Patent Application
20110313302
1. Field of the Invention
The present invention concerns blood pressure measurements, and in particular to an implantable medical device capable of estimating blood pressure of a patient.
2. Description of the Prior Art
Blood pressure measurements are commonly used as diagnostic parameters in cardiac medicine. For example, mean arterial pressure (MAP) representing the average blood pressure in a patient and defined as the average arterial pressure during a single cardiac cycle is commonly used within the diagnostic field. Another common diagnostic parameter is pulse pressure (PP) defining the difference between systolic and diastolic blood pressure and reflects the change in blood pressure seen during a contraction of the heart.
Today blood pressure is measured with various equipment. For instance, arterial pressure is most commonly measured via a sphygmomanometer. Minimum systolic pressure value can be roughly estimated by palpation. The ausculatory method uses a stethoscope and a sphygmomanometer. This comprises an inflatable cuff placed around the upper arm attached to a mercury or aneroid manometer. Also ambulatory blood pressure devices are available on the marked for home monitoring.
However, for some patients it might be advantageous to monitor blood pressure continuously over certain period of times or conduct blood pressure measurements periodically or upon given events. This is in particular true for cardiac patients having an implantable medical device (IMD).
WO 02/28478 discloses a method to estimate the static intracardiac pressure from an intracardiac dynamic pressure sensor. The invention disclosed in this document is based on the finding that there is a relation between the sensed short-term dynamic pressure signal and the intravascular static pressure. A shortcoming with this sensor measuring technique is that the sensor has to be positioned at the relevant intravascular site in order to determine the intravascular static pressure at that site. For instance, pressures relating to the left ventricle must be conducted with the dynamic pressure sensor positioned inside the left ventricle.
U.S. Pat. No. 5,129,394 and U.S. Pat. No. 6,666,826 disclose a pressure sensor provided at the distal end of a left ventricular lead or provided on a sensing catheter introduced through the lumen of a left ventricular lead. An inflatable balloon or an occlusion device is provided proximal to the pressure sensor and is used to occlude the blood flow in a coronary vein in which the pressure sensor is provided. This blood flow occlusion is, according to the documents, a prerequisite in order to obtain an in vivo blood pressure that is proportional to the left ventricular pressure. If no such blood flow occlusion is conducted the result from the pressure measurement is random and not representative of left ventricular pressure.
There is therefore still a need for conducting blood pressure measurements with an IMD that does not have the limitations or shortcomings of the prior art. In particular, it would be beneficial to conduct pressure measurements relating to the left ventricle but not requiring a positioning of the sensing equipment in the ventricle nor needing extra inflating or occluding devices.
It is a general objective to provide an implantable medical device capable of conducting systemic blood pressure measurements.
It is a particular objective to achieve systemic blood pressure measurements having a sensor safely implanted at an implantation site commonly employed for cardiac leads connectable to an implantable medical device.
Briefly, an implantable medical device in accordance with the invention has a lead connecting arrangement configured to be connected to a cardiac lead to be implanted in or in connection with the ventricle of a subject's heart. The cardiac lead includes a cardiomechanic or cardiomechanical sensor configured to generate a deformation signal representative of the myocardial deformation. A derivative calculator of the implantable medical device receives the deformation signal and is configured to generate a deformation rate signal representative of the rate of myocardial deformation by calculating the derivative of the deformation signal with respect to time. A signal filter unit is configured to filter the deformation rate signal to improve signal quality and to get a filtered deformation rate signal. The filtered deformation rate signal is processed by a peak identifier that is configured to identify respective maximum deformation rate values in the filtered deformation rate signal for multiple cardiac cycles. A pressure calculator then calculates a value representative of the systemic blood pressure of the subject based on a combination of the respective maximum deformation rate values identified by the peak identifier.
A further aspect of the embodiments relates to a method for estimating systemic blood pressure of a subject having an implantable medical device connected to a cardiac lead implanted in or in connection with a ventricle of the subject's heart. The cardiac lead has a cardiomechanic sensor configured to generate a deformation signal representative of the myocardial deformation. The method includes generating a deformation rate signal representative of the rate of myocardial deformation by calculating the derivative of the deformation signal with respect to time. The deformation rate signal is filtered to get a filtered deformation rate signal. The method also includes identifying respective maximum deformation rate values in the filtered deformation rate signal for multiple cardiac cycles. These respective maximum deformation rate values are then combined to get a calculated value representative of the systemic blood pressure of the subject.
The embodiments allow measuring and monitoring systemic blood pressure at any time for a subject having an implantable medical device. No dedicated visits to the physician are thereby needed to conduct blood pressure measurements. As a consequence, the implantable medical device can, in real-time, detect various malicious conditions to the subject and the heart, which can be seen as a significant change in systemic blood pressure. Additionally, a dynamic operation of the implantable medical device is possible based on the blood pressure measurements.
In contrast to the prior art, the embodiments allow reliable systemic blood pressure measurements without any need for any blood vessel obstructing equipment or the risk of implanting the pressure sensor in the left ventricle.
Throughout the drawings, the same reference numbers are used for similar or corresponding elements.
The embodiments generally relate to devices and methods capable of estimating systemic blood pressure in a subject, preferably mammalian subject and more preferably a human subject, by means of an implantable medical device (IMD) connectable to an implantable cardiomechanic sensor.
The embodiments are based on the insight that it is possible to use a cardiomechanic sensor configured to generate a deformation signal representative of the myocardial deformation of the cardiac myocardium in the subject's heart together with a special processing of the deformation signal to get a value representative of the systemic blood pressure in the subject.
A key advantage of the embodiments is that systemic blood pressure measurements are possible using a cardiomechanic sensor provided in a coronary vein of the heart. Hence, the embodiments enable systemic blood pressure measurements without the need of having the sensor provided in the left ventricle, which is generally not desired due to an increased risk of the formation of emboli.
The cardiomechanic sensor of the embodiments generates a deformation signal that is representative of the myocardial deformation. There is a relation between myocardial deformation and strain in terms of strain being equal to relative deformation. Hence, the signal from the cardiomechanic sensor could also be representative of the strain of the myocardium.
Without being bound by theory, it is known that the ambient or systemic blood pressure affects the cardiac output and stroke volume through Frank-Starling's law. An increase in contractility of the heart tends to increase stroke volume. Thus, an increased stroke volume is associated with an increased contractility. A positive value of the rate of myocardial deformation or the strain rate is a measure of compliance since the maximum (positive) deformation rate during a cardiac cycle represents the maximum speed at which the myocardium increases in size in the current cardiac cycle. Generally, this maximum increase in size occurs in early diastole or at the very end of systole. A positive deformation indicates an expansion of the myocardium and a negative deformation indicates a contraction. Thus, the speed at which the heart contracts, i.e. contractility, is manifested by a negative deformation rate. The speed at which the heart relaxes, i.e. compliance, is manifested by a positive deformation rate.
If the heart's size has decreased considerably during systole, i.e. has had a negative average deformation, then during diastole there must be a high average deformation. A high average deformation is not necessarily the same as a high peak deformation rate, although it is very likely that an increase in average deformation would also affect the peak deformation rate. In other words, there is a connection between high contractility, generating a high deformation in systole and a high peak deformation rate, indicating a high compliance.
Hence, there is a link between deformation rate as obtained by processing the deformation signal from the cardiomechanic sensor and ambient or systolic blood pressure.
The embodiments therefore use an implantable cardiomechanic sensor generating a deformation signal representative of the myocardial deformation and process the deformation signal to get a deformation rate signal from which a value representative of systemic blood pressure of the subject is obtainable according to the embodiments.
According to the embodiments, the IMD 100 is connectable to at least one cardiomechanic sensor that is implanted in or in connection with a ventricle of the heart 10. The cardiomechanic sensor is advantageously arranged on a cardiac lead 210 configured to be connected to the IMD.
The IMD 100 has a housing, often denoted as can or case in the art. The housing can act as return electrode for unipolar leads, which is well known in the art. The IMD 100 also comprises a lead connector or input/output (I/O) 110 having, in this embodiment, a plurality of terminals 111-116. These terminals 111-114 are configured to be connected to matching electrode terminals of one or more cardiac leads connectable to the IMD 100 and the lead connectors 110. In addition, the lead connector 110 comprises at least one and preferably two terminals 115, 116 arranged to be connected to matching terminals of a cardiac lead equipped with a cardiomechanic sensor. This at least one terminal 115, 116 therefore receives the deformation signal representative of the myocardial deformation generated by the cardiomechanic sensor, denoted cardiomechanic electric sensor (CMES) in the figure.
With reference to
In an alternative implementation, the IMD 100 is not connectable to a right atrial lead 230 but instead to a left atrial lead configured for implantation in the left atrium 16. A further possibility is to have an IMD 100 with an electrode connector 110 having sufficient terminals to allow the IMD 100 to be electrically connectable to both a right atrial lead 230 and a left atrial lead. Though, it is generally preferred to have at least one electrically connectable atrial lead in order to enable atrial sensing and pacing, the IMD 100 does not necessarily have to be connectable to any atrial leads. In such a case, the terminals 111, 112 of the electrode connector 110 can be omitted.
In order to support left chamber sensing and pacing, the lead connector 110 further comprises a left ventricular tip terminal 114 and a left ventricular ring terminal 113, which are adapted for connection to a left ventricular tip electrode 212 and a left ventricular ring electrode 214 of the left ventricular lead 210 implantable in connection with the left ventricle 12, see
The left ventricular lead 210 is equipped with a cardiomechanic sensor 250 according to the embodiments, such as a cardiomechanic sensor 250 of the CMES type. The cardiomechanic sensor 250 can be arranged anywhere along the portion of the left ventricular lead 210 that is in contact with and arranged in connection with the left ventricle 12. Hence, the cardiomechanic sensor 250 could be provided at the most distal end of the left ventricular lead 210, at a position upstream of the distal end, such as between the tip electrode 212 and the ring electrode 214 or upstream of the ring electrode 214. If the left ventricular lead 210 is of a so-called multi-electrode lead, such as quadropolar lead having four ring electrodes, the cardiomechanic sensor 250 could be provided in between two such electrodes.
The CMES material can actually be placed onto one of the ring electrodes or tip electrode. In such a case, the CMES sensor 250 occupies the same or at least a portion of the same lead surface as the electrode. In a preferred embodiment, the electrode is then mainly employed for sensing and preferably not any pacing.
In yet another embodiment, the IMD 100 is connected to both a left ventricular lead and a right ventricular lead and optionally further to at least one atrial lead. In such a case, any of or both the left and right ventricular lead can be equipped with a cardiomechanic sensor.
The cardiomechanic sensor must not necessarily be arranged on the same cardiac lead as the electrodes of the left or right ventricle. In clear contrast, a dedicated sensor lead or catheter can then be used that does not comprise any electrodes but merely the cardiomechanic sensor. Such a sensor lead can then generally be less complex as compared to constructing a cardiac lead having both pacing and/or sensing electrodes and a cardiomechanic sensor. However, using a dedicated sensor lead generally implies that the IMD has to be connected to a further lead and this further lead has to be transplanted into the heart in addition to the normal cardiac lead(s).
It is possible to use an IMD that is not connected to a single cardiomechanic sensor but rather multiple such cardiomechanic sensors. These multiple sensors can all be provided on the same lead or on different leads in or connection with the same ventricle or distributed among both ventricles. In such a case, the further processing of the deformation signals as disclosed herein can then be performed for each of the deformation signals. The final value representative of the systemic blood pressure is preferably an average of respective such values obtained from each of the cardiomechanic sensors. Alternatively, averaging can be performed anywhere in the processing up to the final pressure value.
The cardiomechanic sensor preferably comprises a piezoelectric material. Piezoelectric materials generate a charge when subject to mechanical stress or strain, with the magnitude of charge dependent upon the magnitude of the stress or strain. A sensor that includes such piezoelectric material can be arranged for detection of myocardial deformation and generate raw signals of myocardial deformation.
A cardiomechanic sensor preferably comprises one or more piezoelectric transducers, which convert mechanical motion into electric signals. Such a cardiomechanic sensor 250 of CMES type is illustrated in cross section in
The cardiomechanic sensor 250 is preferably designed and dimensioned to be arranged on a cardiac lead and in particular a ventricular lead. In such a case, the outer diameter of the cardiomechanic sensor 250 can be similar to the outer diameter of the cardiac lead. In some embodiments, one or more electrodes of the cardiac lead can be disposed over at least a portion of the cardiomechanic sensor 250.
The cardiomechanic sensor 250 preferably has a longitudinal passageway or bore 256 to permit routing of electrical connections therethrough.
The conductor 252 and optionally conductor 253 has any suitable biocompatible, electrically conducting material known in the art, for example, titanium, including titanium, platinum, carbon, niobium, tantalum, gold, combinations thereof including alloys of the metallic materials presented above, and titanium nitride.
An elastomer can be disposed over the cardiomechanic sensor 250. The elastomer can then be selected from biocompatible elastomers that are suitable for implantation in the animal body, such as silicones, polyurethanes, ethylene-propylene copolymers, fluorinated elastomers and combinations thereof.
The piezoelectric material 251 of the cardiomechanic sensor 250 can be of a relatively hard material, thereby permitting reliable measurements with only small deflections of the piezoelectric material 251. Preferred such piezoelectric materials include ceramic ferroelectric particles, lead zirconate titanate (PCT), barium titanate, sodium potassium niobate. A non-limiting example of piezoelectric material that can be used is described in U.S. Pat. No. 6,526,984 and has the general formula of Na0.5K0.5NbO3.
Instead of using a piezoelectric material the cardiomechanic sensor can use a conductive polymer that has resistance that changes as a function of deformation. By measuring the resistance of the conductive polymer, the cardiomechanic deformation can be determined. Non-limiting examples of conductive polymers include polyacetylene, polyaniline, polypyrrole.
More information of the design of the cardiomechanic sensor can be found in U.S. Patent Application No. 2009/0312814.
With reference to
The IMD 100 as illustrated in
It is understood that in order to provide stimulation therapy in different heart chambers, the atrial and ventricular pulse generators 140, 143 may include dedicated, independent pulse generators, multiplexed pulse generators, or shared pulse generators. The pulse generators 140, 143 are controlled by a controller 130 via appropriate control signals, respectively, to trigger or inhibit the stimulating pulses.
The IMD 100 also comprises the controller 130, preferably in the form of a programmable microcontroller 130 that controls the operation of the IMD 100. The controller 130 typically includes a microprocessor, or equivalent control circuitry, designed specifically for controlling the delivery of pacing therapy, and may further include RAM or ROM memory, logic and timing circuitry, state machine circuitry, and I/O circuitry. Typically, the controller 130 is configured to process or monitor input signal as controlled by a program code stored in a designated memory block. The type of controller 130 is not critical to the described implementations. In clear contrast, any suitable controller may be used that carries out the functions described herein. The use of microprocessor-based control circuits for performing timing and data analysis functions are well known in the art.
The controller 130 further controls the timing of the stimulating pulses, such as pacing rate, atrioventricular interval (AVI), atrial escape interval (AEI) etc. as well as to keep track of the timing of refractory periods, blanking periods, noise detection windows, evoked response windows, alert intervals, marker channel timing, etc.
A preferred electronic configuration switch 120 includes a plurality of switches for connecting the desired terminals 111-114 to the appropriate I/O circuits, thereby providing complete electrode programmability. Accordingly, the electronic configuration switch 120, in response to a control signal from the controller 130, determines the polarity of the stimulating pulses (e.g., unipolar, bipolar, combipolar, etc.) by selectively closing the appropriate combination of switches (not shown) as is known in the art.
An optional atrial sensing circuit or detector 141 and a ventricular sensing circuit or detector 142 are also selectively coupled to the atrial lead(s) and the ventricular lead(s) through the switch 120 for detecting the presence of cardiac activity in the heart chambers. Accordingly, the atrial and ventricular sensing circuits 141, 142 may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. The switch 120 determines the “sensing polarity” of the cardiac signal by selectively closing the appropriate switches, as is also known in the art. In this way, the clinician may program the sensing polarity independent of the stimulation polarity. The sensing circuits are optionally capable of obtaining information indicative of tissue capture.
Each sensing circuit 141, 142 preferably employs one or more low power, precision amplifiers with programmable gain and/or automatic gain control, band-pass filtering, and a threshold detection circuit, as known in the art, to selectively sense the cardiac signal of interest.
The outputs of the atrial and ventricular sensing circuits 141, 142 are connected to the controller 130, which, in turn, is able to trigger or inhibit the atrial and ventricular pulse generators 140, 143, respectively, in a demand fashion in response to the absence or presence of cardiac activity in the appropriate chambers of the heart.
Furthermore, the controller 130 is also capable of analyzing information output from the sensing circuits 141, 142 and/or a signal sensing unit or data acquisition unit 160 to determine or detect whether and to what degree tissue capture has occurred and to program a pulse, or pulse sequence, in response to such determinations. The sensing circuits 141, 142, in turn, receive control signals over signal lines from the controller 130 for purposes of controlling the gain, threshold, polarization charge removal circuitry, and the timing of any blocking circuitry coupled to the inputs of the sensing circuits 141, 142 as is known in the art.
According to the embodiments cardiac signals are applied to inputs of the data acquisition unit 160 connected to the electrode connector 110. The data acquisition unit 160 is preferably in the form of an analog-to-digital (A/D) data acquisition unit 160 configured to acquire intracardiac electrogram (IEGM) signals, convert the raw analog data into a digital signal, and store the digital signals for later processing and/or transmission to the programmer by a transceiver 190. The data acquisition unit 160 is coupled to the atrial lead and/or the ventricular lead through the switch 120 to sample cardiac signals across any pair of desired electrodes.
The IMD 100 comprises a derivative calculator 131 configured to process the deformation signal obtained from the cardiomechanic sensor through the terminals 115, 116 and the preferred switch 120. The derivative calculator 131 in particular generates a deformation rate signal representative of the rate of the myocardial deformation. Thus, the derivative calculator 131 calculates the derivative with respect to time of the myocardial deformation as represented by the deformation signal. The deformation rate signal will, thus, comprise multiple samples, where each sample has a data element corresponding to the deformation rate at the point in time represented by the particular sample. The derivative calculator 131 can, in a simple embodiment, generate the deformation rate signal by calculating the difference in data elements between successive samples in the deformation signal, i.e. dfri=dsi−dsi−1, where dfri represents sample i in the deformation rate signal and dsi/i−1 represents sample i/i−1 in the deformation signal.
A signal filter unit 150 is arranged in the IMD 100 for filtering the deformation rate signal from the derivative calculator 131. The filter unit 150 is in particular arranged in order to suppress noise and generally improve the quality of the deformation rate signal. The output from the filter unit 150 is a filtered deformation rate signal.
A peak identifier 132 of the IMD 100 is configured to identify respective maximum deformation rate values in the filtered deformation rate signal for multiple cardiac cycles. In a particular embodiment, the peak identifier 132 simply parses through the samples of the filtered deformation rate signal corresponding to a cardiac cycle in order to find the largest value, which corresponds to the maximum rate value for that cardiac cycle. The peak identifier 132 must not necessarily parse through all the samples corresponding to a cardiac cycle in order to identify the maximum deformation rate value. As has previously been discussed, this maximum in deformation rate generally occurs in early diastole or at the very end of systole. Hence, it is generally sufficient if the peak identifier 132 merely parses through the samples corresponding to the cardiac cycle coinciding with end of systole-early diastole, generally occurring around the T-wave as detectable in the IEGM signal.
The peak identifier 132 preferably identifies the maximum deformation rate value in each out of multiple cardiac cycles. These multiple cardiac cycles are preferably consecutive cardiac cycles. However, skipping one or a few cardiac cycles generally does not have any large impact on the processing so it is not a prerequisite that the multiple cardiac cycles are successive cardiac cycles.
When conducting pressure measurements, the IMD 100 generally collects the deformation signal from the cardiomechanic sensor for some defined period of time. In such a case, the peak identifier 132 can advantageously identify a respective maximum deformation rate value in the filtered deformation rate signal for each full cardiac cycle encompassed by the period of time to get as many maximum deformation rate values as possible.
The IMD 100 also has a pressure calculator 133 that is configured to calculate a value representative of the systemic blood pressure of the IMD patient based on a combination of multiple maximum deformation rate values identified by the peak identifier 132. Thus, the pressure calculator 133 co-processes multiple, i.e. at least two, maximum deformation rate values in order to get a blood pressure presenting value. These multiple maximum deformation rate values are preferably identified by the peak identifier from consecutive cardiac cycles. The generated blood pressure representing value is of high diagnostic value and is therefore advantageously stored in a memory 170 of the IMD 100 for later uploading to the data processing unit by means of transmitter 190 or transceiver having connected antenna 195. The blood pressure representing value can also be used for diagnostic purposes by the IMD 100 itself and control the operation of the IMD 100, which is further discussed herein.
The filter unit 150 of the IMD 100 preferably has a low pass filter 153 configured to low pass filter the deformation rate signal to remove or at least suppress noise from the deformation rate signal. Experiments have been successfully conducted with a 4th order low pass 30 Hz filter.
The filter unit 150 can alternatively or preferably in addition has a high pass filter 151 that is configured to high pass filter the deformation rate signal in order to remove or at least suppress any DC component from the deformation rate signal. Experiments have been successfully conducted with a 2nd order high pass 0.1 Hz filter.
Thus, the filter unit 150 preferably both conduct high pass and low pass filtering to remove or suppress the DC component and noise from the deformation rate signal and further to improve the signal quality.
Both the low pass and high pass filters can be either analog or digital. In case of analog filtering, the filter can be switch cap-based or component-based using resistors, capacitors and operational amplifiers. In case of the digital domain, the filter can either have a finite or infinite impulse response, i.e. be either so-called FIR or IIR filters. Typical IIR filters can be Butterworth, Chebyshev, elliptical, Bessel or other.
The IMD 100 preferably includes a cardiac cycle identifier 134 configured to identify a start point and an end point of a cardiac cycle in the filtered deformation rate signal. The cardiac cycle identifier 134 advantageously identifies the start sample corresponding to the start point of the cardiac cycle and the end sample corresponding to the end point of the cardiac cycle in the filtered deformation rate signal. The cardiac cycle identifier 134 preferably identifies the start and end samples based on the signal representative of electric activity of at least a portion of the heart generated by the data acquisition unit 160. It is well known in the art that the start and end points of a cardiac cycle is relatively easily identifiable from an IEGM signal generated by the data acquisition unit 160. The cycle identifier 134 can then simply identify the samples in the filtered deformation rate signal that coincide in time with the start and end points as identified in the IEGM signal. The mapping of the start and end points in the IEGM signal to the start and end sampled in the filtered deformation rate signal is easily performed based on the relation in sampling frequency of the IEGM signal versus the deformation signal.
Once the cycle identifier 134 has identified the start sample and the end sample of a cardiac cycle, the peak identifier 132 can parse through the samples in the filtered deformation rate signal between the start and end samples in order to find the maximum deformation rate for that cardiac cycle.
The cycle identifier 134 preferably identifies respective start and end sample for multiple, preferably consecutive cardiac cycles, in the filtered deformation rate signal. The peak identifier 132 can then identify respective maximum deformation rate values for each of these consecutive cardiac cycles or at least a portion thereof.
The pressure calculator 133 of the IMD 100 preferably calculates the value representative of the systemic blood pressure based on the amplitudes or magnitudes of multiple maximum deformation rate values. Various processings are possible and within the scope of the embodiments. The pressure calculator 133 preferably calculates at least one value representative of the systemic blood pressure as the average of multiple maximum deformation rate values, preferably multiple consecutive maximum deformation rate values. Consecutive maximum deformation rate values correspond to respective maximum deformation rate values for consecutive cardiac cycles. In a particular embodiment, the pressure calculator 133 is configured to calculate a signal representative of the systemic blood pressure. This signal is obtained from a moving average of multiple consecutive deformation rate values:
where psi represents sample i of the signal, fdrsi represents sample j of the filtered deformation rate signal and N is a predefined positive number equal to or larger than two.
Averaging over multiple maximum deformation rate values is generally preferred since it decreases the effects of confounding factors that may impact the deformation signal from the cardiomechanic sensor. These effects can, for instance, be micro movements of the cardiomechanic sensor, beat-to-beat variability in the deformation rate, noise on the sensor, etc. By averaging over a period of time, corresponding to a number of cardiac cycles, these effects be canceled out or at least decrease in magnitude.
Instead of using a simple moving average, the pressure calculator 133 can use a weighted average with different weights for different maximum deformation rate values. In such a case, weights for more recent samples could be set higher as compared to maximum deformation rate values occurring further back in time.
Experimental studies have been successfully conducted with a moving average over 30 cardiac cycles. The number of heart cycles over which the deformation signal may be averaged is preferably in the range of 10 to 180 cycles.
It is also possible, most preferably in case of a continuous deformation measurement, to process the obtained maximum deformation rate values with an exponential averaging method or a so-called IIR filter. An exponentially weighted averaging method is a type of IIR filter well known in the art and it is preferable for implementation into a medical device as it does not require storing of a large set of filter coefficients and requires less calculations.
Although less preferred than using an average of maximum deformation rate values, the pressure calculator 133 could use the median of multiple maximum deformation rate value as the value representative of the systemic blood pressure of the patient.
The value or signal generated by the pressure calculator 133 is representative of the systemic blood pressure of the patient and is, in fact, a relative systemic blood pressure. In a particular embodiment, the IMD 100 can be configured to not only be capable of measuring relative systemic blood pressure but actually generate a value or signal representative of absolute systemic blood pressure, such as MAP and/or PP.
In such a case, the IMD 100 is employed together with a device capable of recording absolute blood pressure, such as MAP and PP. For instance, MAP and PP can be measured using a standard arm cuff blood pressure apparatus. Alternatively, any other well-known device for measuring MAP and/or PP, such as any of the devices mentioned in the background section, can be used. The values representative of systemic blood pressure from the pressure calculator 133 are then transmitted by the transmitter/transceiver 190 to the data processing unit 300 illustrated in
The data processing device 300 then calculates scale or mapping factors based on the relative systemic blood pressure values and the MAP and/or PP values. Two such factors are needed per blood pressure parameter so the pressure measurements conducted by the IMD 100 and the external MAP and/or PP measuring device have to be conducted for at least two different MAPs and/or PPs. This can, for instance, be achieved by letting the patient 20 walk on a treadmill or pedal on a bicycle. The more different MAPs and/or PPs that are tested the better the quality of the mapping factors. The data processing unit 300 can use different well-known optimization techniques in order to derive the mapping factors that allow conversion or mapping of the value representative of relative systemic blood pressure from the pressure calculator into an absolute MAP or an absolute PP value. A non-limiting example of such an optimization technique is the method of least squares and variants thereof. Such a method can then be used to derive the factors of linear polynomial: y=kx+m, where y represents the absolute MAP or PP value, x is the value representative of system blood pressure from the pressure calculator 133 and k and m are the mapping factors determined by the data processing device 300.
The determined mapping factors can then be downloaded into the IMD 100 using the communication device. The mapping factors are received by the receiver/transceiver 190 and stored by the controller 130 in the memory 170. The memory 170 can therefore store one set of mapping factors for conversion into absolute MAP values, one set of mapping factors for conversion into absolute PP values or two sets of mapping factors, where one is used for absolute MAP conversion and the other for absolute PP conversion.
The IMD 100 preferably has an absolute pressure calculator 135 configured to calculate an absolute MAP value (yMAP) representative of an absolute MAP of the patient and/or absolute PP value (yPP) representative of an absolute PP of the patient based on the value calculated by the pressure calculator 133 and a MAP scaling factor (kMAP) and a MAP offset factor (mMAP) or a PP scaling factor (kPP) and a PP offset factor (mPP): yMAP=kMAPx+mMAP and yPP=kPPx+mPP.
The controller 130 is further coupled to the memory 170 by a suitable data/address bus, wherein the programmable operating parameters used by the controller 130 are stored and modified, as required, in order to customize the operation of the IMD 100 to suit the needs of a particular patient. Such operating parameters define, for example, time threshold, pacing pulse amplitude, pulse duration, electrode polarity, rate, sensitivity, automatic features, time interval between pacing pulse of an applied pacing pulse sequence and the previously mentioned mapping factors.
The memory 170 may also advantageously store diagnostic data collected by the IMD 100. The diagnostic data include the IEGM signal from the data acquisition unit 160, the signal from the pressure calculator 133 and optionally absolute MAP and/or PP values or signal from the absolute pressure calculator 135.
Advantageously, the operating parameters of the IMD 100 may be non-invasively programmed into the memory 170 through the transceiver 190 in communication via a communication link with the previously described communication unit of the programmer. The controller 130 activates the transceiver 190 with a control signal. The transceiver 190 can alternatively be implemented as a dedicated receiver and a dedicated transmitter connected to separate antennas or a common antenna, preferably a radio frequency (RE) antenna 195.
The IMD 100 additionally includes a battery 180 that provides operating power to all of the circuits shown in
In
These units can then be implemented as a computer program product stored on the memory 170 and loaded and run on a general purpose or specially adapted computer, processor or microprocessor, represented by the controller 130 in the figure. The software includes computer program code elements or software code portions effectuating the operation of the derivative calculator 131, the peak identifier 132, the pressure calculator 133, the cardiac cycle identifier 134 and absolute pressure calculator 135. The program may be stored in whole or part, on or in one or more suitable computer readable media or data storage means that can be provided in an IMD 100.
In an alternative embodiment, the derivative calculator 131, the peak identifier 132, the pressure calculator 133, the cardiac cycle identifier 134 and absolute pressure calculator 135 are implemented as hardware units either forming part of the controller 130 or provided elsewhere in the IMD 100.
The signal filter unit 150 has been illustrated as separate unit of the IMD 100. In an alternative embodiment, the filter unit 150 is implemented as forming part of the controller 130 in similarity to the other units implemented and run therein.
The above presented figures thereby confirm that the embodiments can be used to accurately generate a value or signal representative of systemic blood pressure in animals and furthermore can be used to obtain accurate representations of absolute MAP and/or PP values.
The embodiments were initially designed to obtain the systemic blood pressure value or signal using a cardiomechanic sensor implanted in connection with the left ventricle and advantageously in a coronary vein of the left ventricle. At this position the cardiomechanic sensor efficiently captures deformation of the myocardium of the left ventricle as the sensor is positioned close to or even in direct contact with the myocardium.
Experiments have also been conducted with the cardiomechanic sensor arranged in the right ventricle. It was then very surprising that the same cardiomechanic sensor as used in the left ventricle and the same signal processing could be employed and still get a very accurate value representative of the systemic blood pressure also with a right ventricular cardiomechanic sensor. It is speculated that this sensor will also capture the deformation of the myocardium partly from the myocardium of the right ventricle but also from myocardial deformations that are transplanted through the blood present in the right ventricle to thereby be captured by the cardiomechanic sensor.
Thus, although arranging the cardiomechanic sensor in connection with the left ventricle is thought to be preferred, the embodiments also cover implantation in or in connection with the right ventricle. Not all patients receive a lead in a left-sided coronary vein, why placing the sensor in the right ventricle may be the only option unless an extra lead is to be implanted.
The pressure value is preferably calculated as an average of the magnitudes or amplitudes of respective maximum deformation rate values for multiple cardiac cycles, preferably multiple consecutive cardiac cycles. Step S4 preferably generating not a single pressure value but rather a signal embodying multiple samples, each having a data element corresponding to the systemic blood pressure at the point in time associated with the particular sample.
An optional additional step of the estimating method involves calculating an absolute value representative of an absolute systemic blood pressure, such as MAP and/or PP, based on the pressure value and a scaling factor and offset factor as previously described.
The pressure value or signal generated according to the embodiments is not only of high diagnostic value itself. It can further be used by the IMD as is briefly discussed below.
One of the most common comorbidities of pacemaker and ICD patients is hypertension. By regularly measuring the absolute MAP or PP, the IN/ID acquires an objective trend of the hyper (or indeed hypo) tension of the patient. This trend can be viewed by the physician, e.g. at follow-ups. By studying the long term trend of the pressure, the effects of antihypertensive (or antihypotensive) drugs can be studied. Thus, the embodiments can aid in drug titration.
For ICD patients inappropriate shocks are a difficult problem. Fast supraventricular tachycardias (SVTs) are often misinterpreted as ventricular tachycardias (VTs) and ventricular therapy, such as shock or anti-tachycardia pacing (ATP) is applied. By studying the MAP and/or PP during the arrhythmia the IMD can conclude whether the arrhythmia is a VT or SVT. VT generally leads to a reduction in or low MAP and/or PP and therefore such a low MAP and/or PP can be used to select a more aggressive anti-arrhythmia treatment by the IMD, such as to shock the patient, than if the MAP and/or PP is high during the arrhythmia event. If the MAP and/or PP is high the probability is high that the patient is conscious and then it is better to try a less painful type of therapy, such as ATP, or to withhold therapy altogether.
Vasovagal syncope has two main forms: a cardioinhibitory and a vasodilatory type. In the first case, the heart rate of the patient is decreased and the blood pressure is thereby decreased. This can today be detected by an IMD without any cardiomechanic sensor and using heart rate monitoring. In the second case, the vascular bed is dilated, which decreases the pressure. This can not be detected today by an IMD that does not have a cardiomechanic sensor according to the embodiments. Thus, embodiments can detect both types of vasovagal syncope by measuring MAP or diastolic blood pressure. By increasing the heart rate by pacing syncope episodes can be avoided.
Thus, the embodiments as disclosed herein can be used to control the delivered therapy by the IMD and diagnosis, for instance detection of syncope, arrhythmia detection and discrimination, drug titration of beta-blockers, detection of physical and emotional stress, long term pressure variability statistics can be collected, detection of pressure alternans, such as alternans of 2:1 patterns, can be an early marker of future severe arrhythmia events.
Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10166535.4 | Jun 2010 | EP | regional |
The present application claims the benefit of the filing date of U.S. Provisional Application 61/383,547, filed Sep. 16, 2010.
| Number | Date | Country | |
|---|---|---|---|
| 61383547 | Sep 2010 | US |
