Abstract:
An implantable pacemaker that uses impedance cardiography to measure intrathoracic impedance and then transmit impedance data to an external PC based analyzer for accurate calculation of cardiac output, and a method for optimizing cardiac resynchronization therapy using the pacemaker are disclosed.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Patent Application No. 60/673,686 filed Apr. 21, 2006. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The invention relates to a Cardiac Resynchronization Therapy (CRT) implantable pacemaker that uses impedance cardiography to calculate cardiac output, and to a method for optimizing cardiac resynchronization therapy using the pacemaker. 
         [0005]    2. Description of the Related Art 
         [0006]    Congestive heart failure and left ventricular dysfunction are the result of multiple and common disease processes, including coronary disease, hypertension, diabetes, cardiomyopathy to name but a few. It will be responsible for more deaths than all other causes combined by 2010, and accounts now for the lion&#39;s share of health care cost in the United States. 
         [0007]    Current therapies available today include surgery, such as bypass or valve surgery, or more recently, ventricular remodeling surgery; medical therapy including medications known as ACE inhibitors, beta blockers, aldosterone antagonists, and so forth; and recently, the application of cardiac resynchronization therapy in selected patients. Cardiac resynchronization therapy, CRT, as it is known, is a pacemaker based technology in which leads are placed in both left and right ventricles in order to synchronize their contraction and thus optimize cardiac function. Cardiac resynchronization therapy systems have been developed by various medical device companies. 
         [0008]    Approximately one fourth of patients receiving cardiac resynchronization therapy fail to respond favorably to this very expensive and labor intensive therapy, and in such patients, attempts are made to optimize the many adjustable parameters of the cardiac resynchronization therapy device. Optimization is conducted by confirming proper position of the leads, but primarily by Doppler analysis of blood flow characteristics during echocardiographic evaluation, a moderately expensive procedure which takes about 30 to 45 minutes to perform, and is occasionally technically inadequate for purpose of optimization. 
         [0009]    Impedance cardiography is a technique by which there is made an indirect measurement of the cardiac output, or volume of blood pumped by the heart in the time of one minute, wherein the cardiac output is calculated from the measured intrathoracic impedance. To this point in time, impedance cardiography is performed much like an electrocardiogram, that is, with wires placed on the skin of the chest and extremities. A tiny current is introduced between two points (a current source and a current sensor) placed some distance from one another on the chest, the impedance of the current is analyzed and is translated into the cardiac output. 
         [0010]    Impedance cardiography is an older science and has historically been slow to evolve due to the fact that the measurement of impedance is affected by many variables, typically resulting in spurious results and therefore unsupported assumptions. Proprietary algorithms have been developed to increase the accuracy of the calculation of cardiac output from impedance measurements, and it is felt that these algorithms have solved issues addressing accuracy of cardiac output calculation. In the future, concepts of impedance cardiography will be incorporated into upcoming models of pacemakers to allow for early detection of a falling intrathoracic impedance, which in and of itself is an indication of impending congestive heart failure. Significantly further benefit will accrue if the calculation of impedance can lead to an accurate calculation of cardiac output, which in turn will lead to more precise manipulation of medical and pacemaker therapies being applied to the management of congestive heart failure. 
         [0011]    Therefore, there exists a need for a method to optimize cardiac resynchronization therapy using a pacemaker that uses impedance cardiography to calculate not only impedance, but cardiac output, accurately. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    The foregoing needs are met by the present invention which provides an implantable pacemaker device with an algorithm capable of calculating cardiac output, in turn allowing for simple, accurate and real-time optimization of cardiac resynchronization therapy without the need for echocardiography. The implantable device is capable of accurate measurement of impedance, which can be interrogated transdermally in a clinic or outpatient setting, much as current pacemakers are interrogated today, by the treating physician (the cardiologist or electrophysiologist). The ability to obtain this data and convert it to cardiac output simply, accurately and cost effectively, would be expected to revolutionize the management of congestive heart failure and possibly impact not only the cost of the disease, but its morbidity and mortality. 
         [0013]    Such an implantable device allows not only for optimization of cardiac resynchronization therapy in patients who fail to respond to therapy, but would allow for optimization of medical therapy, such as dose adjustment of medications, adding or subtracting therapies as cardiac output is positively or negatively impacted; adjusting CPAP or biPAP settings in patients being treated for sleep apnea; monitoring cardiac function easily and inexpensively in patients receiving chemotherapy (much of which is cardiotoxic); risk assessment in any or all post myocardial infarction patients, and so forth. 
         [0014]    It is therefore an advantage of the present invention to provide a method for optimizing cardiac resynchronization therapy using a pacemaker that uses impedance cardiography to calculate cardiac output. 
         [0015]    These and other features, aspects, and advantages of the present invention will become better understood upon consideration of the following detailed description, drawing, and appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0016]      FIG. 1  is a schematic of a cardiac pacing system used with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    In  FIG. 1 , there is shown a cardiac pacing system  10  suitable for use with the present invention. The cardiac pacing system  10  includes a pacemaker  15  having a circuit in electrical communication with a patient&#39;s heart  12  by way of three leads  20 ,  24  and  30  suitable for delivering multi-chamber stimulation and shock therapy. The circuit of the pacemaker  15  is also in communication with an electrode  17  that is located on or near the pacemaker. The pacemaker  15  is implanted subcutaneously in the patient&#39;s body between the skin and upper ribs. The pacemaker  15  provides stimulating pulses from a pulse generator to the heart. 
         [0018]    To sense right atrial cardiac signals and to provide right atrial chamber stimulation therapy, the pacemaker  15  is coupled to an implantable right atrial lead  20  having a right atrial tip electrode  22 , which typically is implanted in the patient&#39;s right atrial appendage. The right atrial lead  20  may also have a right atrial ring electrode  23  to allow bipolar stimulation or sensing in combination with the right atrial tip electrode  22 . 
         [0019]    To sense left ventricular cardiac signals and to provide left-chamber stimulation therapy, the pacemaker  15  is coupled to a coronary sinus lead  24  designed for placement in the coronary sinus region via the coronary sinus ostium so as to place a distal electrode adjacent to the left ventricle. The coronary sinus lead  24  is designed to receive left ventricular cardiac signals and to deliver left ventricular stimulation therapy using a left ventricular tip electrode  26 . 
         [0020]    The pacemaker  15  is also shown in electrical communication with the patient&#39;s heart  12  by way of an implantable right ventricular lead  30  having a right ventricular tip electrode  32 , a right ventricular ring electrode  34 , and a right ventricular coil electrode  36 . Typically, the right ventricular lead  30  is transvenously inserted into the heart  12  so as to place the right ventricular tip electrode  32  in the right ventricular apex so that the right ventricular coil electrode  36  will be positioned in the right ventricle. Accordingly, the right ventricular lead  30  is capable of receiving cardiac signals, and delivering stimulation in the form of pacing and shock therapy to the right ventricle. 
         [0021]    During operation, the pacemaker  15  provides an alternating current signal between the pacemaker  15  and the left ventricular tip electrode  26 . The electrode  17  on or near the pacemaker  15  and a coronary sinus ring electrode  27  (or left ventricular tip electrode  26 , or right ventricular tip electrode  32 , or right ventricular ring electrode  34 , or right atrial tip electrode  22 ) provide signals representative of impedance changes between the pacemaker  15  and the heart to the circuit in the pacemaker  15 . The circuit in the pacemaker  15  includes a microprocessor having software or firmware for storing the impedance data. 
         [0022]    The pacemaker  15  also provides pacing pulses to the atrial tip electrode  22 , the right ventricular tip electrode  32  and the left ventricular tip electrode  26  during operation. In order to maximize cardiac output, the pacing pulse interval between the atrial tip electrode  22  and the right ventricular tip electrode  32  must be optimized, and the pacing pulse interval between the right ventricular tip electrode  32  and the left ventricular tip electrode  26  must be optimized, along with other appropriate programmable parameters. 
         [0023]    The circuit of the pacemaker  15  also includes a receiver capable of receiving interrogation signals (such as radio frequency signals) from an external computing device. The interrogation signals pass through the receiver to the control logic in the pacemaker microprocessor memory. The memory will produce information relating to interrogation signals and generate this data back through the control logic into a transmitter in the pacemaker so that the transmitter transmits this data to the external computing device. For example, the external computing device may send interrogation signals requesting the impedance values from the pacemaker  15 . The interrogation signals pass through the receiver in the pacemaker  15  to the control logic and impedance data is generated back through the transmitter to the external computing device where cardiac output is calculated from the impedance data. As is known, a measure of cardiac output can be obtained by extracting the first time derivative of cyclical impedance changes. Suitable software, including a proprietary algorithm for calculating cardiac output from impedance, is available from Vasamed, Minneapolis, Minn., USA. The cardiac output value may be displayed on the display of the external computing device. In a similar manner, the external computing device may send interrogation signals requesting the current pacing pulse interval between the atrial tip electrode  22  and the right ventricular tip electrode  32 , and the current pacing pulse interval between the right ventricular tip electrode  32  and the left ventricular tip electrode  26 . This pacing pulse interval data may then be displayed on the display of the external computing device (e.g., a laptop computer). 
         [0024]    Having described the components of a cardiac pacing system  10  suitable for use with the present invention, various methods of the invention can be described. 
         [0025]    In one version of the invention, there is provided a method for optimizing cardiac resynchronization therapy using the cardiac pacing system  10 . First, a timing interval between successive right atrial stimulation pulses, which are provided from the pacemaker pulse generator to the right atrial tip electrode  22 , and right ventricular stimulation pulses, which are provided from the pacemaker pulse generator to the right ventricular tip electrode  32 , is stored in a memory location in the pacemaker microprocessor. An alternating current signal is generated between the pacemaker  15  and the left ventricular tip electrode  26 . Impedance changes are sensed between the electrode  17  on or near the pacemaker  15  and the coronary sinus ring electrode  27  (or left ventricular tip electrode  26 , or right ventricular tip electrode  32 , or right ventricular ring electrode  34 , or right atrial tip electrode  22 ) to provide signals representative of impedance. 
         [0026]    Then, impedance values are transmitted to a computing device external to the patient, thus allowing the calculation of cardiac output in the computing device and display of the calculated cardiac output values on a display of the computing device. The timing interval in the pacemaker microprocessor memory location can be adjusted by transmitting signals to the microprocessor from the computing device. Also, the cardiac output values as a function of time may be stored in the microprocessor or on the computing device for analysis. 
         [0027]    In another version of the invention, there is provided a method for optimizing cardiac resynchronization therapy using the cardiac pacing system  10 . First, a timing interval between successive right ventrical stimulation pulses, which are provided from the pacemaker pulse generator to the right ventricular tip electrode  32 , and left ventricular stimulation pulses, which are provided from the pacemaker pulse generator to the left ventricular tip electrode  26 , is stored in the pacemaker microprocessor. An alternating current signal is generated between the pacemaker  15  and the left ventricular tip electrode  26 . Impedance changes are sensed between the electrode  17  on or near the pacemaker  15  and the coronary sinus ring electrode  27  (or left ventricular tip electrode  26 , or right ventricular tip electrode  32 , or right ventricular ring electrode  34 , or right atrial tip electrode  22 ) to provide signals representative of impedance. 
         [0028]    Then, impedance values are transmitted to a computing device external to the patient, thus allowing the calculation of cardiac output in the computing device and display of the calculated cardiac output values on a display of the computing device. The timing interval in the pacemaker microprocessor memory location can be adjusted by transmitting signals to the microprocessor from the computing device. Also, the cardiac output values as a function of time may be stored in the microprocessor or on the computing device for analysis. 
         [0029]    In yet another version of the invention, there is provided a method for adjusting dosage of a medication in a patient having a pacemaker located external to the patient&#39;s heart. In the method, an alternating current signal is generated between the pacemaker  15  and the left ventricular tip electrode  26 . Impedance changes are sensed between the electrode  17  on or near the pacemaker  15  and the coronary sinus ring electrode  27  (or left ventricular tip electrode  26 , or right ventricular tip electrode  32 , or right ventricular ring electrode  34 , or right atrial tip electrode  22 ) to provide signals representative of impedance. Then, impedance signals are transmitted from the microprocessor to an external computing device, and cardiac output values are calculated from impedance signals received in the external computing device. The cardiac output values as a function of time are stored in the microprocessor or in the external computing device and then reviewed by a physician. The dosage of the medication may then be adjusted based on the stored cardiac output values. 
         [0030]    Thus, the present invention provides an implantable pacemaker that uses impedance cardiography to calculate cardiac output, and in so doing, a method for optimizing cardiac resynchronization therapy by interrogating the CRT device, itself, or of a multitude of other therapies. 
         [0031]    Although the present invention has been described with reference to certain embodiments, one skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which have been presented for purposes of illustration and not of limitation. Therefore, the scope of the appended claims should not be limited to the description of the embodiments contained herein.