Abstract:
A programmable pacemaker that allows both noninvasive electrophysiological (&#34;EP&#34;) testing for atrial tachycardias and ventricular pacing support, by allowing operation in the atrial channel to be decoupled from operation in the ventricular is provided. Many patients require EP testing to evaluate a predisposition to tachycardias. Many of these patients also have dual-chamber pacemakers for cardiac support. These systems can be noninvasively coupled to a external programmer enabling the already implanted system to serve as an in vivo EP laboratory. When performing noninvasive atrial EP testing with current dual-chamber pacemakers, the device must first be programmed to a single-chamber triggered mode. The present system allows the pacemaker to maintain ventricular pacing during EP testing.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to programmable implantable pacemakers, and particularly to implantable dual-chamber pacemakers used for performing noninvasive electrophysiological (&#34;EP&#34;) testing. More particularly, the present invention relates to a dual-chamber pacemaker in which the atrial and ventricular channels can operate simultaneously yet independently so that, during noninvasive EP testing for atrial tachycardias, the device can generate bursts of pacing pulses to induce and terminate atrial arrhythmias, while maintaining ventricular pacing support. 
     EP testing is a procedure that is commonly used to evaluate an individual&#39;s susceptibility to cardiac arrhythmias, particularly atrial and ventricular tachycardias. EP testing is also used to determine whether or not a particular patient would respond favorably to various therapies, such as drug therapies and electrical stimulation therapies, that are often used to treat cardiac arrhythmias. 
     These objectives are typically accomplished by inducing arrhythmias at selected locations in the patient&#39;s heart through the use of &#34;bursts&#34; of electrical stimulation. The stimulation bursts are applied to the patient&#39;s heart in a sequence that is known to induce the desired arrhythmia. 
     Once an arrhythmia has been induced, electrical stimulation and/or drug therapy may be used to attempt to revert the arrhythmia. For example, another burst of electrical stimulation may be applied to the patient&#39;s heart in a sequence that is known to be successful in reverting arrhythmias. Alternatively, a drug may be administered to attempt to return the patient&#39;s heart rhythm back to normal. In addition, EP testing can be used to evaluate the effectiveness of preventative measures, such as drugs that are intended to reduce susceptibility to cardiac arrhythmias in patients who may be predisposed to such episodes. Thus, EP testing allows a physician to prescribe a course of therapy that is specifically tailored to each patient&#39;s condition. 
     EP testing traditionally has been an invasive procedure. Specifically, surgery has been required to introduce electrical leads into the patient&#39;s body and to guide the electrode tips of the leads to a desired location in the patient&#39;s heart. The leads are coupled to an external EP stimulator which is used by the physician to control the intensity and sequence of electrical stimulation that is delivered to the patient&#39;s heart. 
     Surgery, of course, is not without risks. In the case of invasive EP testing, certain patients may occasionally experience thrombosis, bleeding or infection. Not surprisingly, alternatives to invasive EP testing were sought. 
     It has been found that noninvasive EP testing can be performed on patients who have received pacemakers to treat bradycardia (slow heart rate). Essentially, the implanted pacemaker, when properly configured, can be used as an in vivo EP testing laboratory. This is typically accomplished by configuring the pacemaker, using an external programming unit, to generate and administer bursts of pacing pulses in a sequence that induces or reverts the desired arrhythmia. Noninvasive EP testing can be used for most tests that are performed through invasive EP testing. 
     A detailed description of some of the different approaches to noninvasive EP testing, and their respective advantages and disadvantages, may be found in Fletcher, R. D. et al., &#34;The Use of the Implanted Pacemaker as an In Vivo Electrophysiology Laboratory,&#34; Journal of Electrophysiology, Vol. 1, No. 5, 1987. In one approach known as &#34;triggered,&#34; an EP stimulator is used to apply external, chest wall stimulation to a patient undergoing EP testing. With this approach, the implanted pacemaker is set to a triggered mode of operation (e.g., AAT for atrial testing or VVT for ventricular testing). The chest wall stimulation is applied in a burst sequence designed to either induce or revert an arrhythmia, but at an energy level that is not uncomfortable for the patient. Each stimulation pulse in the burst triggers the implanted pacemaker to administer a pacing pulse. The pacemaker thus tracks the burst sequence of the EP stimulator. 
     In a second approach known as &#34;indirect,&#34; an EP stimulator is coupled to a pacemaker programmer, which in turn is set up to communicate telemetrically with the implanted pacemaker. The programmer maintains a radio frequency link to the pacemaker during the test, except when the EP stimulator sends a pulse to the programmer. When a pulse is sent, the programmer breaks the radio frequency link which causes the pacemaker (which is typically set to the AAI or VVI mode) to administer a pacing pulse. The EP stimulator can thus cause the pacemaker to deliver pulses in a desired burst sequence by sending signals to the programmer in the appropriate sequence. 
     A third approach known as &#34;direct&#34; does not require an EP stimulator. With this approach, the physician can control the pacemaker burst sequences by using a pacemaker programmer that includes software that is specifically designed for EP testing. When this approach is used, the physician typically first disables pacing support in the chamber that is not to be tested. The physician then uses the programmer to send commands to the pacemaker to cause the pacemaker to administer burst stimulation in a desired sequence to either induce or revert an arrhythmia. 
     Regardless of the approach used, noninvasive EP testing can be a highly desirable alternative to invasive EP testing. Unfortunately, however, atrial EP testing does not accommodate patients who may require back-up ventricular pacing during the EP test, even if the patient is equipped with a dual-chamber pacemaker. This is because the pacemaker is generally set to a single-chamber mode of operation prior to EP testing. This may be done by setting the pacemaker to AAI or AAT mode (depending on the approach being used to perform EP testing), or by setting the pacing parameters in the non-tested chamber (in this case, the ventricle) to prevent effective pacing pulses from being delivered (e.g., by extending the refractory period or by setting the output energy to below threshold). Dual-chamber pacemakers have generally been set to a single-chamber mode during EP testing in order to prevent high-rate pacing in one chamber (e.g., the atrium) during EP testing from causing the other chamber (e.g., the ventricle) to follow the high-rate activity, because inappropriate pacing of the other chamber may cause pacemaker-induced arrhythmia. Thus, with current dual-chamber pacemakers, if the pacemaker is set to perform EP testing in the atrium, then no ventricular pacing (or ventricular sensing) is provided. 
     Patients who lack sufficient AV conduction require back-up ventricular pacing during atrial EP testing. Previously known techniques for providing back-up ventricular pacing during atrial EP testing require the insertion of invasive leads into the patient to pace the ventricle. 
     Thus, it would be desirable for an implanted pacemaker capable of operating in an EP testing mode to provide back-up ventricular pacing support during atrial EP testing. It would also be desirable for the pacemaker to allow operation of the atrial channel involved in EP testing to be decoupled from the operation of the ventricular channel involved in ventricular pacing. It would further be desirable for the pacemaker to allow the physician to set pacemaker parameters for the atrial channel independent of the pacemaker parameters for the ventricular channel, so that the pacemaker can simultaneously perform atrial EP testing and provide optimal ventricular pacing support. 
     SUMMARY OF THE INVENTION 
     The disadvantages and limitations discussed above are overcome by the present invention. In accordance with this invention, a new implantable dual-chamber pacemaker is provided, which can be programmed to simultaneously operate in one mode in an atrial channel (e.g., an atrial EP testing mode) and in a different, independent mode in a ventricular channel (e.g., a ventricular pacing mode). 
     The present invention thus provides an implantable pacemaker that can maintain back-up ventricular pacing support during atrial EP testing. 
     The pacemaker of the present invention includes a control system for controlling the operation of the pacemaker, a set of leads for receiving atrial and ventricular signals and for delivering atrial and ventricular stimulation pulses, a set of amplifiers for amplifying the atrial and ventricular signals, and pulse generators for generating atrial and ventricular stimulation pulses. In addition, the pacemaker includes memory for storing operational parameters for the control system and for storing data acquired by the control system for later retrieval by the medical practitioner using an external programmer. The pacemaker also includes a telemetry circuit for communicating with the external programmer. 
     Unlike previously known dual-chamber pacemakers, the pacemaker of the present invention allows simultaneous operation in an atrial EP testing mode and a ventricular pacing mode. When programmed according to the present invention, the pacemaker can provide ventricular pacing support while EP burst sequences are directed to the atrial chamber under the control of a pacemaker programmer that is configured to control the pacemaker in an EP testing mode. The ventricular channel can be programmed to operate in VVI or VOO mode during atrial EP testing to provide either demand or asynchronous ventricular pacing support with parameters deemed appropriate by the physician. The pacing parameters may be programmed into the pacemaker independent of any EP testing parameters that may be set by the physician. These parameters may include pulse width, pulse amplitude, pacing rate and amplifier sensitivity, among others. 
     In the preferred embodiment of the present invention, the pacemaker is configured to operate as though the atrial channel (operating in EP testing mode) is decoupled from the ventricular channel (operating in single-chamber pacing mode). Atrial events that occur during EP testing do not affect the operation of the ventricular channel. The ventricular channel paces the ventricular chamber independently while the atrial channel is used with the programmer for EP testing according to input provided by the physician. For example, the pacemaker can operate in AAT burst-pacing mode to revert atrial tachycardias in the atrial chamber and simultaneously in VVI mode to provide ventricular pacing support in the ventricular chamber. 
     With respect to VVI mode, the ventricular sensor can be adjusted to avoid inappropriate interpretation of atrial pulses as intrinsic ventricular activity due to cross-talk. Desensitizing the ventricular sensor to atrial activity, in addition to decoupling the channels, helps prevent inappropriate inhibition of ventricular output. 
     In another aspect of the invention, a method of performing EP testing in an atrial chamber of the heart and simultaneously providing ventricular pacing support using an implanted pacemaker is provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference numerals refer to like parts throughout, and in which: 
     FIG. 1 generally depicts a patient undergoing noninvasive EP testing in accordance with the principles of the present invention; 
     FIG. 2 is a block diagram of a pacemaker that can simultaneously perform atrial EP testing and provide ventricular pacing support in accordance with the principles of the present invention; 
     FIG. 3 is a block diagram of a programmer that can be used with the pacemaker of FIG. 1 in accordance with the principles of the present invention; 
     FIG. 4 is a timing waveform annotated with marker channel data illustrating the atrial and ventricular pulses delivered as well as the intrinsic atrial and ventricular contractions sensed by the pacemaker in accordance with the principles of the present invention; and 
     FIG. 5 is a logic flow diagram representing a control routine that may be used by the pacemaker of FIG. 2 to implement simultaneous atrial EP testing and ventricular pacing support in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, a generalized arrangement of a noninvasive EP testing system 10 is described. The EP testing system 10 may be used to simultaneously perform atrial EP testing on, and provide bradycardia pacing support to, a patient 12 in accordance with the principles of the present invention. 
     In broad terms, the EP testing system 10 includes an external programmer 14 and a pacemaker 16 implanted in the patient 12. The programmer 14 communicates with the pacemaker 16 via a telemetry head 18 which is coupled to the programmer using a suitable connecting cable 20. The telemetry head 18 is typically placed on the chest of the patient 12 in the vicinity of the pacemaker 16 in order to insure a reliable RF connection. 
     Preferably, the EP testing system 10 includes at least a pair of electrodes 22 and 24 which are used by the programmer 14 to monitor the electrocardiogram (&#34;ECG&#34;) of the patient 12 undergoing EP testing. The ECG provides the physician with important information concerning the patient&#39;s condition during EP testing. Other important diagnostic information, such as the intracardiac electrogram (&#34;IEGM&#34;) of the patient 12 and marker channel data, may be transmitted from the pacemaker 16 to the programmer 14 via the telemetry head 18 and the cable 20. 
     The EP testing system 10 as shown in FIG. 1 is arranged in the &#34;direct&#34; configuration. In the direct configuration, the programmer 14 can cause the pacemaker 16 to deliver EP testing stimulation bursts upon receiving the appropriate commands from the physician, without the need for an external EP stimulator (not shown). Those of ordinary skill in the art will understand, however, that the principles of the present invention may be practiced using other suitable EP testing configurations, including at least the &#34;indirect&#34; and &#34;triggered&#34; configurations. 
     Referring now to FIG. 2, a pacemaker 160 is described which is suitable for use as the pacemaker 16 of the EP testing system 10 shown in FIG. 1. In accordance with the principles of this invention, the pacemaker 160 is capable of performing atrial EP testing while maintaining ventricular pacing support. 
     The pacemaker 160 is coupled to the patient&#39;s heart 30 by way of a pair of leads 32 and 34, the lead 32 having an electrode 36 which is in contact with one of the atria of the heart 30, and the lead 34 having an electrode 38 which is in contact with one of the ventricles. The lead 32 carries stimulating pulses to the electrode 36 from an atrial pulse generator 40, while the lead 34 carries stimulating pulses to the electrode 38 from a ventricular pulse generator 42. In addition, electrical signals from the atria are carried from the electrode 36, through the lead 32 to an input terminal of an atrial sense amplifier 44. Electrical signals from the ventricles are carried from the electrode 38, through the lead 34 to an input terminal of a ventricular sense amplifier 46. The atrial pulse generator 40, atrial sense amplifier 44 and lead 32 may be viewed collectively as an atrial channel 48. Similarly, the ventricular pulse generator 42, ventricular sense amplifier 46 and lead 34 may be viewed collectively as a ventricular channel 50. 
     Although the pacemaker 160 is shown in FIG. 2 as having two pulse generators (one each for the atrium and the ventricle), it should be noted that the invention can be practiced using other pacemaker embodiments. For example, a single pulse generator may be used with suitable switching circuitry (not shown). 
     Controlling the dual-chamber pacemaker 160 is a control system 52, which is preferably microprocessor-based. The control system 52 also includes a real-time clock (not shown) for providing timing for monitoring cardiac events and for timing the application of stimulation pulses by the pulse generators 40 and 42. 
     The control system 52 receives the output signals from the atrial amplifier 44 over a signal line 54. Similarly, the control system 52 receives the output signals from the ventricular amplifier 46 over a signal line 56. These output signals are generated each time that an atrial event (e.g., a P-wave) or a ventricular event (e.g., an R-wave) is sensed within the heart 30. 
     The control system 52 also generates an atrial trigger signal which is sent to the atrial pulse generator 40 over a signal line 58, and a ventricular trigger signal which is sent to the ventricular pulse generator 42 over a signal line 60. The appropriate trigger signal is generated each time that a stimulation pulse is to be generated by one of the pulse generators 40 or 42. The atrial stimulation pulse is referred to simply as the &#34;A-pulse,&#34; and the ventricular stimulation pulse is referred to as the &#34;V-pulse.&#34; 
     During the time that either an A-pulse or a V-pulse is being delivered to the heart 30, the corresponding atrial amplifier 44 or the ventricular amplifier 46 is typically disabled by way of a blanking signal presented to the appropriate amplifier from the control system 52 over a signal line 62 for the atrial amplifier 44 or a signal line 64 for the ventricular amplifier 46. This blanking action prevents the amplifiers 44 and 46 from becoming saturated with the relatively large stimulation pulses which are present at their input terminals during pacing pulse delivery. This blanking action also prevents residual electrical signals (known as &#34;after-potentials&#34;) present in the muscle tissue as a result of the pacemaker stimulation from being interpreted as atrial or ventricular events. 
     Still referring to FIG. 2, the pacemaker 160 also includes a memory circuit 66 which is coupled to the control system 52 through a suitable data bus 68. The memory circuit 66 allows certain control parameters used by the control system 52 in controlling the operation of the pacemaker 160 to be programmably stored and modified, as required, in order to customize the operation of the pacemaker 160 to suit the needs of a particular patient. These parameters may include, for example, pulse width, pulse amplitude, pacing rate, blanking interval and amplifier sensitivity, among others. 
     Advantageously, the pacemaker 160 of the present invention can store different sets of control parameters in the memory 66 for use in controlling different modes of operation. For example, the parameters for controlling the atrial channel 48 during EP testing may differ from the parameters used to control the atrial channel 48 during bradycardia pacing. In addition, the parameters used by the control system 52 to control bradycardia pacing by the ventricular channel 50 during atrial EP testing may differ from those used to control the ventricular channel at other times. 
     Data sensed during the operation of the pacemaker 160 (e.g., marker channel and IEGM data) may be stored in the memory circuit 66 for later retrieval and analysis. 
     A telemetry circuit 68 is also included in the pacemaker 160. The telemetry circuit 68 is connected to the control system 52 by way of a suitable command/data bus 70. In turn, the telemetry circuit 68 may be selectively coupled to the external programmer 14 (FIG. 1) by means of an appropriate communication link 72. The communication link 72 may be any suitable electromagnetic link such as an RF (radio frequency) channel. 
     Commands may be sent by the medical practitioner to the control system 52 from the external programmer 14 (FIG. 1) through the communication link 72. Similarly, through this communication link 72 and the external programmer 14 (FIG. 1), data (either held within the control system 52 (as in a data latch) or stored within the memory circuit 66), may be remotely transmitted by the pacemaker 160 to the external programmer 14 (FIG. 1). In this manner, noninvasive communication may be established with the implanted pacemaker 160 from a remote location. 
     The operation of the pacemaker 160 is generally controlled by a control program stored in the memory circuit 66 and executed by the control system 52. This control program usually consists of multiple integrated program modules, with each module bearing responsibility for controlling one or more functions of the pacemaker 160. For example, one program module may control the delivery of stimulating pulses to the heart 30, while another module may control the acquisition of atrial and ventricular electrical signals. In effect, each program module is a control program dedicated to a specific function or a set of functions of the pacemaker 160. As described in greater detail below, a particular control program module is used by the control system 52 to enable the pacemaker 160 to perform atrial EP testing while maintaining ventricular pacing support. 
     Referring now to FIG. 3, an external programmer 140 is described which may be used as the external programmer 14 of the EP testing system 10 shown in FIG. 1. In particular, the programmer 140 may be used to program the pacemaker 16 (FIG. 1) to perform noninvasive atrial EP testing while maintaining ventricular pacing support. 
     The programmer 140 is controlled by a control system 80, which is preferably microprocessor-based. The control system 80 executes control program instructions that it reads from a memory circuit 82. Upon start-up, the control system 80 loads the memory circuit 82 with at least a portion of a control program that is stored in a nonvolatile storage unit 84. The storage unit 84 may be, for example, a hard disk, although other suitable nonvolatile storage devices may be used instead. 
     By executing the control program stored in the memory circuit 82, the control system 80 is able to present a series of menus on a display 86. The physician can make selections from the menus through the use of a user interface 88. The user interface 88 may be a keyboard, mouse, touch-screen, light pen, digitizer or any other suitable input device that allows the physician to make choices from the menus displayed on the display 86. 
     Some of the menus presented by the control system 80 on the display 86 allow the physician to configure the pacemaker 160 (FIG. 2) for noninvasive EP testing. The configuration instructions provided by the physician through the user interface 88 are interpreted by the control unit 80 and transmitted to the pacemaker 160 (FIG. 2) through use of a telemetry circuit 90 and a cable 92 (corresponding to the cable 20 of FIG. 1). 
     The programmer 140 is used initially by the physician to reconfigure the pacemaker 160 (FIG. 2) from its ordinary mode of operation (e.g., dual chamber bradycardia pacing) to the EP testing mode of the present invention. The instructions sent by the programmer 140 to the pacemaker 160 (FIG. 2) may include the ventricular pacing mode to be used while atrial EP testing is being performed (e.g., VVI, VVT, VOO). The instructions may also include separate sets of pacing parameters for the atrial channel 48 (FIG. 2) and the ventricular channel 50 (FIG. 2). These parameters may include pulse width, pulse amplitude, pacing rate, blanking interval and amplifier sensitivity, among others. Ventricular pacing parameters may be needed in addition to the atrial EP testing parameters, because it may be desirable to use a different set of parameters for ventricular pacing during EP testing than are used during ordinary dual-chamber operation of the pacemaker 160 (FIG. 2). In particular, if VVI pacing is to be used, it may be desirable to decrease the sensitivity of the ventricular sense amplifier 46 (FIG. 2) in order to avoid sensing of the stimulation pulses delivered by the atrial pulse generator 40 (FIG. 2) during EP testing. 
     The programmer 140 is also used by the physician to define the burst sequences to be used during atrial EP testing. Different burst sequences may thus be administered by the pacemaker 160 (FIG. 2) to induce and revert atrial arrhythmias. 
     The programmer 140 is also used to receive information from the pacemaker 160 (FIG. 2) during noninvasive EP testing. For example, the programmer 140 may receive through the cable 92 and telemetry circuit 90, marker channel and IEGM data sent by the pacemaker 160 (FIG. 2). In addition, the patient&#39;s ECG may be obtained through the electrodes 94 and 96 (corresponding to the electrodes 22 and 24 of (FIG. 1). Advantageously, the data received by the programmer 140 represents cardiac activity in both the atrial chamber undergoing EP testing and the ventricular chamber that receives bradycardia pacing support during EP testing of the atrial chamber. 
     When EP testing is complete, the physician uses the programmer 140 to instruct the pacemaker 160 (FIG. 2) to return to its original, dual-chamber mode of operation. 
     Referring again to FIG. 2, during EP testing, the atrial pulse generator 40 generates pulses as directed by the control system 52 of the pacemaker 160, which in turn is directed by the external programmer 140 (FIG. 3) through the telemetry circuit 72. Bursts of pacing pulses delivered in a prescribed sequence to the atrium of the heart 30 may be used to induce an atrial arrhythmia. Another burst sequence can then be used to revert the arrhythmia to sinus rhythm. 
     Operation of the ventricular channel 50 proceeds according to the mode of operation programmed by the physician using the programmer 140 (FIG. 3). In one embodiment of the present invention, the ventricular channel 50 operates in VOO mode. The ventricular pulse generator 42 provides asynchronous, back-up pacing support regardless of any atrial activity that occurs during EP testing. The ventricular pacing rate is determined by the physician depending on the patient&#39;s needs. A relatively high pacing rate may be required by some patients, whereas for other patients, a high pacing rate can cause pacemaker-induced tachycardia. 
     In an alternative embodiment, the ventricular channel 50 operates in VVI mode, providing ventricular pacing support only as needed by the patient (i.e., in the absence of intrinsic ventricular activity). If the patient&#39;s intrinsic rhythm is faster than the programmed pacing rate of the pacemaker 160, then the ventricular pulse generator 42 is inhibited. 
     The ventricular channel 50 can alternatively operate in VVT mode. In VVT mode, however, the ventricular pulse generator 42 paces the ventricular chamber even if the intrinsic rate is higher than the programmed pacing rate for the ventricular channel 50. 
     VVI pacing is preferred over VVT pacing for several reasons. First, VVI mode conserves energy and thus increases the life of the battery (not shown) of the pacemaker 160. VVT mode depletes the battery more rapidly than VVI mode because VVT mode stimulates the heart even when the intrinsic rate exceeds the programmed pacing rate. Second, VVI mode presents an undistorted, intrinsic QRS complex, which facilitates the evaluation of the patient&#39;s symptoms. In VVT mode, each intrinsic QRS complex is distorted by a triggered, pacing artifact--thus limiting diagnostic utility. Third, VVT mode may cause ventricular pacing at undesired high rates. 
     The energy content of stimulation pulses administered to the atrium during EP testing may be higher than the energy content of atrial pacing pulses. These higher energy pulses may be sensed by the ventricular channel sense amplifier 46 and misinterpreted as intrinsic ventricular activity. The present invention provides at least two ways for addressing this concern. First, the sensitivity of the ventricular channel sense amplifier 46 can be reduced to decrease the likelihood of it sensing atrial events. Thus, the sensitivity of the ventricular channel sense amplifier 46 may be lower during atrial EP testing than it is at other times. Second, a post-atrial blanking signal can be sent to the ventricular channel sense amplifier 46 over the signal line 64 after each atrial pulse during the EP test. The blanking signal causes the ventricular channel sense amplifier 46 to become nonresponsive to cardiac signals for a period of time known as a &#34;blanking interval.&#34; The duration of the blanking interval may be set by the physician using the programmer 140 (FIG. 3). The duration may vary depending upon testing conditions (e.g., the energy content of the atrial stimulation pulses) and the condition of the patient. 
     In an alternative embodiment of the present invention, the ventricular pacing rate can be a function of the atrial rate, rather than independent of atrial events. For example, the ventricular pacing rate can be derived from dividing down the atrial events. 
     FIG. 4 is a timing waveform illustrating an atrial EP test, during which a burst of atrial stimulation pulses is used by the pacemaker 160 (FIG. 2) to terminate an atrial tachycardia. The timing waveform of FIG. 4 is annotated with marker channel data showing a sequence of atrial EP testing stimuli (AS) 100, 102, 104, 106, 108, 110 and 112 and ventricular pacing pulses 114, 116, 118 and 120 as they would be generated by the pacemaker 160 (FIG. 2) during atrial EP testing in accordance with the principles of the present invention. It should be noted that the same principles apply when the pacemaker 160 (FIG. 2) is delivering EP testing stimuli to induce an arrhythmia in the atrium. 
     Through the telemetry circuit 68 (FIG. 2), the external programmer 140 (FIG. 3) provides the instructions necessary for the atrial pulse generator 40 to deliver the sequence of atrial stimuli 102, 104, 106, 108, 110 and 112 to terminate the atrial tachycardia. During the atrial EP test, the ventricular pacing pulses 114, 116, 118 and 120 provide ventricular pacing support as needed by the patient. Intrinsic atrial contractions sensed as P-waves and ventricular contractions sensed as R-waves by the pacemaker 160 (FIG. 2) reset the time intervals for pacing by the atrial channel 48 (FIG. 2) and the ventricular channel 50 (FIG. 2) respectively. In this example, during the atrial EP test the atrial channel 48 (FIG. 2) of the pacemaker 160 (FIG. 2) operates in AAI mode, with the time interval set to 400 ms or a programmed rate of 150 bpm. The ventricular channel 50 (FIG. 2) of the pacemaker 160.(FIG. 2) operates in VVI mode, with the time interval set to 1000 ms or a programmed rate of 60 bpm. 
     Referring now to FIG. 5, a logic flow diagram is described which represents a control routine performed by the control system 30 (FIG. 2) of the pacemaker 160 (FIG. 2) to implement atrial EP testing while maintaining ventricular pacing support in accordance with the principles of the present invention. The routine starts when the control system 52 (FIG. 2) receives a &#34;start EP test&#34; command from the programmer 140 (FIG. 3) through the telemetry circuit 68 (FIG. 2). 
     At step 200, the control system 52 (FIG. 2) sets the ventricular pacing mode to the mode selected by the physician using the programmer 140 (FIG. 3). Preferably, the ventricular pacing mode is set to VVI mode; however, VOO and VVT modes may be used instead. At step 202, the control system 52 (FIG. 2) sets the atrial pulse parameters by storing the parameters provided by the physician using the programmer 140 (FIG. 3) in the memory circuit 66 (FIG. 2). These parameters may include, for example, pulse width, pulse amplitude, blanking interval and amplifier sensitivity, among others. These parameters are used by the control system 52 (FIG. 2) to regulate the EP stimulation pulses generated by the atrial pulse generator 40 (FIG. 2) during EP testing. 
     At step 204, the control system 52 (FIG. 2) sets the ventricular pulse parameters by storing the parameters provided by the physician using the programmer 140 (FIG. 3) in the memory circuit 66 (FIG. 2). These parameters may include pulse width, pulse amplitude and pacing rate, among others. These perimeters are used by the control system 52 (FIG. 2) to regulate the pacing pulses generated by the ventricular pulse generator 42 (FIG. 2) while EP testing is being performed in the atrium. Advantageously, the ventricular pulse parameter used during atrial EP testing may differ from the atrial pulse parameters, and from the ventricular pulse parameters that are used when testing is not being performed by the pacemaker 160 (FIG. 2). 
     At test 206, the control system 52 (FIG. 2) determines if the physician has selected VVI mode at step 200. If VVI mode was selected, the control system 52 (FIG. 2) may receive an instruction to set a post-atrial ventricular blanking interval at step 208. The physician may choose to use this parameter to prevent sensing of atrial EP pulses by the ventricular channel sense amplifier 46 (FIG. 2). The duration of the blanking interval may be set by the physician based on the EP testing conditions and the condition of the patient. At step 210, the sensitivity of the ventricular channel sense amplifier 46 (FIG. 2) may be adjusted to reduce sensing of atrial EP pulses. 
     After step 210, or if VVI mode was not detected at test 206, the control system 52 (FIG. 2) sets the telemetry circuit 68 (FIG. 2) to transmit IEGM and marker data at step 212. Advantageously, the IEGM and marker data provide information identifying cardiac events in both the atrial chamber undergoing EP testing and the ventricular chamber that receives bradycardia pacing support during EP testing, as shown in (FIG. 4). 
     At test 214, the control system 214 (FIG. 2) determines if the physician has transmitted a new EP burst sequence using the programmer 140 (FIG. 2). If so, at step 216, the control system 52 (FIG. 2) stores the parameters that define the burst sequence in the memory circuit 66 (FIG. 2). The burst sequence stored in the memory circuit 66 (FIG. 2) may be a sequence that is known for either inducing or reverting an atrial arrhythmia. It should be noted that test 214 is not performed until a burst sequence has been stored in the memory circuit 66 (FIG. 2). 
     At step 218, the control system 52 (FIG. 2) causes the atrial channel 48 (FIG. 2) to administer the burst sequence defined at step 216 to the atrium in order to attempt to induce or revert an atrial arrhythmia. Simultaneously, the control system 52 (FIG. 2) causes the ventricular channel 50 (FIG. 2) to deliver bradycardia pacing pulses to the ventricle in the manner defined by the steps 200, 204, 208 and 210. Thus, atrial EP testing is performed while the patient continues to receive ventricular pacing support. 
     At test 220, the control system 52 (FIG. 2) determines if the physician, using the programmer 140 (FIG. 3) has indicated that EP testing is complete. If not, the control system 52 (FIG. 2) returns to test 214 to receive a new burst sequence, if the physician chooses to provide one. If testing is complete, step 222 is performed to return the pacemaker 160 (FIG. 2) to its original operating configuration. For example, the pacemaker 160 (FIG. 2) may resume conventional dual-chamber pacing. The pacing pacemakers that were being used prior to the start of EP testing are retained in the memory circuit 66 (FIG. 2) to allow the pacemaker 160 (FIG. 2) to return to its original mode of operation without requiring the physician to reenter those parameters. 
     Thus, a programmable pacemaker that can be used for noninvasive EP testing while maintaining ventricular support is provided, where the EP testing functions in one channel are decoupled from the underlying mode of operation of the other channel in the pacemaker. The implanted pacemaker can be used to perform serial EP studies. The pacemaker can induce and revert atrial arrhythmias by delivering bursts of stimulation pulses to the atrium in sequences that are defined by the physician. The pacemaker of the present invention can also provide ventricular pacing support operating in a single-chamber ventricular mode during atrial EP testing. 
     One skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims that follow.