Abstract:
A relatively compact implantable cardiac medical device includes a wireless communications module, which employs a directional antenna and which is adapted to receive input concerning ventricular wall motion. When the cardiac medical device is anchored to a ventricular wall, transmitter elements of the communications modules are only activated for communication during a detected period of reduced ventricular wall motion. The period of reduced ventricular wall motion may be defined as at least one time interval during which an axis of the directional antenna does not rotate out from a baseline orientation by more than 15 degrees. The communication may be conducted with an external programmer-type device, or with another implanted device, for example, located remote from the heart.

Description:
TECHNICAL FIELD 
       [0001]    The present invention pertains to implantable cardiac devices and more particularly to systems employing the devices and methods for controlling wireless communications thereof. 
       BACKGROUND 
       [0002]    The traditional implantable cardiac monitoring and/or therapy delivery system includes a medical device to which one or more flexible elongate lead wires are coupled. The device is typically implanted in a subcutaneous pocket, remote from the heart, and each of the one or more lead wires extends therefrom to a corresponding cardiac site, either endocardial or epicardial, in order to deliver therapy to, and/or monitor the site. Mechanical complications and/or MRI compatibility issues, which are sometimes associated with elongate lead wires and are well known to those skilled in the art, have motivated the development of relatively compact cardiac medical devices that can be implanted in close proximity to the cardiac site, for example, within the right ventricle (RV) of the heart, so that elongate lead wires are not required. With reference to  FIG. 1 , such a device  100  is illustrated, wherein a fixation member  115  anchors device  100  against the endocardial surface of the RV, for cardiac therapy delivery and/or monitoring, via medical components thereof, for example, a pair of electrodes, a mechanical transducer, and/or any other type of suitable sensor known in the art. Due to size constraints on device  100 , limited space is available, within a hermetic enclosure/shell  101  thereof, for a power supply (i.e. battery) and circuitry (i.e. input/output circuit, a microcomputer circuit, memory, etc.) in support of the medical components. Device  100  is preferably accessible via wireless telemetry, for example, to update the programming of device  100  and/or to collect information from device  100 , so a wireless communications module must also be contained within the limited space and supported by the contained power supply. In order to increase the life of the power supply, the most efficient operation of every component of device  100 , including the communications module, is highly desirable. 
       SUMMARY 
       [0003]    According to embodiments of the present invention, a relatively compact cardiac medical device includes a wireless communications module that employs a directional antenna; the communications module is adapted to receive input concerning ventricular wall motion in order to stabilize telemetry signal strength from the antenna and thereby make communication more efficient. According to methods of the present invention, when such a device is anchored to a ventricular wall, transmitter elements of the communications module are only activated for communication during a detected period of reduced ventricular wall motion. The period of reduced ventricular wall motion may be defined as at least one time interval during which an axis of maximum signal strength for the directional antenna does not rotate significantly out from a baseline orientation, for example, by more than approximately 15 degrees. Wireless communication, according to some embodiments, is conducted with an external programmer-type device, while, according to some alternate embodiments, the communication is conducted with another implanted device, for example, located at a site remote from the heart. 
         [0004]    According to some embodiments, the cardiac medical device includes electrodes to detect the period of reduced ventricular wall motion, while according to alternate embodiments, the device includes a mechanical transducer to detect the period. According to yet further embodiments the device includes a pulse generator, and, when the device is implanted at an apical location of the right ventricle, pacing pulses are applied, according to some methods, in order to create the period of reduced ventricular wall motion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The following drawings are illustrative of particular embodiments of the present invention and therefore do not limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description. Embodiments will hereinafter be described in conjunction with the appended drawings wherein like numerals denote like elements, and 
           [0006]      FIG. 1  is a schematic showing an exemplary cardiac medical device implanted in a right ventricle of a heart; 
           [0007]      FIG. 2A  is a block diagram showing main modules of an implantable cardiac medical device, according to some embodiments; 
           [0008]      FIG. 2B  is a plan view of the exemplary device of  FIG. 1 ; 
           [0009]      FIG. 3  is a heart wall motion schematic diagram; and 
           [0010]      FIG. 4  is schematic diagram illustrating rotation of an antenna axis from a baseline orientation. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The following detailed description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides practical examples, and those skilled in the art will recognize that some of the examples may have suitable alternatives. 
         [0012]      FIG. 1  illustrates device  100  with an axis A overlaid thereon to designate the direction of maximum signal strength from a directional antenna  103 , which, with reference to  FIGS. 2A-B  is part of a wireless communications module  400  contained with shell  101  of device  100 . At the time device  100  is implanted, radiopaque markers (not shown) included in device  100  may be viewed, via fluoroscopy, and/or telemetry signal strength, via antenna  103 , may be monitored, in order to fix device  100  at the implant site in a particular orientation suitable to establish a favorable orientation of axis A.  FIGS. 2A-B  are a schematic block diagram and a plan view, respectively, for device  100 , according to some embodiments.  FIG. 1  further illustrates an external device  200 , for example, an external programmer-type device, such as is known in the art, and another, optional, implanted device  300 , either of which also includes a wireless communications module adapted for entering into wireless communications with device  100 , according to any suitable configuration known in the art. According to preferred methods, a controller  405  of wireless communications module  400  of device  100  activates receiver elements  401  at predetermined/pre-programmed intervals to ‘listen’ for an activation signal from another device, such as device  200  or device  300 , and once such a signal is detected, prepares for communication. If device  200  is a communication head of a programmer that requires positioning to align with axis A, for example, for inductive coupling telemetry, device  100  may transmit a beacon-type signal to help with the alignment of device  200 . According to methods of the present invention, after controller  405  receives an activation signal from device  200 , controller  405  activates transmitter elements  402  of communications module  400 , but only according to input from a ventricular wall motion detector  440  of device  100 . 
         [0013]    With further reference to  FIG. 2B , according to embodiments and methods of the present invention, ventricular wall motion detector  440  provides input to controller  450  of communications module  400 , which, after the aforementioned activation signal is received from device  200 , activates transmitter elements  402  only during a detected period of reduced ventricular wall motion.  FIG. 3  is a schematic diagram showing orthogonal coordinate axes X,Y,Z overlaid on a heart in order to illustrate heart wall motion with each natural contraction of the heart. Those skilled in the art understand that the heart&#39;s intrinsic conduction system causes ventricular myocardium to contract with a twisting, or wringing (generally around axis Z), from the apex toward the base (generally along axis Z), per arrows C 1  and C 2 , to squeeze blood out from the ventricles. With reference back to  FIG. 1 , since device  100  is anchored to the right ventricular wall, each natural ventricular contraction causes axis A to shift and rotate, so that an alignment of axis A with a corresponding axis of device  200  (as well as with that of device  300 ) changes during each contraction and causes a telemetry signal strength delivered via antenna  103  to sinusoidally alternate between approximately 0% and approximately 100%, thereby compromising wireless communication with device  100 . According to some methods, the period of reduced ventricular wall motion includes one or more diastolic intervals between contractions (systolic intervals) of the heart. So, rather than powering up for transmission throughout the aforementioned sinusoidal variation caused by ventricular wall motion during systole, transmitter elements  402  are only powered during diastolic intervals, when the ventricular walls are relatively still for filling. During this period, a lower telemetry signal strength, which means less power consumption, is required from antenna  103 , since the signal strength is relatively stable, thereby increasing the efficiency of outbound communication. 
         [0014]    In addition to, or as an alternative to diastolic intervals, the period of reduced ventricular wall motion may be created by pacing stimulation, for example, delivered from a pulse generator  420  of device  100 , when device  100  is implanted at an apical location, as illustrated in  FIG. 1 , at a rate that is greater (i.e. 10 to 20 beats per minute) than an intrinsic heart rate of the patient. Those skilled in the art understand that the ventricular wall motion, which corresponds to ventricular contractions that are externally stimulated from the apex of the heart, as opposed to those generated, from base to apex, by the heart&#39;s intrinsic conduction system, is reduced in the directions indicated by arrows C 1  and C 2  of  FIG. 3 . Thus pacing stimulation may extend the period of reduced ventricular wall motion into systolic intervals of each cardiac cycle. With reference back to  FIGS. 2A-B , device  100  includes a pair of electrodes  111 ,  112 , by which such pacing stimulation may be applied, wherein electrode  111  is coupled to internal pulse generator circuitry  420  via a hermetic feedthrough, known in the art, and electrode  112  is formed by an exposed conductive portion of shell  101 , according to some embodiments. According to some methods, once an inbound activation signal is received, for example, from device  200  or device  300  ( FIG. 1 ), by controller  405  of wireless communications module  400 , via receiver elements  401 , controller  405  sends a signal to activate pulse generator  420 , in order to create the period of reduced ventricular wall motion via pacing stimulation. It should be noted that electrodes  111 ,  112  may also be employed by ventricular wall motion detector  440  for detection of the period of reduced ventricular wall motion that results from the applied pacing stimulation, as described below. The activation signal to create the period of reduced ventricular wall motion by the applied pacing stimulation is preferably sent by device  200  when the patient is in a clinical setting for a checkup, so that a clinician can monitor the patient&#39;s intrinsic heart rate, for example, to assure that the heart rate is a resting heart rate and stable before the higher rate pacing stimulation is applied. Furthermore, controller  406  of device  100  may have a programmable setting to limit the rate of applied pacing stimulation from the activated pulse generator  420 , according to the patient&#39;s condition, for example, to prevent the stimulation from inadvertently triggering a cardiac arrhythmia. 
         [0015]    According to some embodiments, ventricular wall motion detector  440  includes a mechanical transducer adapted to sense mechanical changes indicative of ventricular wall motion, for example, a pressure sensor for indirect detection of the period of reduced ventricular wall motion (i.e. intraventricular pressure changes over each cardiac cycle), an accelerometer for direct detection of reduced ventricular wall motion, a Doppler sensor to detect blood flow, or an auditory/acoustic sensor to detect heart valve, lung and/or blood flow sounds. According to alternate embodiments, ventricular wall motion detector  440  includes a pair of electrodes, for example, electrodes  111 ,  112  of  FIG. 2B , which are adapted to sense electrical cardiac signals indicative of ventricular wall motion, for example, timing of the QRS complex to find diastolic intervals and/or QRS morphology to identify retrograde conduction resulting from applied pacing stimulation, for example, when the pulse generator is employed to create a period of reduced ventricular wall motion, as described above. According to yet further embodiments a chemical sensor may be employed in device  100 , to provide additional input to controller  406 , for example, of blood pH or blood oxygen saturation that may be indicative of a patient&#39;s physiological condition. 
         [0016]      FIG. 4  is a schematic diagram illustrating a baseline orientation of axis A, designated AB, which corresponds to a best alignment of axis A with the maximum signal strength axis of the communications module antenna of another device, such as device  200  ( FIG. 1 ).  FIG. 4  further illustrates limits of rotation RX and RZ out from AB, about axes X and Z, respectively, within which the period of reduced ventricular wall motion is defined. According to some preferred embodiments, the limits of rotation RX and RZ are no greater than approximately 15 degrees, and rotation within these limits may be correlated to diastolic intervals and/or to extended intervals during pacing stimulation, as detected by ventricular wall motion detector  440 . By means of in vivo experimentation that employed biplane fluoroscopic tracking of radiopaque markers attached to a device similar to device  100 , which was implanted at an apical location (similar to  FIG. 1 ), we have found that, when pacing stimulation was applied, device rotation during ventricular contractions, from a baseline orientation such as AB, is significantly reduced from that which was typical during intrinsic ventricular contractions. 
         [0017]    With reference back to  FIG. 1 , according to some embodiments, third device  300  may be implanted at a site remote from the heart, for example, to monitor and/or deliver therapy. Communication between device  300  and device  100  may be necessary to coordinate therapy delivery, from one or both devices, and/or to transfer data/information from device  100  to device  300 , for example, for storage in a data storage module of a memory of device  300  until predetermined time periods when an external device, such as device  200 , is employed to retrieve the stored data/information. For example, device  300  may be a cardiac defibrillation generator that is implanted in an abdomen of the patient, a neuromodulation generator implanted in the abdomen or pectoral region, or a cardiac monitor implanted in the pectoral region, any of which, in addition to having a more stable axis of maximum wireless communication strength, by virtue of their implant location, may also have a size sufficient to include greater battery capacity and more sophisticated telemetry hardware (relative to device  100 ), for example, capable of long range and/or automated telemetry with an external device, which is known in the art. 
         [0018]    In the foregoing detailed description, the invention has been described with reference to specific embodiments. However, it may be appreciated that various modifications and changes can be made without departing from the scope of the invention as set forth in the appended claims.