Abstract:
A system ( 10 ) for achieving a desired cardiac rate and cardiac rhythm in response to atrial fibrillation in a heart includes an atrial fibrillation (AF) detector ( 40 ) for detecting AF. The system also includes an atrioventricular node vagal stimulator (AVN-VS) ( 30 ) for stimulating vagal nerves associated with an atrioventricular (AV) node of the heart. The system further includes an on-demand pace maker ( 40 ) for providing ventricular pacing stimulation to the heart. A control unit ( 20 ) is operatively connected with the AF detection device, the AVN-VS device, and the on-demand pacing device. The control unit is responsive to AF detection by the AF detector to cause the AVN-VS to stimulate the vagal nerves to help reduce the ventricular rate of the heart. The control unit is further responsive to AF detection by the AF detector to cause the on-demand pace maker to help regulate the ventricular rate of the heart.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a system and method for treating cardiac arrhythmia. More particularly, the present invention relates to a system and method for achieving regular slow ventricular rhythm in response to atrial fibrillation.  
       BACKGROUND OF THE INVENTION  
       [0002]     Cardiac arrhythmia are common and potentially dangerous medical aliments associated with abnormal cardiac chamber wall tissue. Characteristic of cardiac arrhythmia, abnormal regions of cardiac tissue do not follow the synchronous beating cycle associated with normal cardiac tissue. The abnormal cardiac tissue regions conduct electrical activity to adjacent tissue with aberrations that disrupt the cardiac cycle, creating an asynchronous cardiac rhythm. Various serious conditions, such as stroke, heart failure, and other thromboembolic events, can occur as a result of cardiac arrhythmia.  
         [0003]     One particular type of cardiac arrhythmia is atrial fibrillation (AF). Atrial fibrillation is recognized as the most common clinically significant cardiac arrhythmia and increases significantly the morbidity and mortality of patients. Current data estimates that 2.3 million Americans experience AF. Since the prevalence of AF increases with age, and due to the aging population, the number of AF patients is estimated to increase 2.5 times during the next 50 years.  
         [0004]     Atrial fibrillation usually results in a rapid ventricular rate and an irregular ventricular rhythm that produce undesirable negative hemodynamic effects. Long-term uncontrolled rapid ventricular rate could, for example, lead to tachycardia-induced cardiomyopathy. Irregular ventricular rhythm may independently produce detrimental consequences and may cause symptoms in some patients, even when the ventricular rate is controlled. It is therefore desirable to achieve ventricular rate control and ventricular rhythm regularization during AF.  
         [0005]     A variety of medical procedures have been developed to help treat cardiac arrhythmia. Drug therapy is the most common approach to achieve slow ventricular rate in AF patients. Drug therapy may, however, may be ineffective or not well tolerated. Partial ablative procedures, such as AV node modification, have been shown to be effective in reducing ventricular rate in some drug-refractory AF patients. However, due to the risk of AV block associated with AV node modification, this therapy is recommended only when AV nodal ablation with pacemaker implantation is intended. Although AV nodal ablation with right ventricular pacing has been shown to be beneficial in improving symptoms, quality of life, and exercise duration in drug-refractory patients with AF, it creates permanent AV block and results in lifelong pacemaker dependency.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention relates to a system and method for treating cardiac arrhythmia. More specifically, the present invention relates to a system and method for achieving regular slow ventricular rhythm in response to atrial fibrillation. In one particular aspect, the system and method involves the use of AV node selective vagal stimulation and ventricular on-demand pacing to achieve regular slow ventricular in response to atrial fibrillation.  
         [0007]     The present invention also relates to a system for achieving a desired cardiac rate and cardiac rhythm in response to atrial fibrillation in a heart. The system includes atrial fibrillation (AF) detecting means for detecting AF. The system also includes atrioventricular node vagal stimulation (AVN-VS) means for stimulating vagal nerves associated with a atrioventricular (AV) node of the heart. The system also includes on-demand pacing means for providing ventricular pacing stimulation to the heart. The system further includes control means operatively connected with the AF detecting means, the AVN-VS means, and the on-demand pacing means. The control means is responsive to AF detection by the AF detecting means to cause the AVN-VS means to stimulate the vagal nerves to help reduce the ventricular rate of the heart. The control means is further responsive to AF detection by the AF detecting means to cause the on-demand pacing means to help regulate the ventricular rate of the heart.  
         [0008]     The present invention also relates to a system for achieving a desired cardiac rate and cardiac rhythm in response to atrial fibrillation in a heart. The system includes an atrial fibrillation (AF) detector for detecting atrial fibrillation in the heart. The system also includes an atrioventricular node vagal stimulation (AVN-VS) electrode for stimulating vagal nerves associated with an atrioventricular (AV) node of the heart. The system also includes an on-demand pacing electrode for providing ventricular pacing stimulation to the heart. The system also includes a control unit operatively connected with the AF detector, the AVN-VS electrode, and the on-demand pacing electrode. The control unit is operative to determine an AF episode via the AF detector. The control unit is also operative to provide an electrical signal to the AVN-VS electrode to stimulate the vagal nerves to help reduce the ventricular rate of the heart in response to determining an AF episode. The control means is further operative to provide an electrical signal to the on-demand pacing electrode to stimulate the heart to help regulate the ventricular rate of the heart in response to determining an AF episode.  
         [0009]     The present invention further relates to a method for achieving a desired cardiac rate and cardiac rhythm in response to atrial fibrillation in a heart. The method includes the steps of stimulating vagal nerves associated with an atrioventricular (AV) node of the heart to reduce the cardiac rate and applying ventricular pacing stimulation to regulate the ventricular rhythm of the heart. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0010]     The foregoing and other features of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which:  
         [0011]      FIG. 1  is a schematic illustration of a system for treating cardiac arrhythmia in accordance with a first example embodiment of the present invention;  
         [0012]      FIG. 2  illustrates a patient outfitted with the system of  FIG. 1 ;  
         [0013]      FIG. 3  is a schematic block diagram of a process performed by the system of  FIG. 1 ;  
         [0014]      FIG. 4  is a chart illustrating the effectiveness of the present invention in normalizing R-R interval distributions in response to atrial fibrillation;  
         [0015]      FIGS. 5A-5D  are charts depicting ECG traces that illustrate the effectiveness of the present invention in treating atrial fibrillation;  
         [0016]      FIGS. 6A-6H  are charts illustrating the improved hemodynamic performance realized by the present invention in response to atrial fibrillation;  
         [0017]      FIG. 7  is a schematic illustration of a system for treating cardiac arrhythmia in accordance with a second example embodiment of the present invention; and  
         [0018]      FIG. 8  is a schematic illustration of a system for treating cardiac arrhythmia in accordance with a third example embodiment of the present invention. 
     
    
     DESCRIPTION OF EMBODIMENTS  
       [0019]     The present invention relates to a system and method for treating cardiac arrhythmia and, in particular, a system and method for treating atrial fibrillation (AF). Representative of the present invention,  FIG. 1  illustrates an example configuration of a system  10  for combining AV node vagal stimulation (AVN-VS) with on-demand pacing, such as VVI pacing, to achieve a desired ventricular rate with a desired ventricular rhythm in response to atrial fibrillation. For example, the system  10  may combine AVN-VS and on-demand pacing to achieve a relatively slow ventricular rate with a substantially regular ventricular rhythm. In the example embodiment of  FIG. 1 , the system  10  includes a control unit  20 , one or more vagal stimulation (VS) electrodes  30 , and one or more pacing electrodes  40 . The VS electrodes  30  are operatively connected with the control unit  20  via VS leads  32 . The pacing electrodes  40  are operatively connected with the control unit  20  via pacing leads  42 .  
         [0020]     The control unit  20  is operative to provide electrical stimulation signals to the VS electrodes  30  and the pacing electrodes  40  via the leads  32  and  42 , respectively. The control unit  20  modulates or controls the frequency, amplitude, duration/pulse-width of the stimulation signals in accordance with the description set forth below in order to achieve the desired ventricular rate and rhythm. The control unit  20  may also be operative to monitor cardiac activity, such as R waves, via the pacing electrodes  40 .  
         [0021]     Referring to  FIG. 2 , a patient  50  is outfitted with the system  10  of the example embodiment of  FIG. 1 . The pacing electrodes  40  are delivered and implanted in the right ventricle  52  of the patient&#39;s heart  54  via blood vessels, which may be accessed by known means (not shown), such as by cannulization and catheterization of the vessels. In the embodiment of  FIG. 2 , the pacing electrodes  40  are inserted into the left brachiocephalic vein  60 , pass through the superior vena cava  62  and right atrium  64 , and are delivered to the right ventricle  52  (e.g., the right ventricular apex) of the patient&#39;s heart  54 . While this procedure is commonly used to deliver the pacing electrodes  40 , those skilled in the art will appreciate that the pacing electrodes may be delivered to the right ventricle  52  in any suitable alternative manner.  
         [0022]     In the embodiment of  FIG. 2 , the VS electrodes  30  are implanted or otherwise positioned on an AV nodal fat pad  70  of the patient&#39;s heart  54 . The AV nodal fat pad  70  is an epicardial fat pad located adjacent the posterior AV groove of the heart  54 . The AV nodal fat pad  70  is rich with vagal nerves/nerve fibers. The VS electrodes  30  may be positioned on the AV nodal fat pad  70  in a variety of manners. For example, the VS electrodes  30  may be positioned on the AV nodal fat pad  70  via a surgical procedure in which the patient&#39;s chest cavity is opened and the electrodes are positioned on the heart  54  directly. Those skilled in the art will appreciate, however, that the VS electrodes  30  may be positioned on the AV nodal fat pad  70  in an alternative manner, such as via a minimally invasive surgical procedure.  
         [0023]     The control unit  20  may be implanted subcutaneously in the chest area of the patient  50 , as shown in  FIG. 2 . As known in the art, the pacing electrodes  30  may also serve as sensors for monitoring the electrical activity of the heart  54 , such as the R-R intervals of the heart. The control unit  20  may thus monitor the rate or rhythm of the heart  54  via the pacing electrodes  30  and may detect the occurrence of atrial fibrillation. Alternatively, separate electrodes (not shown) may be used to detect AF. The system  10  thus may be an active or “on-demand” system in which ventricular rate and rhythm control is applied in response to detection to an AF episode.  
         [0024]      FIG. 3  illustrates a functional block diagram depicting a process  120  performed by the system  10  of  FIGS. 1 and 2  in accordance with the present invention. Referring to  FIG. 3 , at  100 , the system  10  monitors electrical cardiac activity. At  102  a determination is made as to whether atrial fibrillation is detected. If atrial fibrillation is not detected, the system  10  reverts to  100  and continues to monitor the cardiac electrical activity. The system  10  thus may provide continuous monitoring of cardiac electrical activity for the occurrence of atrial fibrillation.  
         [0025]     If, at  102 , atrial fibrillation is detected, the system  10  proceeds to  104  and applies AVN-VS. In the embodiment illustrated in  FIG. 2 , the control unit  20  delivers electrical stimulation to the AV nodal fat pad  70  via the VS electrodes  30  and lead  32 . This AVN-VS is effective to reduce the ventricular rate during AF. By titrating or modulating the energy delivered to the AV nodal fat pad  70 , the average ventricular rate can be controlled (i.e., lowered) to or toward a desired value.  
         [0026]     At  104 , the control unit may implement an open loop control algorithm for applying the AVN-VS energy. In this example configuration, the AVN-VS energy may be applied at a constant predetermined level or intensity to lower the ventricular rate during AF. The rate to which the ventricular rate is lowered may depend on a variety of factors, such as the ventricular rate at the onset of AF and the particular physiological cardiac conditions of the particular patient. Thus, in an open loop control configuration, the ventricular rate is lowered using AVN-VS without being controlled to a particular desired rate.  
         [0027]     Alternatively, at  104 , the control unit  20  may implement a closed loop control algorithm to control the AVN-VS in order to achieve a particular desired ventricular rate. The closed loop algorithm implemented in the control unit  20  may be any algorithm suited to achieve active feedback control. For example, a classic proportional-integral-derivative (PID) control algorithm may be implemented in the control unit  20 . In this example configuration, the control unit  20  would measure the instantaneous heart rate based on the time interval between two successive heart beats. This instantaneous heart rate would then be compared to the desired ventricular rate to determine an error value, which is provided to the PID control algorithm. Based on the error value, the PID control algorithm calculates an increase or decrease in the intensity of the AVN-VS energy to control or “steer” the ventricular rate toward the desired value. This control loop continues until a zero-error condition is achieved. Through implementation of the closed loop control algorithm, the control unit  20  may thus maintain the ventricular rate at the desired value.  
         [0028]     According to the present invention, with the average ventricular rate is adjusted to the desired value using AVN-VS procedure described above, the system  10  proceeds to  106 , where the ventricular rhythm is normalized via on-demand ventricular pacing (e.g., VVI pacing). Referring to  FIG. 2 , the control unit  20  delivers electrical stimulation to the cardiac tissue of the right ventricle  52  via the pacing electrodes  40  and lead  42 . This on-demand pacing is effective to help normalize the ventricular rhythm brought about by the AF. By controlling the frequency, amplitude, or both the frequency and amplitude with which the energy is delivered to the right ventricle  52 , the ventricular rhythm can be controlled to help maintain a desired ventricular rhythm. Thus, according to the present invention, the system  10  combines the AVN-VS and on-demand ventricular pacing to help achieve a regular slow ventricular rate in response to AF. The system  10  may achieve this purpose while avoiding the use of AVN ablation and medication therapy.  
         [0029]     The description set forth above regarding the process  120  performed by the system  10  is not meant to limit the steps to any sequence or order. For example, while the steps of applying AVN-VS (step  104 ) and on-demand pacing (step  106 ) appear as being performed in a particular sequence, these steps could be performed in any order or simultaneously. In fact, the steps of monitoring cardiac activity  100 , determining the occurrence of AF  102 , applying AVN-VS  104 , and applying on-demand pacing  106  may all be performed simultaneously.  
         [0030]     On-demand ventricular (VVI) pacing, when initiated during AF, helps eliminate R-R intervals longer than the pacing interval. A progressively increasing pacing rate can also eliminate many of the R-R intervals that are shorter than the pacing interval. When the pacing rate is faster than the average intrinsic ventricular rate during AF, it has been found that a regular ventricular rhythm ensues. Thus, when the intrinsic ventricular rate is already excessively rapid during AF, the required pacing rate would be unacceptably high, rendering the on-demand pacing method impractical for clinical applications. The specific pacing rate required to achieve regularization is thus directly linked to the spontaneous ventricular rate during AF. Therefore, regularization at a desired rate level could be achieved if the average intrinsic ventricular rate during AF is slowed down first. According to the present invention, the application of AVN-VS prior to ventricular pacing helps lower the average ventricular rate to a level at which an increase induced by subsequent ventricular pacing remains at an acceptable level.  
         [0031]     On-demand pacing as a rate regularization tool necessitates relatively short pacing intervals. For example, a significant reduction of irregularity during AF can be achieved at a cost of about 2% to 17% increase of ventricular rate. However, because the ventricular rate during AF frequently is quite rapid without medications, such an increase in the ventricular rate may be undesirable. On-demand pacing could thus be viewed as being somewhat limited due to concerns over elevating the ventricular rate to dangerously high levels.  
         [0032]     Instead of using on-demand ventricular pacing as the ventricular rate slowing mechanism, the present invention utilizes neural control of AV transmission during AF (i.e., AVN-VS) to slow the ventricular rate. AVN-VS takes advantage of the rich and selective supply of vagal nerves to the AV node, which exert negative dromotropic effect. By modulating the stimulation amplitude applied during AVN-VS, one can achieve graded ventricular rate slowing in order to reach an optimal hemodynamic response.  
         [0033]     This pacing strategy is mechanistically based on suppression of conduction through the AVN as a result of collision of anterograde and retrograde wavefronts. To begin with, AF itself has been shown to result in random, high-rate bombardment at the AVN inputs, resulting in subsequent concealed conduction of multiple impulses. While the precise mechanism of concealment remains undetermined, it certainly depends on the delayed recovery of nodal excitability as reflected in the “refractory” theory for ventricular response in AF. Whether or not associated with decremental conduction, the coexistence of multiple anterograde impulses should result in collision, intranodal block(s), and electrotonic events that modulate subsequent propagation. As postulated by the “interception” theory and demonstrated in modeling studies, in these conditions retrograde impulses invading the AVN are followed by refractoriness with slow recovery of excitability, setting the stage for electrotonic inhibition of anterograde impulses.  
         [0034]     Therefore, it is most likely that ventricular pacing at a rate equal to or above that present during AF results in a critical degree of collision/annihilation of retrograde and anterograde impulses in the AVN. The major role of the AVN-VS would be to further inhibit the propagation through the AVN during AF. Because a reduced number of atrial impulses would successfully traverse the node, a comparatively lower rate VVI pacing would be necessary to counteract them and achieve the effect of “electrical jam.” Thus, AVN-VS accentuates the VVI effect by permitting full elimination of anterograde propagation of fibrillatory impulses at substantially lower rates.  
       Experimental Data  
       [0035]     To determine the efficacy of the systems and methods described herein, experiments were performed on adult canine specimens (body weight 21-30 kg). The specimens were anesthetized and ventilated with room air supplemented with oxygen as needed to maintain normal arterial blood gases. The left external jugular vein was cannulated, and normal saline was infused at 100 to 200 mL/h to replace spontaneous fluid losses. Standard surface ECG leads I, II, and III were monitored continuously throughout the entire study. Intermittent arterial blood gas measurements were taken, and adjustments of ventilator were made to correct metabolic abnormalities. Body temperature was monitored and maintained at 36 to 37° C. using an electrical heating pad placed under the specimen and operating room lamps.  
         [0036]     Micromanometer-tipped catheters were inserted through cannulated femoral and carotid arteries and advanced to the thoracic aorta and left ventricle (LV), respectively, to record blood pressure and LV pressure. After the chest was opened through a median sternotomy, a cardiac output probe was placed around the aorta and connected to a flow meter to measure aortic flow. Custom-made quadripolar plate electrodes were sutured to the high right atrium and right ventricular apex for bipolar pacing and recording. Atrial pacing was used to induce AF, whereas ventricular VVI pacing was used for rate regularization. A bipolar stimulating electrode was sutured to the epicardial fat pad (the inferior vena cava-left atrium fat pad) that contains parasympathetic neural pathways selectively innervating the AVN. All signals (surface ECG, right atrial and right ventricular ECGs, aortic blood pressure, LV pressure, and aortic flow signals) were properly amplified, filtered, and for purposes of display and recording.  
         [0037]     AVN-VS was delivered to the inferior vena cava-left atrium fat pad by a computer-controlled feedback program to achieve three levels (targets) of average ventricular rate slowing. These targets were defined as 75%, 100%, and 125% of the corresponding spontaneous sinus cycle length (SCL) present before AF was induced. The program implemented a classic, proportional-integral-derivative closed-loop process control in delivering the AVN-VS. To achieve target ventricular rate levels during AF, AVN-VS was delivered as short bursts synchronized with the right ventricular electrogram. After each target rate was achieved, VVI pacing was initiated at a rate equal to the achieved target while maintaining delivery of the AVN-VS.  
         [0038]     After surgical preparation and at least 30 minutes of stabilization, SCL was determined. AF then was induced and maintained by rapid right atrial pacing (20 Hz, 2 ms). After at least 15 minutes of stabilization, the ventricular rate was determined on-line by averaging 500 cardiac cycles collected during AF. Then, while maintaining AF, the feedback computer program was initiated to deliver the AVN-VS and to slow the average ventricular rate to 1 of 3 target levels: 75%, 100%, or 125% of the corresponding SCL. The computerized AVN-VS was considered satisfactory when the targets were reached within 5%. After a given target level was reached and maintained for at least 500 beats, VVI pacing at a cycle length equal to the achieved target was added to the on-going AVN-VS and another 500 beats were collected. The order of the three levels of ventricular rate slowing was randomized. A recovery period of 5 minutes was allowed after each target study, although the AF was maintained uninterrupted.  
         [0039]     AF produced irregular and rapid ventricular responses that resulted in an average ventricular rate substantially faster than the spontaneous sinus rate. The average R-R during AF was 287±36 ms, or 56% of the SCL (SCL=514±57 ms, n=8, P&lt;0.01). AVN-VS successfully slowed average ventricular rate to each of the three target levels (achieved values=74%, 99%, and 123% of SCL, respectively).  
         [0040]      FIG. 4  shows an example of R-R interval distribution during AF (beats 1-500), with average R-R interval of 298 ms (corresponding to 201 bpm) and a standard deviation (SD) of 70 ms. AVN-VS prolonged the R-R intervals (i.e., ventricular rate was slowed). In this case, SCL was 510 ms (corresponding to 118 bpm), and the target was 100% SCL (beats 501-1000). The achieved average R-R interval was 505 ms but with a SD of 141 ms. Thus, compared with AF, AVN-VS reduced the average ventricular rate but not the irregularity.  
         [0041]     When, in addition to AVN-VS, VVI pacing at a cycle length equal to the achieved target level (i.e., 505 ms) was initiated, it not only abolished all R-R intervals longer than the pacing interval but also eliminated all intervals shorter than the pacing interval (beats 1001-1500). Thus, a regular ventricular rhythm was immediately achieved and maintained. This regularity was just as immediately lost when AVN-VS was turned off (beats 1501-2000), indicating that AVN-VS was a crucially needed component of the complex pacing algorithm used to maintain regular ventricular rate.  
         [0042]     ECG traces from the same experiment are shown in  FIGS. 5A-5D .  FIG. 5A  illustrates recordings during AF revealing the irregularity of the right ventricular electrograms (R-R range 242-428 ms in this episode).  FIGS. 5B-5D  illustrate episodes of combined delivery of AVN-VS and VVI pacing. In  FIG. 5B , the AVN-VS intensity was first titrated to maintain the average ventricular rate during AF at a level corresponding to 75% SCL. The concomitant VVI pacing then resulted in constant R-R=379 ms (158 bpm).  FIGS. 5C and 5D  illustrate the outcome at 100% and 125% SCL, respectively. Again, progressively slower but strictly regular rhythms (R-R=505 ms [119 bpm] and 620 ms [97 bpm], respectively) were achieved in each case. The computer-controlled intensity of the brief AVN-VS bursts increased from 1.2 mA in  FIG. 5B  to 3.5 mA in  FIG. 5D  (where small artifacts produced by the brief AVN-VS bursts are indicated on the ECG at  110 ). Thus, by combining AVN-VS with VVI pacing, regular slow ventricular rhythms were achieved at each of the three target levels.  
         [0043]     As set forth below, Table 1 lists the average R-R intervals and the corresponding standard deviation (SD) in each of the eight specimens (along with the composite data) during sinus rate, AF, after rate slowing by AVN-VS alone, and during AVN-VS plus VVI pacing. Note that although average ventricular rates were successfully controlled by AVN-VS alone (achieved average cycle lengths were within 2% of the corresponding targets), the rates still were very irregular, as evidenced by large SD. However, AVN-VS plus VVI pacing resulted not only in rate slowing but also in rhythm regularization (SD=0).  
                                                                     TABLE 1                                       Target 75% SCL   Target 100% SCL   Target 125% SCL            Specimen   SCL   AF   AVN-VS   AVN-VS + VVI   AVN-VS   AVN-VS + VVI   AVN-VS   AVN-VS + VVI               1   510   298 ± 70   379 ± 93   379 ± 0    505 ± 141   505 ± 0   620 ± 189   620 ± 0       2   540   257 ± 46   396 ± 89   396 ± 0    529 ± 121   529 ± 0   660 ± 133   660 ± 0       3   400   267 ± 22   298 ± 61   298 ± 0   398 ± 74   398 ± 0   497 ± 87    497 ± 0       4   480   271 ± 48   356 ± 59   356 ± 0   474 ± 77   474 ± 0   590 ± 105   590 ± 0       5   500   283 ± 21   373 ± 64   373 ± 0   498 ± 10   498 ± 0   625 ± 109   625 ± 0       6   580   304 ± 43   427 ± 98   427 ± 0   570 ± 13   570 ± 0   709 ± 129   709 ± 0       7   530   252 ± 37   397 ± 85   397 ± 0   529 ± 12   529 ± 0   658 ± 156   658 ± 0       8   570   365 ± 68   419 ± 82   419 ± 0   558 ± 14   558 ± 0   699 ± 167   699 ± 0       Composite Mean   514 ± 57   287 ± 36   381 ± 41    381 ± 41   508 ± 54    508 ± 54   632 ± 68     632 ± 68       (% SCL)   (100%)   (56%)   (74%)   (74%)   (99%)   (99%)   (123%)   (123%)                 AF = atrial fibrillation;            SCL = sinus cycle length.             
 
         [0044]      FIGS. 6A-6H  illustrate the Improved hemodynamic responses realized through the application of the AVN-VS plus VVI pacing during AF in accordance with the present invention. In  FIGS. 6A-6H , the hemodynamic parameters are shown as measured during sinus rhythm (SA), during atrial fibrillation (AF), and during AVN-VS plus VVI pacing at 75%, 100%, and 125% of the sinus cycle length (SCL). In  FIGS. 6A-6H , plot points that include an asterisk (*) indicate values that are statistically significant over values experienced during AF (i.e., where P&lt;0.05, as determined using post hoc Tukey&#39;s honestly significant difference test).  
         [0045]     As shown in  FIGS. 6A-6H , during AF, the measured hemodynamic parameters were significantly worsened compared to sinus rhythm. Regular slow rates achieved by AVN-VS plus VVI pacing during AF significantly improved all responses, with the exception that diastolic blood pressure (DSB, see  FIG. 6D ) improved only slightly and without statistical significance. In particular, systolic blood pressure (SBP, see  FIG. 6C ), LV systolic pressure (LVSP, see  FIG. 6E ), LV end-diastolic pressure (LVEDP, see  FIG. 6F ), ±dp/dt (see  FIGS. 6G and 6H ), stroke volume (SV, see  FIG. 6B ), and cardiac output (CO, see  FIG. 6A ) all improved significantly at each of the regular slow rates achieved by AVN-VS plus VVI pacing. Cardiac output, ±dp/dt, and LV end-diastolic pressure were best improved at a rate target corresponding to 100% SCL. This indicates that slowing the average ventricular rate to the level of the spontaneous sinus rhythm provided optimal overall hemodynamic benefits during AF.  
         [0046]     The experimental data set forth above confirms that, with use of selective neural AVN-VS as a first step, subsequent regular VVI pacing at predetermined desired slow rates can be achieved. The slow, regular ventricular rates achieved by AVN-VS plus VVI pacing were associated with pronounced hemodynamic benefits that were rate dependent and permitted an optimal tune-up of the pacing protocol.  
         [0047]     Based on the above, it will be appreciated that the system  10  of the present invention, capable of delivering VVI pacing along with AVN-VS, could achieve not only rate control but also regularization of the ventricular rhythm. The system  10  of the present invention could, for example, be embodied as a pace maker adapted to provide the on-demand pacing and AVN-VS functionality described above. The use of AVN-VS in combination with on-demand pacing may be preferable over ablation procedures and drug therapy, or may be used in addition to drug therapy.  
         [0048]      FIG. 7  illustrates an example configuration of a system  10   a  for combining AVN-VS with on-demand pacing to achieve a desired ventricular rate with a desired ventricular rhythm in response to AF. The system  10   a  of the second embodiment of the invention is similar to the system  10  of the first embodiment, except that the AVN-VS is administered in a manner that differs from that of the first embodiment. Therefore, in  FIG. 7 , reference numbers similar to those used to describe the first embodiment will be used to describe like elements, the suffix “a” being added to the reference numbers in  FIG. 7  to avoid confusion.  
         [0049]     The system  10   a  includes a control unit  20   a , one or more vagal stimulation (VS) electrodes  152 , and one or more pacing electrodes  40   a . The VS electrodes  152  are operatively connected with the control unit  20   a  via VS leads  150 . The pacing electrodes  40   a  are operatively connected with the control unit  20   a  via pacing leads  42   a.    
         [0050]     The control unit  20   a  is operative to provide electrical stimulation signals to the VS electrodes  152  and the pacing electrodes  40   a  via the leads  150  and  42   a , respectively. The control unit  20   a  modulates or controls the frequency, amplitude, duration/pulse-width of the stimulation signals as described herein to achieve the desired ventricular rate and rhythm. The control unit  20   a  may also be operative to monitor cardiac activity, such as R waves, via the pacing electrodes  40   a.    
         [0051]     In  FIG. 7 , a patient  50   a  is outfitted with the system  10   a  of the second embodiment. The pacing electrodes  40   a  are delivered and implanted in the right ventricle  52   a  of the patient&#39;s heart  54   a  via blood vessels in a manner similar or identical to that described above in regard to the first embodiment. According to the second embodiment, VS electrodes  152  are implanted or otherwise positioned for stimulating left vagus nerves  154  of the patient  50   a . In the embodiment of  FIG. 7 , the left vagus nerves  154  are cervical vagus nerves accessed through the patient&#39;s neck  156  via means, such as a catheterization or surgical procedure.  
         [0052]     The control unit  20   a  may monitor electrical cardiac activity, such as R-R intervals, via the pacing electrodes  30   a , in a manner similar or identical to that described above in regard to the first embodiment. This allows the system  10   a  to monitor the rate or rhythm of the heart  54   a  and detect the occurrence of atrial fibrillation. The system  10   a  thus may be an active or “on-demand” system in which ventricular rate and rhythm control is applied in response to detection to an AF episode.  
         [0053]     In operation, the system  10   a  of the second embodiment operates in a manner similar or identical to that of the first embodiment as described above, with the exception that AVN-VS signals are delivered to the left cervical vagus nerve  154  as opposed to the AV nodal fat pad. The functional block diagram of  FIG. 3  thus depicts a process performed by the system  10   a  of  FIG. 7 . More specifically, as shown in  FIG. 3 , the system  10   a  monitors electrical cardiac activity for the occurrence of atrial fibrillation. Upon detecting an AF episode, AVN-VS is applied to reduce the ventricular rate and ventricular (VVI) pacing is applied to help maintain a desired ventricular rhythm. Thus, according to the second embodiment of the present invention, the system  10   a  combines the AVN-VS and on-demand ventricular pacing to help achieve a regular slow ventricular rate in response to AF.  
         [0054]      FIG. 8  illustrates an example configuration of a system  10   b  for combining AVN-VS with on-demand pacing to achieve a desired ventricular rate with a desired ventricular rhythm in response to AF. The system  10   b  of the third embodiment of the invention is similar to the systems  10  and  10   a  of the first and second embodiments, except that AVN-VS is administered in a manner that differs from those of the first and second embodiments. Therefore, in  FIG. 8 , reference numbers similar to those used to describe the first and second embodiments will be used to describe like elements, the suffix “b” being added to the reference numbers in  FIG. 8  to avoid confusion.  
         [0055]     The system  10   b  includes a control unit  20   b , one or more vagal stimulation (VS) electrodes  172 , and one or more pacing electrodes  40   b . The VS electrodes  172  are operatively connected with the control unit  20   b  via VS leads  170 . The pacing electrodes  40   b  are operatively connected with the control unit  20   b  via pacing leads  42   b.    
         [0056]     The control unit  20   b  is operative to provide electrical stimulation signals to the VS electrodes  172  and the pacing electrodes  40   b  via the leads  170  and  42   b , respectively. The control unit  20   b  modulates or controls the frequency, amplitude, duration/pulse-width of the stimulation signals as described herein to achieve the desired ventricular rate and rhythm. The control unit  20   b  may also be operative to monitor cardiac activity, such as R waves, via the pacing electrodes  40   b.    
         [0057]     In  FIG. 8 , a patient  50   b  is outfitted with the system  10   b  of the third embodiment. The pacing electrodes  40   b  are delivered and implanted in the right ventricle  52   b  of the patient&#39;s heart  54   b  via blood vessels in a manner similar or identical to that described above in regard to the first embodiment.  
         [0058]     According to the second embodiment, VS electrodes  172  are implanted or otherwise positioned for stimulating vagal nerve fibers indirectly via various endocardial structures. The embodiment of  FIG. 8  illustrates various different alternative locations for endocardial placement of the VS electrodes  172 . One location for endocardial placement of the VS electrodes  172  is the AV node  180 . With this placement, the VS electrodes  172  apply post-ganglionic vagal stimulation to the AV node  180  directly. Another location for endocardial placement of the VS electrodes  172  is on the inside surface of the atrial wall as identified at  182  in  FIG. 8 . With this placement, the lead tip of the VS electrodes  172  will be in relatively close proximity to the AVN fat pad. Other locations for endocardial placement of the VS electrodes  172  include the interior wall of the superior vena cava  184 , coronary sinus  186 , or right pulmonary artery  188 .  
         [0059]     The control unit  20   b  may monitor electrical cardiac activity, such as R-R intervals, via the pacing electrodes  30   b  in a manner similar or identical to that described above in regard to the first embodiment. This allows the system  10   b  to monitor the rate or rhythm of the heart  54   b  and detect the occurrence of atrial fibrillation. The system  10   b  thus may be an active or “on-demand” system in which ventricular rate and rhythm control is applied in response to detection to an AF episode.  
         [0060]     In operation, the system  10   b  of the third embodiment operates in a manner similar or identical to those of the first and second embodiments as described above, with the exception that AVN-VS signals are delivered to one or more of the endocardial locations set forth above, i.e., the AV node  180 , atrial wall  182 , superior vena cava  184 , coronary sinus  186 , or right pulmonary artery  188 . The functional block diagram of  FIG. 3  thus depicts a process performed by the system  10   b  of  FIG. 8 . More specifically, as shown in  FIG. 3 , the system  10   b  monitors electrical cardiac activity for the occurrence of atrial fibrillation. Upon detecting an AF episode, AVN-VS is applied to reduce the ventricular rate and ventricular (VVI) pacing is applied to help maintain a desired ventricular rhythm. Thus, according to the third embodiment of the present invention, the system  10   b  combines the AVN-VS and on-demand ventricular pacing to help achieve a regular slow ventricular rate in response to AF.  
         [0061]     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.