Abstract:
A method and apparatus for cardiac pacing and, more particularly, for Atrial-His-Ventricular sequential pacing to improve sino-atrial node dysfunction or heart block superior to the His bundle. As a derivative, His-Ventricular sequential pacing can be employed to treat permanent atrial fibrillation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for cardiac pacing and, more particularly, for Atrial-His-Ventricular sequential pacing to improve sinoatrial node dysfunction or heart block superior to the His bundle. As a derivative, His-Ventricular sequential pacing can be employed to treat permanent atrial fibrillation. 
     BACKGROUND OF THE INVENTION 
     The sinoatrial (SA) node represents the natural pacemaker that controls the rhythmic electrical excitation in a normal human heart. At an appropriate time, an electrical impulse arising from the SA node is transmitted to the right and left atrial chambers. This impulse causes muscle tissue surrounding the atrium to depolarize and contract which generates an electrical signal known as a P-wave. The same electrical impulse arising from the SA node also travels to the right and left ventricles through the atrioventricular (AV) node and atrioventricular (AV) bundle. The AV node, situated in the lower portion of the right atrium, receives the impulse to contract. The impulse is then transmitted through the AV bundle comprising Common Bundle of His (His bundle), right and left bundle branches, and Purkinje fibers that cover most of the endocardial surface of the ventricles. The ventricular muscle tissue depolarizes, generating an R-wave, then contracts. This forces blood held in the ventricles through the arteries and to various body locations. This action is repeated in a rhythmic cycle in which the atrial and ventricular chambers alternately contract and pump, then relax and fill. 
     Disturbances of impulse formation by the sinus node and/or AV conduction block due to disease and aging are commonly treated by artificial pacing. An artificial pacemaker is an implantable medical device that monitors the activity of the heart for the occurrence of P- and/or R-waves. When a P- or R-wave is not sensed after a prescribed period of time, the pacemaker electronically generates stimuli in order to force the depolarization of the atria and/or ventricles. A pacemaker-generated stimulus that is delivered to the atria is known as an A-pulse, whereas a stimulus delivered to the ventricles is a V-pulse. 
     Different methods of artificial pacing have been employed including single or bi-ventricular pacing and dual chamber pacing. In single ventricular pacing, a pacing lead connected to an electrode is typically implanted in the apex of the right ventricle to deliver electrical impulses to the ventricular muscle tissue. However, this type of pacing results in the loss of synchronous mechanical contraction of the right and left ventricles due to the interventricular delay in impulse propagation to the left ventricle. This results in an immediate decrease in cardiac output along with potential deterioration of ventricular function over the long term as permanent changes occur in myocardial perfusion and structure. A method to pace both ventricles, bi-ventricular pacing, has been demonstrated to restore substantially simultaneous contraction of both ventricles and is accomplished by placing one pacing lead in the apex of the right ventricle and another pacing lead through the coronary sinus into a vein on the left ventricular wall. The surgical procedure used to implant the pacing lead in the coronary sinus however may be complex and a long pacing lead is needed to connect the electrode to the pulse generator thus requiring a higher voltage resulting in a large drain on the power source generating the pulses. In dual chamber pacing (DDD/R), electrodes connected to pacing leads are placed in the atria (for example the right atrium) and one or both of the ventricles. Under this method, ventricular synchrony can be achieved when electrodes are placed in both ventricles and an optimal delay between the A pulse and V pulse (AV delay) is utilized. However, this still does not produce a similarly coordinated contraction as compared to natural AV bundle activation. 
     Patients with SA node dysfunction or heart block superior to the His bundle, due to their conditions, suffer from a delay in the electrical response from the atria to the ventricles. According to current practice, patients with these conditions typically receive a DDD/R pacemaker. Many of these patients however still have normal ventricular contraction and thus are unnecessarily ventricularly paced due to pacemaker programming restrictions and/or prolonged AV conduction times. Because of this unnecessary ventricular pacing, these patients experience a decrease in cardiac efficiency due to the uncoordinated contraction sequence and eventually exhibit adverse long-term effects. Therefore, it&#39;s desired to develop a pacing system for these patients, that is more tailored to their cardiac condition, without utilizing ventricular pacing means. 
     It has recently been shown in U.S. Pat. No. 5,320,642 (Scherlag) that a pacing lead can be implanted near the His bundle, just below the AV node to pace the ventricles as an alternative to ventricular pacing. When the ventricles are paced through this natural conduction system, the ventricles contract in a more coordinated fashion as compared to pacing the ventricles themselves thus improving cardiac output. Also, patients receiving ventricular pacing who do not need to be ventricularly paced are spared from the long term harmful hemodynamic effects that occur from continuous ventricular pacing. However, there is a risk that the electrode may become dislodged from the His bundle or that the patient may subsequently experience heart block inferior to the His bundle. Therefore, it&#39;s desired to utilize this more natural His bundle pacing system in order to obtain a more coordinated contraction sequence, yet retain the option to ventricularly pace should the His bundle pacing lead fail or heart block below the His bundle occur. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a pacing apparatus and method for sequentially pacing the atria, His bundle and ventricles to provide synchronous mechanical contractions to improve cardiac output and prevent long term hemodynamic effects caused by unnecessary ventricular pacing. The pacing apparatus includes pacing leads extending from a pacer that lead to prescribed positions in the atria, AV septum and ventricle of the heart. The pacing leads have electrodes attached to their distal ends for measuring and delivering electrical impulses. The pacing leads leading to the atria and ventricles are programmed to deliver electrical pulses, on a demand mode basis, if natural electrical signals are not measured within a predetermined period of time. The pacing lead leading to the AV septum delivers continuous electrical pulses to the His bundle immediately following the sensing of atrial activity. These electrical pulses then travel through the right and left bundle branches and to the Purkinje fibers causing the ventricles to depolarize and contract. By utilizing this natural conduction system, the ventricles contract in a more coordinated manner as compared to ventricular pacing thus improving cardiac output and performance. 
     In one embodiment of the present invention, the pacing apparatus is provided for patients who suffer from sino-atrial (SA) node dysfunction or heart block superior to the His bundle. Because these patients typically have normal ventricular contraction, continuously pacing the His bundle results in electrical pulses being sent from the atria to the ventricles to travel more quickly thus allowing the ventricular pacing lead to remain dormant during the pacing cycle. The ventricular pacing is provided only for emergency situations such as, for example, inter alia, if the His bundle pacing lead should fail or if heart block inferior to the His bundle should occur. 
     In another embodiment of the present invention, the pacing apparatus is provided for patients who suffer from permanent atrial fibrillation. The His bundle pacing lead is implanted following AV node ablation to provide electrical pulses to the ventricles causing depolarization and contraction. Again, a ventricular pacing lead is provided but remains dormant and is activated only in emergency situations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of one embodiment of the pacing system in accordance with this invention whereby pacing leads are provided, the leads being shown with electrodes positioned in the heart. 
     FIG. 2 is a block diagram of one embodiment of the implantable pacemaker system. 
     FIG. 3 is a logic control diagram for pacing patients suffering from sinoatrial node dysfunction or superior His bundle heart block. 
     FIG. 4 is a logic control diagram for pacing patients suffering from heart block at the AV node. 
     FIG. 5 is a logic control diagram for pacing patients suffering from permanent atrial fibrillation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. 
     Referring to FIG. 1, there is shown a schematic representation of a pacing system  10  in accordance with one embodiment of the present invention. The pacing system  10  comprises an implantable pacemaker (pacer)  11 , from which three leads,  12 A (atrial),  12 H (AV bundle) and  12 V (ventricle) extend. It is recognized that actual lead packaging may vary, but at least three functional conductive leads must originate in pacer  11  to direct energy to various locations in the cardiac tissue. The pacing leads  12 A,  12 H and  12 V, in this embodiment, enter the heart through the superior vena cava  13 . Each pacing lead provides electrodes at or proximate to their distal ends for pacing and sensing electrical stimuli. A plurality of electrodes can be placed at the distal ends of each lead for measuring and delivering pulses, however for simplicity it&#39;s assumed each lead has one electrode for the following discussion. Pacing lead  12 A is anchored or otherwise positioned generally in the right atrium just below the SA node  14  for contacting the right atrium. Pacing lead  12 H is positioned distal to the blocked or slowly conducting AV node and in the AV septum  15  for contacting the AV bundle comprising His bundle  16 , right and left bundle branches  17 , and the Purkinje fibers  18 . The His bundle  16  is a structure of cardiac muscle through which all impulses from the atria are conducted to the ventricles of the heart. Pacing lead  12 V is positioned so that its distal end is anchored in the apex of the right ventricle  19 . The electrodes are stabilized or fixed in their respective positions in a manner well known in the art. 
     Referring then to FIG. 2, a block diagram of pacer  11  is illustrated. The pacer is adapted to interface with the atria, His bundle and ventricles of the heart. Those portions of pacer  11  that interface with the atria, His bundle and ventricles, and the corresponding portions of control system  35  are commonly referred to as the atrial, His bundle and ventricular channels, respectively. 
     Pacer  11  generates electrical pulses that are transmitted through pacing leads  12 A,  12 H and  12 V. Control system  35  controls pacer  11 . The control system includes timing circuitry and a microprocessor for carrying out logical steps in analyzing received signals, and determining when pace pulses should be initiated, with particular sequences and locations comprising part of the present invention. The leads  12 A,  12 H, and  12 V carry the stimulating pulses to electrodes  20 ,  21  and  22  from an atrial pulse generator A-PG, His bundle pulse generator H-PG and a ventricular pulse generator V-PG  27 , respectively. An electrical pulse generated by A-PG is known as an A-pulse; an electrical pulse generated by H-PG is known as an H-pulse; and an electrical pulse generated by V-PG is known as a V-pulse. Further, natural electrical signals from the atria (P-waves) are carried from the electrode  20 , through lead  12 A, to the input channel of an atrial channel sense amplifier P-AMP. Likewise, natural electrical signals from the ventricles (R-waves) are carried from electrode  22 , through the lead  12 V, to the input terminal of a ventricular sense channel amplifier R-AMP. Thus, when a P-wave or R-wave is generated by the heart, it is sensed by electrodes  20  or  22  and amplified by P-AMP or R-AMP. The control system  35  receives the output signals from P-AMP over signal line  25 . The control system  35  also receives the output signals from R-AMP over signal line  26 . The control system  35  also generates trigger signals when needed that are sent to A-PG, H-PG and V-PG over signal lines  27 ,  28  and  29  respectively. These trigger signals are generated each time that an electrical pulse is to be generated by the respective pulse generator. 
     Referring now to FIG. 3, there is shown a logic control flow diagram for controlling the system of this invention to pace a patient with SA node dysfunction or AV conduction block superior to the His bundle. The assumption is that the right and left ventricles are functioning normally but that sinus signals from the SA node to the AV Bundle are not occurring or are being delayed from the atria to the ventricles within the AV node. As shown at  51 , the pacer monitors the right atrium and measures a P-wave generated by the SA node. As shown in  54 , once a P-wave is sensed, the pacer transmits an H-pulse through the pacing lead to an electrode located distal to the AV node to stimulate the His bundle. The H-pulse is transmitted through the His bundle to the right and left bundle branches of the heart and then to the Purkinje fibers to cause ventricular depolarization. Then, the pacer senses the right ventricle for an R-wave. As shown in  55 , if an R-wave is not sensed following atrial activity plus some predescribed period of time, the right ventricle is electrically paced with a V-pulse generated by the pacer. In  56 , a subsequent P-wave is sensed in the right atrium. As shown in  57 , if a P-wave is not sensed following ventricular activity plus some prescribed period of time, the pacer generates an electrical stimulus, or A-pulse, to the right atrium. An H-pulse is generated immediately following atrial activity and sent to the electrode located near the AV bundle and the right ventricle is again monitored for ventricular depolarization as the cardiac cycle continues. The advantage to this system is that both the atrial and ventricular pacing leads remain dormant throughout the cardiac cycle unless a P-wave or R-wave is not sensed within a prescribed period of time. This eliminates the harmful hemodynamic effects suffered by patients who are currently unnecessarily ventricularly paced. In addition, using the natural conduction system of the SA node, the His-bundle, right and left bundle branches and Purkinje fibers as a pacing pathway generates a more natural synchronous mechanical ventricular contraction to improve cardiac output. 
     In another embodiment of the invention, shown in logic control flow diagram FIG. 4, it&#39;s assumed that the patient suffers only from heart block located in the AV node. In  61 , the pacer measures a P-wave generated by the SA node. To increase conduction time from the SA node to the ventricles, an electrode is placed distal to the AV node to stimulate the His bundle. Then, in  64 , the pacer sends an H-pulse to this electrode immediately following the sensing of a P-wave. As shown in  65 , the ventricular pacing lead remains dormant throughout this cycle unless an R-wave is not sensed following atrial activity plus a prescribed period of time, after which the pacer would generate a V-pulse to force the ventricles to contract. 
     Referring to FIG. 5, there is shown a logic control flow diagram for controlling the system of this invention to pace a patient with permanent atrial fibrillation. It&#39;s assumed that the patient suffers from a fast and unreliable ventricular response. According to current practice, these patients are treated by ablating the AV node and placing a permanent single-chamber pacemaker with the lead tip in the right ventricle to restore normal ventricular rhythm. However, this can cause hemodynamic deterioration and death in some patients. In order to provide these patients with improved long-term pacing, AV node ablation can be followed by permanent His bundle pacing. As shown in  71 , AV node ablation is preceded by implanting a pacemaker distal to the AV node to stimulate the His bundle. Continuous H-pulses are transmitted to the His bundle via the electrode located at the AV bundle, to cause the ventricles to depolarize as shown in  73  and  74 . Because there is a risk that the AV bundle lead could become dislodged or that the patient could suffer heart block inferior to the AV bundle lead, a second pacing lead is anchored in the apex of the right ventricle to provide V-pulses when necessary. As shown, if an R-wave is not sensed within a prescribed time following an H-pulse, a V-pulse is generated by the pacer and transmitted to the right ventricle causing the ventricles to contract. The subsequent H-pulse is generated according to the operating pacing rate to continue the cardiac cycle. Thus, a V-pulse is generated only on an emergency basis, i.e. when heart block occurs inferior to the His bundle. 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.