Drift mitigation for cardiac pressure measurement implant

Abstract

A method includes receiving, from a pressure-sensing implant (24), digital values that are calculated by circuitry of the implant and are indicative of measurements of ambient pressure sensed in the implant. A rate, at which the digital values drift over time in representing the ambient pressure, is estimated. Drifting of the digital values is compensated for based on the estimated rate.

Description

[0001] DRIFT MITIGATION FOR CARDIAC PRESSURE MEASUREMENT IMPLANT CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application 63 / 745,333, filed lanuary 15, 2025, whose disclosure is incorporated herein by reference.

[0002] FIELD OF THE INVENTION

[0003] The present invention relates generally to medical devices, and particularly to methods and systems for calibration of cardiac pressure measurement implants.

[0004] BACKGROUND OF THE INVENTION

[0005] The use of implants that measure cardiac blood pressure is highly effective in treating and managing patients that suffer from various cardiological conditions. For example, Vectorious Medical Technologies (Tel Aviv, Israel) produces a Left Atrial Pressure (LAP) measurement system referred to as V-LAP, comprising a cardiac implant.

[0006] U.S. Patent 10,105,103, entitled "Remotely Powered Sensory Implant", describes an implant including an antenna and circuitry. The antenna is configured to communicate with an external unit using inductive coupling of a magnetic field. The circuitry is configured to receive electrical energy from the magnetic field via the antenna for powering the circuitry, to receive measurements commands from the external unit, to produce data for transmission to the external unit, and to modulate a load impedance applied to the antenna as a function of the data so as to transmit the data.

[0007] U.S. Patent 10,687,716, entitled "Drift Compensation for Implanted Capacitive Based Pressure Transducer", describes an apparatus including a pressure sensor and calibration circuitry. The pressure sensor is configured to be fitted in a living organ in which an ambient pressure varies as a function of time, and to sense the ambient pressure using a capacitance that varies in response to the ambient pressure, so as to produce a time-varying waveform. The calibration circuitry is configured to apply to the pressure sensor a calibration voltage that modifies the capacitance and thus the time-varying waveform, to process the time-varying waveform so as to isolate and measure a contribution of the calibration voltage to the waveform, and to calibrate a dependence of the capacitance on the ambient pressure using the measured contribution of the calibration voltage.

[0008] U.S. Patent 11,206,988, entitled "Power-Efficient Pressure-Sensor Implant" describesan apparatus including a sensor, configured to vary a capacitance of the sensor in response to a parameter. The apparatus further includes (a) conversion circuitry, configured to convert an input capacitance into an output that is indicative of the input capacitance, (b) a set of calibrationcapacitors, (c) a first switching unit having multiple first-switching-unit settings that (i) connect respective subsets of the calibration capacitors to the conversion circuitry, and further (ii) have respective first-switching-unit effects on the output, and (d) a second switching unit connected to the sensor and having multiple second-switching-unit settings that (i) connect the sensor to the conversion circuitry, and further (ii) have respective second-switching-unit effects on the output that are the same as the first-switching-unit effects.

[0009] SUMMARY

[0010] An embodiment of the present invention that is described herein provides a method including receiving, from a pressure-sensing implant, digital values that are calculated by circuitry of the implant and are indicative of measurements of ambient pressure sensed in the implant. A rate, at which the digital values drift over time in representing the ambient pressure, is estimated. Drifting of the digital values is compensated for based on the estimated rate.

[0011] In some embodiments, estimating the rate is performed, at least in part, while the implant is operating in a calibration setup prior to implantation. Estimating the rate may include compensating for errors in pressure and / or temperature measurement errors of the calibration setup.

[0012] Additionally or alternatively, estimating the rate is performed, at least in part, while the implant is operating in a body of a subject. In some embodiments the rate is re-estimated, in the body of the subject, in daily intervals. In some embodiments, compensating for the drifting is performed, at least in part, while the implant is operating in a body of a subject.

[0013] In a disclosed embodiment, estimating the rate includes calculating multiple rate coefficients for respective temperature-pressure combinations, and compensating for the drifting includes (i) determining a rate coefficient based on a pressure and a temperature sensed by the implant, and (ii) correcting the current pressure sensed by the implant using the rate coefficient.

[0014] In some embodiments, the measurements of the ambient pressure are capacitance measurements of a capacitive pressure sensor in the implant, the method further includes receiving from the implant reference digital values that are calculated by the circuitry of the implant and are indicative of capacitance measurements of one or more reference channels in the implant, and estimating the rate is based on both the digital values and the reference digital values. In an example embodiment, one of the reference channels includes a reference capacitor. In an example embodiment, one of the reference channels includes a selectable bank of internal capacitors.In some embodiments, estimating the rate includes calculating calibration coefficients separately in two or more temperature sub-ranges. Additionally or alternatively, estimating the rate may include calculating calibration coefficients separately in two or more pressure subranges.

[0015] There is additionally provided, in accordance with an embodiment that is described herein, an apparatus including a memory and one or more processors. The memory is configured to store digital values received from a pressure-sensing implant, the digital values having been calculated by circuitry of the implant and are indicative of measurements of ambient pressure sensed in the implant. The one or more processors are configured to estimate a rate at which the digital values drift over time in representing the ambient pressure, and to compensate for drifting of the digital values based on the estimated rate.

[0016] The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Fig. l isa schematic illustration of a system for intracardiac blood pressure measurement using a cardiac implant, in accordance with an embodiment of the present invention;

[0019] Fig. 2 is a block diagram showing circuitry contained inside the implant of Fig. 1, in accordance with an embodiment of the present invention;

[0020] Fig. 3 is a block diagram that schematically illustrates a drift calibration system, in accordance with an embodiment of the present invention;

[0021] Fig. 4 is a flow chart that schematically illustrates a method for drift calibration, in accordance with an embodiment of the present invention; and

[0022] Fig. 5 is a flow chart that schematically illustrates a method for estimating an actual pressure applied to the cardiac implant, for use in drift calibration, in accordance with an embodiment of the present invention.

[0023] DETAILED DESCRIPTION OF EMBODIMENTS

[0024] OVERVIEW

[0025] Embodiments of the present invention that are described herein provide improved techniques for calibration of drift in cardiac pressure-sensing implants.

[0026] In some embodiments, a cardiac implant comprises a capacitive pressure sensor that senses the ambient blood pressure in the heart of a subject, e.g., in the left atrium. The implant further comprises an Application-Specific Integrated Circuit (ASIC) whose circuitry, amongother tasks, converts the capacitance of the sensor into a pressure-indicative digital value. For example, the circuitry may comprise a capacitance-to-frequency (c-to-f) converter and a Logic Processing Unit (LPU). The c-to-f converter generates a signal whose frequency depends on the capacitance of the sensor. The LPU, possibly among other tasks, counts the number of cycles of the signal over a defined time period. The resulting pressure-indicative digital value, denoted "COUNT", is indicative of the capacitance of the sensor and thus of the ambient blood pressure.

[0027] In some embodiments, the implant further comprises one or more reference channels in addition to the capacitive pressure sensor. One reference channel may comprise a reference capacitor, e.g., a capacitor that is similar in structure to the pressure sensor but is not sensitive to the ambient blood pressure. In an alternative embodiment, the reference capacitor may comprise a capacitor component having very small temperature sensitivity. The reference capacitor should typically be located adjacent to the pressure sensor, so that both will share similar physical conditions (e.g., temperature, pressure and stray capacitance). Typically, the electronic properties of the reference capacitor are similar to those of the pressure sensor at a certain temperature and pressure. Another reference channel may comprise a bank of high-accuracy capacitors internal to the ASIC (referred to as "leaps").

[0028] Any of the reference channels can be switched to the input of the c-to-f converter instead of the capacitive pressure sensor, in which case the COUNT values generated by the LPU are referred to as reference digital values. The description that follows will sometimes refer to a "real channel" (the channel comprising the pressure sensor, which yields the pressure-indicative digital values) and to one or more reference channels (which yield the reference digital values).

[0029] The implant transmits both the pressure-indicative digital values and the reference digital values (referred to collectively as "digital values") to an external unit. One or more processors (e.g., in the external unit, in a production-line tester, in a cloud-based server and / or in a mobile device) use the pressure-indicative digital values and the reference digital values to estimate the ambient blood pressure.

[0030] In practice, however, the relationship between the actual ambient blood pressure and the resulting pressure-indicative digital value (COUNT value) may drift over time. Such a drift may be caused, for example, by variations in hardware properties, e.g., in the pressure sensor, reference channels, c-to-f converter and / or LPU. Since the drift continues over time in the months and years after implantation, it may considerably degrade the accuracy of the blood pressure measurements. Unless accounted for, drift may be the bottleneck that limits the useful lifetime of the implant.In some embodiments, the one or more processors carry out novel calibration processes that estimate and compensate for the drift. These processes are described in detail below. For the sake of clarity, the description herein often refers to the disclosed techniques as being carried out by "a processor". In various embodiments, the disclosed techniques can be carried out by one or more processors, e.g., in the implant, in the external unit, in a mobile device, in a cloud server, in a production line tester or calibration device, or in any other suitable system. When the disclosed techniques are carried out by two or more processors, the process can be divided between the processors in any suitable manner.

[0031] Calibration processes described herein are typically performed throughout the implant's life cycle, before and after implantation in a living subject. The disclosed calibration processes estimate and compensate for drifts in the pressure-indicative digital values generated by the implant, from the production stage until years after implantation. As such, the disclosed techniques enhance the accuracy of cardiac pressure measurements, and also increase the useful lifetime of cardiac implants.

[0032] SYSTEM DESCRIPTION

[0033] Fig. 1 is a schematic illustration of a system 20 for intracardiac blood pressure measurement, in accordance with an embodiment of the present invention. System 20 comprises an implant 24, an external unit 32 and a cloud-based server 33. System 20 is used for measuring Left Atrial Pressure (LAP) in the left atrium of a heart 28 of a patient 30.

[0034] Implant 24 is implanted in heart 28 of subject 30, e.g., on the septum between the left and right atria. The terms "subject" and "patient" are used interchangeably herein. External unit 32 uses magnetic field induction to supply electrical power to implant 24, and to receive signals indicative of blood pressure measurements and other information from the implant. The external unit also uses the magnetic field to transmit commands to the implant, e.g., commands to switch between measurement channels. External unit 32 transmits the measurements to server 33 for processing. Systems of this sort are described, for example, in U.S. Patents 10,105,103, 10,687,716 and 11,206,988, cited above.

[0035] Fig. 2 is a block diagram showing circuitry contained inside implant 24, in accordance with an embodiment of the present invention.

[0036] Implant 24 comprises an antenna 34, configured to (i) provide power supply to the implant circuitry by drawing energy from the magnetic field generated by external unit 32, (ii) receive commands that are modulated onto the magnetic field by external unit 32, and (iii)transmit data (e.g., pressure measurements) from implant 24 to external using 32 using load modulation.

[0037] For measuring blood pressure, implant 24 comprises a capacitive pressure sensor 22, configured to vary its capacitance in response to the ambient pressure within the heart of subject 30 (i.e., the subject's intracardiac pressure). Sensor 22 typically comprises a Micro-Electro-Mechanical Systems (MEMS) sensor.

[0038] Implant 24 further comprises a reference capacitor 26 used for calibration purposes. Reference capacitor 26 is typically insensitive to the ambient pressure. In some embodiments, reference capacitor 26 is of a similar (or identical) design as pressure sensor 22, with the exception that the reference capacitor is not sensitive to the actual blood pressure. In other embodiments, reference capacitor 26 differs in design from pressure sensor 22, but has similar (or identical) electrical properties at a certain temperature-pressure operating point (e.g., room temperature and barometric pressure). In yet other embodiments, reference capacitor 26 is a component capacitor having low temperature sensitivity (although other components of the reference channel may be temperature sensitive). The reference capacitor may be exposed to the ambient blood pressure, but since it is not sensitive to pressure, its capacitance does not vary with LAP.

[0039] In addition, the ASIC in implant 24 comprises a bank of high-accuracy Internal Capacitors (leaps) 76. A switching unit 74 is configured to select a combination of one or more of leaps 76. The selected leaps are connected in parallel to one another, such that the total capacitance is equal to the sum of the capacitances of the leaps in the chosen combination. leaps 76 are also used for calibration, as will be described below. In an example embodiment, a set of 127 different capacitances (denoted leap 1 -leap 127) can be chosen by switching unit 74. Icapl denotes the smallest capacitance in the set. leap 127 denotes the largest capacitance in the set.

[0040] Implant 24 further comprises a capacitance-to-frequency (c-to-f) converter 44. Converter 44 comprises an oscillator whose oscillation frequency depends on the capacitance that is input to the converter. Thus, converter 44 outputs a "sensor clock out" signal whose frequency is a function of the input capacitance. In other words, converter 44 converts the input capacitance into an output frequency.

[0041] A selector switch 72 selects the input capacitance that is applied to the input of c-to-f converter 44. Switch 72 selects between (i) pressure sensor 22, (ii) reference capacitor 26 and (iii) the combination of leaps 76 chosen by switching unit 74.

[0042] A Logic Processing Unit (LPU) 40 performs various processing tasks of implant 24. Among other tasks, LPU 40 receives the "sensor clock out" signal from c-to-f converter 44, andcounts the number of cycles over a predefined time period. The resulting integer number, referred to as "COUNT", is indicative of the capacitance applied to the input of c-to-f converter 44. LPU 40 also controls selector switch 72 and switching unit 74, i.e., (i) chooses which channel is applied to the input of c-to-f converter 44, and (ii) when the chosen channel is that of leaps 76, sets the combination of leaps 76 that will be measured.

[0043] Implant 24 transmits the COUNT values to external unit 32 by modulating the load impedance of antenna 34. In the example of Fig. 2, LPU 40 outputs a serial digital word representing the COUNT value on a serial line 50. The bits of the digital word drive a buffer 52, whose output controls a modulation switch 42.

[0044] When modulation switch 42 is closed, the load on antenna 34 is increased. When modulation switch 42 is open, the load on antenna 34 is decreased. The modulation in the load of the antenna causes variations in the amount of energy consumed by the implant from the magnetic field. External unit 32 senses these variations, and reconstructs the COUNT value transmitted by the implant. The value of COUNT is in turn indicative of the capacitance connected to the input of c-to-f converter 44 (the capacitance of sensor 22, of reference capacitor 26 or of the chosen combination of leaps 76).

[0045] As noted above, the magnetic field generated by external unit 32 is also used for powering implant 24. The voltage induced in antenna 34 by the magnetic field is referred to in the figure as a "main supply voltage". A voltage regulator 46 converts a high voltage supply, which is derived from the main supply voltage, into a direct current (DC) sensor-supply voltage, which supplies sensor 22, capacitor 26 and leaps 76. LPU 40 may operate on a DC voltage Vcc that is supplied by a Low Dropoff Oscillator (LDO) 38, which rectifies and regulates the main supply voltage.

[0046] In some embodiments, implant 24 further comprises a voltage clamper 51, which reduces variations in the main supply voltage that are caused by variations in the amount of current being drawn. The voltage clamper may comprise, for example, a Zener diode.

[0047] A buffer 56 derives a "main clock / data in" signal from the raw signal received from the external unit. The "main clock / data in" signal provides a clock signal to LPU 40, and further communicates data from the external unit. For example, via the "data in" signal, the external unit may request particular information from the LPU, which the LPU then provides, e.g., by selecting the appropriate input to capacitance-to-frequency converter 44, and then modulating the load of the antenna in response to the "sensor clock out" signal, as described above.DRIFT AND DRIFT CALIBRATION

[0048] To measure the blood pressure sensed by sensor 22, LPU 40 of implant 24 counts the number of cycles ("COUNT") of the signal generated by c-to-f converter 44 over a defined time period. In some embodiments, LPU 40 derives the time period from the frequency of the external AC magnetic field generated by external unit 32. Since the magnetic field frequency is independent of the implant's temperature and ambient pressure, the derived time period is independent of temperature and pressure, as well.

[0049] In an example embodiment, the magnetic field frequency is 6.78 MHz. LPU 40 divides the cycle period of this frequency by 216, to produce a fixed (pressure and temperature insensitive) time period of 9.6662 mS. To perform a COUNT measurement, LPU 40 counts the number of c-to-f cycles (the number of cycles of the signal produced by c-to-f converter 44) over this 9.6662 mS time period.

[0050] A typical range of COUNT values for pressure sensor 22, across the relevant temperature and pressure ranges, is between 75,000-100,000. A typical range of COUNT values for reference capacitor 26 is between 75,000-85,000. For leaps 76, the COUNT values may be lower, e.g., down to 40,000.

[0051] All the numerical values listed above are in no way limiting, and they are given purely by way of example. Alternatively, any other suitable signal frequency, counting period and COUNT value ranges can be used.

[0052] In an example pressure measurement flow, LPU 40 sets switch 72 to connect the real channel to the input of c-to-f converter 44, and then counts a COUNT value that is indicative of the pressure sensed by sensor 22. Then, LPU 40 sets switch 72 to connect the channel of reference capacitor 26 to the input of c-to-f converter 44, and counts a COUNT value that is indicative of the capacitance of capacitor 26. Then, LPU 40 sets switch 72 to connect the channel of switching unit 74 to the input of c-to-f converter 44, sets switching unit 74 to select any desired number of leaps 76, and counts the COUNT values that are indicative of the capacitances of the desired leaps. The various COUNT values are transmitted to the external unit, and the processor uses them to calculate the ambient blood pressure.

[0053] As noted above, the relationship between the actual ambient blood pressure in the heart and the COUNT value measured by LPU 40 may drift over time. The rate of drift may vary for different temperatures, for different pressures, between different measurement channels, as well as from one implant to another. In various embodiments, various methods are used forestimating and compensating for such drifts. Example drift calibration methods and systems are described below.

[0054] Drift estimation following production., before and after implantation

[0055] In some embodiments of the present invention, implants 24 undergo an extensive calibration process following production, e.g., over a calibration period of a month or several months. During this period, a batch of implants is placed in a calibration chamber and subjected to controlled temperature and pressure conditions. For each implant 24, a processor records the COUNT values for the real channel and for the reference channels of the reference capacitor and one or more of the leaps (i.e., the pressure-indicative digital values and the reference digital values) over time, for various temperature and pressure operating points.

[0056] A calibration system may use the information recorded during this process to estimate the drift per implant.

[0057] Fig. 3 is a block diagram that schematically illustrates a drift calibration system 80, in accordance with an embodiment of the present invention. System 80 comprises an interface (I / F) 84, a processor 88 and a memory 92.

[0058] Interface 84 receives COUNT values that are generated by a given implant 24 during the calibration period. In an example embodiment, the following COUNT values are received and recorded as a function of time, per {temperature, pressure} operating point:

[0059] ■ COUNT (real) - Pressure-indicative digital values for the real channel that includes pressure sensor 22.

[0060] ■ COUNT(ref) - Reference digital values for the reference channel that includes reference capacitor 26.

[0061] ■ One or more COUNT (leap) - Reference digital values for the reference channel that includes Internal Capacitors (leaps) 76. COUNT(Icap) may be recorded for a single selected combination of leaps, or for multiple (e.g., all) combinations of leaps. In an example embodiment, three combinations are recorded, one of which being leap 127 (the combination having the maximal capacitance).

[0062] Processor 88 calculates drift rates (also referred to as rate coefficients, drift coefficients or slope coefficients) based on the COUNT values. Memory 92 is used for storing the COUNT values, the drift rates and / or any other relevant information.

[0063] A given drift coefficient corresponds to a specific measurement channel ("real", "ref1or a certain leap combination), a specific temperature and a specific pressure, and is indicative ofthe rate at which the COUNT value of that channel changes when the implant is placed at that temperature and pressure.

[0064] In an embodiment, for a given {temperature, pressure}, processor 88 calculates two drift coefficients:

[0065] ■ mReal: A drift coefficient for the real channel that includes pressure sensor 22. ■ mRef: A drift coefficient for the reference channel that includes reference capacitor 26.

[0066] In some embodiments, the drift in COUNT(Icapl27) is used as a timeline, or timescale, for estimating the drifts in the channels of pressure sensor 22, or reference capacitor 26and / or of other leaps. For example, a given drift coefficient mReal (for a certain temperature and pressure) indicates the amount of drift in COUNT (Real) for a given amount of drift in COUNT(Icapl27). Similarly, a given drift coefficient mRef (for a certain temperature and pressure) indicates the amount of drift in COUNT(Ref) for a given amount of drift in COUNT(Icapl27). This technique eliminates the need for measuring elapsed time for the sake of drift estimation. leap 127 was found to be best suited for this task, since its capacitance was found to be virtually insensitive to temperature.

[0067] After implant 24 is implanted and operates in a living patient, the calculated drift coefficients can be used for compensating for the actual drift in the implant's measurements. Processor 88 may perform drift compensation itself, or it may send the calculated drift coefficients to another processor or system for performing drift compensation.

[0068] Fig. 4 is a flow chart that schematically illustrates a method for drift calibration, in accordance with an embodiment of the present invention. The method begins at a recording stage 100, with processor 88 recording the COUNT values COUNT(real), COUNT(ref) and COUNT(Icapl27) over time, for various temperature and pressure operating points. As noted above, in the present context the term "reference channel" includes both the channel of reference capacitor 26 and the channel of leaps 76. At a coefficient calculation stage 104, processor 88 uses the COUNT values to calculate drift coefficients mReal and mRef for each {temperature, pressure} operating point. As explained above, the drift rates indicated by mReal and mRef are relative to the amount of drift in COUNT(Icapl27).

[0069] Stages 100 and 104 above are typically performed in the factory, before implant 24 is implanted in subject 30. At some future point in time, processor 88 or a different processor uses the drift coefficients to compensate for drift. The compensation (stages 108-116 below) may beperformed, for example, just before implantation of implant 24, immediately after implantation, or months or years after implantation in-vivo.

[0070] At a measurement stage 108, the processor receives actual COUNT (real) values and COUNT(Ref) values measured by implant 24 in heart 28 of subject 30. At a compensation stage 112, the processor corrects the measured COUNT(real) and COUNT(Ref) values using the applicable drift coefficients to remove the drift. In an example embodiment, the processor performs the following operations:

[0071] ■ Measure the current temperature (or estimate the temperature based on factory calibration using the internal capacitors) and estimate the current pressure at the implant. (Accurate measurement of the pressure felt by the implant is complex at this point. An example method is depicted in Fig. 5 below.)

[0072] ■ Choose the drift coefficients mReal and mRef corresponding to these temperature and pressure (or interpolates between multiple coefficients if needed).

[0073] ■ Calculate the amount of drift in COUNT(Icapl27) since factory calibration (i.e., the difference between (i) COUNT(Icapl27) at the end of factory calibration and (ii) COUNT(Icapl27) at the present time).

[0074] ■ Multiply each of the mReal and mRef drift coefficients by the amount of drift in COUNT(Icapl27). The result is indicative of the actual drifts in COUNT(real) and COUNT(ref).

[0075] ■ Subtract the estimated drifts from (or add the estimated drifts to) the respective measured COUNT(real) and COUNT(ref).

[0076] At a pressure calculation stage 116, the processor calculates the ambient blood pressure in heart 28 using the resulting, drift-compensated, COUNT(real) and COUNT(ref).

[0077] Re-estimation of drift coefficients in patient body

[0078] As noted above, the drift coefficients mReal and mRef are indicative of the rate at which the respective COUNT (real) and COUNT (ref) values change (relative to the drift in COUNT(Icapsl27)) for a given temperature and a given pressure. The values of mReal and mRef are calculated as part of the factory calibration process of implants 26. In practice, however, the drift rate (and therefore mReal and mRef) in a patient's heart sometimes differs from the drift rate at the factory calibration setup.Thus, in some embodiments the processor re-estimates the drift coefficients mReal and mRef after implantation, and uses the re-estimated drift coefficients for subsequent drift compensation in-vivo.

[0079] In an example flow, the processor continues to use the drift coefficients that were calculated at factory calibration for an initial time period after implantation. The processor (or a human) waits until a significant amount of drift has occurred (e.g., until COUNT(Icapl27) has changed by more than a predefined number), and then the processor re-estimates the drift coefficients mReal and mRef.

[0080] The re-estimation typically follows a similar flow to that of Fig. 4 above, with the exception that the measurements are taken for a single {temperature, pressure} operating point. From that point, the processor performs drift compensation using the updated values of mReal and mRef (normalized by COUNT(Icap(127))).

[0081] In an alternative embodiment, the processor updates the mReal and mRef drift coefficients in daily intervals following implantation. The continuous updating process may begin immediately after implantation, or a defined period thereafter. The daily updates maintain the implant's pressure measurements highly accurate. This technique eliminates the need to detect that the actual drift was significant (e.g., in excess of 5 mmHg). In one embodiment, the processor performs the daily updates only for mRef and not for mReal. This embodiment is computationally simpler, since the COUNT (Ref) are typically independent of pressure (and therefore mRef update requires a relatively small number of backward measurements).

[0082] Estimating actual pressure felt by implant

[0083] After implantation, estimating the drift rate is more challenging because of the larger uncertainly in the exact pressure felt by sensor 22 in the heart. During factory calibration, the ambient temperature and pressure are typically measured with high accuracy using sensors other than pressure sensor 22 in implant 24. In vivo, on the other hand, the only pressure measurements available are those obtained from the implant, which are subject to drift and other impairments. The method below reduces this uncertainty to a minimum.

[0084] Fig. 5 is a flow chart that schematically illustrates a method for estimating the actual pressure applied to the cardiac implant in vivo, for use in drift calibration, in accordance with an embodiment of the present invention.

[0085] The method begins with implant 24 measuring the current COUNT (Real) value (the current pressure-indicative digital value), at a COUNT(Real) measurement stage 120. The implant transmits the COUNT(Real) value to external unit 32.At a temperature determination stage 124, the processor measures or otherwise determines the current temperature (denoted To) at the location of the implant. In some embodiments, the processor measures To using a temperature sensor. In other embodiments, the processor assumes a certain temperature To, e.g., an average body temperature of 37 °C.

[0086] At a possible counts calculation stage 128, the processor calculates multiple pairs of drift-corrected COUNT(Real) and COUNT(Ref) values, for a range of hypothetical ("possible") pressures. The processor defines a range of possible pressures, e.g., from 640 to 840 mmHg. For each possible pressure P in the range, the processor calculates (i) a drift-corrected COUNT (Real) value using the drift coefficient mReal(P,To) and (ii) a drift-corrected COUNT (Ref) value using the drift coefficient mRef(P,To).

[0087] At an actual pressures calculation stage 132, the processor calculates an actual pressure based on each pair of drift-corrected COUNT (Real) value and drift-corrected COUNT (Ref) value. In other words, the processor calculates a respective "actual" pressure for each "possible" pressure in the range.

[0088] At a best match selection stage 136, the processor finds the actual pressure that is closest to the corresponding possible pressure. This actual pressure is a highly accurate estimate of the ambient pressure that is currently felt by sensor 22 of implant 24.

[0089] The processor may use this actual pressure in subsequent in vivo drift compensation. For example, when estimating up-to-date drift coefficients mReal and mRef as a function of temperature and pressure in vivo, the processor may use the actual pressure calculated using the method of Fig. 5.

[0090] In some embodiments, the overall range of operating temperatures of the implant is divided into two or more temperature sub-ranges, and the processor may carry out the disclosed techniques (e.g., recording of COUNT values, calculation of calibration coefficients) separately in two or more temperature sub-ranges. This may apply to processes conducted before and / or after implantation of the implant. Calculating the coefficients separately in different temperature sub-ranges improves the calibration accuracy and the resulting pressure measurements. Adjacent sub-ranges may or may not overlap one another. In one non-limiting implementation, the overall temperature range of 34°C-40°C is divided into two sub-ranges, one sub-range spanning 34°C-38°C and the other spanning 36°C-40°C. Alternatively, any other suitable number of sub-ranges, and any other suitable temperature values, can be used.In such embodiments, to calibrate a certain measured pressure, the processor first determines the temperature sub-range in which the current temperature of the implant falls, and then uses the calibration coefficients associated with that sub-range.

[0091] If the temperature of the implant falls in the overlap between two sub-ranges, the processor may use any suitable technique for calculating calibration coefficients based on the calibration coefficients of both sub-ranges. For example, the processor may average the coefficients of the two sub-ranges, and use the averaged coefficients to compensate for the pressure. The average may be weighted by the distance of the measured implant temperature from the edges of the two sub-ranges.

[0092] Additionally or alternatively, the processor may divide the overall range of pressure values into two or more pressure sub-ranges, and carry out the disclosed techniques separately in two or more pressure sub-ranges. Adjacent pressure sub-ranges may or may not overlap. When two adjacent pressure sub-ranges overlap, the processor may handle pressure measurements that fall in the overlap region in a similar manner to temperatures that fall in an overlap between temperature sub-ranges.

[0093] In some embodiments, the processor may apply both multiple temperature sub-ranges and multiple pressure sub-ranges.

[0094] In some embodiments, once a temperature and / or pressure sub-range has been selected, the processor may verify that the temperature or pressure calculated using the corresponding coefficients indeed fall in the selected sub-range. If not, i.e., if the calculated temperature or pressure falls in the other sub-range, the processor may carry out an additional calculation using the coefficients of the other sub-range, and re-verify. This iterative process may continue until converging to one of the sub-ranges.

[0095] The configurations of system 20 and 80, and of implant 24, as illustrated in Figs. 1-3, are example configurations chosen purely for the sake of conceptual clarity. In alternative embodiments, any other suitable configuration can be used. The disclosed implant 24, external unit 32, server 33 and calibration system 80, may be implemented using suitable hardware, such as in one or more Application-Specific Integrated Circuits (ASICs) or Field-Programmable Gate Arrays (FPGAs), using software, using hardware, or using a combination of hardware and software elements. Elements that are not mandatory for understanding of the disclosed techniques have been omitted from the figure for the sake of clarity.

[0096] Certain elements of the disclosed implant 24, external unit 32, server 33 and / or calibration system 80, may be implemented using one or more general-purpose processors, programmed in software to carry out the functions described herein. The software may bedownloaded to the processor or processors in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and / or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory.

[0097] Although the embodiments described herein mainly address drift mitigation in cardiac pressure-sensing implants, the methods and systems described herein can also be used in other applications, such as for drift mitigation in other types of physical sensors, e.g., accelerometers, gyros, temperature sensors, flow meters and others. In all these cases, when the sensor reading is transduced into electrical signals, the sensor accuracy may degrade over time due to aging of the electronic circuitry. Such a drift can be mitigated using the disclosed techniques.

[0098] It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.

Claims

CLAIMS1. A method, comprising:receiving, from a pressure-sensing implant, digital values that are calculated by circuitry of the implant and are indicative of measurements of ambient pressure sensed in the implant;estimating a rate at which the digital values drift over time in representing the ambient pressure; andcompensating for drifting of the digital values based on the estimated rate.

2. The method according to claim 1, wherein estimating the rate is performed, at least in part, while the implant is operating in a calibration setup prior to implantation.

3. The method according to claim 2, wherein estimating the rate comprises compensating for errors in pressure and / or temperature measurement errors of the calibration setup.

4. The method according to claim 1, wherein estimating the rate is performed, at least in part, while the implant is operating in a body of a subject.

5. The method according to claim 4, and comprising re-estimating the rate, in the body of the subject, in daily intervals.

6. The method according to claim 1, wherein compensating for the drifting is performed, at least in part, while the implant is operating in a body of a subject.

7. The method according to any of claims 1-6, wherein estimating the rate comprises calculating multiple rate coefficients for respective temperature-pressure combinations, and wherein compensating for the drifting comprises (i) determining a rate coefficient based on a pressure and a temperature sensed by the implant, and (ii) correcting the current pressure sensed by the implant using the rate coefficient.

8. The method according to any of claims 1-6, wherein:the measurements of the ambient pressure are capacitance measurements of a capacitive pressure sensor in the implant;the method further comprises receiving from the implant reference digital values that are calculated by the circuitry of the implant and are indicative of capacitance measurements of one or more reference channels in the implant; andestimating the rate is based on both the digital values and the reference digital values.

9. The method according to claim 8, wherein one of the reference channels comprises a reference capacitor.

10. The method according to claim 8, wherein one of the reference channels comprises a selectable bank of internal capacitors.

11. The method according to any of claims 1-6, wherein estimating the rate comprises calculating calibration coefficients separately in two or more temperature sub-ranges.

12. The method according to any of claims 1-6, wherein estimating the rate comprises calculating calibration coefficients separately in two or more pressure sub-ranges.

13. An apparatus, comprising:a memory, configured to store digital values received from a pressure-sensing implant, the digital values having been calculated by circuitry of the implant and are indicative of measurements of ambient pressure sensed in the implant; andone or more processors, configured to estimate a rate at which the digital values drift over time in representing the ambient pressure, and to compensate for drifting of the digital values based on the estimated rate.

14. The apparatus according to claim 13, wherein the one or more processors are configured to estimate the rate, at least in part, while the implant is operating in a calibration setup prior to implantation.

15. The apparatus according to claim 14, wherein, in estimating the rate, the one or more processors are configured to compensate for errors in pressure and / or temperature measurement errors of the calibration setup.

16. The apparatus according to claim 13, wherein the one or more processors are configured to estimate the rate, at least in part, while the implant is operating in a body of a subject.

17. The apparatus according to claim 16, wherein the one or more processors are configured to re-estimate the rate, in the body of the subject, in daily intervals.

18. The apparatus according to claim 13, wherein the one or more processors are configured to compensate for the drifting, at least in part, while the implant is operating in a body of a subject.

19. The apparatus according to any of claims 13-18, wherein the one or more processors are configured to estimate the rate by calculating multiple rate coefficients for respective temperature-pressure combinations, and to compensate for the drifting by (i) determining a rate coefficient based on a pressure and a temperature sensed by the implant, and (ii) correcting the current pressure sensed by the implant using the rate coefficient.

20. The apparatus according to any of claims 13-18, wherein:the measurements of the ambient pressure are capacitance measurements of a capacitive pressure sensor in the implant; andthe one or more processors are further configured to (i) receive from the implant reference digital values that are calculated by the circuitry of the implant and are indicative of capacitance measurements of one or more reference channels in the implant, and (ii) estimate the rate is based on both the digital values and the reference digital values.

21. The apparatus according to claim 20, wherein one of the reference channels comprises a reference capacitor.

22. The apparatus according to claim 20, wherein one of the reference channels comprises a selectable bank of internal capacitors.

23. The apparatus according to any of claims 13-18, wherein the one or more processors are configured to estimate the rate by calculating calibration coefficients separately in two or more temperature sub-ranges.

24. The apparatus according to any of claims 13-18, wherein the one or more processors are configured to estimate the rate by calculating calibration coefficients separately in two or more pressure sub-ranges.

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