Device for analyzing the function of a heart

0001The present invention generally relates to a device for analyzing the function of a heart and particularly to a device that comprises means for accurately determining the heart's condition.

0002In the monitoring, diagnosis and treatment of a heart's function, accurate determination of the heart's current condition, with minimal risk of erroneous interpretations, is important. Automatic monitoring of the heart is a valuable asset in the treatment of heart disease so a therapeutic measure can be instituted without delay when necessary.

0003The electrocardiogram (ECG) is one heart variable which is an indicator of a heart's function. Sensing the ECG in order to obtain a measurement signal which can be evaluated in establishing the condition of the heart is prior art.

0004US-A-4,417,306 describes an apparatus which monitors and stores heart signals. The apparatus senses the ECG, and the ECG signal must have a predesignated slope, amplitude, duration and course to be accepted as a heart beat. The QRS complex is the main segment sensed, i.e. the electrical signals which occur in the heart when there is a ventricular beat (ventricular systole).

0005US-A-4,453,551 describes an apparatus designed to detect ventricular fibrillation (VF). The apparatus senses the ECG signal from the heart, digitizes it and amplifies it to a predetermined amplitude. The amplified signal can then be analyzed in different ways to ascertain whether or not VF is present. For example, the statistical distribution of gradients or the frequency of the maximum negative gradient can be analyzed.

0006EP-A-0 220 916 describes an apparatus designed to detect the presence of ventricular tachycardia (VT) and VF and to supply treatment to terminate these conditions. The apparatus senses the heart's ECG at a plurality of points on the heart and determines the sequence in which the signals are detected at the different measurement points. In VT and VF, the sequence deviates from the normal pattern in different ways.

0007One way to graphically elucidate the electrocardiogram by plotting the voltage in a recorded electrocardiogram against the time derivative of the voltage is described in an article entitled "Phase Plane Plot of Electrograms as a Marker of Ventricular Electrical Instability During Acute Ischemia: Initial Experimental Results and Potential Clinical Applications", published in the journal PACE, Vol 15, part II, November 1992, pp. 2188-2193. This procedure produces a curve corresponding to the ECG signal. The article shows that there is a relationship between changes in parts of the curve during acute ischemia and the development of ventricular fibrillation. The authors of the article feel that a presentation of an electrocardiogram in graphical form can be an excellent complement to traditional, real-time presentation. Therein is described how a parameter signal (dV/dt) is generated from a measurement signal (V(t)), and related values of the measurement signal and parameter signal are analyzed. The related values correspond to coordinates forming a curve in a coordinate system, with the measurement signal and parameter signal as coordinate axes.

0008US-A-3 616 791 describes a device for analysing the function of a heart according to the preamble of claim 1.

0009The invention aims at achieving a device which analyzes the function of the heart in a reliable, efficient but still simple manner.

0010The invention also aims at achieving a device which can be used for diagnosing heart defects, monitoring heart functions and adapting therapy to make treatment as safe and effective as possible.

0011These and other aims are achieved according to the invention as embodied in a device according to claim 1.

0012According to the invention the predetermined number of conditions corresponds to a predetermined number of areas in the coordinate system, whereby an evaluation unit determines the sequence in which the curve passes the predetermined number of areas.

0013A normally working heart (as in a healthy heart) is hemodynamically stable, and a virtually identical sequence of related values is generated from one heart cycle to another, so different conditions are satisfied in the same sequence cycle after cycle. Pathological changes and other abnormal conditions in a heart affect the satisfying of different conditions in a distinct way and can therefore be easily identified. The conditions can consist of any kind of mathematical relationship. Certain relationships for different, known changes in a specific individual can also be used.

0014Having the related values correspond to coordinates in a coordinate system results in a clarified analysis. The generated curve becomes virtually identical, from one heart cycle to another, as long as the heart functions in a constant way.

0015Defining different areas which the curve can pass in various heart conditions and determining which areas the curve passes and in which sequence the areas are passed make it possible to identify different arrhytmias and anomalies. The curve changes even when certain other specific cardiac events occur, such as retrograde conduction and extrasystoles. Spontaneous and stimulated heart beats give rise to different curves, and the device can be used for detecting both spontaneous heart beats and stimulated heart beats.

0016A simultaneously filed patent application, application No. 0 607 511, describes a device which analyzes heart-related signals by utilizing a two- or multidimensional representation of the signals.

0017According to a preferred embodiment of the device, the means for generating at least one parameter signal from the measurement signal may comprise a derivator which derives the measurement signal.

0018With the derivative of the measurement signal as a parameter, related values are obtained which, for a normal heart, form an essentially closed curve with a small loop inside a larger loop in the coordinate system. This curve changes when e.g. a VT or a VF is present.

0019With an integrated signal, integration occurring over a plurality of heart cycles, the system is more stable, and the curve does not wander outside the predetermined decision areas. When the integration interval is shorter than a heart cycle, a parameter signal is produced, which with the measurement signal forms a curve in the corresponding way as the measurement signal and the measurement signal's derivative. Integration produces simultaneous filtration of noise.

0020With a plurality of lines, the coordinate system is subdivided into a plurality of areas which can be made larger or smaller. On the basis of the comparators' output signals, the course of the curve can be followed throughout each heart cycle and compared with various predetermined courses in order to determine the condition of the heart. The lines can be parallel to one another, perpendicular to one another, arise from a common point, arise from a plurality of points etc.

0021In order to be able to shift the curve in relation to the coordinate system and additionally form a plurality of different lines, it would be an advantage if the evaluation unit further comprised a reference signal generator which generates a reference signal serving as the input signal for at least one comparator.

0022According to a preferred embodiment of the invention the device comprises a first timer for measuring the time in which the related values satisfy at least one specific condition.

0023This supplies information in addition to that provided by condition satisfaction itself or, when a curve is described, the course of the curve in relation to the measurement signal-parameter signal (MS-PS) diagram, thereby increasing the possibility of attaining reliable identification of various conditions in the heart.

0024Alternately, or as an additional complementary condition, the device may comprise a second timer to measure the time elapsing from when the relevant values satisfy a first specific condition until they satisfy a second specific condition.

0025It would be an advantage if the device comprised a pulse generator for generating and emitting stimulation pulses to the heart according to the state of the heart.

0026Hereby, the device can supply a therapeutic measure when necessary.

0027The invention will be described in greater detail with the aid of six Figures, whereby <ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 shows, in block diagram form, an embodiment of the device according to the invention;</li><li>FIG. 2 shows a spontaneous heart signal and a stimulated heart signal respectively;</li><li>FIG. 3 illustrates, in diagram form, how the device works;</li><li>FIG. 4 shows a block diagram of a comparator unit in the device;</li><li>FIGS. 5, 6 illustrates, in constitutional graphs, two different heart sequences.</li></ul>

0028The pacemaker 1 in FIG. 1 is connected to a heart 7, whereby a tip electrode 3 and a ring electrode 4 are placed in the right ventricle of the heart 2 and connected, via a first electrode conductor 5 and a second electrode conductor 6 , to a pulse generator 7 in the pacemaker 1. The pulse generator is also connected to pacemaker can 20 which functions as an indifferent electrode, i.e. the pulse generator 7 can either emit stimulation pulses between the tip electrode 3 and the ring electrode 4 or between the tip electrode 3 and the pacemaker can 20. A detector 8 is connected in parallel to the first electrode conductor 5 and the second electrode conductor 6 . The detector 8 senses the heart's electrical activity, i.e. the ECG, and sends the measurement signal to a signal shaper 9 . The signal shaper 9 first filters the signal in a band-pass filter 10. The band-pass filter 10 primarily eliminates high frequency noise which could otherwise overwhelm subsequent signal components. After filtration, the filtered signal is sent to an amplifier 11, to a derivator 12 and to an integrator 13 which integrates the signal for a plurality of heart cycles. In this manner, three parameter signals are formed from the single measurement signal. The signal shaper 9 contains a reference generator 14 which generates a reference signal. The reference generator 14 is connected to pacemaker electronic circuitry 15 which includes e.g. a battery and microprocessor for controlling the pacemaker. The four signals, at least two of which constituting coordinates forming a curve in a coordinate system, are sent to a comparator unit 16 . The comparator unit 16 comprises a plurality of comparators representing different lines in the coordinate system. Each comparator generates an output signal when the curve is on a specific side of the line the comparator represents. So a plurality of output lines, X1, X2 , ... , Xn , runs from the comparator unit 16 to a sequence analyzer 17 which senses which comparator emits an output signal and the sequence in which this occurs. A plurality of sequence signal lines, Y1, ... , Ym , runs from the sequence analyzer 17 to pacemaker electronic circuitry 15 in which the microprocessor decides whether any action should be taken on the basis of the signal from the sequence analyzer 17.

0029A physician, can, with the aid of a programming unit 19 , communicate with pacemaker electronic circuitry 15 , via a telemetry unit 18, in order to e.g. change the lines the comparators in the comparator unit 16 represent or in order to retrieve stored information on detected sequences and the treatment given.

0030FIG. 2 shows an example of two different signals which can be detected in the pacemaker 1. A spontaneous heart signal 25 is shown at the top. Here, the spontaneous heart signal 25 only shows the QRST complex in the heart signal 25, i.e. the signals generated by ventricular depolarization in systole and repolarization in diastole.

0031A stimulation pulse 26, resulting in a stimulated heart signal 27 , is shown at the bottom. Again, only the ventricular signal is shown. As a direct comparison shows, the stimulated heart signal 27 lacks the positive R wave found in the spontaneous heart signal 25 , while the negative part of the stimulated heart signal 27 is simultaneously more pronounced than the S wave in the spontaneous heart signal 25 . The repolarization wave in the stimulated heart signal 27 is larger than the T wave in the spontaneous heart signal 25.

0032In FIG. 3, the proportional value is plotted against the derivative for every point in time. The spontaneous heart signal 25 then generates the curve 30 , and the stimulated heart signal 27 generates the curve 31.

0033The signal lines the comparators represent have also been entered into the coordinate system. The comparators will be described in greater detail in conjunction with FIG. 4. As can be seen, the morphological difference between the spontaneous heart signal 25 and the stimulated heart signal 27 is depicted with greater clarity in the PD-coordinate system than in the real time diagrams in FIG. 2.

0034As noted above, the comparator unit 16 contains a plurality of comparators. FIG. 4 shows one way of devising the comparator unit 16. With four comparators 32, 33, 34 , 35 , four different limit conditions are created which correspond to lines 45, 46, 47, and 48 in the PD diagram in FIG. 3. Input signals to the comparator unit 16 consist of the proportional signal in signal line 36 , the integrated signal in signal line 37 , the reference signal in signal line 38 and the derived signal in signal line 39. In the first comparator 32 , the proportional signal is led to the negative input, via first resistor 40a . The integrated signal is also sent, via a potentiometer 41a, to the negative input. To the positive input are connected the derived signal, via a second resistor 40b, and the reference signal, via a second potentiometer 41b . The output signal from the first comparator 32 has been designated X3, and the following conditions must be satisfied for the first comparator 32 to emit an output signal: <maths id="math0001"><math display="block"><mi mathvariant="normal">D</mi><mo mathvariant="normal">+</mo><msub><mi mathvariant="normal">C</mi><mi mathvariant="normal">b</mi></msub><mo mathvariant="normal">⋅</mo><msub><mi mathvariant="normal">V</mi><mi>ref</mi></msub><mo mathvariant="normal">⋅</mo><mi mathvariant="normal">P</mi><mo mathvariant="normal">-</mo><msub><mi mathvariant="normal">C</mi><mi mathvariant="normal">a</mi></msub><mo mathvariant="normal">⋅</mo><mi mathvariant="normal">l</mi><mo mathvariant="normal">></mo><mn mathvariant="normal">0.</mn></math><img file="EP0613655B2_D0001.tif" /></maths> where the proportional signal is generally designated P, the derived signal is generally designated D, the integrated signal is generally designated I, the reference signal is generally designated V_ref, the resistors 40a, 40b etc. all have the same value set at one, and the potentiometers' value in relation to the resistors is designatedC_a for the first potentiometer 41a, C_b for the second potentiometer 41b etc.

0035Thus, the first line can be written: <maths id="math0002"><math display="block"><mi mathvariant="normal">D</mi><mo mathvariant="normal">=</mo><mi mathvariant="normal">P</mi><mo mathvariant="normal">+</mo><msub><mi mathvariant="normal">C</mi><mi mathvariant="normal">a</mi></msub><mo>⁢</mo><mi mathvariant="normal">l</mi><mo mathvariant="normal">-</mo><msub><mi mathvariant="normal">C</mi><mi mathvariant="normal">b</mi></msub><mo>⁢</mo><msub><mi mathvariant="normal">V</mi><mi>ref</mi></msub><mo mathvariant="normal">,</mo></math><img file="EP0613655B2_D0002.tif" /></maths> which produces the line 45 in the PD diagram in FIG. 3. So the first comparator 32 generates an output signal when the curve 30 is above the line 45.

0036In the corresponding manner, the proportional signal is connected, via a resistor 40c and a first inverter 43a , to the negative input in the second comparator 33 . The integrated signal is also connected, via a third potentiometer 41c and the first inverter 43a, to the negative input. To the positive input are connected the derived signal, via a fourth resistor 40d, and the reference signal, via a fourth potentiometer 41d. The output signal from the second comparator 33 has been designated X2, and the condition D + C_dV_ref - (- P - C_cl) >0 must be satisfied for an output signal to be received from the second comparator 33 , i.e. the line D=-P-C_cl-C_dV_ref. This is line 46 in the PD diagram.

0037For the comparator 34 , the proportional signal is analogously connected to the negative input, via a fifth resistor 40e and a second inverter 43b , and the integrated signal, via a fifth potentiometer 41e and the second inverter 43b . To the positive input, the derived signal is connected, via a sixth resistor 40f , and reference signal, via a sixth potentiometer 41f. Output X4 from the third comparator 34 produces an output signal when the curve is above line 47 in the PD diagram.

0038The fourth comparator 35 has its negative input signal connected to virtual ground 42, and to the positive input are connected the derived signal, via a seventh resistor 40g , and the reference signal, via a seventh potentiometer 41g. The output X1 produces an output signal when the curve is above line 48 in the PD diagram.

0039As shown by the curves in the PD diagram in FIG. 3, the spontaneous curve 30 encloses both a first point 49 at the intersection of lines 45 and 46 and a second point 50 at the intersection of lines 45, 47 and 48 , whereas the stimulated signal 31 only encloses the second point 50. So determining by the sequence analyzer 17 of whether the generated curve encloses both the first point 49 and the second point 50 is sufficient to distinguish spontaneous heart signals from stimulated heart signals and to determine whether the signal is spontaneous or stimulated.

0040More generally, the areas formed between the lines 45, 46, 47, and 48 can be said to represent different states, since the combination of output signals from the comparators 32 , 33 , 34 , 35 are unique to each area. If, for example, the origin in the PD diagram is viewed, conditions for emission of a signal by the first comparator 32 , the third comparator 34 and the fourth comparator 35 , corresponding to lines 45 , 47 , and 48 respectively, are satisfied, resulting in a signal state of 1011 for signal outputs X1 X2 X3 X4 . If each state described by a curve is recorded, a number of different heart conditions can be identified. The function of the sequence analyzer 17 can thereby be described with constitutional graphs, as in FIGS. 5 and 6.

0041FIG. 5 shows eight states, corresponding in principle to the states which can occur in the abovedescribed embodiment. The intersection of lines 46 and 48 (not shown) gives rise to an additional area and state which is not shown in the FIG.

0042The spontaneous curve shown in FIG. 3 will commence in state 1011 in the constitutional graph, then passes line 46 over to state 1111 and then, after passing line 45, go over to state 1101 etc., traversing the entire sequence back to state 1011.

0043FIG. 6 shows the sequence between states covered by the stimulated signal 31 . It also begins in state 1011 but subsequently crosses line 45 and goes directly to state 1011, thereafter following the same sequence as the spontaneous signal 30.

0044On the basis of the possible sequences a signal is able to follow, the sequence analyzer 17 can be devised to identify specific state change sequences, thereafter emitting a signal to pacemaker electronic circuitry 15 in the pacemaker 1. The lines 45 - 48 can be shifted in different ways with the potentiometers 41a - 41g so as to adapt conditions to different patients. Each sequence, or series of changes in state, corresponds to a specific morphology for the input signal, and since morphology changes in different ways in different heart conditions, such as tachyarrhythmias and extrasystoles, fast and reliable identification of the heart's current condition is achieved. If necessary, therefore, suitable therapy can be instituted immediately. Especially with patients suffering from different types of tachyarrhythmia, a pacemaker with an analyzer according to the invention can easily identify the different types and institute the therapeutic measure most appropriate to the condition in question.

0045To increase reliablity in the identification of different heart conditions, the sequence analyzer 17 can be equipped with a timer which measures the time a generated curve is in a specific state.

0046Since every condition is unique, the sensing of every condition transition is not always necessary, only a few of which needing to be executed in a specific way (e.g. come from a specific state or take a specific amount of time to pass between two specific states).