System and method for alleviating the effects of pacemaker crosstalk

Methods and apparatus are provided for alleviating the effects of crosstalk in an implantable stimulation device. An autoblanking approach is provided whereby the total blanking interval is made up of an absolute blanking interval followed by a retriggerable relative blanking interval. The implantable stimulation device sensing circuitry is disabled during the absolute blanking interval and enabled during the relative blanking intervals. If a signal is detected during a relative blanking interval, a successive relative blanking interval is initiated. If no signal is detected, then relative blanking terminates. Further, an approach for combining safety standby pacing with autoblanking is provided. If autoblanking terminates before the crosstalk sensing interval reaches a maximum blanking interval, then safety standby pacing is cancelled. Another aspect of the invention relates to monitoring the amount of time in a safety standby sensing window during which signals are detected. If signals are detected longer than a predetermined time, then a safety standby stimulation pulse is provided, otherwise, safety standby pacing is inhibited.

FIELD OF THE INVENTION 
This present invention relates to methods and apparatus for alleviating the 
effects of crosstalk in dual-chamber pacemakers, and more particularly to 
methods and apparatus for providing improved autoblanking and safety 
standby pacing. 
BACKGROUND OF THE INVENTION 
A variety of pacemakers are presently available that apply electrical 
pulses to a patient's heart in order to maintain a healthy heart rhythm. 
Modern pacemakers contain sensing circuitry for monitoring the various 
heartbeat signals produced by a patient's heart and for controlling the 
operation of the pacemaker accordingly. 
"Single-chamber" pacemakers sense heartbeat signals and apply stimulation 
pulses within a single-chamber of the heart, either an atrial or 
ventricular chamber, depending on the patient's individual condition. 
Other pacemakers, known as "dual-chamber" pacemakers, are capable of 
sensing and pacing within both the atrial chamber and the ventricular 
chamber. Although dual-chamber pacemakers are more complex than 
single-chambered pacemakers, they afford the physician considerable 
flexibility in treating the cardiac conditions of different patients. 
In a normally functioning heart, the heart's sinus node generates 
electrical pulses at a heart rate appropriate for the body's current level 
of activity. The stimulus from the sinus node initially propagates to the 
atrial chambers, causing the associated atrial heart tissue to contract. 
The stimulus then propagates to the ventricles, causing the ventricular 
heart tissue to contract. The stimulation pulse generated by the sinus 
node and the subsequent atrial and ventricular contractions forms a 
complete heart contraction. 
Dual-chamber pacemakers can be configured to operate in a variety of modes 
to ensure that the heart beats properly. In order to operate in the 
various pacing modes, dual-chamber pacemakers contain atrial sensing 
circuitry for monitoring heartbeat signals that occur in an atrium (e.g., 
the right atrium) and ventricular sensing circuitry for monitoring 
heartbeat signals in a ventricle (e.g., the right ventricle). 
The signal that accompanies a natural atrial contraction is known as a 
P-wave. The normally occurring ventricular heartbeat signal is the R-wave. 
In one mode of pacing--known as demand pacing--the pacemaker applies 
stimulation pulses only if naturally occurring heartbeat signals are not 
detected within certain predetermined time intervals. If, for example, no 
P-wave is detected by the atrial channel sensing circuitry during an 
expected interval, then the pacemaker will apply a stimulation pulse to 
the atrium. But if a P-wave is detected within this time period, the 
atrial stimulation pulse is inhibited (not applied). Similarly, because a 
ventricular beat is expected to follow an atrial event (whether a 
naturally occurring or paced event), if an R-wave is not detected shortly 
following the atrial event, the pacemaker will apply a ventricular 
stimulation pulse. If an R-wave is detected during this time, however, 
then the ventricular stimulation pulse is inhibited. Demand pacing is 
physiologically beneficial for the patient, because it prevents 
competition between the pacemaker and the naturally occurring rhythm of 
the patient's heart. 
Successful detection of naturally occurring cardiac events is crucial for 
effective demand pacing and for the operation of dual-chamber pacemakers 
in general. However, sometimes pacing signals generated by, for example, 
the atrial channel of the pacemaker may be detected by the sensing 
circuitry in the ventricular channel and erroneously identified as a 
naturally occurring event. This phenomena is commonly referred to as 
"crosstalk". 
There are at least two types of crosstalk. The first type occurs when 
stimulation pulses from one chamber of the heart are conducted through the 
body and sensed on the opposite pacemaker channel, e.g., when the atrial 
and ventricular electrodes are closely coupled and/or the stimulation 
pulse amplitude is very large and/or the residual polarization signals 
following the stimulation pulse are significant. Another type of crosstalk 
occurs when conductor traces within the integrated circuits themselves are 
closely coupled. In either case, crosstalk typically occurs when the 
stimulation pulse is large. 
For example, when a high atrial stimulation pulse is applied to the heart, 
this high output pulse may be detected by the ventricular sense amplifier 
which will identify any signal of appropriate amplitude as an R-wave and 
behave in accordance with that interpretation. Measures need to be taken 
to ensure that known, but physiologically inappropriate, signals are 
managed properly. Misidentification of crosstalk as an R-wave may inhibit 
the application of physiologically useful ventricular stimulation pulses. 
Consequently, crosstalk signals should be taken into account for proper 
pacemaker design and operation. 
As used hereinafter, "crosstalk" shall include both types described above, 
i.e., crosstalk due to paced signals which are sensed between chambers, 
whether internal to the circuitry or conducted through the body, and 
include the residual polarization signals. 
One approach for alleviating the effects of crosstalk is to disengage the 
sensing circuitry in the channel in which the crosstalk is anticipated for 
a predetermined length of time, known as a blanking interval. Thus, 
immediately following application of an atrial stimulation pulse, the 
ventricular sensing circuitry is turned off for the duration of the 
blanking interval. Because the ventricular sensing circuitry is inactive 
during this interval, the operation of the pacemaker will be unaffected by 
crosstalk. 
Although, with this approach, the blanking interval may periodically be 
adjusted by a physician, for practical purposes the interval is fixed. As 
a result, the blanking interval is generally chosen to be "long enough" to 
ensure that crosstalk will not affect the pacemaker under a variety of 
conditions. However, a blanking interval that is too long increases the 
probability that a natural cardiac event (e.g., a premature ventricular 
contraction, or PVC) occurring during the blanking interval will not be 
sensed. This phenomenon is sometimes referred to as "blanking-induced 
undersensing". That is, natural cardiac events, such as R-waves or PVC's, 
are not detected due to a blanking interval that was too long. Missing a 
naturally occurring cardiac event may cause the pacemaker to apply an 
inappropriate stimulus to the heart. For example, if the pacemaker fails 
to detect a PVC because it occurred during the blanking interval, the 
pacemaker will fail to inhibit application of the ventricular stimulation 
pulse at the end of the AV delay. Thus, the pacemaker will apply a 
ventricular stimulation pulse, which pulse is not only unnecessary, but 
may be applied during the so-called "vulnerable period" of the cardiac 
cycle (near the apex of the T-wave). Pulses applied during the vulnerable 
period may induce an undesirable repetitive cardiac rhythm in an 
electrically unstable heart. 
A technique known as "autoblanking" allows the crosstalk blanking interval 
to be reduced automatically. Autoblanking is described in 
commonly-assigned U.S. Pat. No. 4,974,589. As described in the '589 
patent, the blanking interval is made up of one or more relatively short 
"basic" blanking intervals, each consisting of an "absolute" blanking 
interval and a "relative" blanking interval. During the absolute blanking 
intervals, the sensing circuitry is not enabled, which prevents crosstalk 
(e.g., the stimulation pulse) from being detected. During the relative 
blanking intervals, the sensing circuitry is enabled. If a signal is 
detected during a relative blanking interval (e.g., as a result of 
residual polarization), the signal is presumed to be crosstalk and the 
pacemaker initiates another basic blanking interval. Eventually, when no 
signal is detected for the entire length of a relative blanking interval, 
then it is presumed that the crosstalk signal has ended. Normal pacemaker 
function are allowed to resume and no further basic blanking intervals are 
initiated. With this approach, the total blanking interval length is equal 
to the sum of the lengths of the basic blanking intervals. Generally, this 
length is less than the length of a typical fixed blanking interval. 
The autoblanking technique described in the '589 patent is generally 
satisfactory. However, the basic blanking intervals that are repeatedly 
retriggered in the approach of the '589 patent each contain an absolute 
blanking interval. During such absolute blanking intervals, it is not 
possible to monitor the cardiac signals for potential crosstalk. It is 
only during the relative blanking intervals that crosstalk is monitored. 
As a result, the approach of the '589 patent is not as efficient as it 
might be, because even if no signals occur for the entire length of one of 
the retriggered absolute blanking intervals (a situation that would 
indicate that the signal is no longer likely to be crosstalk so that 
normal operation can be resumed), it is not until an additional length of 
time--equal to a relative blanking interval--has elapsed that it is 
possible to terminate blanking. 
Another approach for alleviating the effects of crosstalk is called safety 
standby pacing. With this approach, the duration of the conventional 
absolute blanking interval can be reduced to about 13 ms. This shorter 
blanking interval is immediately followed by an interval called the 
crosstalk detection window which is approximately 50 ms in length. 
After an atrial stimulation pulse is applied, the ventricular sensing 
circuitry is disengaged for the duration of the blanking interval. During 
the crosstalk detection window, the ventricular sensing circuitry is 
active, but if a signal is detected in the window, the ventricular 
stimulation pulse is not inhibited. Because the signal detected in the 
crosstalk detection window could be an R-wave, the ventricular stimulation 
pulse is not applied at the programmed AV interval (e.g., 250 ms), but 
rather is applied after an abbreviated interval (e.g., 120 ms). 
Allowing the ventricular stimulation pulse to be applied even if a signal 
is detected during the crosstalk detection window ensures that the output 
of the pacemaker will not be inadvertently inhibited by crosstalk. Thus, 
in the presence of high grade AV block, the patient would not be left 
asystolic. And, if the signal detected during the crosstalk detection 
window is a native R-wave, then applying the ventricular stimulation pulse 
at the abbreviated interval will typically cause the pulse to fall within 
a period in which the ventricular tissue is physiologically refractory 
(unresponsive to stimulus). It is therefore generally safe for the 
stimulation pulse to be applied at this point in the cardiac cycle. 
Applying the ventricular stimulation pulse at the abbreviated interval 
following an R-wave also ensures that the pulse is not applied during the 
heart's vulnerable period. 
However, even with safety standby pacing the absolute blanking interval 
must still be chosen fairly conservatively. If the blanking interval is 
too short, crosstalk will repeatedly be detected during the crosstalk 
detection window. Each time crosstalk is detected, the pacemaker will 
apply the ventricular stimulation pulse at the abbreviated interval. While 
it is generally safe to apply the stimulation pulse at this point (because 
the ventricle is refractory), such a stimulation pulse results in a 
shortened, less hemodynamically optimum AV delay and, further, there may 
be circumstances in which this pulse could induce an undesirable heart 
arrhythmia. Furthermore, continued application of stimulation pulses at 
the abbreviated interval may cause concern on the part of medical 
personnel monitoring the patient's condition, because such abbreviated 
intervals are not commonly observed. 
Furthermore, with known safety standby pacing systems, regardless of the 
length of the blanking interval, once signals are detected in the 
crosstalk detection window, release of the safety standby stimulation 
pulse is committed, even if there is a high probability that the detected 
signal is an R-wave. What is needed is a system which could inhibit the 
safety standby pacing output pulse if it is found that the signal sensed 
in the crosstalk detection window corresponds to a naturally occurring 
cardiac event. 
SUMMARY OF THE INVENTION 
In accordance with the principles of the present invention, methods and 
apparatus are provided for minimizing the undesirable effects of standard 
crosstalk protection circuits and techniques, while still protecting the 
patient from the adverse consequences of crosstalk. 
One aspect of the present invention relates to an improved autoblanking 
technique that minimizes the total blanking interval. Immediately 
following (or coincident with) release of a stimulation pulse on a first 
channel (e.g., the atrial channel), the sensing circuitry for the second 
channel (e.g., the ventricular channel) is disabled for a period known as 
an absolute blanking interval. Contrary to existing techniques, the 
present invention repeatedly initiates a relative blanking interval 
following the initial absolute blanking period. During these relative 
blanking intervals the sensing circuitry is enabled. Signals detected 
during the relative blanking intervals are presumed to be crosstalk. When 
a signal is detected during one relative blanking interval, a subsequent 
relative blanking interval is initiated. After a complete relative 
blanking interval passes with no detected signals, blanking terminates. In 
addition, if no relative blanking interval passes without detected 
signals, blanking is still terminated when the total length of the 
blanking intervals reaches a predetermined maximum blanking interval. 
An advantage of the present invention over previously known techniques (in 
which each retriggering of a blanking interval necessarily retriggers the 
application of an absolute and a relative blanking interval) is that the 
length of the total blanking interval is generally reduced. 
Another aspect of the present invention relates to implementing an improved 
method of safety standby pacing in combination with autoblanking. The 
pacemaker performs autoblanking as long as the crosstalk persists. 
However, when the crosstalk sensing interval reaches a maximum blanking 
interval, blanking is terminated and a safety standby sensing window of 
(e.g., approximately 10-20 ms) is initiated. 
During the safety standby sensing window, the ventricular channel is 
enabled so that the pacemaker can process signals on the ventricular 
channel. If crosstalk continues into the safety standby sensing window, a 
ventricular stimulation pulse is applied to the heart at an abbreviated AV 
interval (e.g., 100-120 ms). If an R-wave also occurs during the safety 
standby sensing window, the ventricular stimulation pulse is not applied 
during the terminal (or vulnerable) phase of the T-wave. Further, the 
ventricular stimulation pulse is applied when the heart is refractory, so 
it will not result in an evoked response. If no R-wave occurs during the 
safety standby sensing window, then the ventricular stimulation pulse 
would have been properly timed. Thus, when the crosstalk sensing interval 
reaches the maximum blanking interval in length, the full functionality of 
conventional safety standby pacing is maintained. 
The primary difference between the present invention and the prior art is 
that in those instances in which the crosstalk sensing interval does not 
extend to the maximum blanking interval: (a) the length of the total 
blanking interval is reduced (which reduces blanking-induced 
undersensing), and (b) safety standby pacing is not invoked (which reduces 
the likelihood of confusion on the part of a medical personnel observing 
the operation of the pacemaker). 
Another feature of the present invention relates to an improved safety 
standby pacing approach that may be used with either a fixed blanking 
interval or with autoblanking. With this approach, blanking is initiated 
simultaneously with the release of the atrial stimulation pulse. 
Regardless of the blanking technique that is employed, a safety standby 
sensing window is initiated upon the completion of blanking. During the 
safety standby sensing window, the pacemaker monitors not only the 
occurrence or non-occurrence of any sensed signals, but also determines 
the maximum length of time for which signals are present. Thus, when an 
R-wave occurs during the safety standby sensing window, the pacemaker 
determines that a signal was detected in the sensing window beginning at, 
for example, a time T.sub.1. The pacemaker also determines when the signal 
ends, for example, at time T.sub.2. Based on this type of measurement, the 
pacemaker determines the amount of time for which signals were detected 
during the safety standby sensing window. The pacemaker can either 
determine the total time period during which signals were detected, or can 
determine the length of the longest continuous signal that was detected. 
Regardless of the method for determining the length of time (T) for which a 
signal was present during the safety standby sensing window, at some time 
prior to the time at which a safety standby stimulation pulse would 
otherwise be applied, a determination is made whether the time T is 
greater than a predetermined time limit corresponding to the width of an 
R-wave (e.g., approximately 50 ms). Because the blanking interval has 
terminated, two likely origins of signals in the safety standby pacing 
detection window are noise (i.e., crosstalk) and native ventricular beats. 
If the time T is greater than the predetermined interval (or continuous), 
then the signal is likely to be noise. Therefore, a safety standby 
stimulation pulse is applied. Applying a safety standby stimulation pulse 
ensures that the patient receives a needed stimulation pulse. If the time 
T is approximately equal to the predetermined interval, then the signal is 
likely to correspond to a native R-wave. Accordingly, no safety standby 
stimulation pulse is generated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A pacemaker 10 constructed in accordance with the present invention is 
shown in FIG. 1. The overall operation of the pacemaker 10 is governed by 
control circuitry 12, which is preferably microprocessor-based. An atrial 
channel electrode 14 is connected to a patient's atrium by a suitable lead 
(not shown). When it is desired to apply an atrial output pulse, the 
control circuitry 12 generates a control signal on a control line 16, 
which causes atrial stimulation pulse generating circuitry 18 to provide 
an output pulse at the electrode 14. Cardiac signals from the atrial 
channel are sensed using an atrial sense amplifier 20 that is enabled and 
disabled using a control line 22 from the control circuitry 12. The atrial 
channel signals are further processed by atrial channel input circuitry 
23, which preferably contains conventional pacemaker bandpass and 
threshold detection circuitry. 
The input and output circuitry for the ventricular channel generally 
operates in the same way as the atrial channel circuitry. Ventricular 
channel output pulses are generated at electrode 24 by ventricular 
stimulation pulse generating circuitry 26 in response to control signals 
received via a control line 28. The electrode 24 is connected to a 
patient's ventricle using an appropriate lead (not shown). Ventricular 
channel input signals are received via a ventricular sense amplifier 30. 
The control circuitry 12 enables and disables the ventricular sense 
amplifier 30 via a control line 32. Ventricular channel input circuitry 34 
preferably contains conventional pacemaker bandpass and threshold 
detection circuitry. 
When the patient experiences a normal ventricular contraction, an R-wave is 
typically sensed by the ventricular sense amplifier 30. The R-wave is 
provided to the ventricular channel input circuitry 34. The typical output 
of the ventricular channel input circuitry 34 is a pulse 36, which 
indicates to the control circuitry 12 the time at which the R-wave was 
detected. Similarly, when atrial events are detected, the atrial 
stimulation pulse generating circuitry 18 provides a corresponding pulse 
40 to the control circuitry 12. 
In normal operation of the pacemaker 10, the control circuitry 12 receives 
pulses, such as pulses 36 and 40, whenever cardiac events are detected. 
The pacemaker 10 applies corresponding atrial and ventricular stimulation 
pulses to the heart via the atrial and ventricular channels, as needed. 
The immediate effects of these pulses are generally confined to the atrium 
or ventricle to which the pulse is applied. occasionally, however, there 
can be crosstalk between the two channels (e.g., stimulation pulses and 
residual depolarization signals from one channel are sensed, either 
internally within the pacemaker or conducted within the heart, by the 
opposite channel). 
With atrial channel to ventricular channel crosstalk, atrial stimulation 
pulses result in crosstalk signals on the ventricular channel that are 
detected by the ventricular sense amplifier 30. In order to avoid the 
potentially adverse effects of crosstalk, the control circuit 12 disables 
the ventricular sense amplifier upon release of an atrial stimulation 
pulse. The period, for which the ventricular sense amplifier is disabled, 
is known as the absolute blanking interval. 
Ventricular channel to atrial channel crosstalk may also result in 
crosstalk on the atrial channel, which can be detected by atrial sense 
amplifier 20. At the time of the ventricular output pulse, the atrial 
sense amplifier is typically already disabled because the atrial channel 
has begun its refractory period. However, if sensing were enabled on the 
atrial channel during this refractory period, the atrial channel could 
likewise be disabled for an absolute blanking interval. 
Thus, in the following description, illustrative embodiments of the 
invention are described in the context of atrial channel to ventricular 
channel crosstalk, although the invention is equally applicable in the 
context of ventricular channel to atrial channel crosstalk. 
In accordance with one feature of the present invention, the blanking 
interval is made up of an initial absolute blanking interval (e.g., 12 
ms), during which the sense amplifiers 20 and 30 are preferably disabled, 
followed by a retriggerable relative blanking interval (e.g., 4-8 ms 
each), during which signals may be sensed, but are presumed to be 
crosstalk. If a signal is sensed during a first relative blanking 
interval, the pacemaker 10 automatically restarts another relative 
blanking interval. When a complete relative blanking interval passes 
without any sensed signal, blanking is terminated. In order to minimize 
the phenomenon known as "blanking-induced undersensing" (i.e., in which 
natural cardiac events are not detected because of a lengthy crosstalk 
interval), blanking is also terminated whenever the total length of the 
crosstalk sensing interval reaches a predetermined maximum blanking 
interval. 
The autoblanking approach of the present invention is illustrated in FIGS. 
2-5. As shown in FIG. 2, the release of an atrial stimulation pulse 42 
results in an evoked P-wave 44. corresponding crosstalk 46 appears on the 
ventricular channel. At the same time that the atrial stimulation pulse 42 
is released, the ventricular sense amplifier 30 is disabled by the control 
circuitry 12 via control signals on the control line 32. The control 
circuitry 12 disables the ventricular sense amplifier 30 for the duration 
of an absolute blanking interval 48, which is preferably about 12 ms. 
After the completion of the absolute blanking interval 48, a first relative 
blanking period 50 is initiated during which the control circuitry 12 
enables the ventricular sense amplifier 30, so that the ventricular 
channel input circuitry 34 can process any signals received at the 
electrode 24. The first relative blanking period 50 is preferably about 4 
ms in length. If no signals are detected during the relative blanking 
interval 50, as in FIG. 2, then control circuitry 12 terminates blanking 
at the end of the relative blanking interval 50. The total blanking 
interval is equal to the sum of the absolute blanking interval 48 and the 
first relative blanking interval 50. 
Sometimes crosstalk (e.g., residual polarization potentials) persists for a 
longer period of time. As shown in FIG. 3, an atrial stimulation pulse 52 
results in crosstalk 54. The ventricular sense amplifier 30 is disabled 
for the duration of an absolute blanking interval 56. In contrast to the 
situation of FIG. 2, ventricular crosstalk persists during a first 
relative blanking interval 58. The crosstalk 54 is therefore detected 
during the first relative blanking interval 58. Because the blanking 
interval 58 is a relative blanking interval, any detected signal is 
presumed to be crosstalk. When a signal is detected during a relative 
blanking interval, the control circuitry 12 extends, or retriggers, the 
blanking period by another relative blanking interval 60, which is 
preferably approximately 4 ms in duration. 
The example illustrated in the blanking interval trace 63 of FIG. 3 uses 
relative blanking intervals of a fixed length (e.g., 4 ms). With this 
approach, the control circuitry 12 waits until the entire 4 ms length of 
the relative blanking interval 58 passes, before commencing the relative 
blanking interval 60. With a clock rate of approximately 32 KHz, the 4 ms 
relative blanking period 58 lasts approximately 128 clock cycles. 
The blanking interval trace 65 in FIG. 3 illustrates an example using 
variable length relative blanking intervals. When the ventricular channel 
input circuitry 34 detects the crosstalk 54 during the first relative 
blanking interval 59, the control circuitry 12 extends the blanking 
interval by a second relative blanking interval 61, which preferably has a 
maximum nominal length of approximately 4 ms. In contrast to the fixed 
relative blanking interval example (described above), the control 
circuitry 12 commences the second relative blanking interval 61 
immediately after the crosstalk 54 is no longer detected on the 
ventricular channel. The first relative blanking interval 59 is therefore 
terminated before reaching the end of its nominal 4 ms duration. This 
approach may be preferable to the fixed relative blanking interval 
approach in that it further minimizes the length of the total blanking 
interval. 
Preferably, a clock rate of approximately 32 KHz is used for generating the 
necessary clock signals in the pacemaker 10. As typically is the case in 
digital systems, control signals such as the enabling or disabling control 
signals for the sense amplifiers 20 and 30 are preferably generated in 
synchronization with the signals from the clock (contained in the control 
circuitry 12 of FIG. 1). Generating control signals in synchronization 
with the clock has a negligible effect on the length of the variable 
relative blanking interval, because the clock period for a 32 KHz clock 
(31 .mu.s) is negligible compared with the length of a typical relative 
blanking period. 
The process of extending the blanking interval by additional relative 
blanking intervals (whether of fixed or variable length) preferably 
continues until (1) no signals are detected for an entire relative 
blanking interval or (2) the total duration of the blanking interval 
reaches a predetermined maximum blanking interval. The maximum blanking 
interval preferably can be adjusted to any suitable length between zero 
and the length of the AV delay. In the arrangement of FIG. 3, the blanking 
interval is terminated prior to reaching the predetermined maximum 
blanking interval because no signals were detected on the ventricular 
channel for the final relative blanking interval. 
One advantage of the autoblanking arrangement of the present invention over 
the autoblanking arrangement described in the above-mentioned U.S. Pat. 
No. 4,974,589, is that with the present approach, only the relative 
blanking interval is retriggered. With the arrangement of the '589 patent, 
whenever a signal is detected during a relative blanking interval, both a 
new absolute blanking interval and a new relative blanking interval are 
retriggered. But because no signals can be detected during the absolute 
blanking intervals of the '589 patent, the total length of the blanking 
interval may be unnecessarily long. 
With the present autoblanking approach, as soon as a complete relative 
blanking interval passes without a signal being detected, blanking is 
terminated. Thus, for given absolute and relative blanking intervals, the 
present autoblanking technique will generally result in shorter total 
blanking intervals than the technique of the '589 patent. Shorter total 
blanking intervals are desirable, because they reduce the risk of 
blanking-induced undersensing. 
The autoblanking approach of FIGS. 2 and 3 is further illustrated in the 
flowcharts of FIGS. 4 and 5. Autoblanking using fixed length relative 
blanking intervals (trace 63, FIG. 3) is illustrated in FIG. 4. At step 
200, an atrial stimulation pulse is applied to the patient's heart via the 
atrial stimulation pulse generating circuitry 18. The ventricular sense 
amplifier 30 is then disabled at step 202 for the length of the absolute 
blanking interval. At step 206, the ventricular sense amplifier 30 is 
enabled. At step 208, a relative blanking interval is initiated. For the 
duration of the relative blanking interval, the ventricular sense 
amplifier 30 and the ventricular channel input circuitry 34 monitor the 
signals on the ventricular channel (steps 210, 212). At test 214, the 
control circuitry 12 determines whether any signals were detected during 
the relative blanking interval. If a signal was detected, then control 
returns to step 208 and another relative blanking interval is initiated. 
If not, then an entire relative blanking interval has passed without a 
signal being detected and autoblanking is therefore terminated at step 
216. 
Autoblanking using variable length relative blanking intervals (trace 65, 
FIG. 3) is further illustrated in the flowchart of FIG. 5. Similar to the 
description above for the fixed interval approach, at step 200, an atrial 
stimulation pulse is applied to the patient's heart. The ventricular sense 
amplifier 30 is then disabled at step 202 for the length of the absolute 
blanking interval. At step 206, the ventricular sense amplifier 30 is 
enabled. At step 208, a relative blanking interval is initiated. At step 
210 the ventricular sense amplifier 30 and the ventricular channel input 
circuitry 34 monitor the signals on the ventricular channel (step 210). 
However, in this embodiment, the control circuitry 12 does not wait until 
the end of the relative blanking interval before checking to see if a 
signal has been detected. Once a signal is detected during the relative 
blanking interval, at step 218, then another test is done at step 220 to 
determine when the signal is no longer present. If the signal is still 
present, the system continues to monitor until the signal is gone (steps 
210, 218 and 220). 
Once the signal is no longer present, then control passes to step 208 and 
another relative blanking interval is initiated. The ventricular channel 
is monitored using the ventricular sensing amplifier 30 and the 
ventricular channel input circuitry 34 at step 210. 
If the signal ceases to be present at step 218, then a determination is 
made as to whether the end of the relative blanking interval has been 
reached at step 222. The system will continue to monitor the relative 
blanking interval until the relative blanking interval has ended without 
detecting the presence of the crosstalk signal (steps 210, 218 and 222), 
whereby autoblanking is then terminated at step 224. 
Another feature of the present invention relates to safety standby pacing. 
Pacemakers capable of conventional safety standby pacing are well known. 
With such pacemakers, a special sensing window follows the predetermined 
absolute blanking interval. During the absolute blanking interval, all 
input sensing circuitry is disabled to avoid the effects of crosstalk. 
During the special sensing window, the detection of a signal triggers the 
generation of a ventricular stimulation pulse at an abbreviated AV 
interval, commonly in the range of 100 ms to 120 ms. Further, because the 
pulse is applied at an abbreviated interval, it serves as a marker to a 
physician that safety standby pacing is being invoked. This allows the 
physician to programmably shorten the length of the absolute blanking 
interval to more thoroughly eliminate blanking-induced undersensing, while 
still protecting the patient from ventricular output inhibition if 
cross-talk was present. Often the signal that is detected during the 
special sensing window corresponds to a normal cardiac event such as an 
R-wave. In that case, applying the ventricular stimulation pulse at the 
abbreviated interval ensures that the ventricular stimulation pulse is not 
applied during the terminal phase of a native T-wave, which is a period of 
the cardiac cycle in which applied pulses can cause undesirable rhythms. 
The ventricular stimulation pulse is typically applied during a period of 
the cardiac cycle in which the heart is physiologically refractory and 
therefore is not affected by the applied pulse. Despite the advantages of 
safety standby pacing, conventional safety standby pacing has engendered 
concern on the part of physicians. Application of the ventricular 
stimulation pulse at the abbreviated interval is not an infrequent 
occurrence (that is, it is a relatively frequent occurrence in response to 
late cycle PVC's), so when it does take place physicians can become 
confused. 
In accordance with the present invention, an improved safety standby pacing 
mode is implemented in conjunction with, preferably, the autoblanking mode 
described above. The combination ensures that the blanking period is as 
short as possible (thereby minimizes blanking-induced undersensing) and, 
further, safety standby pacing is only invoked in the event that the total 
crosstalk blanking interval reaches a predetermined maximum blanking 
interval. 
If the crosstalk blanking interval is terminated prior to reaching the 
maximum blanking interval, there is a strong likelihood that crosstalk has 
not occurred, because the atrial stimulation pulse, residual polarization 
and any evoked response that led to the crosstalk have ended. Safety 
standby pacing is therefore not needed. Inhibiting safety standby pacing 
in instances where the blanking interval is shorter than the maximum 
blanking interval reduces the number of times that the ventricular 
stimulation pulse is applied at the reduced AV interval in response to 
known and appropriate physiological signals. Thus, this type of safety 
standby pacing arouses the concern of a physician less frequently than 
conventional safety standby pacing. 
The safety standby pacing approach of the present invention is illustrated 
in FIGS. 6-8. As shown in FIG. 6, an atrial stimulation pulse 62 results 
in an evoked response 64. Corresponding crosstalk 66 appears on the 
ventricular channel. Using either an autoblanking approach such as 
described in the above-mentioned U.S. Pat. No. 4,974,589, or, preferably, 
as described above in connection with the crosstalk blanking technique of 
the present invention, the pacemaker performs relative blanking as long 
the signal continues to be detected. However, when a crosstalk sensing 
interval 68 reaches the maximum blanking interval, blanking interval is 
terminated. A safety standby sensing window 70 of, for example, 
approximately 10-20 ms in length, follows the crosstalk sensing interval 
68. During the safety standby sensing window 70, the control circuit 12 
enables the ventricular sense amplifier 30, so that the ventricular 
channel input circuitry 34 can process signals on the ventricular channel. 
In the example shown in FIG. 6, the crosstalk 66 continues into the safety 
standby sensing window 70. Following detection of a signal during the 
safety standby sensing window 70, the control circuitry 12 applies a 
ventricular stimulation pulse 72 at an abbreviated AV interval (e.g., 
100-120 ms). Although an R-wave 74 also occurred during the safety standby 
sensing window, the ventricular stimulation pulse 72 is not applied during 
the terminal phase of the T-wave (not shown in FIG. 6). Further, the 
ventricular stimulation pulse 72 is applied when the heart is refractory, 
so it does not result in an evoked response. If the R-wave 74 had not 
occurred during the safety standby sensing window 70, then the ventricular 
stimulation pulse 72 would have been properly timed to protect against 
blanking-induced undersensing. Thus, under the circumstances shown in FIG. 
6, in which the crosstalk sensing interval 68 reaches the maximum blanking 
interval in duration, the full functionality of conventional safety 
standby pacing is maintained. 
In those instances in which the crosstalk sensing interval does not extend 
to the maximum blanking interval, the length of the total blanking 
interval is reduced, which reduces blanking-induced undersensing. Further, 
safety standby pacing is not invoked, which reduces the likelihood of 
confusion on the part of a physician observing the operation of the 
pacemaker. 
The operation of the pacemaker when the crosstalk sensing interval 
terminates before the maximum blanking interval is shown in FIG. 7. A 
crosstalk sensing interval 80 is initiated upon application of an atrial 
stimulation pulse 76. Crosstalk 78 is detected during the relative 
blanking intervals of the crosstalk sensing interval 80, preferably as 
described in the preferred embodiments of FIGS. 3-5. When crosstalk 78 is 
no longer detected, the crosstalk sensing interval 80 terminates. Because 
the crosstalk sensing interval 80 is often shorter than would be the case 
if a prior art blanking interval was used, blanking-induced undersensing 
is reduced. Further, as shown in FIG. 7, it is not necessary to invoke a 
safety standby pacing (illustrated as a time interval in the dotted area 
81). 
The safety standby pacing approach of the present invention is further 
illustrated in the flowchart of FIG. 8. Crosstalk sensing is carried out 
at step 234 using any suitable technique, such as the approach illustrated 
in FIGS. 2-5 or the autoblanking technique of the above-mentioned U.S. 
Pat. No. 4,974,589. Following crosstalk sensing, the control circuitry 12 
compares the length of the total crosstalk sensing interval to a 
predetermined maximum blanking interval at step 236. If the crosstalk 
sensing interval is determined to be less than the maximum blanking 
interval, then safety standby pacing is not invoked, and the cycle ends at 
step 238. If, however, the crosstalk sensing interval reaches the maximum 
blanking interval, then a safety standby sensing window is initiated at 
step 240. The control circuitry 12 monitors the ventricular channel until 
the safety standby window has ended (steps 242 and 244). The control 
circuitry 12 then determines whether a signal was detected during the 
safety standby sensing window at test 246. If no signal is detected during 
the entire safety standby sensing window then a safety standby stimulation 
pulse is not applied and the cycle ends at step 238. If it is determined 
at test 246 that a signal was detected during the safety standby sensing 
window, then, at step 248, a ventricular stimulation pulse is applied to 
the patient's heart at an abbreviated AV interval. After the ventricular 
stimulation pulse is applied, the cycle ends at step 238. 
Another feature of the present invention relates to an improved safety 
standby pacing approach that may be used with either a fixed or automatic 
blanking routines. This approach is described in connection with FIGS. 
9-11. As shown in FIG. 9, an atrial stimulation pulse 82 results in 
crosstalk 84 on the ventricular channel. Simultaneous with the release of 
the atrial stimulation pulse 82, the control circuitry 12 initiates a 
crosstalk sensing interval 86 on the ventricular channel. Regardless of 
the blanking technique that is employed, a safety standby sensing window 
88 is initiated whenever the crosstalk sensing interval 86 is completed 
or, as in the preferred embodiment, the crosstalk sensing interval reaches 
the MBI. During the safety standby sensing window 88, the control 
circuitry 12 monitors not only for the absence or presence of any sensed 
signals, it also determines the maximum length of time for which signals 
are present. Thus, when an R-wave 90 occurs during the safety standby 
sensing window 88, the control circuitry 12 determines that a signal was 
detected in the sensing window, and that the signal began at time T.sub.1. 
The control circuitry 12 also determines that the signal ended at time 
T.sub.2. Based on this type of measurement, the control circuitry 12 
determines the amount of time for which signals were detected during the 
safety standby sensing window 88. The control circuitry 12 can use any 
suitable method for determining the amount of time signals are detected. 
For example, the control circuitry 12 can determine the total time period 
during which signals were detected. Alternatively, the control circuitry 
12 can determine the length of the longest continuous signal that was 
detected. In FIG. 9, the time T is the time elapsed between the detection 
of the R-wave 90 at time T.sub.1 and its completion at time T.sub.2. 
Regardless of the method for determining the length of time T for which a 
signal was present during the safety standby pacing detection window 88, 
at some time prior to time T.sub.3, a determination is made whether the 
time T is greater than a predetermined time limit corresponding to an 
R-wave (e.g., approximately 50 ms). Because the crosstalk blanking 
interval 86 has terminated, two likely origins of signals in the safety 
standby pacing detection window are noise and native ventricular beats. If 
the time T is greater than the predetermined time limit, then the signal 
is likely to be noise and a safety standby stimulation pulse is applied at 
the abbreviated AV interval (e.g., approximately 100 ms to 120 ms) at time 
T.sub.3. Applying a safety standby stimulation pulse ensures that the 
patient receives a needed stimulation pulse in the case of AV block. If an 
R-wave, such as R-wave 90 in FIG. 9, has a measured time, T, approximately 
equal to the predetermined time limit, then the system assumes that the 
signal is a native ventricular beat. Accordingly, application of the 
safety standby stimulation pulse at time T.sub.3 is inhibited. 
The situation in which a safety standby stimulation pulse is applied 
because the duration (time T) of the signal detected during the safety 
standby sensing window is greater than the predetermined time interval is 
shown in FIG. 10. An atrial stimulation pulse 92 generates crosstalk 94. 
The control circuitry 12 blanks out the crosstalk using either a fixed or 
variable length blanking interval. At the termination of the crosstalk 
window 96, the control circuitry 12 initiates a safety standby sensing 
window 98. During the safety standby sensing window 98, the control 
circuitry 12 enables the ventricular sense amp 30. As shown in FIG. 10, 
noise is present and the duration, T, of the signal 100 is greater than 
the predetermined time interval. Therefore, the system assumes that the 
signal is probably noise. Accordingly, at time T.sub.3, a safety standby 
stimulation pulse 102 is generated. 
A flowchart of the safety standby pacing approach described in connection 
with FIGS. 9 and 10 is shown in FIG. 11. At step 234, a crosstalk sensing 
routine is performed, preferably the preferred embodiments described in 
FIGS. 3-5. In the preferred embodiment, the safety standby sensing window 
is not evoked unless the crosstalk sensing interval is greater than or 
equal to the Maximum Blanking Interval, MBI, as described in conjunction 
with FIG. 8. Thus, as determined at step 236, a crosstalk interval less 
than the MBI will terminate the program at step 256. A crosstalk interval 
greater than the MBI will initiate a safety standby sensing window at step 
240. At step 250, the maximum length of time, T, for which signals were 
present during the safety standby sensing window is determined, using any 
suitable technique. At step 252, the control circuitry 12 compares the 
time, T, to a predetermined time limit. If the time, T, is greater than 
the predetermined time limit than the detected signals are likely to be 
noise. Since it is difficult to determine whether an R-wave occurred in 
the presence of noise, a safety standby stimulation pulse is applied at 
step 254 and the cycle ends at step 256. If T is determined at step 252 to 
be less than or equal to the time limit (i.e., corresponding to an 
R-wave), at step 258 the application of a safety standby stimulation pulse 
is inhibited. The cycle ends at step 256. 
Thus, methods and apparatus for alleviating crosstalk in a pacemaker are 
provided in which pacemaker pulses applied to a patient's heart on a first 
pacemaker channel can result in crosstalk on a second pacemaker channel. 
One skilled in the art will appreciate that the present invention can be 
practiced by other than the described embodiments, which are presented for 
the purposes of illustration and not of limitation, and the present 
invention is limited only by the claims that follow.