Method and a Device for Monitoring Heart Function

In general, the present invention addresses the problem of monitoring heart function.

Correct monitoring of heart function is important in order to be able to produce, for example:

systems for monitoring the behaviour of the cardiocirculatory system and of the heart muscle, particularly for diagnostic and therapeutic purposes, and

monitoring devices which can be implanted for diagnostic and therapeutic purposes, possibly in conjunction with heart stimulators (pacemakers), particularly of the so-called "rate responsive" type, in which the characteristics of the stimulation activity can be varied in dependence on one or more parameters related to the physical activity of the person wearing the pacemaker, and with defibrillators.

Naturally, the possible fields of use mentioned above should in no way be interpreted in a limiting sense.

Over the years, various tecnhiques have been proposed for solving the problem of monitoring heart function.

For example, many techniques are based on the monitoring of electrical parameters of the activity of the heart. For example, this applies to electrocardiographic (ECG) equipment and to many pacemakers currently produced. The main disadvantage of these solutions is that the electrical parameters detected provide a very incomplete and, in many cases, inadequate description of the behaviour of the heart and of its functioning.

Other solutions provide for the use of sensors for monitoring chemical parameters: however, these solutions suffer from the main disadvantage that their operation is based on surface phenomena so that after a fairly short period of use they may no longer be capable of ensuring correct operation: this excludes the use of these solutions, for example, if the sensor is to be made suitable for implantation in the wearer's body for a long time.

Other solutions are based on techniques such as echocardiography, cardiac catheterization, and seismographic auscultation of the cardiac tones (see, for example, patent No. U.S. Pat. No. 4,989,611 in this connection). These techniques have the disadvantage that they are usually suitable only for interventions of short duration which can be performed only in acute phases. Moreover, the results of the monitoring are usually fairly approximate.

Finally, there are known solutions in which quantities derived directly from the measurement of ventricular pressure or, in general, quantities related to the contractility of the heart are used as parameters indicative of heart function. All these solutions have the disadvantage that, as an essential requisite for correct operation, they require the sensors to be able to remain permanently exposed to the effect of the parameter monitored: this is difficult to ensure, particularly when the sensor may interact with the body tissues so as to hinder or even prevent correct operation. It is very difficult, if not impossible, particularly when the sensors concerned are to be connected directly to the pulsating wall of the heart, to correlate the signal produced thereby (which is indicative primarily of the radial contractile activity of the heart muscle) to the essential fluido-dynamic parameters of the behaviour of the heart.

In fact, it should not be forgotten that the main function required of heart muscle is to act as a pump to bring about the movement of the blood within the cardiocirculatory system with predetermined flows and flow-rates.

For clarity, it should also be noted that the foregoing relates primarily to the monitoring of heart function: this monitoring action should not be confused with another, often related, function, that is, the monitoring of the physical activity or the metabolic needs of the patient: this field is the subject of intense research activity, to which numerous patent documents such as, for example, European Application EP-A-0 383 732 bear witness.

The object of the present invention is to provide a solution which enables heart function to be monitored correctly and which radically overcomes the problems of the solutions of the prior art.

According to the present invention, this object is achieved by virtue of a method and a device for monitoring heart function having the specific characteristics recited in the following claims.

As already stated, the solution according to the invention is suitable for application of a wholly general character, particularly as regards the production of diagnostic and/or therapeutic systems, possibly in conjunction with pacemakers and defibrillators. Naturally, those listed are only some of the possible fields of use of the invention.

The invention will now be described, purely by way of non-limiting example, with reference to the appended drawings, in which:

FIG. 1 shows schematically a possible configuration of a device operating according to the invention,

FIG. 2 is a diagram showing the physical principle upon which the invention is based, and

FIG. 3 comprises three graphs, one above another, indicated a), b) and c), indicative of the behaviour of various heart-function monitoring signals.

FIG. 4 is an extended block diagram showing components of the processing device according to the invention.

By way of premise, it is considered useful to mention some theoretical principles upon which the invention is based. In fact, the invention provides for the use of the momentum and/or velocity of the heart masses, as well as the variations thereof over time, as primary parameters for identifying, detecting and monitoring heart activity.

As a first approximation, the heart and the connected vascular system may be considered as an isolated system in which movements of muscular and fluid masses take place in definite cycles and at definite intervals. The total mass is constant and is distributed within a volume which coincides approximately with the heart muscle and the main vessels.

At any moment, in the absence of external forces, the total momentum Q of the system is zero; this quantity is given by: ##EQU1## in which u(r) represents the velocity of the mass contained in the volume element dV, in which .rho.(r) is the density at the generic point r and the integral is extended to the volume V containing the system.

The movement of a certain mass must correspond to a movement of other masses which renders the total momentum zero.

Thus, considering, for example, the systolic phase, if m.sub.1 .multidot.v.sub.1 represents the momentum resulting from the discharge of blood, the heart mass as a whole will have an equal and opposite momentum m.sub.2 v.sub.2, that is

The velocity v.sub.2 of the centre of masses of the heart mass, for example, is consequently a function of the velocity of the blood and of the mass of blood discharged.

Moreover, if, for example, a ventricular chamber (a subsystem) from which a quantity of blood is expelled within a certain period is considered, the following equation applies: ##EQU2## in which F.sub.t is the external force acting on the subsystem, M is the variable mass of the subsystem, v is the velocity of its centre of the masses and u the flow velocity relative to the centre of masses of the subsystem.

In this particular case, if the mono-directional model of FIG. 2 is considered as a first approximation, it may be stated simply that: ##EQU3## in which .epsilon. is the cross-section of the discharge orifice and .rho. is the density of the blood.

In this case, F.sub.t represents a force acting on the ventricular chamber which causes a retrograde acceleration and hence a movement; in reality, this movement is opposed and damped by the resilient biasing forces exerted by the tissues surrounding and supporting the heart.

More generally, it is found that the velocity of the blood discharged and the mass discharged are functions of the velocity of the system and of the external forces applied. Naturally, the phenomena which occur in the circulatory system are more complex and difficult to describe; however, for the purposes of the present invention it is necessary neither to seek a description of the physical model, nor to define the system under examination precisely.

In fact, it follows from the basic laws set out above and from the fact that the individual events of the heart cycle are separated in time and take place in ways which are known from physiology, that for each individual event there is:

a direction along which the component of the momentum corresponding to the event is substantially directed within the time interval considered, and

an element of heart volume which moves in the direction specified substantially like the centre of the masses of the subsystem involved in the event.

If the position, the velocity, or the acceleration of the element in the direction indicated is measured by a transducer as a function of time (the three quantities being correlated by integration and differentiation operations) these are functions of the velocity and of the mass of blood in movement as well as of the resilient biasing forces.

Variations in the characteristics of each individual event in comparison with normal situations, modifications of the transportation dynamics and the pumping of blood, and hence diagnostic and clinical monitoring signals can consequently be detected from a record which gives one of the mechanical quantities mentioned, or a quantity derived therefrom, as a function of time.

In the drawing, the heart muscle is generally indicated C, the left ventricle LV and the right ventricle RV thereof being shown specifically, separated by the interventricular septum IV.

The so-called atrioventricular septum which separates the two ventricles LV and RV from the respective atria LA and RA is generally indicated AV. The aorta which branches out from the left ventricle through the aortic valve AA is indicated A. The other valves of the heart muscle are indicated MV1 (the pulmonary valve), MV2 (the tricuspid valve) and MV3 (the mitral valve) respectively.

A mechanical/electrical sensor for implantation is generally indicated 1. In the embodiment illustrated, the sensor 1 is located on the interventricular septum IV in a position near the atrioventricular septum AV. It is introduced into the right ventricle RV and kept in contact with the interventricular septum IV by a catheter 2 which extends through the upper vena cava and the tricuspid valve MV2 to reach a supply/processing module 3 which is preferably implanted in the patient's body. The condition in which it is implanted in the body is shown schematically by means of the cutaneous barrier, indicated B, separating the portion of the system which is implanted in the body from any accessory elements such as a receiver/transmitter module 4 and a signal analyser 5 situated outside the patient's body.

The use of the transmitter-receiver module 4 and of the analyser 5 is generally envisaged when the system according to the invention is used for diagnostic purposes, that is, for monitoring the wearer's heart function externally.

When the solution according to the invention is used in combination with a heart stimulator (a pacemaker) or with a defibrillator which can be implanted, the module 3 usually comprises transmitter/receiver means which can transmit signals indicative of the heart activity monitored to the exterior, towards the module 4, and can also receive function-programming signals from the exterior, also by means of the module 4.

This is all in accordance with criteria widely known in the art which do not need to be described in detail herein and, moreover, are not relevant for the purposes of putting the invention into practice: this relates, in particular, to the methods adopted for the transcutaneous transmission of the signals between the modules 3 and 4. In particular, this transmission may take place by means of electromagnetic waves, ultrasound waves, or other suitable vectors.

Naturally, for use in combination with a pacemaker or a defibrillator, it is also possible to do without the transcutaneous transmission. In this case, the system is independent and autonomous and wholly implanted in the wearer's body.

The main characteristic of the sensor 1 used in the device according to the invention is that it is made sensitive primarily to the momentum (and hence, in particular, to the velocity) of the heart mass as a whole, seen theoretically as a vectorial quantity concentrated at the centre of masses of the heart mass. Theoretically, since, as is well known, the dynamic behaviour of the heart mass is such as to give rise to continuous variations in the position of its instantaneous centre of mass.

It is usually preferable to position the implanted sensor 1 on the interventricular septum IV and/or on the atrioventricular septum AV, particularly as far as its application to systems for the diagnostic monitoring of heart activity is concerned. The interventricular and atrioventricular septa IV and AV are in fact the sites within the heart mass which are least exposed to the actual contractile activity of the external walls of the heart muscle. If the momentum or velocity of the heart mass is to be monitored as the main parameter of the activity of the heart, positioning in regions which are not subject to the beating of the heart itself is thus generally preferable.

On the other hand, for use in combination with a pacemaker or a defibrillator, it may be advantageous (according to a known solution) for the sensor also to be able to perform the electrical stimulation function. In this case, positioning on the heart wall the contractile activity of which is to be stimulated (typically the wall of the right ventricle RV) is to be considered greatly preferable.

The monitoring of the sensor then tends to be disturbed by the signals induced by the contractile activity of the heart wall, particularly the signals which express the acceleration of the heart wall in a generally radial direction. In this case, the processing device 3 usually has filter means which can clear the signal produced by the sensor 1 of the components which relate to the physiological parameters considered to be sources of disturbance.

It is also possible to consider using, as the sensor, a sensor with spatially non-uniform monitoring characteristics and hence, if it is to be implanted on a ventricular wall, the use of a sensor the direction of maximum sensitivity of which does not coincide with the direction of radial contraction, but, on the contrary, coincides with the general direction (approximately tangential) in which the net pumping action of the blood masses is brought about by the portion of the heart muscle concerned.

It is also possible to consider the use of several sensors the output signals of which are used in combination so as to obtain a final signal which is free of disturbance components.

As far as the production of the sensor 1 is concerned, the use of various technological solutions may be considered: however, all these make use of sensors available in the art.

A family of sensors which has been found particularly advantageous for this purpose is the accelerometric sensors, for example, based on piezoelectric ceramics, possibly integrated with seismic masses.

In this case, the signal relating to the velocity of the heart mass (and the respective momentum signal) can be obtained from the accelerometric signal produced by the sensor 1 by integrator means (analog or digital) in the signal-processing unit 3.

Amongst other things, the accelerometric sensors of the type specified above have the advantage that they can conveniently be produced with one directional component having more accentuated sensitivity, consequently enabling the undesired components (radial acceleration) already to be excluded when the signal is detected.

Naturally, the sensor 1 may also be produced by different technologies and may thus be consituted by a piezoresistive, capacitive, inductive, magnetic (for example, Hall-effect) sensor, or the monitoring may be achieved by evaluating variations in the impedance of a conductive fluid which deforms as a result of the acceleration, or by flow-rate measurements. In particular, as already stated, the use of several sensors in combination: for example, an accelerometric sensor and a flow-rate sensor, may be considered.

As well as enabling the sensor 1 to be introduced into the implant position within the heart muscle and enabling the monitoring signals of the sensor 1 to be picked up by the processing device 3, the catheter 2 also enables the electrical stimulation pulses to be conveyed towards the sensor/electrode, if the sensor 1 is also used for performing the stimulation fuction. This is all in accordance with criteria which are widely known in the art and do not need to be described specifically in the present description.

The processing/transmission module 3 may be produced according to the microeletronic technologies normally used for producing the units for piloting the pacemakers in current use.

In particular, the module 3, FIG. 4, usually comprises an input interface for picking up the signals coming from the sensor 1 possibly converting them from analog to digital form and subsequently processing them in order to obtain a velocity (momentum) signal which is adequately cleared of spurious components. For example, this result can be obtained by integrating an accelerometric signal which is cleared of the acceleration components which are not relevant for the purposes of identifying the velocity of the heart mass, which constitutes the quantity which is important for the purposes of the monitoring action.

The monitoring signals thus obtained, where appropriate, in conjunction with other physiological parameters monitored by other sensors, not shown, may either be processed locally within the unit 3 for immediate feedback to the sensor 1 which acts as a stimulation electrode (in the typical configuration in combination with a "rate responsive" pacemaker or with a defibrillator) or may be sent to a transmission unit included in the module 3 for transmission (by means of electromagnetic waves, ultrasound, etc.) to the module 4. The signals coming from the sensor 1 (which are indicative of cardiac events such as the opening and closure of the mitral valve, the opening and closure of the aortic valve, the amount of ventricular ejection, the rapid ventricular filling or delayed ventricular filling during the atrial systole, the cardiac flow-rate, etc.) presented to the analyser 5 enable the heart function of the wearer to be monitored in real time. In particular, it is also possible to consider the incorporation in the module 3 of memory means which can record the data relating to the heart function displayed by the wearer over a certain period of time for transmission to the exterior, as a result of an interrogation made by means of the transmitter 4 during checking visits.

Moreover, it is possible to consider the direct piloting, by means of the unit 3, of a device worn by the patient for the dosed release of drugs, or the configuration of the unit 3 as an alarm unit which indicates to the patient (or to a medical team supervising the patient, even remotely) the fact that some parameters of the heart function are to be considered anomalous and/or such as to require intervention.

Conversely, the analyser 5 and the module 4 may also be used for programming functions so as to send to the module 3 indications relating to the modification of the behaviour of the module 3 and of the system of which it forms part.

Acute recordings were made on animals by positioning an accelerometric sensor 1 in the right ventricle so as to maximize the signal due to the movements of the cardiac masses and transmitting the signal obtained, by telemetry. The integral of the signal, which is representative of the velocity curve associated with new cardiac events, is given in the time graph of FIG. 3c in comparison with homologous signals (that is, relating to the same events and related to the same time scale on the abscissa) expressed in the form of an electrocardiographic trace (the graph of FIG. 3a) and of an accelerometric signal (the graph of FIG. 3b).

It can readily be seen that the velocity signal permits macrosopic observation of the main phases corresponding to the total aortic flow and to the diastolic filling in correspondence with the events monitored by the electrocardiographic signal, taking account of the delay of mechanical contraction with respect to the electrical activation.

It should be noted that the wave of the electrical activation of the ventricles is that defined as the QRS wave. The expulsion of the blood starts from this wave and lasts until the repolarisation wave T. After the wave T is the diastolic ventricle-filling phase.