Active implantable medical device with cycle to cycle capture detection

Abstract

The invention relates to a device incorporating an endocardial acceleration (EA) sensor. A capture test circuit of the device collects a sampled EA signal and extracts a limited series of EA measurements during a duration of a predetermined temporal window opened after delivery of a pacing pulse. An indicator value based on an average of absolute values of successive EA measurements of the series of EA measurements is calculated at an end of the temporal window. The indicator value is compared to a predetermined discrimination threshold to determine the presence or absence of a capture according to whether the indicator value lies above or below the predetermined discrimination threshold. The indicator value is very robust to noise and particularly efficient in terms of computing, which reduces, in large proportions, consumption of the digital processor and thus of the capsule.

Claims

1. An active implantable medical device comprising: a housing comprising: a ventricular stimulation circuit adapted to deliver stimulation pulses to a ventricle of a patient; an acceleration sensor, capable of generating an endocardial acceleration (EA) signal; and a ventricular capture detection circuit adapted to detect a presence or absence of a contraction of the ventricle after an application of a stimulation pulse, comprising a processor configured to: collect a series of successive EA measurements during a duration of a predetermined temporal window which opens after the application of the stimulation pulse, calculate an indicator value based on an average of absolute values of series of the successive EA measurements when the predetermined temporal window ends; and compare the indicator value to a predetermined discrimination threshold; and determine a presence or absence of ventricular capture depending on whether the indicator value is above or below the predetermined discrimination threshold.

2. The device of claim 1, wherein the indicator value is calculated independently of peak-to-peak values of the EA signal.

3. The device of claim 1, wherein the duration of the predetermined temporal window is between 75 and 350 ms.

4. The device of claim 1, wherein the predetermined temporal window begins between 5 and 100 ms after delivery of the stimulation pulse.

5. The device of claim 1, wherein the processor is further configured to disable the ventricular capture detection circuit after the temporal window ends until the temporal window begins for a next cardiac cycle.

6. The device of claim 5, wherein the ventricular capture detection circuit is disabled between two successive collections of the EA signal generated by the sensor.

7. The device of claim 1, wherein the indicator value is calculated from a constant number of successive EA measurements from one cardiac cycle to another.

8. The device of claim 1, wherein the indicator value is calculated by summing absolute values of the series of successive EA measurements.

9. The device of claim 1, wherein the indicator value is calculated by summing absolute values of respective differences between: i) the series of successive EA measurements and ii) a value which is an average of the series of successive EA measurements.

10. The device of claim 1, wherein the indicator value is calculated by summing absolute values of respective differences between: i) the series of successive EA measurements and ii) a constant base line.

11. The device of claim 1, wherein the indicator value is calculated by summing absolute values of respective differences between: i) the series of successive EA measurements and ii) a value of a first EA measurement of the series of successive EA measurements.

12. The device of claim 1, wherein the processor is further configured to determine a discrimination threshold, wherein determining the discrimination threshold comprises: controlling the ventricular stimulation circuit to deliver a series of stimulation pulses with maximum energy; calculating an average of the indicator values of the series of stimulation pulses with the maximum energy; and applying a reduction factor to the average of the indicator value; and issuing a value for the discrimination threshold.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features, characteristics and advantages of the present disclosure will become apparent to a person of ordinary skill in the art from the following detailed description of preferred embodiments of the present invention, made with reference to the drawings annexed, in which like reference characters refer to like elements and in which:

(2) FIG. 1 is an overall perspective view of a leadless capsule.

(3) FIG. 2 is a longitudinal sectional view of the leadless capsule of FIG. 1 showing the main internal components which the leadless capsule may include.

(4) FIG. 3 is a series of timing diagrams illustrating, an electrogram (EGM) signal, the analysis windows for the capture test and the endocardial acceleration EA signal.

(5) FIGS. 4a and 4b are timing diagrams showing the shape of the EA signal delivered by a sensor more precisely, respectively a capacitive sensor and a piezoelectric sensor.

(6) FIG. 5 is a series of timing diagrams illustrative of the known method of operating a capture test.

(7) FIG. 6 is a flow chart outlining the main steps of the capture detection method according to an embodiment of the invention.

(8) FIG. 7 is a series of timing diagrams illustrative of the method of FIG. 6, to determine the discrimination threshold between capture and no capture.

(9) FIG. 8 is a series of timing diagrams corresponding to an exemplary implementation of the invention, after the discrimination threshold has been determined.

DETAILED DESCRIPTION

(10) An exemplary embodiment of a device of the invention will now be described.

(11) Regarding its software aspects, the invention may be implemented by appropriate programming of controlling software of a known cardiac pacemaker, for example an endocardial leadless capsule.

(12) These devices include a programmable microprocessor provided with circuits for shaping and delivering stimulation pulses to implanted electrodes. Using telemetry, it is possible to transmit software that will be stored in memory and executed by the device to implement the functions of an embodiment of the invention which will be described below. The adaptation of these devices to implement the functions of the disclosure is within the reach of a skilled-in-the-art person and will not be described in detail. In particular, software stored in memory and executed can be adapted and used to implement the functions of the disclosure which will be described below.

(13) A method of an embodiment of the invention may be implemented primarily by software, through appropriate algorithms performed by a microcontroller or a digital signal processor. For the sake of clarity, the various processing applied will be decomposed and schematized by a number of separate functional blocks in the form of interconnected circuits. However, this representation is only illustrative, these circuits including common elements in practice may correspond to a plurality of functions generally performed by the same software.

(14) FIGS. 1 and 2 show, in perspective and in longitudinal section, an example of a leadless capsule with the various elements the leadless capsule may include.

(15) In these figures, reference 10 designates the leadless capsule generally, formed as a cylindrical tubular body 12 of axis enclosing various electronic and power circuits of the leadless capsule. Typically, the dimensions of such a leadless capsule are a diameter of about 6 mm and a length of about 25 mm.

(16) At a distal end 14, the leadless capsule 10 is provided with a helical anchoring screw 16 for fixing the leadless capsule into the tissue, for example against a wall of a cardiac cavity. The helical anchoring screw can optionally be an active, electrically conductive screw for collecting cardiac depolarization potentials and/or the application of stimulation pulses. A proximal region 18 of the capsule 10 has a rounded, atraumatic end 20 and is provided with gripping means 22 and 24 suitable for implantation or removal of the leadless capsule.

(17) As shown in FIG. 2, the leadless capsule 10 incorporates a battery 26 typically with a volumetric energy density on the order of 0.8 to 2 Wh/cm.sup.3, an electronic module 28, a front electrode 30, and optionally a side electrode 32. Feedthroughs such as 34 are used to connect the electrodes 30 and 32 to the electronic module 28.

(18) The electronic module 28 includes electronics for controlling various functions of the leadless capsule 10, for storing collected signals, etc. The electronic module 28 may include a microcontroller and an oscillator generating the necessary clock signals for operation of the microcontroller and communication. The electronic module 28 may also contain an analog/digital converter and a digital storage memory. The electronic module 28 may also contain a transmitter/receiver circuit for exchanging information with other implantable devices by human body communication HBC (e.g., intracorporeal communication).

(19) The leadless capsule 10 also typically includes an endocardial acceleration (EA) sensor 36 capable of generating a signal representative of a mechanical activity of the myocardium, for example a sensor in the form of a microaccelerometer interfaced with the electronic module 28.

(20) FIG. 3 shows a series of timing diagrams illustrating, an electrogram (EGM) signal, analysis windows W.sub.DET for the capture test and the EA signal.

(21) After each stimulation (marker V on the EGM indicates stimulated depolarization), the measurement of the EA signal generated by the accelerometer is activated during a window WET.sub.DET which is open either immediately after the issuance of the stimulation pulse, or with a delay on the order of 5 to 100 ms. The length F of the window W.sub.DET is between 75 and 350 ms. Controlling the start time of the capture window W.sub.DET and the duration of the capture window is achieved by a sequencing circuit of the microcontroller and by the embedded software which controls the electronic circuits of the leadless capsule 10.

(22) The sensor 36 measuring the EA signal can be a 1D, 2D or 3D accelerometer sensor. Preferably, the sensor is a piezoelectric or capacitive sensor, but other types of sensor (optical, resistive, inductive, etc.) capable of generating a signal correlated to the displacement, velocity or acceleration of the cardiac walls may be used.

(23) Depending on the type of sensor used, the EA signal may or may not contain a DC component.

(24) The EA signal generated by a capacitive MEMS sensor (integrated microelectromechanical component) has a general shape illustrated FIG. 4a, with a DC component depending on the orientation of the leadless capsule 10 relative to the direction of gravity. In contrast, the EA signal delivered by a piezoelectric accelerometer shown in FIG. 4b provides a signal with a baseline equal to zero, since by design it ensures the filtering of a DC component.

(25) In either case, the capture test measuring circuit is active only for the duration of the acceleration measuring window, the circuit being totally or partially switched off (muting) the rest of the cardiac cycle. If the latency of the sensor is less than the time between two successive acceleration measurements, it is possible to switch off the circuit and the sensor between two successive measurements of the EA signal.

(26) In the case of a piezoelectric sensor and its interface circuit, the energy consumption is on the order of 100 to 200 nW. If the piezoelectric sensor is activated only for the duration of the W.sub.DET window, corresponding to 20 to 50% of the cardiac cycle length, the average consumption of the piezoelectric sensor can be reduced to a value of about 50 to 100 nW.

(27) In the case of a MEMS capacitive sensor, the energy consumption on the order of 300 to 600 nW can be reduced in the same method as above to a value in the range of 150 to 300 nW if the measurement circuit is not activate for the duration of the detection window W.sub.DET.

(28) FIG. 5 shows a series of timing diagrams explaining a known method to operate the capture test. Represented in these timing diagrams, are successively: V markers of stimulated depolarization; An endocardial acceleration EA signal; The EA signal after sampling during the detection periods W.sub.DET; and A calculated value of a peak of endocardial acceleration PEA1, i.e., the value of a difference between a maximum and a minimum (in algebraic value) of the EA signal values sampled during the detection window.

(29) The parameter PEA1 is compared with a predetermined threshold and, for example at the fifth cardiac cycle, if this value is less than the threshold, absence of capture is determined.

(30) The parameter PEA1 which is based on a minimum-maximum difference, is very sensitive to measurement noise and to physiological noises generated, for example, by a patient's breathing or sudden movements that result in corresponding movements of the sensor.

(31) Because the parameter PEA1 is very sensitive the capture test is typically not based on an analysis of this single parameter, but is combined with other representative parameters, as in the case of the multivariate analysis described by EP 2412401 A1 cited above.

(32) An embodiment of the invention, proposes to make a capture test from a single indicator which i) requires a minimum of numerical calculations in order to save the energy consumed by a device and ii) is robust to noise, so as to minimize the risk of false capture detections (false positives), which could affect the reliability of the capture test.

(33) An embodiment of the invention uses an indicator value of an average of absolute values of successive measurements of the EA signal sampled during the detection window.

(34) Denoting by x.sub.i, with i=1, . . . , N, where N is the number of acceleration measurements delivered by the capsule sensor, and in the case of a piezoelectric sensor (wherein the EA signal varies around a baseline equal to zero), such an indicator value according to an embodiment of the invention can be calculated by:

(35) ${\mathrm{MEAN}}_{\mathrm{ABS}1}=\frac{1}{N}\underset{i=1}{\overset{N}{.Math.}}.Math.x_i.Math.$
N may be between 20 and 50 depending on the sampling frequency.

(36) Without impairing the suitability of the indicator, one can avoid the division operation of 1/N, which may be costly in computation time, when using the same number N of samples in each cardiac cycle to calculate the representative indicator MEAN.sub.ABS1, which gives:

(37) ${\mathrm{MEAN}}_{\mathrm{ABS}1}^{}=\underset{i=1}{\overset{N}{.Math.}}.Math.x_i.Math..$

(38) Calculating the representative indicator MEAN.sub.ABS1 is therefore limited to a simple summation of N successive values taken by the signal EA during the window W.sub.DET (even if the window contains a number of samples greater than N).

(39) In the case of a MEMS capacitive sensor, the baseline of the acceleration signal depends on the orientation of the capsule relative to the vertical direction. The gravitational acceleration component (severity) is present in the EA signal independently of the acceleration induced by the cardiac motion and results, as explained above in connection with FIG. 4a, in a non-zero baseline present in the EA signal.

(40) The DC component must be removed in the calculation of the indicator value, which then takes the form:

(41) ${\mathrm{MEAN}}_{\mathrm{ABS}2}=\frac{1}{N}\underset{i=1}{\overset{N}{.Math.}}.Math.x_i-m.Math.$
where m represents the average value of the baseline of the EA signal.

(42) To simplify the calculation, it is possible to use as an approximation of the value of the baseline of the first value x.sub.1 measured during the detection window W.sub.DET:

(43) ${\mathrm{MEAN}}_{\mathrm{ABS}3}=\underset{i=2}{\overset{N}{.Math.}}.Math.x_i-x_1.Math..$

(44) The indicator MEAN.sub.ABS determined by one of the preceding methods is associated with a criterion of presence/absence of capture for use in a capture verification algorithm. The criteria used may be a simple comparison with a discrimination threshold determined in advance.

(45) FIG. 6 is a flowchart summarizing different stages of the capture test.

(46) After stimulation (step 100), the device collects N measurements of the EA signal (values x.sub.i) successively sampled within the detection window W.sub.DET (step 102). The MEAN.sub.ABS indicator value is then calculated by summing absolute values of the measured values x.sub.i or by summing the absolute values of differences between the measured values x.sub.i and a constant value, reflecting a shift of a baseline with respect to an origin (step 104).

(47) The calculated MEAN.sub.ABS indicator value is then compared with a predetermined threshold (step 106). If the indicator is above the predetermined threshold, then it is determined that there is a presence of capture (step 108); otherwise, it is determined that there is an absence of capture (step 110).

(48) The predetermined threshold used to discriminate between the presence and absence of capture is preferably not a fixed threshold but a threshold calculated automatically, to reflect specific circumstances of a particular patient and a possible evolution of his/her clinical condition over a long term.

(49) A discrimination threshold may be determined during a preliminary initialization phase, as follows.

(50) The device triggers M successive stimuli (typically M=3, 5 or 10 stimuli) with parameters set to deliver a maximum energy, for example a pulse voltage of 5 to 7 V and a pulse width of 1 to 2 ms.

(51) For each stimulation i, i=1, . . . , M, the device calculates the value of the indicator MEAN.sub.ABS.

(52) A stimulation threshold Cap.sub.Threshold is then determined by:

(53) ${\mathrm{Cap}}_{\mathrm{Threshold}}=\frac{}{M}\underset{i=1}{\overset{M}{.Math.}}{\mathrm{MEAN}}_{{\mathrm{ABS}}_i}$
where is a predetermined reduction factor, for example =, or .

(54) Cap.sub.Threshold is the value of the discrimination threshold to be applied to each subsequent capture test.

(55) FIG. 7 illustrates an example of determining the discrimination threshold according to this technique, with M=5 stimuli during the initialization phase.

(56) The top timing diagram shows the signal EA obtained as a result of the 5 stimuli, and the bottom timing diagram shows the five corresponding values of the parameter MEAN.sub.ABS(i). The five values are averaged and a reduction factor = is applied to the calculated average, giving a threshold value of approximately 0.185 g (g being the acceleration of gravity).

(57) FIG. 8 shows an example of using the EA signal for a capture test according to the teachings of the present disclosure. The successive following elements are shown in FIG. 8: V markers of stimulated depolarization; An EA signal; The EA signal after collecting during detection periods W.sub.DET; and Calculated parameter MEAN.sub.ABS(i).

(58) It can be seen that at the fifth stimuli, at t=3.75 s, the EA signal collected during the corresponding collecting window has very low amplitude.

(59) The value of the MEAN.sub.ABS(5) indicator calculated for this window is about 0.04 g while MEAN.sub.ABS was approximately 0.4 g for the other stimuli, which caused a capture. The discrimination threshold in this case was set to 0.2 g, and it can be seen that it was possible to clearly distinguish the cycles where a capture is present from the cycles where a capture is not present, with excellent immunity to various noise likely to interfere with the EA signal.

(60) Note that the indicator MEAN.sub.ABS is very simple to calculate with a microcontroller, because it is just a sum of N numerical integers.

(61) It is also robust to noise because the summation operation is equivalent to operating a low-pass filter, which greatly reduces the incidence of noise.

(62) Finally, the capture test criterion is particularly simple to implementa simple comparison between two numeric values to separate a capture zone from an absence of capture zonewith a very large economy of calculation methods.