A heart stimulating device for avoiding problems with fusion beats

Abstract: A heart stimulating device for avoiding problems related to fusion beats, including a pulse generator (8, 9) for delivering stimulation pulses to a patient's heart and having a basic escape interval, filtered sensing means (3, 4, 5) adapted for sensing QRS indications in signals IEGM signals, control means (7) adapted for controlling the pulse generator means (8, 9), and non-filtered sensing means (3, 6) adapted for sensing QRS indications in IEGM signals. The control means (7) activates the non-filtered sensing means preceding an end of the basic escape interval and prolongs said basic escape interval by a predetermined extension interval if the non-filtered sensing means (3, 6) senses a QRS indication.

Description: FIELD OF THE INVENTION The present invention relates to a heart stimulating device having a capability to inhibit stimulation in case of natural activity of a patient's heart. The invention particularly concerns the capability of detecting intrinsic events in order to favour intrinsic heart activity and to avoid fusion beats. Fusion beat generally refers to a situation where atrial or ventricular depolarization starts from two different positions in the heart. Such a situation arises when a natural heart beat occurs simultaneously with an electrical stimulation pulse from a heart stimulating device and they both contribute to the depolarisation of a heart chamber. In a demand pacemakers or other heart stimulating devices that aim to electrically stimulate a patient's heart only in the absence of a normal intrinsic function, fusion beats or pseudofusion beats present a particular problem, since intrinsic events are to be favoured and stimulation energy is to be saved until really needed. That way the longevity of a battery powered heart stimulating device is improved. Some terminology used in this disclosure is explained below. IEGM: An abbreviation for intracardiac electrogram. IEGM signals are emitted by active cardiac tissue and sensed through electrodes placed on or within the heart. QRS or QRS complex: The ventricular depolarization as seen on the electro cardiogram or in the IEGM signals. Intrinsic: Inherent or belonging to the heart itself. An intrinsic beat is a naturally occurring heartbeat. R wave: an intrinsic ventricular event. R refers to the entire intrinsic QRS complex. Evoked response: The electrical activation of the myocardium by a pacemaker output pulse. The ability of cardiac tissue to respond depends on its activity state. Fusion beat: the pacemaker impulse appears close to a spontaneous QRS complex and partly affects the ventricular depolarization. Pseudofusion beat: the pacemaker impulse appears within a spontaneous QRS complex and does not affects the ventricular depolarization. May be referred to as simply a fusion beat in contexts where problems related thereto are similar to those of regular fusion beats. Hysteresis: A programmable feature in some demand pacemakers which allows programming of a hysteresis escape rate lower than the programmed base rate. Hysteresis may be accomplished by prolonging the pacing interval following a sensed intrinsic beat. Escape interval, basic interval, or basic escape interval: The period, typically in the order of 1000 milliseconds, between a sensed intrinsic cardiac event or a stimulation pulse output and the next pacemaker output pulse. DESCRIPTION OF PRIOR ART In today's pacemakers of inhibition type, e.g., in Pacesetter® REGENCY™ pacemakers with AUTOCAPTURE™, IEGM signals are sensed via a lead and electrode arrangement. Intrinsic and stimulated QRS indications in the IEGM signals are monitored by sensing means in the pacemaker. As long as intrinsic QRS complexes are detected at an acceptable rate by the sensing means, the pacemaker inhibits the stimulation pulses. At each detected intrinsic QRS, a timer is started in the pacemaker. If no new QRS is detected within a predetermined basic interval of the timer, a stimulation pulse to the heart is output by a pulse generator in the pacemaker. The QRS detections are performed after a band pass filter that delays the IEGM signals. If an intrinsic QRS occurs immediately before the end of a basic escape interval, it will not be noticed before the next stimulation pulse is transmitted. When that stimulation pulse is transmitted, the pacemaker will have difficulties in detecting the QRS and, as a result of non-detection, the pacemaker may react by increasing output energy although not needed. These problems become more severe if the intrinsic heart rhythm is somewhat faster than that of the pacemaker. A continuous state of this type could be avoided by using so-called hysteresis. Document US-A-5 351 696 discloses a heart stimulating device according to the preamble of claim 1. However, the above referred situation is problematic in the prior art and may lead to a higher energy consumption rate than necessary since the heart stimulating device outputs pulses that are not required. Also, the energy of the stimulation pulses may erroneously be set higher than necessary. An excessively high energy consumption will cause premature depletion of the battery of the stimulation device resulting in higher risk and inconvenience to the patient. Means for safely distinguishing between a true capture failure and a fusion beat are thus highly desirable. SUMMARY OF THE INVENTION It is a general object of the present invention to provide a solution to the above-mentioned shortcomings of the prior art. A particular object is to avoid fusion beats more effectively. Another object of the invention is to provide means for avoiding excessive energy in the stimulation pulses. These objects are attained by a device according to claim 1. Further objects and advantages of the present invention will be evident from the dependent claims and the embodiment description below. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a block diagram of a heart stimulating device according to one embodiment of the invention including sensing inputs, a sense amplifier, detection means, logic means, pulse output means, and charge means. Fig. 2 shows two examples of IEGM signals input to the device of Fig. 1, illustrating situations where a fusion beat occurs due to filter delay in sensing an R wave and where stimulation is inhibited according to the invention as a result of a detected competing intrinsic event, respectively. A PREFERRED EMBODIMENT OF THE INVENTION To describe an embodiment of a heart stimulating device according the invention, references are made to Fig. 1. The described type of device is operable to inhibit stimulation to favour intrinsic events. The device has sensing inputs 1, 2 that feed IEGM signals to a sense amplifier 3. The sensing inputs 1, 2 are typically connected to a lead (not shown) and a cardiac electrode (not shown) which provide the IEGM signals to the heart stimulating device. The sense amplifier provides amplified IEGM signals to three detection means 4, 5, 6. A filtered first R wave detection means 4 is operable to detect intrinsic QRS complexes. It could also have the ability to detect P waves if processing thereof is to be performed by the heart stimulating device. A filtered evoked response (ER) detection means 5 is operable to detect stimulation-evoked QRS complexes. A non-filtered second R wave detection means 6 is operable to detect intrinsic QRS complexes. Each of the detection means 4, 5, 6 is connected to a logic means 7 to which it indicates a detected QRS complex. In order to obtain a reliable detection of QRS complexes, possible noise such as myopotentials and external electromagnetic disturbance has to be prevented from influencing the detection. For this purpose, the filtered first R wave detection means 4 includes a band pass filter for filtering the amplified IEGM signals before they are used for detection of QRS complexes. In the band pass filter there is however inevitably a delay or transfer time for the signals. In today's pacemakers such a delay may be 20 ms. Although the said filter delay time may be the most significant one, all unintended signal delays in the heart stimulating device could be a problem when aiming to avoid fusion beats or the negative consequences thereof. An example of such a delay is the transfer time of the stimulation inhibition signal. The logic means controls a pace pulse generator 8 and a back-up pulse generator 9 which are operable to transmit stimulation pulses to a patient's heart. The pulse generators 8, 9 may be connected to via the same lead (not shown) to the same cardiac electrode (not shown) as the sense amplifier 3. A charge means 10 is connected to both pulse generators 8, 9 to provide them with the necessary stimulation energy. The logic means 7 includes timing means in order to stimulate the patient's heart at an appropriate rate when needed and to distinguish between the different detections made in the detection means. The logic means 7 may also deactivate any of the detection means 4, 5, 6 during selected time intervals. The delay in the filtered first R wave detection means 4 implies that a decision by the logic means 7 to initiate a stimulation pulse can only be based on signals representing a situation up until present minus the filter delay. In order for the heart stimulating device to maintain a selected pacing rate, the logic means 7 has to decide, after a certain time interval has passed since a detected QRS complex, whether a stimulation pulse should be transmitted or not. The said time interval will be referred to as a basic interval. Thus, when stimulation control is based on filtered signals, an intrinsic heart beat occurring within a filter delay time from the end of a basic interval will not lead to inhibition of a stimulation pulse. The non-filtered R wave detection means 6 gives an earlier detection of a QRS complex, but suffers from a higher sensitivity to noise. As will be further described below a combination of the two R wave detection means 4, 6 is particularly advantageous. References will now also be made to fig. 2 which includes an upper IEGM curve including one stimulation-evoked and one intrinsic QRS complex and a lower IEGM curve including first a stimulation-evoked QRS complex and then a fusion beat, i.e., an intrinsic event wherein a stimulation pulse has been transmitted. The dotted line indicates an expected curve shape in the absence of an intrinsic heart activity. Fig. 2 further shows a time line on which points in time t 0 , t 1 , t 2 , t 3 , t 4 , and t 5 have been marked with dashed lines to indicate a transmission of a stimulation pulse to evoke a QRS (t 0 ), an end point for detecting evoked response subsequent to a stimulation (t 1 ), an end of a basic interval simultaneously with an intrinsic QRS complex (t 2 ), the activation of the non-filtered detection means (t 3 ), the instant when the non-filtered detection means detects the intrinsic event (t 4 ), and an end of a prolonged basic interval (t 5 ), respectively. In order to illustrate the operation of the inventive heart stimulating device a sequence to be described is assumed to start by the transmission of a stimulation pulse at to. Then the evoked response detection means 5 is activated after a predetermined delay to detect evoked response until t 1 . If no evoked response has been detected, the heart stimulating device generally emits a back-up pulse. At t 2 , which is the end of the basic interval ranging from to to t 2 , the upper IEGM curve indicates an intrinsic event and an inhibition of stimulation, while the lower curve indicates a fusion beat. According to the invention, the non-filtered detection means is active for only about 20 ms from t 3 until the end of the basic interval at t 2 or only until a detection of an intrinsic event at t 4 . If an intrinsic event is detected in the non-filtered detection means between t 3 and t 2 , the basic interval is prolonged until t 5 , so that the IEGM signal containing the intrinsic QRS indication will have time to travel through the filter of the filtered R wave detection means. For this reason, an extension interval from t 2 to t 5 can be set equal or greater than the filter delay time. Alternatively, the point in time t 5 when the prolonged escape interval ends can be set by t 4 , i.e., the point in time at which the non-filtered detection means detects an intrinsic QRS. A subsequent basic interval is started at t 2 and the length thereof may be set equal to the preceding one in order to maintain a pacing rate or may be set so that hysteresis is achieved which favours intrinsic events. The safe operation of the inventive heart stimulating device is effectively maintained since a basic interval would be prolonged mainly in exceptional situations and the prolongation would be very small compared to the length of a typical heart cycle. A typical length of the basic interval is 850 ms. It could have an adjustment range between 500 and 1333 ms. The band-pass filter delay, which is to be compensated for, in the filtered R wave detection means is typically 20 ms. Consequently, following a detection in the non-filtered R wave detection means of an intrinsic QRS, the basic interval should typically be 20 ms or slightly more immediately after the end of the basic interval or, if a varying timing is not unsuitable, after the detection in the non-filtered R wave detection means. The activation time of the non-filtered R wave detection means has to precede the "blind" interval of the corresponding filtered means before the end of the basic interval. Therefore, it will typically be at least 20 ms or slightly more before the end of the basic interval. However, since the non-filtered R wave detection means only controls the prolongation of the basic interval and not the stimulation inhibition itself, it could be longer. For energy saving reasons, it is kept short. It should be noted that fig. 2 is not drawn to scale but rather illustrates a relative timing of typical phenomena monitored through the IEGM signals. Further, it is to be understood that this embodiment description includes merely illustrative examples of the application of the invention. Thus, many further variations and modifications may be made without departing from the scope of the invention as defined by the appended claims.

Claims: A heart stimulating device comprising: pulse generator means (8, 9) adapted for delivering stimulation pulses to a patient's heart and having a basic escape interval, filtered sensing means (3, 4, 5) adapted for sensing QRS indications in IEGM signals, control means (7) adapted for controlling the pulse generator means (8, 9), characterized by said device further comprising: non-filtered sensing means (3, 6) adapted for sensing QRS indications in the IEGM signals, said control means (7) being adapted to activate said non-filtered sensing means preceding an end of said basic escape interval and adapted to prolong said basic escape interval by a predetermined extension interval if said non-filtered sensing means (3, 6) senses a QRS indication when activated. The heart stimulating device of claim 1, wherein said extension interval is fixed and approximately equal to or greater than a filter delay in said filtered sensing means (3, 4, 5). The heart stimulating device of claim 1, wherein said control means is adapted to determine the length of said extension interval depending on the instant at which the non-filtered sensing means (3, 6) senses a QRS indication. The heart stimulating device of claim 3, wherein the extension interval is approximately equal to a filter delay time in the filtered sensing means (3, 4, 5) minus a remainder of the basic escape interval at the instant at which the non-filtered sensing means (3, 6) senses a QRS indication. The heart stimulating device any one of claims 1-4, wherein said control means is adapted to determine the length of said basic escape interval depending on whether the passed event was intrinsic or stimulated-evoked. The heart stimulating device any one of claims 1-5, wherein said control means (7) is adapted to activate the non-filtered sensing means (3, 6) after at least 95% of the basic escape interval has passed. The heart stimulating device any one of claims 1-5, wherein said control means (7) is adapted to activate the non-filtered sensing means (3, 6) 20 ms or less before the basic escape interval has ended.

Inventors: OBEL MARTIN, OBEL, MARTIN
Applicants: ST JUDE MEDICAL, ST. JUDE MEDICAL AB
Application year: 1997
Main classification: A61N1
IPC codes: A61N1