Patent Application
20240167903
The present disclosure relates to measurement of fluid pressure within flexible tubing. In one aspect, the present disclosure relates to a magnetic system for measuring fluid pressure within tubing of an administration set connected to an infusion pump.
Programmable infusion pumps are used to carry out controlled delivery of liquid food for enteral feeding and medications for various purposes, for example pain management. In a common arrangement, an infusion pump receives a disposable administration set comprising a cassette removably received by the pump, and flexible tubing connected to the cassette for providing a fluid delivery path through the pump. In some cases, the cassette is omitted and the tubing is loaded in the pump without a cassette. The administration set may include a pumping segment of tubing acted upon by a pumping mechanism of the pump to force fluid through the tubing to a patient.
Infusion pumps of the type described above may include one or more pressure sensors arranged to measure fluid pressure within the tubing of the administration set. Pressure sensing is an important safety feature because an unexpected variation in fluid pressure may indicate an unsafe condition, such as an occlusion within the tubing that is blocking delivery of food or medication to the patient. In a common arrangement, a pump may have an upstream pressure sensor situated upstream from the pumping mechanism and a downstream pressure sensor situated downstream from the pumping mechanism. If an occlusion occurs at a location upstream from the pressure sensor, a vacuum condition is created and causes radial contraction of the tubing at the sensor location. Conversely, if an occlusion occurs at a location downstream from the pressure sensor, fluid pressure builds and causes radial expansion of the tubing at the sensor location. Various types of pressure sensors are known for measuring pressure by detecting radial contraction and radial expansion of the tubing using a variety of transducer technologies, including optical, magnetic, ultrasonic, and load cell transducers.
With specific regard to magnetic pressure sensors, it is known to provide a magnet arranged to move in response to contraction and expansion of the tubing, and a corresponding Hall effect sensor arranged to generate an output voltage signal proportional to the strength of the magnetic field of the magnet. As the magnet moves closer to the Hall effect sensor, the voltage signal increases, and as the magnet moves away from the Hall effect sensor, the voltage signal decreases. Magnetic pressure sensors of the type described above are economical to manufacture and incorporate into an infusion pump.
However, tubing radial expansion at pressures safe for a patient is also small. This small tubing expansion causes a correspondingly small magnetic field change reported by the Hall sensor. Additionally, the magnetic field generated by a permanent magnet will vary due to changes in temperature. For example, typical sintered NdFeB magnets have temperature coefficients of Br (α)−0.09˜−0.12%/° C. (this value will be specific to each type of magnetic material). Although the variance due to a change in temperature is small, it can be large enough to cause pressure measurement errors unacceptable for use with infusion pumps. Measuring temperature and applying temperature compensation is a method that can be considered to mitigate such errors. However, measuring a temperature directly on a magnet is impractical, and remote measurement will not eliminate temperature errors completely. Inaccurate measurements may lead to false occlusion alarms that are disruptive to the patient's infusion protocol and to the medical staff. Inaccurate pressure measurements may also result in a missed occlusion alarm when an occlusion is actually present, a situation that may have serious safety consequences for the patient.
Another issue with the simple magnetic pressure sensor described above is that magnetic field strength decreases as the inverse square of the distance (1/r2). This nonlinear relationship results in the need for additional calculation in order to linearize the sensor output.
What is needed is a magnetic pressure sensor system for measuring fluid pressure within flexible tubing that has improved temperature compensation and better linearity compared to the existing sensors.
The present disclosure relates to an apparatus and method for determining a fluid pressure within tubing such as flexible tubing loaded in an infusion pump. For example, determining fluid pressure may allow for detection of occlusions blocking flow through the tubing during delivery of a solution to a patient. More particularly, the present disclosure relates to a fluid pressure sensor wherein a location of a magnet relative to a multidimensional magnetic sensor varies as a result of tube expansion and contraction associated with fluid pressure changes, and output signal information generated by the multidimensional magnetic sensor is used to determine a direction of a magnetic field vector (magnetic field angle) across the multidimensional magnetic sensor corresponding to the fluid pressure. The magnetic field angle is used as an indicator of the fluid pressure. The multidimensional magnetic sensor may be configured to measure magnetic field projections on at least two perpendicular planes.
In some embodiments, an infusion pump includes a pressure sensor as summarized above for measuring fluid pressure within tubing loaded in the infusion pump. The magnet of the pressure sensor has a magnetic field and may be arranged to move relative to the multidimensional magnetic sensor in response to radial contraction and radial expansion of the tubing. The multidimensional magnetic sensor is positioned such that changes in a location of the magnet relative to the multidimensional magnetic sensor provide corresponding changes in the magnetic field strengths detected by the multidimensional magnetic sensor along at least two dimensions. The corresponding field strengths enable determination of the magnetic field angle as an indicator of the fluid pressure.
The relative movement between the magnet and the multidimensional magnetic sensor may be generally linear along a movement axis. For example, the magnet may be affixed to a distal end of a deflectable holder, and a proximal end of the holder may be fixed relative to the multidimensional magnetic sensor, such that movement of the magnet caused by expansion and contraction of the tubing is generally linear along the movement axis. The distal end of the holder carrying the magnet may be resiliently deflectable relative to the proximal end of the holder, and may be spring biased toward engagement with the tubing when the tubing is connected to the infusion pump. In another embodiment, the holder may be arranged such that expansion and contraction of the tubing deflects the holder at a location along the holder between the proximal and distal ends of the holder, such that the distal end of the holder carrying the magnet undergoes an enlarged displacement greater than a radial expansion or radial contraction of the tubing.
The infusion pump may further include a printed circuit board, and the multidimensional magnetic sensor of the first pressure sensor may be mounted to the printed circuit board. The infusion pump may include a processor in electronic communication with the multidimensional magnetic sensor. The processor may be configured to determine a fluid pressure within the tubing based on the output signal from the multidimensional magnetic sensor. The infusion pump may include an alarm configured to generate an alarm signal if the fluid pressure within the tubing exceeds a predetermined threshold.
The infusion pump may have two pressure sensors for measuring fluid pressure within the tubing at a location along the tubing upstream from a pumping mechanism of the infusion pump and at a location along the tubing downstream from the pumping mechanism, respectively.
In another aspect, a method of measuring fluid pressure within flexible tubing of is provided. The method includes providing a multidimensional magnetic sensor and a magnet having a magnetic field, wherein the multidimensional magnetic sensor and the magnet are arranged such that a location of the magnet relative to the multidimensional magnetic sensor varies as a function of the fluid pressure within the flexible tubing, and a direction of a magnetic field vector of the magnetic field at the multidimensional magnetic sensor is dependent upon the location of the magnet relative to the multidimensional magnetic sensor. The method further includes detecting by the multidimensional magnetic sensor at least two magnetic field strengths respectively corresponding to strength of the magnetic field projected along each of at least two dimensions, and determining the direction of the magnetic field vector of the magnetic field at the multidimensional magnetic sensor based on the detected at least two magnetic field strengths. The determined direction of the magnetic field vector of the magnetic field at the multidimensional magnetic sensor is indicative of the fluid pressure within the tubing.
For a fuller understanding of the nature and objects of the disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which:
The present disclosure may be embodied in a pressure sensor for measurement of fluid pressure with flexible tubing. The present disclosure may be embodied in an infusion pump operable to pump fluid through flexible tubing connected to the infusion pump, and in other applications where measurement of fluid pressure with flexible tubing is desired.
The multidimensional magnetic sensor 46 is configured to detect the magnetic field strength of magnet 44 projected along at least two dimensions, for instance two orthogonal spatial dimensions. For example, multidimensional magnetic sensor 46 may be embodied as a two-dimensional (“2D”) magnetic sensor or a three-dimensional (“3D”) magnetic sensor. In some embodiments, the multidimensional magnetic sensor 46 may be two or more magnetic sensors, each arranged to measure a magnetic field strength along a different dimension from the other magnetic sensor(s) (e.g., the respective magnetic sensors may detect magnetic field strengths projected along orthogonal spatial dimensions).
In accordance with the present disclosure, multidimensional magnetic sensor 46 and magnet 44 are arranged such that the magnetic field of magnet 44 impinges on multidimensional magnetic sensor 46, and a direction of a magnetic field vector of the magnetic field at the multidimensional magnetic sensor 46 is dependent upon the location of magnet 44 relative to multidimensional magnetic sensor 46. In a departure from known magnetic pressure sensing systems, the present disclosure uses a direction of the magnetic field vector, not the strength of the magnetic field, as a correlate of the fluid pressure. This has the benefit of eliminating the need for a temperature compensation mechanism. Each dimensional magnetic strength component used to determine the direction of the magnetic field vector is affected by temperature according to the same coefficient associated with the material used for the magnet. Therefore, the temperature effects on each dimensional component cancel one another, and the direction of the magnetic field vector (also referred to as the “magnetic field angle”) is not changed by the temperature of the magnet.
The multidimensional magnetic sensor 46 may be positioned such that changes in a relative location of the magnet 44 provide corresponding changes in the magnetic field strengths detected by the multidimensional magnetic sensor 46 along at least two dimensions, and a direction of a magnetic field vector of the magnetic field at the multidimensional magnetic sensor 46 is dependent upon the location of the magnet 44 relative to the multidimensional magnetic sensor 46. For example, the multidimensional magnetic sensor 46 may be positioned so that the movement axis 45 of the magnet 44 does not cross a center of the multidimensional magnetic sensor 46 (i.e., the multidimensional magnetic sensor 46 is not centered on the movement axis 45). For example, the multidimensional magnetic sensor 46 may be considered to not be centered on the movement axis 45 when a measurement axis of the magnetic sensor (i.e., an axis along one of the measurement dimensions) is not coaxial with the movement axis 45 of magnet 44. In such an arrangement, the magnetic field of magnet 44 impinges on the multidimensional magnetic sensor 46 at a magnetic field angle corresponding to a direction of the magnetic field vector. The magnetic field angle, which changes according to the location of magnet 44 relative to multidimensional magnetic sensor 46, may be determined by, for example, measuring two perpendicular magnetic field strength projections using the multidimensional magnetic sensor 46.
As illustrated in
The fluid pressure may be calculated based on the magnetic field angle α. For example, the fluid pressure may be calculated according to:
P=K1α+K2 (2)
wherein K1 and K2 are constants determined by calibration measurements using known fluid pressures.
In the illustrated embodiment, pump 10 is a rotary peristaltic pump having a motor-driven rotor 30 acting as a pumping mechanism, wherein pumping segment 28 is wrapped around rotor 30 and is engaged by angularly spaced rollers on rotor 30 as the rotor rotates to provide peristaltic pumping action forcing liquid through the tubing of administration set 12. As may be understood by reference to
The infusion pump includes a downstream pressure sensor 34 for measuring fluid pressure within the tubing (for example, within the pumping segment 28 of the tubing). Pressure sensor 34 is shown in detail in
Alternatively, the multidimensional magnetic sensor 46 may be arranged to move relative to the magnet 44 in response to radial contraction and expansion of tubing 28.
In a departure from the arrangement illustrated in
With regard to detection of the magnetic field and measurement of fluid pressure using magnetic field angle α, pressure sensor 34 of infusion pump 10 functions in the same way as pressure sensor 134 described above.
In some embodiments, the infusion pump 10 includes a second pressure sensor 32. Pressure sensor 32 may be arranged as an upstream pressure sensor (i.e., on an “upstream” or “inflow” side of the pumping mechanism 30), and pressure sensor 34 may be arranged as a downstream pressure sensor (i.e., on a “downstream” or “outflow” side of the pumping mechanism). Upstream pressure sensor 32 may be substantially the same as downstream pressure sensor 34. For example, the second pressure sensor may include a multidimensional magnetic sensor and a magnet arranged to move relative to the multidimensional magnetic sensor in response to radial contraction and expansion of the tubing, wherein the multidimensional magnetic sensor is configured to detect a magnetic field along at least two dimensions.
In some embodiments, the infusion pump may have an alarm (i.e., alarm circuit) configured to generate an alarm signal if the fluid pressure meets and/or exceeds a predetermined threshold. The alarm signal may be an audible alarm (buzzer, speaker, horn, etc.), a visible alarm (e.g., strobe, indicator light, flag, etc.), an electronic alarm signal (e.g., a digital alarm flag, alarm sequence, etc.), or any other alarm signal suitable to a particular application.
In some embodiments, the infusion pump may further comprise a processor, for example on printed circuit board 40, in electronic communication with each multidimensional magnetic sensor 46. The processor may be configured to determine fluid pressures based on the output signals from each multidimensional magnetic sensor 46.
The processor may be any suitable processing device or devices made up of one or more integrated circuits, one or more circuits made up of discrete components, or combinations of these. The processor may be configured to run and/or execute a set of instructions or code. For example, the processor can be a general purpose processor, a central processing unit (CPU), an accelerated processing unit (APU), a field-programmable gate array (FPGA), an application specific integrated circuit (ASIC), a digital signal processor (DSP), and/or the like. In some instances, the processor includes one or more modules and/or components. Each module/component executed by the processor can be any combination of hardware-based module/component (e.g., a field-programmable gate array (FPGA), an application specific integrated circuit (ASIC), a digital signal processor (DSP), etc.), software-based module (e.g., a module of computer code stored in the memory and/or in the database, and/or executed at the processor, etc.), and/or a combination of hardware- and software-based modules. Each module/component executed by the processor is capable of performing one or more specific functions/operations as described herein. In some instances, the modules/components included and executed in the processor can be, for example, a process, application, virtual machine, and/or some other hardware or software module/component. The processor can be any suitable processor (or more than one processor) configured to run and/or execute those modules/components.
The processor may be in communication with and/or include a memory. The memory can be, for example, a random-access memory (RAM) (e.g., a dynamic RAM, a static RAM), a flash memory, a removable memory, and/or so forth. In some instances, instructions associated with performing the operations described herein (e.g., calculating magnetic field angle and fluid pressure) can be stored within the memory and/or a storage medium (which, in some embodiments, includes a database in which the instructions are stored) and the instructions are executed at the processor.
With reference to
Although the present disclosure has been described with respect to one or more particular embodiments, it will be understood that other embodiments of the present disclosure may be made without departing from the scope of the present disclosure.
