hydrostatic head of pressure, is positioned at the same elevation as the patient’s heart.

The signals from both sensors are normally displayed in real-time on a pressure analyser/display unit. In addition to providing the cardiologist with instantaneous display of the patient’s status, the unit may also calculate the ratio of the two pressures, which is used in FFR as a measure of the severity of a coronary stenosis.

The proximal pressure sensor provides an effective means of measuring aortic pressure when there is no flow of liquid along the guide catheter, so that the prevailing conditions at the tip of the guide catheter (i.e. the distal end) are replicated at the proximal sensor. However, the guide catheter can also be used to deliver saline, drugs (in liquid form), such as adenosine to stimulate maximal blood flow in the coronary artery under investigation, or a contrast agent for facilitating the imaging of the coronary artery using, for example, an x-ray imager. In a similar way, the guide catheter may also be used to extract liquid samples, for diagnostic purposes, for example.

When a liquid is being injected along the catheter, the proximal pressure sensor measurement no longer bears a simple relationship to the true aortic pressure and, to avoid this erroneous situation, the proximal sensor is often isolated, normally by means of a valve, from the catheter during this part of a procedure.

This interruption in monitoring of aortic blood pressure can be disadvantageous. For example, if a vasodilator drug such as adenosine is being introduced, while the proximal sensor is in effect disabled, the exact time of maximal blood flow (hyperemia) through the stenosis may occur when the pressure measurements for FFR are not being taken. As a result, the resultant data may have been obtained in less than ideal circumstances.

The generally accepted view is that if a drug were to be delivered down the guide catheter while measurements were being taken by the proximal pressure sensor, there would need to be an additional lumen, such as a micro-catheter introduced either parallel to or over the pressure wire, so as to provide a passage for the drug to be injected, whilst leaving an outer passage (the annulus between the micro catheter and the interior of the guide catheter) along which the pressure is transmitted to the proximal sensor. However, such an approach significantly increases the complexity of the apparatus.

In another application, where it is desired to measure blood flow rate in individual vessels by the principle of thermodilution, a microcatheter is used to introduce a liquid such as saline at the point of interest.

Summary of the Invention

According to a first aspect of the invention, there is provided a device for use with a fluid delivery apparatus, the fluid delivery apparatus comprising a fluid deliver}' conduit, a fluid flow path extending at least in part through the fluid delivery conduit and having a distal end for connection with a system for conveying or containing fluid, a proximal pressure sensor for measuring a fluid pressure at a proximal end of the fluid flow path, and compensation means for determining a fluid pressure in the system based on an output of the proximal pressure sensor and a pressure drop along the fluid flow path. The device comprises an elongate insert for insertion into the fluid delivery conduit. The insert comprises sensing means arranged to provide an output for use in determining the pressure drop along the fluid flow path.

In this way, the pressure in the system can be derived from the output of the proximal pressure sensor even when fluid is flowing along the fluid delivery conduit. Furthermore, because the sensing means is disposed on an insert that, in use, is positioned within the fluid delivery conduit, the sensing means can, if appropriate, respond accurately to changes in fluid behavior in the conduit, such as changes in temperature or viscosity.

The insert may be flexible. Preferably, the device comprises a wire, preferably a pressure wire or guide wire, or a pressure catheter. The device may be particularly suitable for use in catheterisation apparatus, for example coronary or cardiac catheterisation apparatus. To this end, the fluid delivery apparatus may comprise catheterisation apparatus and the fluid delivery conduit may comprise a guide catheter, and the insert may be arranged to extend through the guide catheter in use. Fluids may be passed through the guide catheter when a suitable flow line is connected to its proximal end.

Preferably, the sensing means is disposed at least 100 mm from a distal end of the insert. In this way, the sensing means can remain within in the fluid delivery conduit even when the distal end of the insert extends out of the distal end of the conduit, for example during a coronary catheterisation procedure. For example, the sensing means may be disposed at least 300 mm from the distal end of the insert.

The sensing means may comprise one or more sensors. The sensing means may thus comprise a plurality of sensors.

The sensing means may comprise one or more flow sensors. For example, the sensing means may comprise a thermal flow sensor. In such an arrangement, the sensor may comprise a heated element and means for measuring the temperature of the element and for regulating the input power to the element, such that it can be maintained at a constant temperature. The heated element, which is typically miniaturized, is cooled by fluid passing across it within the fluid delivery conduit, and the increase in power required to maintain the element at a constant temperature thereby provides a measure of the flow rate in the conduit.

In another example, the flow sensor comprises an ultrasound Doppler flow sensor. In this arrangement, the Doppler sensor determines the rate of flow of fluid along the delivery conduit by means of the Doppler effect.

The flow rate information can be used, together with knowledge of the effective internal diameter of the fluid delivery conduit and the flow conditions within the same, to determine the pressure drop along the length of the fluid delivery conduit. The effective internal diameter takes into account the effect of any other devices that may be inserted through the fluid delivery conduit alongside the insert. The flow conditions could comprise either laminar or turbulent flow conditions, a transition between the two, or combination of the two.

The sensing means may comprise one or more pressure sensors. In one example, in which the device is used with catheterisation apparatus of the kind having a proximal pressure sensor, the output(s) of the pressure sensor(s) (disposed inside the catheter in use) and the proximal pressure sensor may be analysed to yield information which is indicative of the pressure drop along the catheter and the associated flow path and which can be used to compensate for the effect of the pressure drop on the output of the proximal sensor.

Preferably, the sensing means comprises two or more pressure sensors situated at spaced apart positions along the insert. Thus pressure differences can be measured between the corresponding two or more locations along the insert, providing information from which the pressure drop along the length of the flow path can be determined. It will also be apparent that, if the flow path is of constant internal diameter, knowledge of the internal diameter of the conduit, and of any other devices deployed in it alongside the insert, is not necessarily required to determine the pressure drop. Likewise, knowledge of the physical properties of the fluid in the conduit, such as its viscosity, is also unnecessary. Furthermore, by using multiple sensors disposed in the conduit, in use, the measured pressure drop is not affected by changes in the position of the insert with respect to the conduit in use.

In long conduits with relatively slow moving fluids of moderate viscosity, the prevailing flow conditions are invariably fully developed laminar, and so the pressure drop along the length of the delivery conduit is linear with respect to distance along the flow path. Preferably, therefore, the sensing means comprises two pressure sensor elements.

Accordingly, the sensing means may comprise first and second pressure sensors arranged to measure the pressure of fluid in the fluid delivery conduit at first and second positions respectively, wherein the first and second positions are spaced apart along the insert. The first and second positions may be spaced apart by a distance of between 10 mm and 1000 mm. Preferably, the first and second positions are spaced apart by a distance of between 300 mm and 500 mm.

In another embodiment, the sensing means comprises a differential pressure sensor. The differential pressure sensor may comprise a common pressure sensitive element having first and second faces arranged to couple, in use, to the fluid in the fluid delivery conduit at first and second positions respectively, wherein the first and second positions are spaced apart along the insert. The first and second faces may be arranged to couple to the fluid in the fluid delivery conduit at the first and second positions respectively by way of a coupling medium. The coupling medium may comprise fluid from the fluid delivery conduit and/or air. In another example, the coupling medium comprises a medium that is immobilised and yet can transmit pressure fluctuations, such as a gel.

The or each pressure sensor may employ optical interferometry. For instance, established techniques such as Fabry-Perot or low coherence interferometry may be used, such as that described in US Patent Application Publication No. US 2006/061768. In this case, the connection between the sensing element of the respective pressure sensor and the instrumentation connected to the proximal end of the device is by means of an optical fibre.

The or each pressure sensor may comprise a piezo-resistive element. For example, the pressure sensors may include pressure sensor element(s) comprising silicon based piezo-resistive transducers of identical design. When two or more pressure sensors of this type are provided, the piezo-resistive elements may be connected together in a

Wheatstone bridge configuration. In this case, the pressure sensor element may together form a circuit with four interconnections.

Preferably, the insert comprises an outer sheath for housing the sensing means. The sheath may include opening means through which, in use, the sensing means are coupled to fluid in the fluid delivery conduit. When the sensing means comprises a plurality of sensors, the opening means preferably comprises a respective aperture for each of the pressure sensors. When the sensing means comprises a differential pressure sensor having first and second faces, a respective aperture associated with each face may be provided.

The device may further comprise a tip pressure sensor located adjacent a distal end of the insert. When the sensing means comprises one or more pressure sensors, the tip pressure sensor and the or each pressure sensor of the sensing means may share common voltage supply conductors. In embodiments in which the sensing means comprises one or more pressure sensors, each of these elements of the sensing means constitutes a respective additional pressure sensor. Preferably, the tip pressure sensor operates using the same principles and shares common supply voltage conductors with the additional pressure sensors. In the case where two additional pressure sensors are provided, the pressure sensors of the sensing means and the tip pressure sensor may together form a circuit with five interconnections.

Preferably, the device is a pressure wire or pressure catheter in which the tip pressure sensor is used for measuring pressure in the system. In one embodiment, for example, the device may comprise an elongate flexible insert with a pressure sensor at or near its distal tip which, in use, can extend through and beyond the distal end of the fluid delivery conduit connected to the system, to enable the sensor to be exposed to fluid in the region of the system beyond said distal end of the conduit. The sensing means is preferably situated at a region of the insert proximal to the tip pressure sensor. Preferably the device is part of apparatus having a fluid delivery conduit which is a coronary or cardiac catheter.

The compensation means may be a processor, such as a digital processor. The compensation means may instead be an analogue processor or analogue circuit. The compensation means may optionally be implemented as a dedicated device or in a suitably-programmed general purpose computer.

The device of the first aspect of the invention may be used with other types of catheter, for example diagnostic catheters or microcatheters for purposes such as blood flow rate measurement by means of thermodilution such as is described in US Patent Application Publication No. US 2007/0078352.

In a second aspect, the present invention resides in a combination of a device according to the first aspect of the invention, and a fluid delivery apparatus comprising a fluid delivery conduit, a fluid flow path extending at least in part through the fluid delivery conduit and having a distal end for connection with a system for conveying or containing fluid, a proximal pressure sensor for measuring a fluid pressure at a proximal end of the fluid flow path, compensation means for determining a fluid pressure in the system based on an output of the proximal pressure sensor and a pressure drop along the fluid flow path determined from the output of the sensing means.

In another aspect, the invention resides in catheterisation apparatus for use in the field of cardiology having a catheter which acts as a fluid delivery conduit and a device according to the first aspect of the invention, the sensing means of the device being operable to determine a pressure drop caused by the flow of fluid along a flow path at least part of which is through the catheter, or to provide an output for use in compensating for an effect of said drop on a measurement of fluid pressure in the system which measurement is taken at a position proximal of the catheter.

In a further aspect of the present invention, there is provided fluid delivery apparatus for delivery of fluid to or extraction of fluid from a system for containing or conveying fluid, the apparatus comprising a device according to the first aspect of the invention, a fluid delivery conduit for connection to the system at a distal end of the conduit, pressure monitoring means for monitoring the pressure of fluid in the system and comprising a proximal pressure sensor arranged for fluid communication with the system via the conduit, and compensation means for use in compensating for, or reducing the effect of, a flow of liquid through the conduit on an output of the pressure monitoring means, wherein the compensation means comprises the sensing means of the device.

The apparatus or the combination may further comprise a pressure analyser/display unit for displaying the pressure of fluid in the system. The compensation means may be at least partially contained within a casework enclosure of the analyser/display unit. Alternatively, the compensation means may be external to the analyser/display unit.

In another aspect, the invention lies in apparatus comprising a device according to the first aspect of the invention, a fluid delivery conduit, a proximal pressure sensor and compensation means, which includes the sensing means of the device, and is arranged to compensate for the effect of the flow of fluid through the conduit or flow path on the proximal sensor and the display of the compensated proximal pressure sensor output on a pressure analyser/display unit. The compensation means may achieve this by the appropriate addition or subtraction of the information provided by the sensing means.

The invention also extends to an associated method by which the sensing means of a device in accordance with the first aspect of the invention is used to determine the pressure drop along the fluid delivery conduit or flow path, and in which the compensating means uses the pressure drop signal to compensate for errors in the output of the proximal pressure sensor by subtracting or adding the same from/to said output signal in the case of fluid delivery or extraction, respectively.

In a still further aspect of the invention, a kit of parts is provided, comprising a fluid delivery conduit having a distal end for connection with a system for conveying or containing fluid, a proximal pressure sensor for fluid connection to the system by way of the fluid delivery conduit, compensation means for determining a fluid pressure in the system based on an output of the proximal pressure sensor and a pressure drop along a fluid flow path, and a device according to the first aspect of the invention.

The present invention also extends to a method of measuring the pressure in a system by means of a proximal pressure sensor in fluid communication with the system via a fluid flow path that includes a fluid delivery conduit, the method comprising determining a pressure drop along said flow path by means of a device in accordance with the first aspect of the invention deployed in said conduit and compensating for the same by subtracting or adding said pressure drop from/to the output of the proximal pressure sensor. The compensation may be performed by a compensation means operating in conjunction with a pressure analyser/display unit.

In a further aspect of the invention, there is provided a method for compensating for the effect of fluid flow in a fluid delivery conduit on a measurement of fluid pressure in a system for conveying or containing fluid, wherein the system is fluidly connected to a distal end of the fluid delivery conduit and wherein the measurement of fluid pressure is taken at a position proximal to the fluid delivery conduit. The method comprises measuring at least one parameter associated with the fluid flow using at least one sensor disposed within the fluid flow in the fluid delivery conduit, determining a pressure drop compensation value from the at least one measured parameter associated with the fluid flow, and applying the pressure drop compensation value to the measurement of fluid pressure.

Preferably, the or each sensor is provided on an insert disposed in the fluid deliver}' conduit, and the method may further comprise inserting the insert in the fluid delivery conduit.

In another aspect of the present invention, there is provided a method for simultaneous determination of blood pressure at a site in a patient and delivery or extraction of fluid at the site. The method comprises guiding a distal end of a catheter to the site, the catheter having a lumen for fluid flow during delivery or extraction of fluid, inserting an insert in the lumen, the insert comprising sensing means for measuring at least one parameter associated with the fluid flow within the lumen, applying a fluid flow through the lumen for delivery or extraction of fluid, measuring the pressure of fluid at a proximal position with respect to the catheter, determining a pressure drop compensation value from the at least one measured parameter associated with the fluid flow, and applying the pressure drop compensation value to the measurement of fluid pressure to determine the blood pressure at the site.

In a further aspect of the invention, there is provided an apparatus for use in the simultaneous determination of blood pressure at a site in a patient and delivery or extraction of fluid at the site, comprising a catheter having a lumen for fluid flow during delivery or extraction of fluid, an insert for insertion in the lumen and comprising sensing means for measuring at least one parameter associated with the fluid flow within the lumen, pressure sensing means for measuring the pressure of fluid at a proximal position with respect to the catheter when a distal end of the catheter is positioned at the site, in use, and compensation means for determining a pressure drop compensation value from the at least one measured parameter associated with the fluid flow, and for applying the pressure drop compensation value to the measurement of fluid pressure to obtain a measurement of blood pressure at the site.

The devices, apparatus, combinations and methods of the invention may be suitable for use in cardiac catheterisation procedures, including, but not limited to, angioplasty. Aspects of the invention may be suitable for use in FFR procedures. Aspects of the invention are particularly suitable for use in a method of determining blood pressure and/or blood flow in a coronary artery.

Preferred and/or optional features of each aspect of the invention may be used, alone or in combination, with the other features of the invention also.

Brief Description of the Drawings

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic view of a known pressure wire device.

Figure 2 is a diagrammatic representation of the miniaturised pressure sensors frequently used in known pressure wires.

Figure 3 illustrates the circuit within which the two sensing elements on the miniaturised pressure sensor are connected.

Figure 4 is a diagrammatic view of a device according to the present invention.

Figure 5 is an electrical wiring schematic diagram of the sensors in the device of Figure 4.

Figure 6 shows the device of Figure 4 in use with coronary catheterisation equipment together with compensation means.

Detailed Description of Embodiments of the Invention A known pressure wire device 1, as represented in Figure 1, is typically about 175 cm in length and 360 pm in diameter. It generally consists of several regions providing a combination of different functions and different flexural stiffnesses to facilitate navigation into and around the arterial system. These regions are labeled A to E in Figure 1, and have the following primary purposes: A. Electrical connection rings 2 are provided at the proximal end of the wire. These make electrical connection to, either a cable system that is plugged into a pressure analyser/display unit, or wireless system that provides the same function. B. The longest region, typically occupying over 80% of the length of the pressure wire This region has moderate stiffness since it does not protrude beyond the tip of the guide catheter and so does not need to bend around small radii within the arterial system. This region comprises a stainless steel tube or sheath 3 of about 360 μιη outside diameter and 300 μιη internal diameter. This tube has an external polymer coating 4, and internally, carries three insulated electrical conductors 5 each of about 30 μιη diameter. C. This region is about 30 cm in length and has increased flexibility to facilitate improved navigation. Its outer tube 6 is a medically compliant polyimide material and internally it contains a solid stainless steel support wire 7 of about 100 pm diameter and the same three electrical conductors 5 referred to above. D. This section is just a few mm in length, but contains the pressure sensor 8 which is mounted in, either a formed section of the end of the support wire, or a separate component, shaped in the form of a “boat” 9. This is enclosed in a very short length of tube 10 which protects the sensor, but also contains a small aperture 11 through which the face of the sensor is exposed to the external environment. The three electrical conductors 5 connecting to the sensor pass between the boat 9 and the outer tube 10. E. The extreme distal tip of the pressure wire includes a fine wire 12 of about 80 pm diameter formed into a helix around a central wire 13 of about the same diameter. The tip can be formed by the operator to impart a directional behaviour to the wire for navigational purposes. Furthermore, the wire 12 is made from a material, such as platinum, that is radiopaque so that it is visible under x-ray during an interventional procedure.

The regions B, C, D and E together form an insert portion of the device 1, for insertion into a fluid delivery conduit as will be explained in more detail below.

The miniaturised pressure sensor 8 used in this known device, as illustrated in Figure 2, is generally as described below.

The sensor 8 is manufactured from silicon using chemical etching or machining techniques to form a thin membrane region 14. The membrane is bonded over an evacuated cavity 15 that remains so throughout the lifetime of the device. The vacuum so produced causes the membrane 14 to deflect in response to the pressure difference across its two faces, and hence to also deflect further in response to small changes in the applied external pressure. It also includes two sensing elements. The first element 16 is located on the membrane 14 for sensing deflection of the membrane 14 caused by pressure changes; the second element 17 is located in the surrounding material, and is very similar to the first element 16 so that it responds to environmental changes such as temperature changes in the same way as the first element 16, but is immune to changes in pressure.

It will be understood that the three-wire configuration of the sensor element shown in Figure 3, in which the two sensing elements 16, 17 are connected as shown, is suitable for connection to a Wheatstone bridge circuit in such a way that extraneous changes, such as temperature, are cancelled out.

The pressure wire’s external connecting cable (not shown) includes two further resistors that complete the Wheatstone bridge circuit and hence provides a four-wire configuration at its connector to the associated pressure analyser/display unit. One of the connectors on the cable may also include an EEPROM chip to provide each device with a unique identification number.

Figure 4 provides a schematic illustration of a pressure wire or insert device 100 according to one embodiment of the present invention. The construction of this device follows generally similar principles to that described for the prior art device in Figure 1, and like reference numerals are used for like features. However, in addition to the tip pressure sensor 8, sensing means comprising two additional pressure sensors 18, 19 are disposed along the length of the shaft of the pressure wire in the region corresponding to region B described in Figure 1. The additional sensors are absolute pressure sensors that may be of the similar design and construction as the tip sensor 8 as described above with reference to Figure 3. Similarly, the individual sensing elements 16’, 17’, 16”, 17” shown in Figure 5 correspond directly to those in the tip sensor 8, with the elements 16’ and 17’ being the elements of the sensor 18 and the references 16”, 17” denoting the elements of the sensor 19. However, since the quantity of interest is the difference between the outputs of the two sensors, it will be appreciated that the temperature compensation function of the said second sensor elements 17’, 17” is not necessary since the said first sensing elements 16’, 16” can be connected in order to compensate for each other in this respect.

The difference between the output signals of the two sensors 18, 19 can be used to provide a measure of the pressure drop along a guide catheter, which forms a fluid delivery conduit, and associated flow line when the device 100 is inserted into the guide catheter in a coronary catheterisation procedure. A difference signal can be acquired by means of a first differential amplifier (not shown). The gain of the first amplifier can be arranged so that the difference signal has the same sensitivity (in terms of voltage per unit of pressure) as the output from the proximal pressure sensor. Once scaled appropriately in this way, it can be added or subtracted to/from the output of the proximal sensor, by means of a second differential amplifier, in order to make the appropriate compensation for the pressure drop error caused by the flow of fluid.

In a similar fashion to the tip sensor 8, the two additional sensors 18, 19 are mounted within the stainless steel tube or sheath 3, but face the external environment through corresponding apertures 21, 22 in the stainless steel tube or sheath 3.

During assembly of the pressure wire, the additional sensors 18, 19 are bonded into two locations that are pre-formed into the support wire 7 at specific locations corresponding to the position of the two apertures 21, 22 in the side wall of the tube 3. The electrical conductors 5 are attached to the three devices according to the wiring schematic shown in Figure 5. Finally the assembly built onto the support wire is assembled into the outer sheath tube 3 and locally bonded, with for example a UV curing adhesive, at the specific locations.

The design distance D between the locations of the additional sensors 18, 19 is selected so that they are sufficiently separated for there to be a measureable pressure difference between them when fluid is flowing in the fluid delivery conduit. The inventors have found that a distance D of around 300-500 mm is optimal. The additional pressure sensors are sufficiently in-board of the ends of the pressure wire so that during normal use they are contained within the length of the delivery conduit.

To that end, both of the additional sensors are situated at region B of the wire, with the first additional sensor 18 being at least 10 cm from the tip E of the pressure wire.

The wiring schematic shown in Figure 5 illustrates that all three pressure sensors share common +V and 0V conductors. It will be appreciated that the two additional sensors 18, 19 are wired so as to form a second Wheatstone bridge and hence that the output signals from these two sensors are suitable for direct connection to a differential amplifier. It will also be noted that, in this embodiment, the present invention is a five-wire device, and so five connections rings 2 can be seen in Figure 4.

In an alternative embodiment (not shown), the two discrete additional pressure sensors 18, 19 are replaced by a single differential pressure sensor. The differential pressure sensor is located within the tube 3 at a position between the two apertures 21, 22. The differential pressure sensor does not include a vacuum cavity 15 but instead has a first face arranged to be fluidically coupled to fluid in the fluid delivery conduit at the first aperture position and a second face arranged to be fluidically coupled to fluid in the fluid delivery conduit at the second aperture position, in use.

To achieve fluidic coupling between the sensor faces and the fluid in the fluid delivery conduit, a coupling medium may be present within the tube 3 between the sensor faces and the respective apertures 21, 22, The coupling medium may be fluid from within the fluid delivery conduit or a combination of fluid from within the fluid delivery conduit and air, or alternatively may be a gel-like fluid injected into the cavity of the tube 3 during manufacture.

Figure 6 illustrates the device within the context of the coronary catheterisation environment together with compensation means. In this instance, the device 100 performs the role of a pressure wire, in that it has a pressure sensor 8 located at its tip for measuring the pressure downstream of coronary stenosis. A guide catheter 20 provides access, through an axial port 40, for the pressure wire 100 and other devices into the coronary arteries of the patient’s heart. Through a side port 41, the guide catheter provides access for drug or other liquids to be injected into the patient’s arterial system, for example from an infusion pump 31, a saline bag 44, an auxiliary input 45 and/or a contrast agent syringe 46. Fluid can also be extracted from the patient’s arterial system through the side port 41, for example for sampling blood. The tubing 30 connected to the side port 22 also provides fluid communication, by way of a two-valve manifold 32, to the pressure monitoring means, which in this embodiment is a proximal pressure sensor 23. Liquid delivered to the patient by the apparatus travels along a flow path that includes the sensor 23, the tubing 30 and the guide catheter 20.

The electrical output signals 42, 43 from the distal tip sensor 8 and pressure drop sensing means 18, 19 of the pressure wire 100, together with the electrical output signal 24 from the proximal pressure sensor 23 are connected to a pressure analyser/display unit 26. In addition a signal compensating processor 25 or compensation means combines the pressure drop signal 40 with the proximal pressure signal 24 in order to compensate for the pressure drop along the path from the proximal sensor 23 to the distal end of the guide catheter 20. The compensating processor 25 can either be located within the casework 27 of the display unit 26, or be a discrete modular unit external to the display unit 26.

In use, the catheter 20 is inserted into the patient so that the distal end of the catheter 20 is, in effect, connected to the blood in a coronary artery under investigation. The proximal pressure sensor 23 and saline bag 44 are typically supported on a drip stand (not shown), with the proximal pressure sensor 23 situated at the same elevation as the patient's heart to prevent errors in the measured blood pressure of the patient as a consequence of a difference in hydrostatic pressure between the proximal sensor 23 and the sensor 8 on the tip of the device 100.

It will be appreciated that the valves in the manifold 32 can be set so that the proximal pressure sensor 23 is coupled to the environment in the region of the distal end of the catheter 20 via the liquid in the catheter 20 and in the tubing 30. Since liquids are substantially incompressible, the proximal pressure sensor provides an effective means of measuring aortic pressure while there is no flow of liquid in the catheter 20, since the prevailing conditions at the distal end of the catheter 20 will be replicated at the sensor 23.

In these conditions, the pressure analyser and display 26 can simultaneously show the aortic pressure measured by the proximal pressure sensor 23 and also the pressure measured by the sensor 8 at the tip of the device 100, for example on the other side of a stenosis.

These are real time signals that display pulsatile blood pressure and include details of important clinical significance, such as the artefact known as the "dichrotic notch" associated with the closure of the aortic heart valve.

When it is necessary to flow fluid through the catheter during the procedure, such as during the injection of contrast agent from the syringe 46, operation of the infusion pump 31 or when a vaso-dilator drug such as adenosine is injected into the catheter 20 through the auxiliary inlet 45, the pressure at the proximal pressure sensor 23 no longer reflects the aortic pressure, by virtue of the effect of the flow of liquid in the apparatus. Conventional practice, therefore, would suggest that in these circumstances the proximal pressure sensor 23 should be isolated from fluid in the catheter 20 or at least not used for pressure measurement during fluid flow.

However, the device 100 of the present invention allows continued use of the proximal pressure sensor 23 to monitor aortic pressure when various procedures, involving the flow of liquid along the catheter 20, are being performed. This is particularly beneficial because, potentially, the most important time to be monitoring a patient could be precisely when such a procedure is under way, for example when a drug or other liquid is being infused or injected.

The present inventors have appreciated that, even though there might be flow of liquid along the catheter 20, the oscillating pressure pulses in the blood vessel where the catheter tip is located (such as the aorta) will still be transmitted through the catheter 20. Those pressure signals will be superimposed onto the pressures associated with any flowing liquid, and hence can still be detected by the proximal sensor 23.

The clarity of the superimposed pressure signal may deteriorate if pressures within the liquid in the catheter 20 are chaotic, such as during turbulent flow. However, the delivery of most liquids through typical catheters is generally such that prevailing flow conditions are laminar. In such cases, the pressure pulse signal can be preserved.

Accordingly, in the apparatus of Figure 6, the proximal pressure sensor 23 remains in fluid communication with the catheter 20 even during fluid flow, and the first and second additional sensors 18, 19 of the device 100 are used to determine the pressure drop in the fluid flow path between the proximal pressure sensor 23 and the distal end of the catheter 20.

In particular, in this example, the first and second additional sensors 18, 19 of the device 100 provide a measurement of the pressure drop in the catheter 20 over the distance between the sensors 18, 19. The signal compensating means 25 analyses the output from the additional sensors 18, 19 as to determine the effect of the flow of liquid along the catheter 20 on the relationship between the pressure measured by the proximal pressure sensor 23 and the actual aortic pressure at the distal end of the catheter 20.

An example of the type of analysis that may be conducted by the compensating processor 25 is as follows. At the typical flow rates involved, the flow conditions in the guide catheter 20 are predominantly laminar, in which case the pressure drop along the catheter will be directly proportional to the flow rate, Q, i.e.

(1)

The convention adopted in this document is that Q is positive for liquid being delivered to the patient (so that Q would be negative in the case of liquid sampling from the patient), and hence a positive APcatheter denotes an elevated pressure at the proximal end of the catheter 20.

The pressure detected at the proximal pressure sensor 23 (Pmeasured) will be equal to the sum of the actual aortic blood pressure (BP) plus the pressure drop along the flow path from the proximal pressure sensor 23 to the end of the catheter 20, which is assumed in this example to be equal to the catheter pressure drop, i.e.:

(2)

Hence, in order to communicate the actual aortic pressure (BP) to the analyser/display, the signal compensating processor 25 needs to generate an output signal, as follows:

(3)

To determine the pressure drop along the catheter, APcatheter, the processor 25 first determines the pressure difference APsensor between the first and second additional sensors 18, 19. Assuming laminar flow, both APsensor and APcatheter vary linearly with flowrate, and hence with one another. The relationship between APsensor and APcatheter can be determined empirically or otherwise in advance for the apparatus in use, so that the processor 25 can readily determine APcatheter from the output of the sensors 18, 19. In this way, the output signal BP’ can be generated.

In this example, the compensating processor 25 includes a microprocessor to perform the above analysis. Many types of small processor are suitable, although the inventors have found a PIC microcontroller to be ideal for the purpose.

It will be appreciated that, in addition to the pressure drop that occurs along the catheter 20, pressure drops in other components (such as the side port 22, the tubing 30 and the manifold 32) may contribute to the overall pressure drop along the flow path between the proximal pressure sensor 23 and the distal end of the catheter 20. In such cases, it will be understood that the term APcatheter in the above analysis can be taken to represent the overall pressure drop along the flow path.

In a variant (not shown) of the device, the sensing means comprises a single additional pressure sensor. In this case, the pressure drop between the additional pressure sensor of the device and the proximal pressure sensor can be used to determine the pressure drop along the flow path.

In the above-described embodiments, the device 100 is provided with sensing means in the form of one or more pressure sensors. However, it will be appreciated that other sensors could be used for the sensing means. For example, the sensing means could comprise one or more flow sensors to measure the flow rate in the catheter 20, instead of (or in addition to) the pressure drop. The relationship between the measured flow rate Q and the pressure drop along the flow path APcatheter, in accordance with equation (1), can be empirically determined for the type of catheter in use and stored in the processor.

Further modifications and variations of the above-described embodiments are also possible without departing from the scope of the invention as defined in the appended claims.

Claims

1. A device for use with a fluid delivery apparatus, the fluid delivery apparatus comprising a fluid delivery conduit, a fluid flow path extending at least in part through the fluid delivery conduit and having a distal end for connection with a system for conveying or containing fluid, a proximal pressure sensor for measuring a fluid pressure at a proximal end of the fluid flow path, and compensation means for determining a fluid pressure in the system based on an output of the proximal pressure sensor and a pressure drop along the fluid flow path, the device comprising an elongate insert for insertion into the fluid delivery conduit, the insert comprising sensing means arranged to provide an output for use in determining the pressure drop along the fluid flow path.

2. A device according to claim 1, comprising a pressure wire, pressure catheter or guide wire.

3. A device according to claim 2, in which the fluid delivery apparatus comprises catheterisation apparatus and the fluid delivery conduit comprises a guide catheter, and wherein the insert is arranged to extend through the guide catheter in use.

4. A device according to any preceding claim, wherein the sensing means is disposed at least 100 mm from a distal end of the insert.

5. A device according to any preceding claim, wherein the sensing means comprises one or more flow sensors.

6. A device according to claim 5, wherein the sensing means comprises a thermal flow sensor.

7. A device according to claim 5, wherein the sensing means comprises a Doppler ultrasound flow sensor.

8. A device according to any of claims 1 to 4, wherein the sensing means comprises one or more pressure sensors.

9. A device according to claim 8, wherein the sensing means comprises first and second pressure sensors arranged to measure the pressure of fluid in the fluid delivery conduit at first and second positions respectively, wherein the first and second positions are spaced apart along the insert.

10. A device according to claim 9, wherein the first and second positions are spaced apart by a distance of between 10 mm and 1000 mm

11. A device according to claim 10, wherein the first and second positions are spaced apart by a distance of between 300 mm and 500 mm

12. A device according to claim 8, wherein the sensing means comprises a differential pressure sensor.

13. A device according to claim 12, wherein the differential pressure sensor comprises a common pressure sensitive element having first and second faces arranged to couple, in use, to the fluid in the fluid delivery conduit at first and second positions respectively, wherein the first and second positions are spaced apart along the insert.

14. A device according to claim 13 wherein the first and second faces are arranged to couple to the fluid in the fluid delivery conduit at the first and second positions respectively by way of a coupling medium.

15. A device according to claim 14 wherein the coupling medium comprises fluid from the fluid delivery conduit and air.

16. A device according to claim 14 wherein the coupling medium comprises a gel.

17. A device according to any of claims 8 to 16 wherein the or each pressure sensor employs optical interferometry.

18. A device according to any of claims 8 to 16 wherein the or each pressure sensor comprises a piezo-resistive element.

19. A device according to claim 18 when dependent on any of claims 9 to 11, wherein the piezo-resistive elements of the first and second pressure sensors are connected in a Wheatstone bridge configuration.

20. A device according to any preceding claim, wherein the insert comprises an outer sheath for housing the sensing means.

21. A device according to claim 20, wherein the sheath includes opening means through which, in use, the sensing means are coupled to fluid in the fluid delivery conduit.

22. A device according to claim 21, wherein the sensing means comprises a plurality of sensors, and wherein the opening means comprises a respective aperture for each of the sensors.

23. A device according to any preceding claim, further comprising a tip pressure sensor located adjacent a distal end of the insert.

24. A device according to claim 23 when dependent on any of claims 8 to 19, wherein the tip pressure sensor and the or each pressure sensor of the sensing means share common voltage supply conductors.

25. A device according to any preceding claim, wherein the insert is flexible.

26. In combination, a device according to any preceding claim, and a fluid delivery' apparatus comprising a fluid delivery conduit, a fluid flow path extending at least in part through the fluid delivery conduit and having a distal end for connection with a system for conveying or containing fluid, a proximal pressure sensor for measuring a fluid pressure at a proximal end of the fluid flow path, compensation means for determining a fluid pressure in the system based on an output of the proximal pressure sensor and a pressure drop along the fluid flow path determined from the output of the sensing means.

27. A combination according to claim 26, wherein the combination is for use in cardiac catheterisation procedures.

28. A combination according to claim 27, wherein the combination is for use in FFR procedures.

29. A combination according to claim 27 or claim 28, wherein the combination is for use in a method of determining blood flow in a coronary artery.

30. Fluid delivery apparatus for delivery of fluid to or extraction of fluid from a system for containing or conveying fluid, the apparatus comprising a device according to any of claims 1 to 25, a fluid delivery conduit for connection to the system at a distal end of the conduit, pressure monitoring means for monitoring the pressure of fluid in the system and comprising a proximal pressure sensor arranged for fluid communication with the system via the conduit, and compensation means for use in compensating for, or reducing the effect of, a flow of liquid through the conduit on an output of the pressure monitoring means, wherein the compensation means comprises the sensing means of the device.

31. The apparatus of claim 30 or the combination of any of claims 26 to 29, further comprising a pressure analyser/display unit for displaying the pressure of fluid in the system.

32. The apparatus or combination of claim 31, wherein the compensation means is at least partially contained within a casework enclosure of the analyser/display unit.

33. The apparatus or combination of claim 31, wherein the compensation means is external to the analyser/display unit.

34. A kit of parts comprising: a fluid delivery conduit having a distal end for connection with a system for conveying or containing fluid; a proximal pressure sensor for fluid connection to the system by way of the fluid delivery conduit; compensation means for determining a fluid pressure in the system based on an output of the proximal pressure sensor and a pressure drop along a fluid flow path; and a device according to any of claims 1 to 25.

35. A method of measuring the pressure in a system by means of a proximal pressure sensor in fluid communication with the system via a fluid flow path including a fluid delivery conduit, the method comprising determining a pressure drop along said flow path by means of a device in accordance with any of claims 1 to 25 deployed in said conduit and compensating for the same by subtracting or adding said pressure drop from/to the output of the proximal pressure sensor.

36. A method in accordance with claim 35 in which said compensation is performed by a compensation means operating in conjunction with a pressure analyser/display unit.

37. A method for compensating for the effect of fluid flow in a fluid delivery conduit on a measurement of fluid pressure in a system for conveying or containing fluid, wherein the system is fluidly connected to a distal end of the fluid delivery conduit and wherein the measurement of fluid pressure is taken at a position proximal to the fluid delivery conduit, the method comprising: measuring at least one parameter associated with the fluid flow using at least one sensor disposed within the fluid flow in the fluid delivery conduit; determining a pressure drop compensation value from the at least one measured parameter associated with the fluid flow; and applying the pressure drop compensation value to the measurement of fluid pressure.

38. A method according to Claim 37, wherein the or each sensor is provided on an insert disposed in the fluid delivery conduit.

39. A method according to Claim 38, further comprising inserting the insert in the fluid delivery conduit.

40. A method for simultaneous determination of blood pressure at a site in a patient and delivery or extraction of fluid at the site, the method comprising: guiding a distal end of a catheter to the site, the catheter having a lumen for fluid flow during delivery or extraction of fluid; inserting an insert in the lumen, the insert comprising sensing means for measuring at least one parameter associated with the fluid flow within the lumen; applying a fluid flow through the lumen for delivery or extraction of fluid; measuring the pressure of fluid at a proximal position with respect to the catheter; determining a pressure drop compensation value from the at least one measured parameter associated with the fluid flow; and applying the pressure drop compensation value to the measurement of fluid pressure to determine the blood pressure at the site.

41. Apparatus for use in the simultaneous determination of blood pressure at a site in a patient and delivery or extraction of fluid at the site, comprising: a catheter having a lumen for fluid flow during delivery or extraction of fluid; an insert for insertion in the lumen and comprising sensing means for measuring at least one parameter associated with the fluid flow within the lumen; pressure sensing means for measuring the pressure of fluid at a proximal position with respect to the catheter when a distal end of the catheter is positioned at the site, in use; and compensation means for determining a pressure drop compensation value from the at least one measured parameter associated with the fluid flow, and for applying the pressure drop compensation value to the measurement of fluid pressure to obtain a measurement of blood pressure at the site.