Abstract:
An implantable medical device (IMD) having a therapy circuit for delivering atrial pacing and a control circuit for detecting a return to sinus rhythm. The control circuit determines the duration of an atrial arrhythmia preceding the return to sinus rhythm, and controls the therapy circuit to deliver transient atrial pacing based on the atrial arrhythmia duration.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to implantable medical devices (IMDs) that use a method for pacing a heart after an arrhythmia of a sustained duration to reduce the risk of stroke. 
     Arrhythmias are disorders of the regular beating of the heart. Irregular rhythms originating in one of the upper chambers of the heart, or atria, are called atrial arrhythmias, which include both atrial fibrillation and atrial flutter. One particular problem that can arise from a sustained period of atrial arrhythmia is the formation of blood clots, or thrombosis. 
     During atrial fibrillation, the heart&#39;s upper chambers quiver instead of beating effectively. Because blood is not pumped completely out of the atria during atrial fibrillation, blood may pool inside the atria, and a blood clot, or thrombus, may form on an atrial wall. When the normal heartbeat, or sinus rhythm, resumes, either spontaneously or through medical intervention, there is a danger that a blood clot that has formed in the left atrium will be dislodged. If a blood clot breaks free, it may become lodged in a blood vessel and cause a blockage, or thromboembolism, which may lead to stroke. 
     Since blood clots take time to form, there is a particular danger of thrombosis following arrhythmias of a sustained duration of time. Doctors seeking to treat patients by converting an irregular heart rhythm to a normal heart rhythm (cardioversion) must determine how long the arrhythmia has been taking place. If the patient&#39;s tachyarrhythmia has been for a sustained duration of time, usually defined as 48 hours or more, doctors may prescribe a blood thinning drug, such as heparin or warfarin, to dissolve any blood clots that may have formed. Cardioversion therapy is thus postponed until completion of the pharmacologic regimen. 
     A problem associated with cardioversion is early recurrence of atrial fibrillation (ERAF), which, as the name implies, means that the atrial fibrillation returns shortly after a sinus rhythm is achieved. Currently, implantable devices that provide high-energy shock therapy for termination of atrial fibrillation (such as the GEM III AT made by Medtronic, Inc.) and low-energy therapies for termination of atrial fibrillation (such as the GEM III AT and AT500 made by Medtronic, Inc.) are limited by ERAF. The risk of ERAF can be minimized by applying short-term, or transient, overdrive pacing. Overdrive pacing is a type of pacing therapy that paces the right atrium at a rate that is higher than the intrinsic heart rate. 
     Currently devices like the GEM III AT, AT500 and Kappa 900 (another device made by Medtronic, Inc.) include a feature called Post Mode Switch Overdrive Pacing (PMOP) that provides transient, high-rate overdrive pacing when a sinus rhythm is detected after an atrial arrhythmia. PMOP may decrease the risk of ERAF, but it is not intended to address two other problems that arise after cardioversion of an arrhythmia: stunned atrial myocardium and spontaneous echo contrast (SMOKE). 
     When a person suffers an arrhythmia for a sustained duration of time, the contractibility of the atria is depressed. This phenomenon is known as “stunned atrial myocardium”. During this period of time between the end of an arrhythmia and some later time when the patient has fully recovered, there is still a high risk of thrombosis. Spontaneous echo contrast (SMOKE) is a clinical parameter that is associated with this risk of thrombosis. It has been found that during this period of atrial myocardial stunning, applying high-rate atrial overdrive pacing resulted in a lower incidence of SMOKE, and therefore, presumably a lower risk of stroke. Sanders et al., “Reversal of Atrial Mechanical Dysfunction After Cardioversion of Atrial Fibrillation” (Circulation. 2003; 108:1976-1984). 
     Another type of atrial pacing is paired pacing (or coupled pacing), which is a pacing therapy that augments the contractility of the chambers of the heart. After termination of an arrhythmia, the heart requires a period of time until it fully recovers. Particularly, the cells of the heart exhibit a reduced capacity to contract, and as a result of this depressed contractility there is an increased risk of stroke. Paired pacing is the application of a pair of closely coupled pulses: an initial pulse and a second pulse just outside of the refractory period of the beat evoked by the initial pulse. Paired pacing increases the contractility of atrial tissue and reduces the time needed for atrial contractility to return to its normal state. 
     Hemodynamic sensor feedback has been used along with paired pacing, such as the device and method disclosed in U.S. Pat. No. 5,213,098. Also, a recent study demonstrated that atrial paired pacing may augment the function of the left ventricle function by improving atrial function and therefore augmenting ventricular filling. Gaasch et al., “Potentiation of Atrial Contractility by Paired Pacing Augments Ventricular Preload and Systolic Performance” (J Cardiac Fail. 2003; 8: 141-6). 
     Implantable devices that are capable of applying high rate overdrive pacing or paired pacing are known in the art. Using sensors to provide physiological feedback during pacing is also known. However, there is currently no implantable device that applies overdrive pacing or paired pacing in order to reduce the risk of stroke that occurs after a sustained period of atrial fibrillation. Also, there is also no implantable device that uses feedback from hemodynamic sensors to determine appropriate pacing parameters. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is an IMD that uses transient atrial pacing after cardioversion of arrhythmias of a sustained duration of time to augment atrial mechanical function, reduce the risk of thrombosis in the left atrium, and reduce the risk of stroke. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of electronic components in an implantable medical device that is configured to apply pacing and use sensor feedback. 
         FIG. 2  is a flow diagram illustrating the manner of using transient overdrive pacing after cardioversion of arrhythmias of a sustained duration of time to reduce the risk of stroke according to an embodiment of the invention. 
         FIG. 3  is a flow diagram illustrating the manner of using transient paired pacing after cardioversion of arrhythmias of a sustained duration of time to reduce the risk of stroke according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of electronic components of an implantable medical device (IMD)  100  that is configured to apply pacing and use sensor feedback. Microprocessor  102  controls the IMD in response to programmed instructions read from a storage device such as read-only memory (ROM)  104  via data/address bus  110 . IMD  100  also includes random access memory (RAM)  104 , which may be used to store physiological signal data, such as data obtained from tachyarrhythmia episodes. This data may be used by microprocessor  102  for therapy delivery and diagnostic purposes, as will be discussed in more detail in connection with  FIGS. 2 and 3 . This data may also be transferred to an external device via telemetry circuit  108  and antenna  109 . Telemetry circuit  108  and antenna  109  may also be used to transfer information to the IMD. 
     Microprocessor  102  is coupled to timing/control circuitry  112  and controls timing/control circuitry  112  to deliver pacing pulses to a patient at the appropriate times. These pacing pulses are delivered via output circuits  114  and  116 . Timing/control circuit  112  is also coupled to sense amplifiers  118  and  120 . Switch matrix  122  selectively couples the pace output circuits  114  and  116  and sense amplifiers  118  and  120  with electrodes  124 ,  126 ,  128  and  130 . 
     Defibrillation circuit  132  is able to deliver electrical cardioverting therapy via high voltage capacitors and a charging circuit (not shown). Timing/control circuitry  112  controls the time that defibrillation circuit  132  delivers the electrical cardioverting therapy to an atrium via high-voltage electrode  134  after an atrial arrhythmia is detected by electrode pair  124  and  126 . Defibrillation circuit  132  could be further coupled to one or more additional high-voltage electrodes such as electrode  136  positioned on or within a ventricle. 
     Sensor signal processor  138  receives signals from sensors  140  and  142 , which are positioned in the body. Sensors  140  may, for example, be located in the heart to detect parameters such as the oxygen content of the blood, or the carbon dioxide content of the blood. Sensor  142  may, for example, be a hemodynamic sensor that detects parameters such as blood flow, chamber pressure or chamber volume. It may be placed in the heart or in another part of the body, such as systemic or pulmonary arteries and veins. 
     Sensor signal processor  138  is connected to microprocessor  102  via data/address bus  110 . Microprocessor  102  uses the information contained in the signals sent from sensor signal processor  138  to make decisions as to the therapy to deliver to the patient. It may also use data stored in RAM  106 , such as the length of a tachyarrhythmic episode, to make decisions as to the therapy to deliver to the patient. Pacing, such as overdrive pacing or paired pulse pacing, may be delivered to the patient via timing/control circuit  112 . The present invention, as discussed in detail with respect to  FIG. 2  and  FIG. 3 , relates to the manner in which microprocessor  102  makes decisions regarding therapy to be delivered after cardioversion of an arrhythmia of sustained duration in order to reduce the risk of stroke. 
       FIG. 2  shows a flow chart for a process  200  of applying transient, constant rate atrial overdrive pacing after cardioversion of an arrhythmia of a sustained duration to augment atrial mechanical function and reduce the risk of stroke. The process illustrated in  FIG. 2  begins at start box  202 . Because the process is applied after termination of atrial arrhythmia, decision box  204  determines whether an arrhythmia has terminated. If the arrhythmia has not terminated, the process returns to start box  202 ; if the arrhythmia has terminated, the process moves on to decision box  206 . 
     Decision box  206  determines whether the episode of atrial arrhythmia occurred for a sustained duration of time. The definition of sustained duration of time could be selected and entered by the caregiver, or alternatively, it could be pre-determined and programmed into the device. The duration of time may be defined as the length or percentage of time of a single episode of atrial arrhythmia, or alternatively, as the cumulative length of time of multiple IMD detected arrhythmia during a certain period of time, such as 80%, or 22 hours out of the last 24 hours. If the measured duration of time of the arrhythmia is less than the amount of time defined as a sustained duration of time, the process returns to decision box  204 . If the measured duration of time of arrhythmia meets or exceeds the threshold amount of time defined as a sustained duration of time, the process advances to step  208 . 
     Step  208  determines the overdrive pacing rate. The overdrive pacing rate of the device may be a stored setting, or it could be selected and entered into the device by a caregiver, or it may be variable based on feedback from sensors connected to the device. For example, the device may apply a series of incremented overdrive pacing rates to the patient. During the time that the incremented overdrive pacing rates are applied, a sensor measures a relevant hemodynamic parameter, such as atrial pressure. Hemodynamic information could also be measured directly by the IMD, or information could be transmitted to the device from another IMD or other device via telemetry. After the series of incremented overdrive pacing rates has been applied, the device selects the overdrive pacing rate that corresponds to the minimized value of the mean atrial pressure as the optimal rate. Other hemodynamic variables could be in used in place of, or in addition to, atrial pressure to select the pacing rate, such as arterial pressure, ventricular pressure, mitral valve blood flow, aortic blood flow, left ventricular volume, left ventricular dimensions, respiratory rate, thoracic impedance, blood oxygen content, carbon dioxide content, or myocardial acceleration. 
     Next, step  210  determines the duration of the atrial overdrive pacing. The pacing therapy can be continued for a defined period of time, which may be programmed into the device or entered into the device by the caregiver. Alternatively, the pacing duration may be a variable time, such as a percentage of the time between sensed depolarizations during the arrhythmia or a period of time determined by feedback from a sensor. For example, the absolute right or left atrial pressure could be measured after termination of an atrial arrhythmia, or the change in right or left atrial pressure could be measured after termination of an atrial arrhythmia. If either the absolute pressure or the change in pressure exceeds some threshold valve, then overdrive pacing or paired pacing is applied. If the threshold is not met, then therapy is withheld. 
     After the rate and duration of the atrial overdrive pacing are determined, constant rate overdrive pacing is applied at the determined rate for the determined duration in step  212 . Step  214  then uses sensor feedback in the manner described with respect to steps  208  and  210  to determine whether the pacing therapy is optimal. If the pacing therapy is not optimal, the process returns to step  208  to re-determine the overdrive rate and step  210  re-determine the overdrive duration. This cycle of steps  208 ,  210 ,  212  and  214  is repeated until optimal pacing parameters are found. 
       FIG. 3  shows flow chart  300  for the process of applying transient atrial paired pacing after cardioversion of an arrhythmia of a sustained duration of time to augment atrial mechanical function and reduce the risk of stroke. The process illustrated in  FIG. 3  begins at start box  302 . Because the process is applied after termination of atrial arrhythmia, decision box  304  determines whether an arrhythmia has terminated. If the arrhythmia has not terminated, the process returns to start box  342 ; if the arrhythmia has terminated, the process moves on to decision box  306 . 
     Decision box  306  determines whether the episode of atrial arrhythmia occurred for a sustained duration of time. The definition of sustained duration of time could be selected and entered by the caregiver, or alternatively, it could be pre-determined and programmed into the device. The duration of time may be defined as the length or percentage of time of a single episode of atrial arrhythmia, or alternatively, as the cumulative length of time of multiple IMD detected arrhythmia during a certain period of time, such as 80%, or 22 hours out of the last 24 hours. If the measured duration of time of the arrhythmia is less than the amount of time defined as a sustained duration of time, the process returns to decision box  304 . If the measured duration of time of arrhythmia meets or exceeds the threshold amount of time defined as a sustained duration of time, the process advances to step  308 . 
     Next, step  310  determines the coupling interval. Paired pacing involves the application of a pair of closely coupled pulses: an initial pulse and a second pulse just outside of the refractory period of beat evoked by the initial pulse. The coupling interval is the amount of time between the first and second pulse, or the time between the sensed atrial event and the coupled pace. As with the paired pacing rate, the coupling interval may be pre-set in the device, or it may be based on the paired pacing interval during sinus rhythm or determined by using feedback from a sensor in the same way as described with respect to determining the paired pacing rate in box  308 . 
     Next, step  312  determines the duration of the atrial paired pacing. The pacing therapy could be continued for a defined period of time, which may be programmed into the device or entered into the device by the user. Alternatively, the pacing duration may be a variable time, either a percentage of the duration of the arrhythmia or a period of time determined by feedback from a sensor. Hemodynamic information could also be measured directly by the IMD, or information could be transmitted to the device from another IMD or other device via telemetry. For example, the absolute right or left atrial pressure could be measured after termination of atrial arrhythmia, or the change in right or left atrial pressure could be measured after termination of atrial arrhythmia. If either the absolute pressure or the change in pressure exceeds some threshold valve, then paired pacing therapy is applied. If the threshold is not met, then therapy is withheld. Other hemodynamic variables could be in used in place of, or in addition to, atrial pressure to select the pacing rate, such as arterial pressure, ventricular pressure, mitral valve blood flow, aortic blood flow, left ventricular volume, left ventricular dimension, respiratory rate, thoracic impedance, blood oxygen content, carbon dioxide content, or myocardial acceleration. 
     After the rate, coupling interval and duration of the atrial paired pacing are determined, atrial paired pacing is applied at step  314 . Step  316  then uses sensor feedback in the manner described with respect to steps  308 ,  310  and  312  to determine whether the pacing therapy is optimal. If the pacing therapy is not optimal, the process returns to step  308  to re-determine the overdrive rate, step  310  to re-determine the coupling interval and step  312  re-determine the paired pacing duration. This cycle of steps  308 ,  310 ,  312 ,  314  and  316  is repeated until optimal pacing parameters are found. 
     The present invention paces a heart after termination of an arrhythmia of sustained duration in order to reduce the risk of stroke. The invention may be implemented using existing implantable medical devices, which are capable of applying overdrive pacing and paired pacing, and which include sensors that can monitor hemodynamic variables. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.