Abstract:
A method of detecting and classifying cardiac arrhythmias, comprising: receiving a hemodynamic wave signal from at least one hemodynamic sensor by a processor; receiving a cardiac electrical wave signal from a cardiac stimulation device by the processor; integrating, by the processor, the received hemodynamic wave signal and the received cardiac electrical wave signal; determining if a heart arrhythmia is present via examination of the regularity of the hemodynamic signal; and if a heart arrhythmia is determined, classifying the arrhythmia according to a time correlation between the hemodynamic signal and the electrical signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present patent application claims the benefit of priority to British Patent Application Serial Number GB 0516220.1, filed Aug. 8, 2005, entitled “CARDIAC STIMULATION WITH HEMODYNAMIC SENSOR GUARD”. This application is a divisional patent application of U.S. patent application Ser. No. 12/063,245, filed Feb. 8, 2008, entitled “CARDIAC STIMULATION WITH HEMODYNAMIC SENSOR GUARD”, published Aug. 5, 2010, under publication number US 2010/0198285, which was a national stage entry of International Patent Application Serial Number PCT/IL2006/000906, filed Aug. 6, 2006, entitled “CARDIAC STIMULATION WITH HEMODYNAMIC SENSOR GUARD”, published Feb. 15, 2007, under publication number WO 2007/017863; all of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to cardiac pacemakers, defibrillators and implanted hemodynamic sensors and more specifically to cardiac pacing timing cycles, safety and reliability operation of implanted pacemakers and defibrillators. 
       BACKGROUND OF THE INVENTION 
       [0003]    Implanted cardiac stimulation devices, i.e. pacemakers and intracardiac cardioverter defibrillators deliver therapy to patients suffering from various heart-diseases (Clinical Cardiac Pacing and Defibrillation, 2nd edition, Ellenbogen, Kay, Wilkoff, 2000). Dual chamber pacemakers stimulate the right ventricle with synchrony to the sensed atrial event, and with a given delay to the atrioventricular (AV) delay. Cardiac Resynchronization Therapy (CRT) devices, e.g., biventricular pacemakers, stimulate both ventricles and also synchronize the stimulation in correspondence with the sensed atrial event signal. With all implanted cardiac stimulation devices the management of the stimulation according to the accurate sensing and interpreting the electrical activity of the heart in the atria and in the ventricles is crucial for the variety of stimulation methods and algorithms. 
         [0004]    U.S. Pat. No. 6,901,291 discloses a method to distinguish between valid and invalid cardiac senses. U.S. Pat. No. 6,862,471 discloses a method for blanking T-waves from atrial cardiac signals. U.S. Pat. No. 6,912,418 discloses a system for distinguishing electrical events originating in the atria from far-field electrical events originating in the ventricles. U.S. Pat. No. 6,731,978 discloses a prioritized rule based method for detection, classification and treatment of arrhythmias. 
         [0005]    Generally, these methods are related to the timing cycles in cardiac stimulation. The timing cycles are very important for proper operation of the implanted stimulation device and to the safety and well being of the patient. Under conditions of cardiac arrhythmias, these timing cycles become even more important and complicated to manage. In the prior art the only information used on-line are the electrical signals sensed by the implanted electrodes, i.e. the intra cardiac electrograms (IEGMs). The difficulty associated with depending solely on the IEGMs stems from the fact that the sensed signals are local and weak, having an amplitude of only several milli-volts with strong coupling from ventricle to atria and ventricle to ventricle. Hence the IEGMs can be misinterpreted by the device controller which can lead to inappropriate pacing. 
         [0006]    Implanted cardiac hemodynamic sensors are known in the art and are further developed by several manufactures, mainly for minimal invasive electrophysiological studies. In the last years few cardiac pacemaker devices included such hemodynamic sensors for hemodynamic monitoring and for creating a pacemaker system with a rate response according to hemodynamic sensors. 
         [0007]    U.S. Pat. No. 6,865,419 proposes to use an implanted pressure sensor in order to measure the mean pulmonary arterial pressure. U.S. Pat. No. 6,882,882 proposes to use an implanted pressure sensor in order to adjust the atrioventricular delay online. 
         [0008]    US Patent Application 2003/0158492 A1 presents a method for detection and treatment of myocardial ischemia based on monitoring of both the electrical and the mechanical activity of the heart in order to detect and verify more reliably the myocardial ischemia occurrence. When myocardial ischemia is detected and verified the patent suggests selecting between few options of therapy. 
         [0009]    U.S. Pat. No. 5,458,622 presents a method for detection, classification and treatment of pathological tachyarrhythmia from physiologic tachyarrhythmia based on implanted activity sensor. 
         [0010]    European Patent Application 0488512 presents an implantable hemodynamically responsive cardioverting/defibrillating pacemaker. The implantable device uses the information of the implanted hemodynamic sensor during tachycardia to select and control therapeutic means delivered such as antitachycardia pacing, cardioversion or defibrillation options. 
         [0011]    US Patent Application 2002/0103442 presents a system and a method for determining mean pulmonary arterial pressure (MPAP) using a pressure sensor located within a ventricle of a heart. The MPAP approximates the mean pressure of the pulmonary artery that can be used to optimize CRT device timings for example. 
         [0012]    European Patent Application 0541338 presents an implantable monitor/stimulator that monitors and assesses indices of cardiac function and automatically executes physician-selected mode of therapy with ability to increase the strength of contractility of the heart. 
         [0013]    The publications noted above use a hemodynamic sensor as a mean to detect specific heart conditions (US 2003/0158492), detect and treat cardiac arrhythmias (EP 0488512 and U.S. Pat. No. 5,458,622) or improve hemodynamic function (US 2002/0103442 and EU 0541338). The publications noted above do not suggest use of an implanted hemodynamic sensor in order to prevent inappropriate cardiac stimulation or to detect erroneous interpretation of cardiac events. 
         [0014]    There are two additional patent applications of the inventor of the present invention, contents of which are hereby incorporated herein by reference: PCT Serial Number PCT/IL2004/000659, filed Jul. 20, 2004, entitled “ADAPTIVE RESYNCHRONIZATION THERAPY SYSTEM”, published Jan. 1, 2005, under publication number WO 2005/007075; and PCT Serial Number PCT/IL2006/000571, filed May 15, 2006, entitled “VENTRICLE PACING DURING ATRIAL FIBRILLATION EPISODES”, published Nov. 30, 2006, under publication number WO 2006/126185. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention is related to a general approach that combines information from hemodynamic sensors with the information from intracardiac electrograms (IEGMs) for making a more reliable and safer cardiac stimulation operation. The approach of the present invention takes advantage of the fact that implanted hemodynamic sensors are being developed and implemented in implanted cardiac pacemakers and defibrillators in order to improve hemodynamic performance and to be used for future closed loop implanted devices. Since such sensors have become available, an object of the present invention is to further use the complementary data from the hemodynamic sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended drawings in which: 
           [0017]      FIG. 1A  is a schematic description of the interaction between a pacing device and cardiac electrical sensors in the prior art; 
           [0018]      FIG. 1B  is a schematic description of the interaction of a pacing device and sensors in the present invention; 
           [0019]      FIG. 1C  is a flow diagram describing the manner of introducing hemodynamic signal regularity into the process of the present invention; 
           [0020]      FIG. 2A  is a graph showing several heartbeats in a typical ventricle pressure sensor signal; 
           [0021]      FIG. 2B  is a graph showing a typical impedance (ventricular volume) sensor signal giving indication regarding the measure of the cyclic change in the ventricular volume; 
           [0022]      FIG. 3  is a timing diagram that shows how a pressure signal is used to prevent false stimulation during a systole cycle; 
           [0023]      FIG. 4  is a timing diagram that shows how a digital pressure signal is used to detect a far field R wave that appears in an atrial lead signal during a diastole cycle; 
           [0024]      FIG. 5  is a timing diagram that shows how a digital pressure signal is used to detect a far field R wave on an atrial lead electrogram during a diastole cycle; 
           [0025]      FIG. 6  is a timing diagram that shows how a digital ventricle pressure signal is used to identify a loss of atrial event sensing during a diastole cycle; 
           [0026]      FIG. 7  is a timing diagram that shows how a digital ventricle pressure signal is used to identify a loss of ventricle event sensing during a diastole cycle; 
           [0027]      FIG. 8  is a timing diagram that shows the manner in which a loss of the digital ventricular pressure signal waveform is used to detect a capture loss of a ventricle contraction after ventricle stimulation; 
           [0028]      FIG. 9  is a timing diagram that shows the manner in which a ventricular pressure signal is used to identify ventricular tachyarrhythmia; 
           [0029]      FIG. 10  is a timing diagram that shows the manner in which a digital ventricle pressure signal is used to identify atrial fibrillation; and 
           [0030]      FIG. 11  is a timing diagram that shows the manner in which a digital ventricle pressure signal is used to identify atrial fibrillation. 
       
    
    
       [0031]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
         [0033]    In accordance with some embodiments of the present invention hemodynamic sensor signals are used to validate the electrical stimulation produced by a pacemaker or a defibrillator. To explain the application of the invention in very general terms, reference is first made to  FIGS. 1A-C . In some prior art systems as shown in  FIG. 1A , electrical sensor  22  senses electrical currents in the heart muscle  24 . Processor  26  reads electrical sensor  22  and accommodates to the heart&#39;s condition by generating conditioned pacing and delivering it to the heart muscle  24  using a pulse generator  28 . In an embodiment of the present invention shown schematically in  FIG. 1B , processor  26  generates pacing to the heart muscle  24  but accepts conditioning signals arising from a combination of sources. Thus, electrical sensor  22  senses the electrical currents in the heart muscle  24 , whereas hemodynamic sensor  36  senses parameters of the blood circulation  38 . Processor  26  receives and integrates the output signals of both types of sensors and generates conditioned pulsing stimuli through pulse generator  28  as to produce a safer and more physiologically sensible pacing. The procedure for conditioning the pacing device in accordance with the present invention is explained infra, by describing examples of specific conditions. The heart muscle pacing electrodes are also used as electrical sensors providing information as to the dynamic voltage difference between the electrodes. Hemodynamic sensors are used for the detection or monitoring of the condition of the blood stream. For example, blood pressure, blood flow, blood oxygenation level, and blood impedance as a measure of the ventricle volume. Hemodynamic sensors obtain the dynamic information required. This however does not preclude the heart itself as a site of monitoring for hemodynamic sensors, but unlike electrical sensors per se, the hemodynamic sensors in the heart collect information regarding the blood itself rather than information related to the heart muscle. In accordance with the present invention, information about the mechanical activity of the heart obtained from an implanted hemodynamic sensor, such as a ventricle pressure sensor, is used to validate the electrophysiological information obtained from implanted electrodes that sense only the electrical activity in the heart chambers heartbeat after heartbeat. The present invention can be implemented for preventing false stimulation, validating the sensed signals and classifying arrhythmias, using the time correlation between the electrical and mechanical heart activities. 
         [0034]    In  FIG. 1C  a flow diagram shows the manner in which the applicability and significance of the hemodynamic signal is examined. The regularity of the hemodynamic signal is examined in step  40 . If the hemodynamic signal is regular, the system in step  42  continues to rely on it as a validating signal. If the signal is irregular, the heart condition is defined as arrhythmic in step  44  and an arrhythmia classification routine is invoked in step  46 . Pacing therapy may or may not consequently be delivered according to the heart condition. In general terms, if the hemodynamic signal is regular it supplies complementary information related to the heart condition to be used to detect physiologically improper pacing signals. 
         [0035]      FIG. 2A  depicts a typical schematic ventricular pressure signal having systolic and diastolic heart cycles. The pressure wave signal  50  is very high in magnitude during the systolic cycle, with typical maximal value reaching above 100 mmHg. The pressure wave signal  50  is also highly repeatable with only small deviations from one heartbeat to the next. The pressure wave rising edge is due to the mechanical contraction of the ventricle triggered by the electrical depolarization wave (known as the R wave) and it sustains its high level during the systolic cycle. The pressure wave falling edge is due to the onset of the next diastolic cycle and it is accompanied by an electrical re-polarization wave (known as T wave). The pressure level falls near 0 mmHg at the end of the diastolic cycle. Hence the pressure wave obtained by hemodynamic pressure sensor is highly correlated to the physiological electrical activity and at the same time clearly distinguishes between systolic and diastolic cycles during each heartbeat. If a threshold value  52  is defined at the half of the maximal value of the systolic pressure wave, a digital wave can be produced that has a logic “1” level during systolic cycle and logic “0” level during diastolic cycle. This type of digital wave generated in the pacemaker or defibrillator device controller is used in the present invention for prevention of false heart stimulation and erroneous interpretation of signals, as well as for classifying arrhythmias as described in more detail below. 
         [0036]      FIG. 2B  shows a typical impedance signal that reflects the ventricle volumes for several heartbeats. The impedance wave signal,  56 , monotonically rises during the diastolic cycle due to the ventricle passive and active blood stream into the ventricle. Arrows  58 , designate the diastolic cycle time period. During the systolic cycle the ventricle contracts and the blood is ejected with a decrease in ventricle volume. Hence the impedance sensor signal, as a ventricle pressure signal, can be used for obtaining the systolic cycle timing and diastolic cycle timing and with this data to perform detection validation and classification of arrhythmia combined with the intra cardiac electrograms (IEGMs) as will be explained below. 
         [0037]      FIG. 3  shows a timing diagram illustrating the manner in which a digitized ventricle pressure signal  62  is used to prevent false stimulation during a systolic cycle; thus preventing a dangerous ventricle arrhythmia. Ventricular pacing is delivered subsequently to a programmed atrioventricular (AV) delay  68  measured from the last atrial event  64 , as shown in the atrial electrogram  66 , in both dual chamber pacemakers and biventricular pacemakers. According to the present invention if a systolic cycle has already started and hence the digital pressure wave is at logic high level, the stimulation of the ventricle at the end of AV delay  68 , as scheduled at time  70 , is aborted during this heart cycle, as shown by the quiescent electrogram  66  at time  70 . 
         [0038]    Very similarly to the description above, if an impedance sensor is used as a hemodynamic sensor, stimulation during a systole may be duly aborted thus avoiding a dangerous ventricle arrhythmia. As explained above, regarding the use of a pressure sensor signal, the impedance signal can be used to obtain the digital diastolic and systolic cycles and be used to prevent inappropriate pacing during the systolic cycle. 
         [0039]    The prevention of untimely ventricular stimulation is extremely significant at such points in time in which the ventricle has just concluded its refractory period. In this type of situation, the heart is receptive and thus inappropriate stimulation may cause a dangerous ventricular tachyarrhythmia. 
         [0040]    In the prior art, prevention of untimely stimulations is brought about referencing only sensed local electrical activity, that might be a false sensing event, or a far field event, etc. 
         [0041]    In  FIG. 4  a timing diagram shows the manner in which a digital pressure signal  80  is used to detect a far field T wave (known as ventricle re-polarization wave) on the atrial lead that may be mistakenly interpreted as a new atrial event (known as P wave). The re-polarization, T wave in the ventricle lead, occurs immediately after the falling edge of the ventricle pressure wave, hence it is expected to appear a few milliseconds after the initial ventricular pressure falling edge. Thus a typical time zone can be defined including the typical time delay between onset of the T wave and the preceding pressure wave fall. The ventricle T wave is also known to generate an electrical signal in the atrial electrogram  82 , due to coupling, that occasionally crosses the detection threshold level  84 , and hence may be interpreted erroneously as a P wave  86 , whereas event  88  is a valid atrial event. Hence the digital pressure signal  80  facilitates the screening and abortion of untimely, erroneously interpreted far field T waves. 
         [0042]      FIG. 5  shows a timing diagram depicting the manner in which the digital pressure signal  96  is used to detect a far field R wave on the atrial lead electrogram  98  during a diastolic cycle. The ventricle depolarization wave, known as the R wave, triggers the rising edge of the pressure wave, hence it is expected to appear a few milliseconds prior to the rising edge. As a result, a typical time zone can be defined including the typical time delay between the onset of the R wave and the pressure wave rising edge. The R wave is also known to generate an electrical signal in the atrial electrogram, due to coupling, that occasionally crosses the detection threshold level  100 , and could therefore be detected falsely as a new P wave  102 . In contrast, event  104  is a valid atrial event. Hence the present invention facilitates the detection and the rejection of a far field R wave occurring when the pressure wave is at low level, i.e. in a diastolic cycle, just prior to its rising edge in the R wave far field time zone. 
         [0043]    To summarize  FIGS. 4 and 5 , the hemodynamic sensor can be used to detect far field R and T waves. Since dual chamber pacemakers and biventricular pacemakers use the sensed atrial event as the trigger for internal counters that define the time intervals for stimulating the ventricles, such as the AV interval, it is of utmost importance for proper and reliable device operation to exercise the ability to differentiate between an actual atrial event that is correlated to the P wave and the far field R and T waves. 
         [0044]      FIG. 6  shows a timing diagram clarifying the manner in which the digital pressure signal  108  is used to identify a loss of atrial event sensing during a diastolic cycle; this circumstance is known as under-sensing. When the weak atrial event signal  110  does not reach the threshold level  112  it may not be detected in the atrial electrogram  114 . For the sake of clarity, a stronger atrial event  116  is shown surpassing the threshold voltage  112 . If the pressure wave appears normally in the following systolic cycle, the atrial wave detection failed, i.e. an under-sensing event occurred, but the overall hemodynamic performance is appropriate and there is no need for intervention. The absence of atrial event in this case can be recorded and stored for device interrogation during follow-ups. If, conversely, a pressure wave does not appear in a predefined time interval, the loss of atrial event is not an under-sensing event and device stimulation may be necessary (for instance stimulation of the atria). 
         [0045]      FIG. 7  shows a timing diagram depicting the manner in which a digital ventricle pressure signal  108  is used to detect loss of a ventricular event  118  in the ventricular electrogram  114 , during a diastolic cycle, when the ventricle event electrogram threshold detection level  112  is not reached. If the pressure wave appears normally in the following systolic cycle, the ventricle event detection failed but the overall hemodynamic performance is appropriate. The absence of a ventricular event, such as an under-sensing event, can be counted and stored for device interrogation during follow-ups. If, conversely, a pressure wave does not appear normally in a predefined time interval, the loss of ventricle event is not an under-sensing event and therapy may be needed (for instance stimulation of the ventricle). Additionally, note that with the present invention if a pressure wave appeared, meaning a systolic cycle is ongoing, pacing of the ventricle will be prevented as described above. 
         [0046]    PCT Serial Number PCT/IL2006/000571, filed May 15, 2006 entitled, “VENTRICLE PACING DURING ATRIAL FIBRILLATION EPISODES”, and published Nov. 30, 2005 under publication number WO 2006/126185 is hereby incorporated herein by reference. In this publication the inventor of the present invention discloses a neural network architecture that performs a temporal pattern recognition applied to the hemodynamic sensor signal. The pattern recognition neural network architecture can be further used for detection of a capture loss and for detection and classification of arrhythmias. Reference is now made to  FIG. 8 , which shows a timing diagram depicting the manner in which loss of digital ventricular pressure signal waveform  130  is used to detect a capture loss of the ventricle contraction subsequent to ventricle stimulation in the ventricle electrogram  132 . Following a ventricular stimulation at a specific point in time, designated by arrow  136  on the time axis, an evoked response  138  is expected, the ventricle is expected to contract within a typical delay time and the ventricle contraction is expected to generate a sharp rise of the pressure wave as shown in graph  130 . In contrast, when a ventricular stimulation designated by arrow  140  is followed by an evoked response  142 , and loss of a sharp rise of the pressure wave as shown on graph  130 , a loss of capture is detected, and the evoked electrical response may be a lead depolarization due to stimulation and not an actual evoked response. The absence of a sharp pressure wave rising edge, that creates a loss of detection of the digital pressure wave, is a direct indication of a mechanical loss of capture, and hence an algorithm for detection of capture loss events can be performed using the digital pressure waveform. The advantage of the present invention method for detection of capture loss events is that a direct measurement of the ventricle mechanical behavior is taken, based on the pressure wave signal which is more reliable and repeatable compared to the weak electrical signals. Additionally, phenomena such as lead polarization due to stimulation that may be mistakenly interpreted as an evoked response are counteracted if the pressure wave is used for detection of loss of capture rather than relying solely on electrical activity. 
         [0047]    In  FIG. 9 , a timing diagram shows the manner in which the ventricular pressure signal  150  is used to identify ventricular tachyarrhythmia as sensed in the ventricle electrogram  152 . Ventricular events  154  are all sensed with a high rate (low time interval between adjacent events) while the pressure wave is irregular. The reason for the irregular pressure wave is due to low maximal systolic pressure values in a few heartbeats reaching below the pressure wave detection threshold  156 . The irregular pressure wave is a very strong indication for a ventricular tachyarrhythmia that, as previously mentioned, is in sharp contrast to the pressure wave of a normal sinus rhythm which has an almost perfect periodic waveform. With detection of an irregular hemodynamic sensor signal, an arrhythmia classification routine is invoked and therapy can be delivered accordingly. 
         [0048]    In  FIG. 10  a timing diagram shows the manner in which the digital ventricular pressure signal  160  is used to identify atrial fibrillation as seen in the atrial electrogram  162 . When the detected atrial event appears in the far field R wave zone  192 ,  164 , or far field T wave zone  194   166 , the events are defined as far field events. When the detected atrial events appear in atrial fibrillation (AF) zone I  196 ,  168  or AF zone II  198 ,  170 , the events are defined as possible atrial fibrillation events, and an algorithm that counts several such consecutive atrial fibrillation events can determine the onset of atrial fibrillation. 
         [0049]    In  FIG. 11  a timing diagram shows the manner in which the ventricle digital pressure signal  180  is used to validate the onset of sinus tachyarrhythmia, seen in atrial electrogram  182 . Correlation between the systolic and diastolic cycles in the pressure wave and the atrial sensed events  184 , is preserved but the heart rhythm is very high, above a predefined cut-off rate that can be programmed, for example to 180 beats per minute. In this example the correlation between the pressure wave and the detected atrial events is used to classify a sinus tachyarrhythmia. 
         [0050]      FIGS. 9-11  show three examples of how the complementary information received from the hemodynamic sensor in addition to the IEGM&#39;s can be used to classify and differentiate cardiac arrhythmias. Such approach can be used for example also to differentiate between ventricular tachyarrhythmia (VT) and Supra ventricular tachyarrhythmia (SVT) and to deliver appropriate therapy accordingly since VT will affect significantly the signal coming from the hemodynamic sensor and will not affect the atrial electrogram, while SVT that is triggered in the atria will affect both atrial IEGM and the hemodynamic sensor as was shown in  FIG. 11 . 
         [0051]    It will be appreciated that the present invention is not limited by what has been described hereinabove and that numerous modifications, all of which fall within the scope of the present invention, exist. For instance, the hemodynamic sensor guard approach presented in this patent might be used with additional aspects of cardiac pacing by people skilled with cardiac pacemakers and defibrillators design beyond the examples given here, and such applications that use complementary data from hemodynamic sensors and implanted electrocardiograms as presented here are part of this invention. The pressure sensor waveform is one example of a hemodynamic sensor that measures the mechanical behavior of the ventricles beat after beat. The present invention is not limited to the use of ventricle pressure sensors and can be applied to other implanted hemodynamic sensors, such as ventricle impedance sensors or ventricle wall accelerometers. It is also important to note that only three examples of arrhythmias&#39; classification were discussed with respect to examples as shown in  FIGS. 9-11 . The present invention can be used for an elaborate classification of different arrhythmias based on the combined and complementary information from IEGMs and hemodynamic sensors. 
         [0052]    It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described herein above. Rather the scope of the invention is defined by the claims which follow: