The present invention relates to implantable pacemakers, and more particularly to an automatically adjustable blanking circuit and method for use within a dual chamber implantable pacemaker. Such an adjustable blanking circuit automatically adjusts, during each pacemaker cycle, the blanking period of one channel of the pacemaker to avoid crosstalk from the other channel of the pacemaker.
A dual chamber pacemaker provides stimulation pulses to, and/or senses electrical activity in, both the atrium and ventricle of a heart. Such pacemakers utilize a lead having one or more electrodes for making electrical contact with each heart chamber (typically the right atrium or right ventricle). The same electrode advantageously serves as the medium for both stimulating the cardiac tissue and sensing cardiac activity (muscle contraction, or depolarization of muscle tissue, evidenced by an electrical signal) within that heart chamber. The circuits associated with a particular heart chamber, both for stimulating and sensing, are referred to as a channel. Thus, a dual chamber pacemaker includes two channels, one for providing stimulation pulses to and/or sensing electrical activity within the atrium, and the other for providing stimulation pulses to and/or sensing electrical activity within the ventricle.
Most modern pacemakers are programmable, allowing the operating mode of the pacemaker to be programmably set to a desired mode depending upon the particular needs of the patient. The operating mode is typically designated by a three letter code, where the first letter of the code indicates the chamber of the heart in which pacing occurs ("A"=atrium; "V"=ventricle; "D"=both the atrium and the ventricle); the second letter of the code indicates the chamber of the heart in which sensing occurs; and the third letter indicates the mode of response of the pacemaker ("T"=triggered; "I"=inhibited; "D"=double, i.e., atrial triggered and ventricular inhibited or atrial triggered/ inhibited and ventricular inhibited).
One operating mode for a dual chamber pacemaker, for example, is the DDD mode, wherein stimulation and sensing occur in both the atrium and ventricle, and wherein the response mode of the pacemaker may be either inhibited or triggered, as required. In such mode, the atrial channel senses whether a P-wave occurs (indicating contraction of the atrium) within a prescribed time period. If so, then an atrial stimulation pulse (hereafter an "A-pulse") is inhibited from being delivered to the atrium. If not, then the A-pulse is delivered to the atrium, thereby triggering an atrial contraction. Similarly, the ventricular channel senses whether an R-wave occurs (indicating contraction of the ventricle) within a prescribed time period. If so, then a ventricular stimulation pulse (hereafter a "V-pulse") is inhibited from being delivered to the ventricle. If not, then the V-pulse is delivered to the ventricle, thereby triggering a ventricular contraction. In this way, each chamber of the heart has a prescribed time period in which contraction should occur. If contraction does not occur within the prescribed time period, then stimulation pulses are delivered in order to trigger contraction. Such operation is termed "demand" pacing, because stimulation pulses are provided only on demand, that is, only as needed. In order for a dual chamber pacemaker to properly perform its function of providing stimulation pulses on demand, it is imperative that it be able to properly sense P-waves and R-waves. That which is sensed by the atrial channel is usually assumed to be a P-wave; and that which is sensed by the ventricular channel is usually assumed to be an R-wave. However, it is not uncommon for an A-pulse, or the P-wave resulting from an atrial contraction caused by an A-pulse, to couple over to the ventricular channel sensing circuits. Similarly, it is quite common for a V-pulse, or the R-wave resulting from a ventricular contraction caused by a V-pulse, to couple over to the atrial channel sensing circuits. Such cross coupling of an electrical signal from one channel of a dual channel pacemaker to the other channel is referred to as "crosstalk." Modern dual chamber pacemakers must utilize some means for handling crosstalk if reliable operation is to be maintained.
For example, after an A-pulse has been delivered to the heart, the atrial muscles contract, causing the desired atrial evoked response from the heart. However, accompanying such muscle contractions are electrical signals that may be sensed through crosstalk by the ventricular sensing circuits. If of sufficient magnitude, these crosstalk signals will be interpreted by the ventricular sensing circuits as spontaneous ventricular activity (e.g., an R-wave), when in fact no R-wave has occurred, thereby causing the pacemaker to inhibit the delivery of a V-pulse, even though such V-pulse may be needed.
The problem of crosstalk is discussed generally in, e.g., C. D. Johnson, "Atrial Synchronous Ventricular Inhibited (VDD) Pacemaker-Mediated Arrhythmia Due to Atrial Undersensing and Atrial Lead Oversensing of Far-Field Ventricular Afterpotentials of Paced Beats: Crosstalk", Pace, Vol. 9, pp. 710-19 (Sept.-Oct. 1986). A discussion of a particular type of crosstalk involving the generation of marker signals may be found in Barold et al., "Crosstalk Due to Activation of Atrial Sense Marker Function of DDD Pulse Generators," Pace, Vol. 10, pp. 293-301 (March-April 1987).
The most common technique used in dual chamber or two-channel pacemakers to deal with crosstalk is to utilize a "blanking period" or "blanking interval" in order to blank out or otherwise disable the sensing circuits of one channel immediately after a stimulation pulse has been delivered to the other channel. During the blanking period following delivery of an A-pulse on the atrial channel, for example, the ventricular sensing circuits of the ventricular channel are made absolute refractory (meaning that no sensing of any kind can occur). Hence, any crosstalk (or other noise) signals that occur during the blanking interval are not sensed. While various circuit arrangements are known in the art for effectuating a desired blanking interval, see, e.g., U.S. Pat. No. 4,462,407 (separate input/output circuits for each channel powered by respective isolated capacitors) and U.S. Pat. No. 4,470,418 (switched bipolar leads), the most common circuit arrangement is to simply disable the sensing circuit of one channel by removing power from the sensing amplifier of that channel during the blanking interval. A difficulty still remains, however, in determining the correct length of the blanking period.
In theory, the blanking interval or period associated with the ventricular channel should be made as short as possible in order to allow the ventricular sense circuits to sense ventricular activity (e.g., an R-wave) during as much of the A-V interval as possible. (The "A-V interval" is the maximum period allowed by the pacemaker between contraction of the atrium and contraction of the ventricle. The A-V interval commences with delivery of an A-pulse or the sensing of a P-wave, and lasts a prescribed time thereafter. During this prescribed time, an R-wave must be sensed in order to inhibit delivery of a V-pulse at the conclusion of the prescribed time.) On the other hand, the ventricular blanking period or interval should be made sufficiently long to effectively block out any residual crosstalk signals (resulting from the atrial contractions triggered by the A-pulse). Unfortunately, the optimum blanking interval for one patient may not be the optimum blanking interval for another patient, nor for even the same patient at different times. That is, if some of the basic pacemaker parameters are changed, or if other changes occur that somehow affect the manner in which the patient's heart responds to a stimulation pulse, the amount of crosstalk that occurs can be significantly altered. For example, a blanking interval of 13 msec may be sufficient for a programmed A-pulse amplitude of 2 volts; but a blanking interval of 50 msec may be needed if the A-pulse amplitude is increased to 7 volts. What is needed, therefore, is a reliable technique for making the blanking interval of one channel be as short as possible while at the same time having it be sufficiently long so as to prevent crosstalk detection of the stimulation pulse by the other channel.
Prior art approaches dealing with the aforementioned problem (of finding an optimum value for the blanking interval) have focused on making the blanking interval programmable. Siemens-Pacesetter, Inc. of Sylmar, Calif., for example, allows the ventricular blanking period in its AFP pacemaker to be programmed to 13, 25, 38 or 50 msec. See also, Barold et al, "Programmability in DDD Pacing," Pace, Vol. 7, pp. 1159-64 (Nov.-Dec. 1984), wherein the desirability of having a programmable blanking interval is suggested. A programmable blanking interval, however, is totally dependent upon skilled human intervention in order to assure that the correct value is chosen.
Further, because only a few blanking interval values are typically available from which to programmably choose a desired blanking interval value, the optimum blanking interval value for a particular patient will likely not be available. Moreover, even if the correct blanking interval value is available for use by a given patient at one particular time, this same blanking interval value may not be the correct value at a subsequent time. Changes made to other programmable parameters of the pacemaker, physiological changes within the patient, as well as the passage of time, can all influence the proper blanking interval value that should be used. Hence, what is needed is a technique for periodically adjusting the blanking interval value to an optimum value over a wide range of possible values without the need for highly skilled human intervention.
Because of the dependence of the correct blanking interval value on other programmable parameters of the pacemaker, it is known in the art to automatically change the programmed value of the blanking interval to a more appropriate value if another programmable parameter of the pacemaker is changed. See, e.g., Kersschot et al., "Atrial Pacing Bigeminy: A Manifestation of Crosstalk," Pace, Vol. 8, pp. 402-07 (May-June 1985), wherein it is suggested that the blanking interval value be automatically reprogrammed if the programmed values of the atrial output and/or ventricular sensitivity are changed. Hence, if either the A-pulse amplitude or width is reprogrammed, for example, such values can impact the optimum blanking interval value. Thus, it is known in the art to include in the programmer a table of correlated values, so that when certain key parameters of the pacemaker are reprogrammed, such as the A-pulse amplitude or width, an appropriate blanking period value associated with that key parameter is also automatically reprogrammed. Unfortunately, however, the values used in such "correlation tables" may not be valid for all conditions. That is, one correlation table may be needed at one heart rate, and another correlation table may be needed at another heart rate. Moreover, the cost and complexity of having to continually switch and update correlation tables can quickly overshadow the benefits derived from making the automatic reprogramming changes in the first place. Thus, what is clearly needed is a simple and inexpensive way to automatically adjust the blanking interval value to an optimum value for the patient regardless of the patient's heart rate or other factors that may create the need to change the optimum blanking interval value.
The present invention advantageously addresses the above and other needs.