Implantable device with automatic sensing adjustment

An implantable cardiac pacemaker and a method of its operation. The pacemaker delivers pacing pulses to a first chamber of a patient's heart and senses depolarizations of a second chamber of the patient's heart. The amplifier for sensing depolarizations of the second chamber of the patient's heart defines a base sensing threshold and, responsive to delivery of a pacing pulse to the first chamber of the patient's heart for defines an increased sensing threshold greater than the base sensing threshold. The increased sensing threshold persists for a defined period of time following delivery of a pacing pulse to the first chamber of the patient's heart and thereafter gradually decreases from the increased sensing threshold to the base sensing threshold.

BACKGROUND OF THE INVENTION 
The present invention relates to medical stimulators and leads generally, 
and more particularly to implantable pacemakers, cardioverters and 
defibrillators. 
In the context of implantable pacemakers or other stimulators which 
stimulate and sense electrical activity in multiple chambers of the heart, 
it has been conventional to provide a blanking period for the amplifier 
associated with one chamber of the heart, during delivery of a pacing 
pulse to another chamber of the heart. An earlier example of this feature 
may be found in U.S. Pat. No. 4,312,355 issued to Funke. It is also 
conventional to provide a blanking period for the sense amplifier coupled 
to the chamber being paced, during delivery of the pacing pulse. 
Particularly in the context of devices which detect tachyarrhythmias, 
amplifiers have been developed which automatically adjust the effective 
sensing threshold, in order to facilitate sensing of the relatively lower 
amplitude depolarization wave forms that may be associated with 
tachyarrhythmias without sensing the repolarization wave forms associated 
with depolarizations occurring during normal sinus rhythm. The adjusting 
of the effective sensing threshold may be accomplished by adjusting the 
gain of the amplifier and comparing the amplified signal to a fixed 
threshold and/or by adjusting the threshold level of the detector 
associated with the amplifier, which adjustments should be understood to 
be equivalent alternatives in the context of the present invention. One 
such auto adjusting amplifier is disclosed in U.S. Pat. No. 5,117,824 
issued to Keimel et al, incorporated herein by reference in its entirety. 
An alternative implementation of an auto adjust amplifier is disclosed in 
U.S. Pat. No. 5,269,300 issued to Kelly et al., also incorporated herein 
by reference in its entirety. In these references, following a detected 
depolarization, the amplifier is automatically adjusted so that the 
effective sensing threshold is set to be equal to a predetermined portion 
of the amplitude of the sensed depolarization, and the effective sensing 
threshold decays thereafter to a lower or base sensing threshold. 
Following delivery of a pacing pulse, in the system disclosed in the 
Kiemel et al patent, no adjustment is made to the sensing threshold, while 
in the Kelly et al. patent, following delivery of a pacing pulse the 
effective sensing threshold is set to a preset value and remains at this 
value for a defined period of time, after which the threshold decays to 
the lower or base value. 
Simply employing such auto adjusting amplifiers in the context of a device 
which paces and senses in multiple chambers of the heart does provide a 
useful and workable device. However, this approach does not address the 
difficulties which arise when a depolarization in one chamber of the heart 
occurs during a blanking period initiated in response to delivery of a 
pacing pulse to the opposite chamber of the heart. In this circumstance, 
the depolarization signal may go unsensed, in turn interfering with 
detection of an ongoing tachyarrhythmia. 
SUMMARY OF THE INVENTION 
The present invention addresses this problem of sensing in one chamber 
following pacing in another chamber by automatically adjusting the 
effective sensing threshold in the chamber not being paced to a predefined 
amplitude selected to be large enough to prevent sensing of the pacing 
pulse delivered to the chamber being paced, while still allowing sensing 
of depolarizations in the chamber not being paced. The effective sensing 
threshold is set at this defined level for a period of time following the 
pacing pulse delivered to the paced chamber. The blanking period of the 
chamber not being paced is preferably minimized to include substantially 
only the delivered pacing pulse and the fast recharge pulse thereafter. 
In a preferred embodiment of the invention following setting of the sensing 
threshold at the defined level for a predetermined period, the effective 
sensing threshold is allowed to decay to a lower or base value. In a most 
preferred embodiment of the present invention, the automatic adjustment of 
the threshold of the sense amplifier associated with a first chamber of 
the heart following delivery of a pacing pulse to a second chamber of the 
heart is embodied in an amplifier which also adjusts its effective sensing 
threshold of the amplifier after sensing and/or pacing in the first 
chamber. In such embodiments, the decay time of the effective sensing 
threshold following a pacing pulse delivered to the second chamber of the 
heart is preferably less than the decay time of the effective sensing 
threshold following sensing and/or pacing in the first chamber. 
In some preferred embodiments, the adjustment of the sensing threshold in a 
first chamber of the heart in response to delivery of the pacing pulse to 
a second chamber of the heart is only undertaken if the defined lower or 
base sensitivity threshold level associated with the amplifier is less 
than a preset value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a defibrillator and lead set according to the present 
invention. The ventricular lead includes an elongated insulative lead body 
16, carrying three concentric coiled conductors, separated from one 
another by tubular insulative sheaths. Located adjacent the distal end of 
the lead are a ring electrode 24, an extendable helix electrode 26, 
mounted retractably within an insulative electrode head 28, and an 
elongated coil electrode 20. Each of the electrodes is coupled to one of 
the coiled conductors within the lead body 16. Electrodes 24 and 26 are 
employed for cardiac pacing and for sensing ventricular depolarizations. 
At the proximal end of the lead is a bifurcated connector 14 which carries 
three electrical connectors, each coupled to one of the coiled conductors. 
The defibrillation electrode 20 may be fabricated from platinum, platinum 
alloy or other materials known to be usable in implantable defibrillation 
electrodes and may be about 5 cm in length. 
The atrial/SVC lead includes an elongated insulative lead body 15, carrying 
three concentric coiled conductors, separated from one another by tubular 
insulative sheaths, corresponding to the structure of the ventricular 
lead. Located adjacent the J-shaped distal end of the lead are a ring 
electrode 21 and an extendable helix electrode 17, mounted retractably 
within an insulative electrode head 19. Each of the electrodes is coupled 
to one of the coiled conductors within the lead body 15. Electrodes 17 and 
21 are employed for atrial pacing and for sensing atrial depolarizations. 
An elongated coil electrode 23 is provided, proximal to electrode 21 and 
coupled to the third conductor within the lead body 15. Electrode 23 
preferably is 10 cm in length or greater and is configured to extend from 
the SVC toward the tricuspid valve. In one preferred embodiment tested by 
the inventors, approximately 5 cm of the right atrium/SVC electrode was 
located in the right atrium, with the remaining 5 cm located in the SVC. 
At the proximal end of the lead is a bifurcated connector 13 which carries 
three electrical connectors, each coupled to one of the coiled conductors. 
The coronary sinus lead includes an elongated insulative lead body 6, 
carrying one coiled conductor, coupled to an elongated coiled 
defibrillation electrode 8. Electrode 8, illustrated in broken outline, is 
located within the coronary sinus and great vein of the heart. At the 
proximal end of the lead is a connector plug 4 which carries an electrical 
connector, coupled to the coiled conductor. The coronary sinus/great vein 
electrode 8 may be about 5 cm in length. 
An implantable pacemaker/cardioverter/defibrillitor 10 is shown in 
combination with the leads, with the lead connector assemblies 4, 13 and 
14 inserted into the connector block 12. Optionally, insulation of the 
outward facing portion of the housing 11 of the 
pacemaker/cardioverter/defibrillator 10 may be provided using a plastic 
coating, for example parylene or silicone rubber, as is currently employed 
in some unipolar cardiac pacemakers. However, the outward facing portion 
may instead be left uninsulated, or some other division between insulated 
and uninsulated portions may be employed. The uninsulated portion of the 
housing 11 optionally serves as a subcutaneous defibrillation electrode, 
used to defibrillate either the atria or ventricles. 
FIG. 2 is a functional schematic diagram of an implantable 
pacemaker/cardioverter/defibrillator in which the present invention may 
usefully be practiced. This diagram should be taken as exemplary of one 
type of device in which the invention may be embodied, and not as 
limiting, as it is believed that the invention may usefully be practiced 
in a wide variety of device implementations, including cardiac pacemakers 
which do not provide high voltage cardioversion and defibrillation 
therapies. 
The device as illustrated is provided with an electrode system including 
electrodes as illustrated in FIG. 1. The correspondence to the illustrated 
electrodes is as follows. Optional electrode 310 corresponds to electrode 
11, and is the uninsulated portion of the housing of the implantable 
pacemaker/cardioverter/defibrillator. Electrode 320 corresponds to 
electrode 20 and is a defibrillation electrode located in the right 
ventricle. Electrode 311 corresponds to electrode 23, and is located in 
the right atrium and SVC. Electrode 318 corresponds to electrode 8 and is 
a defibrillation electrode located in the coronary sinus and great vein. 
Electrodes 324 and 326 correspond to electrodes 24 and 26, and are used 
for sensing and pacing in the ventricle. Electrodes 317 and 321 correspond 
to electrodes 17 and 19 and are used for pacing and sensing in the atrium. 
Electrodes 310, 311, 318 and 320 are coupled to high voltage output circuit 
234. High voltage output circuit 234 includes high voltage switches 
controlled by CV/defib control logic 230 via control bus 238. The switches 
within circuit 234 control which electrodes are employed and which are 
coupled to the positive and negative terminals of the capacitor bank 
including capacitors 246 and 248 during delivery of the defibrillation 
pulses. 
Electrodes 324 and 326 are located on or in the ventricle and are coupled 
to the R-wave amplifier 200, which preferably takes the form of an 
automatically adjusted amplifier according to the present invention, 
providing an adjustable sensing threshold as a function of the measured 
R-wave amplitude and providing an increased sensing threshold following 
pacing pulses delivered to the atrium. Operation of Amplifier 200 is 
controlled by pacing circuitry 212 via control lines 201. A signal is 
generated on R-out line 202 whenever the signal sensed between electrodes 
324 and 326 exceeds the present sensing threshold. 
Electrodes 317 and 321 are located on or in the atrium and are coupled to 
the P-wave amplifier 204, which preferably also takes the form of an 
automatically adjusted amplifier according to the present invention, 
providing an adjustable sensing threshold as a function of the measured 
P-wave amplitude and providing an increased sensing threshold following 
pacing pulses delivered to the ventricle. Operation of Amplifier 204 is 
controlled by pacing circuitry 212 via control lines 205. A signal is 
generated on P-out line 206 whenever the signal sensed between electrodes 
317 and 321 exceeds the present sensing threshold. The operation of 
amplifiers 204 and 206 is discussed in more detail below in conjunction 
with FIGS. 3, 4 and 5. 
Switch matrix 208 is used to select which of the available electrodes are 
coupled to wide band (2.5-100 Hz) amplifier 210 for use in digital signal 
analysis. Selection of electrodes is controlled by the microprocessor 224 
via data/address bus 218, which selections may be varied as desired. 
Signals from the electrodes selected for coupling to bandpass amplifier 
210 are provided to multiplexer 220, and thereafter converted to multi-bit 
digital signals by A/D converter 222, for storage in random access memory 
226 under control of direct memory access circuit 228. Microprocessor 224 
may employ digital signal analysis techniques to characterize the 
digitized signals stored in random access memory 226 to recognize and 
classify the patient's heart rhythm employing any of the numerous signal 
processing methodologies known to the art. 
The remainder of the circuitry is dedicated to the provision of cardiac 
pacing, cardioversion and defibrillation therapies, and, for purposes of 
the present invention may correspond to circuitry known in the prior art. 
An exemplary apparatus is disclosed of accomplishing pacing, cardioversion 
and defibrillation functions follows. The pacer timing/control circuitry 
212 includes programmable digital counters which control the basic time 
intervals associated with DDD, VVI, DVI, VDD, AAI, DDI and other modes of 
single and dual chamber pacing well known to the art. Circuitry 212 also 
controls escape intervals associated with anti-tachyarrhythmia pacing in 
both the atrium and the ventricle, employing any anti-tachyarrhythmia 
pacing therapies known to the art. 
Intervals defined by pacing circuitry 212 include atrial and ventricular 
pacing escape intervals, the refractory periods during which sensed 
P-waves and R-waves are ineffective to restart timing of the escape 
intervals and the pulse widths of the pacing pulses and all interval 
associated with the automatic adjustments of effective sensing thresholds 
discussed in more detail below. The durations of these intervals are 
determined by microprocessor 224, in response to stored data in memory 226 
and are communicated to the pacing circuitry 212 via address/data bus 218. 
Pacer circuitry 212 also determines the amplitude of the cardiac pacing 
pulses under control of microprocessor 224. 
During pacing, the escape interval counters within pacer timing/control 
circuitry 212 are reset upon sensing of R-waves and P-waves as indicated 
by a signals on lines 202 and 206, and in accordance with the selected 
mode of pacing on time-out trigger generation of pacing pulses by pacer 
output circuitry 214 and 216, which are coupled to electrodes 317, 321, 
324 and 326. The escape interval counters are also reset on generation of 
pacing pulses, and thereby control the basic timing of cardiac pacing 
functions, including anti-tachyarrhythmia pacing. The durations of the 
intervals defined by the escape interval timers are determined by 
microprocessor 224, via data/address bus 218. The value of the count 
present in the escape interval counters when reset by sensed R-waves and 
P-waves may be used to measure the durations of R-R intervals, P-P 
intervals, P-R intervals and R-P intervals, which measurements are stored 
in memory 226 and used to detect the presence of tachyarrhythmias. 
Microprocessor 224 operates as an interrupt driven device, under control of 
a stored program in its read only memory and is responsive to interrupts 
from pacer timing/control circuitry 212 corresponding to the occurrence 
sensed P-waves and R-waves and corresponding to the generation of cardiac 
pacing pulses. These interrupts are provided via data/address bus 218. Any 
necessary mathematical calculations to be performed by microprocessor 224 
and any updating of the values or intervals controlled by pacer 
timing/control circuitry 212 take place following such interrupts. 
For example, in response to a sensed or paced ventricular depolarization or 
R-wave, the intervals separating that R-wave from the immediately 
preceding R-wave, paced or sensed (R-R interval) and the interval 
separating the paced or sensed R-wave from the preceding atrial 
depolarization, paced or sensed (P-R interval) may be stored. Similarly, 
in response to the occurrence of a sensed or paced atrial depolarization 
(P-wave), the intervals separating the sensed P-wave from the immediately 
preceding paced or sensed atrial contraction (P-P interval) and the 
interval separating the sensed P-wave from the immediately preceding 
sensed or paced ventricular depolarization (R-P interval) may be stored. 
Preferably, a portion of the memory 226 (FIG. 4) is configured as a 
plurality of recirculating buffers, capable of holding a preceding series 
of measured intervals, which may be analyzed in response to the occurrence 
of a pace or sense interrupt to determine whether the patient's heart is 
presently exhibiting atrial or ventricular tachyarrhythmia. 
Detection of atrial or ventricular tachyarrhythmias, as employed in the 
present invention, may correspond to tachyarrhythmia detection algorithms 
known to the art. For example, presence of atrial or ventricular 
tachyarrhythmia may be confirmed by means of detection of a sustained 
series of short R-R or P-P intervals of an average rate indicative of 
tachyarrhythmia or an unbroken series of short R-R or P-P intervals. The 
suddenness of onset of the detected high rates, the stability of the high 
rates, or a number of other factors known to the art may also be measured 
at this time. Appropriate ventricular tachyarrhythmia detection 
methodologies measuring such factors are described in U.S. Pat. No. 
4,726,380, issued to Vollmann, U.S. Pat. No. 4,880,005, issued to Pless et 
al. and U.S. Pat. No. 4,830,006, issued to Haluska et al., all 
incorporated herein by reference in their entireties. An additional set of 
tachycardia recognition methodologies is disclosed in the article "Onset 
and Stability for Ventricular Tachyarrhythmia Detection in an Implantable 
Pacer-Cardioverter-Defibrillator" by Olson et al., published in Computers 
in Cardiology, Oct. 7-10, 1986, IEEE Computer Society Press, pages 
167-170, also incorporated herein in its entirety. However, one of the 
advantages of the present invention is that it is believed practicable in 
conjunction with most prior art tachycardia detection algorithms. Atrial 
fibrillation detection methodologies in particular are disclosed in 
Published PCT Application Ser. No. US92/02829, Publication No. WO92/18198, 
by Adams et al., and in the article "Automatic Tachycardia Recognition", 
by Arzbaecher et al., published in E, May-June, 1984, pp. 541-547, both 
of which are incorporated by reference in their entireties. 
Because the accurate detection of arrhythmias using measured intervals 
between R-waves and P-waves is dependent on accurate sensing of the 
occurrences of these depolarization signals, the automatic effective 
sensing threshold adjustment provided by the present invention is 
particularly valuable in the context of ant-tachyarrhythmia devices. 
However, the improved sensing accuracy is also valuable in the context of 
anti-bradycardia pacemakers as well, particularly in the context of 
mode-switching features intended to prevent such pacemakers from pacing 
the heat at inappropriately high rates. 
In the event that an atrial or ventricular tachyarrhythmia is detected, and 
an anti-tachyarrhythmia pacing regimen is desired, appropriate timing 
intervals for controlling generation of anti-tachyarrhythmia pacing 
therapies are loaded from microprocessor 224 into the pacer timing and 
control circuitry 212, to control the operation of the escape interval 
counters therein and to define refractory periods during which detection 
of R-waves and P-waves is ineffective to restart the escape interval 
counters. 
Alternatively, circuitry for controlling the timing and generation of 
anti-tachycardia pacing pulses as described in U.S. Pat. No. 4,577,633, 
issued to Berkovits et al. on Mar. 25, 1986, U.S. Pat. No. 4,880,005, 
issued to Pless et al. on Nov. 14, 1989, U.S. Pat. No. 4,726,380, issued 
to Vollmann et al. on Feb. 23, 1988 and U.S. Pat. No. 4,587,970, issued to 
Holley et al. on May 13, 1986, all of which are incorporated herein by 
reference in their entireties may also be used. 
In the event that generation of a cardioversion or defibrillation pulse is 
required, microprocessor 224 employs an escape interval counter to control 
timing of such cardioversion and defibrillation pulses, as well as 
associated refractory periods. In response to the detection of atrial or 
ventricular fibrillation or tachyarrhythmia requiring a cardioversion 
pulse, microprocessor 224 activates cardioversion/defibrillation control 
circuitry 230, which initiates charging of the high voltage capacitors 246 
and 248 via charging circuit 236, under control of high voltage charging 
control lines 240 and 242. The voltage on the high voltage capacitors is 
monitored via VCAP line 244, which is passed through multiplexer 220 and 
in response to reaching a predetermined value set by microprocessor 224, 
results in generation of a logic signal on Cap Full (CF) line 254, 
terminating charging. Thereafter, timing of the delivery of the 
defibrillation or cardioversion pulse is controlled by pacer 
tiring/control circuitry 212. Following delivery of the fibrillation or 
tachycardia therapy the microprocessor then returns the device to cardiac 
pacing and awaits the next successive interrupt due to pacing or the 
occurrence of a sensed atrial or ventricular depolarization. 
One embodiment of an appropriate system for delivery and synchronization of 
ventricular cardioversion and defibrillation pulses and for controlling 
the timing functions related to them is disclosed in more detail in 
commonly assigned U.S. Pat. No. 5,188,105 by Keimel, issued Feb. 23, 1993, 
incorporated herein by reference in its entirety. Embodiments of 
appropriate systems for delivery and synchronization of atrial 
cardioversion and defibrillation pulses and for controlling the timing 
functions related to them are disclosed in more detail in U.S. Pat. No. 
5,269,298 by Adams et al., issued Dec. 14, 1993 and in U.S. Pat. No. 
4,316,472 by Mirowski et al., issued Feb. 23, 1982, both incorporated 
herein by reference in their entireties. 
However, any known cardioversion or defibrillation pulse control circuitry 
is believed usable in conjunction with the present invention. For example, 
circuitry controlling the timing and generation of cardioversion and 
defibrillation pulses as disclosed in U.S. Pat. No. 4,384,585, issued to 
Zipes on May 24, 1983, in U.S. Pat. No. 4,949,719 issued to Pless et al., 
cited above, and in U.S. Pat. No. 4,375,817, issued to Engle et al., all 
incorporated herein by reference in their entireties may also be employed. 
In the illustrated device, delivery of the cardioversion or defibrillation 
pulses is accomplished by output circuit 234, under control of control 
circuitry 230 via control bus 238. Output circuit 234 determines whether a 
monophasic or biphasic pulse is delivered, the polarity of the electrodes 
and which electrodes are involved in delivery of the pulse. Output circuit 
234 also includes high voltage switches which control whether electrodes 
are coupled together during delivery of the pulse. Alternatively, 
electrodes intended to be coupled together during the pulse may simply be 
permanently coupled to one another, either exterior to or interior of the 
device housing, and polarity may similarly be pre-set, as in current 
implantable defibrillators. An example of output circuitry for delivery of 
biphasic pulse regimens to multiple electrode systems may be found in the 
above cited patent issued to Mehra and in U.S. Pat. No. 4,727,877, 
incorporated by reference in its entirety. 
An example of circuitry which may be used to control delivery of monophasic 
pulses is set forth in commonly assigned U.S. Pat. No. 5,163,427, by 
Keimel, issued Nov. 17, 1992, also incorporated herein by reference in its 
entirety. However, output control circuitry as disclosed in U.S. Pat. No. 
4,953,551, issued to Mehra et al. on Sep. 4, 1990 or U.S. Pat. No. 
4,800,883, issued to Winstrom on Jan. 31, 1989 both incorporated herein by 
reference in their entireties, may also be used in conjunction with a 
device embodying the present invention for delivery of biphasic pulses. 
In the event that, as in FIG. 1, both atrial and ventricular defibrillation 
are available, ventricular defibrillation may be accomplished using higher 
pulse energy levels than required for atrial defibrillation and may employ 
the same or a different electrode set. For example, electrodes 310, 311, 
318 and 320 or only electrodes 311, 318 and 320 may be employed for atrial 
defibrillation. Electrodes 311, 320 and 310 might be employed for 
ventricular defibrillation, with electrode 311 (right atrium/SVC) coupled 
to electrode 310 (device housing). Alternatively, electrodes 310, 318 and 
320 may be employed, with electrode 318 (coronary sinus/great vein) 
coupled to electrode 310. As a further alternative, electrodes 311, 310, 
318 and 323 might all be employed for ventricular defibrillation, with 
electrodes 310, 311 and 323 coupled in common. As yet another alternative, 
only electrodes 310 and 320 might be employed for ventricular 
defibrillation. added or substituted for either of electrodes 311 or 318 
for treating ventricular fibrillation. 
One particularly desirable embodiment of the invention employs only the 
right atrial/SVC electrode 311, the coronary sinus/great vein electrode 
318 and the right ventricular electrode 320. During atrial defibrillation, 
electrodes 320 and 318 are coupled in common with one another, and the 
atrial defibrillation pulse is delivered between these electrodes and 
electrode 311. During ventricular defibrillation, electrodes 311 and 318 
are coupled in common with one another, and the ventricular defibrillation 
pulse is delivered between these electrodes and electrode 320. This 
particular set of electrodes thus provides optimized defibrillation pulse 
regimens for both atrial and ventricular defibrillation, by simply 
switching the connection of the coronary sinus/great vein electrode. 
In modern implantable cardioverter/defibrillators, the particular therapies 
are programmed into the device ahead of time by the physician, and a menu 
of therapies is typically provided. For example, on initial detection of 
an atrial or ventricular tachycardia, an anti-tachycardia pacing therapy 
may be selected and delivered to the chamber in which the tachycardia is 
diagnosed or to both chambers. On redetection of tachycardia, a more 
aggressive anti-tachycardia pacing therapy may be scheduled. If repeated 
attempts at anti-tachycardia pacing therapies fail, a higher level 
cardioversion pulse may be selected thereafter. Therapies for tachycardia 
termination may also vary with the rate of the detected tachycardia, with 
the therapies increasing in aggressiveness as the rate of the detected 
tachycardia increases. For example, fewer attempts at anti-tachycardia 
pacing may be undertaken prior to delivery of cardioversion pulses if the 
rate of the detected tachycardia is above a preset threshold. The 
references cited above in conjunction with descriptions of prior art 
tachycardia detection and treatment therapies are applicable here as well. 
In the event that atrial or ventricular fibrillation is identified, the 
typical therapy will be delivery of a high amplitude defibrillation pulse, 
typically in excess of 10 joules in the case of ventricular fibrillation 
and about 1 joule or less in the case of atrial defibrillation. Lower 
energy levels will be employed for cardioversion. As in the case of 
currently available implantable pacemakers/cardioverter/defibrillators, 
and as discussed in the above-cited references, it is envisioned that the 
amplitude of the defibrillation pulse may be incremented in response to 
failure of an initial pulse or pulses to terminate fibrillation. Prior art 
patents illustrating such pre-set therapy menus of anti-tachyarrhythmia 
therapies include the above-cited U.S. Pat. No. 4,830,006, issued to 
Haluska, et al., U.S. Pat. No. 4,726,380, issued to Vollmann et al. and 
U.S. Pat. No. 4,587,970, issued to Holley et al. 
FIG. 3 is a functional block diagram of amplifier 200 illustrated in FIG. 
2. This diagram illustrates the basic functional components of the 
amplifier and their interconnection to the pacer timing and control 
circuitry 212. Signals from the ventricular electrodes 324 and 326 first 
pass through blanking switches 350, which operate to disconnect the 
amplifier from the electrodes during delivery of an atrial pacing pulse, 
during the duration of a ventricular input blanking signal on line VINB, 
which extends through the delivered atrial pacing pulse and during the 
fast recharge period thereafter. Depolarization signals passing through 
blanking switches 350 are amplified by preamp 352 and then pass through a 
first high pass filter 354. The high pass filtered signal is passed 
through an adjustable gain amplifier 356 which amplifies the signal by one 
of eight available multiplication factors under the control of digital 
signals on lines SEN:0, SEN:1 and SEN:2. The degree of amplification 
determines the effective lower or base sensing threshold, as discussed 
below. 
The amplified signal is passed on through a first low pass filter 358, a 
second low pass filter 360, a second high pass filter 362 and an absolute 
value circuit 364 which produces at its output the absolute value of the 
previously filtered and amplified signal. In response to a blanking signal 
on line VHP2BLK, passage of signals through high pass filter 362 is 
prohibited for defined periods of time following delivery of atrial and 
ventricular pacing pulses, providing an additional blanking function. The 
duration of blanking in conjunction with a delivered atrial pacing pulse 
is preferably the same as blanking interval defined by the blanking signal 
on line VINB following delivery of an atrial pacing pulse. In conjunction 
with the delivery of a ventricular pacing pulse, the blanking period is 
substantially greater, for example, sixty or more milliseconds. 
The output of the absolute value circuit 364 is provided to the detector 
circuit 366 which compares it to a defined sensing threshold to determine 
whether an R wave is to be detected or not. If the signal exceeds the 
threshold, detector circuit 366 provides an output on R-OUT line 202, 
which is provided to pacer timing and control circuit 212 (FIG. 2). The 
sensing threshold defined by detector circuit 366 is variable, and is 
adjusted in response to sensed ventricular events, delivered ventricular 
pacing pulses and is adjusted atrial pacing pulses. The detector 366 
defines a basic or lower sensing threshold which is normally in effect, 
and a variable sensing threshold effective after sensed ventricular events 
and delivered atrial and ventricular pacing pulses. In order to be 
detected as an R wave, the signal from absolute value circuit 364 must 
exceed the greater of the lower or base sensing threshold and the variable 
threshold, as discussed in more detail below. Control of the effective 
sensing threshold following delivered atrial pacing pulses is controlled 
by signals on lines VBDET, VTCBLK and VREF. Adjustment of the effective 
sensing threshold following a delivered pacing pulse is accomplished by 
means of a reference voltage applied to the input of the detector 366 by 
the line VPRFR. Adjustment of the sensing threshold following a sensed R 
wave is a function of the amplitude of the sensed R wave as reflected by 
the output of absolute value circuit 364. 
FIG. 4 illustrates detector 366 in more detail, and illustrates the manner 
in which the detector defines the various effective sensing threholds 
employed by the device. The amplified filtered R-wave signal from absolute 
value circuit 364 (FIG. 3) is applied to a comparator 414 which, in 
conjunction with the remainder of the illustrated circuitry defmes an 
adjustable sensing threshold. If the signal from the absolute value 
circuit exceeds the currently effective sensing threshold, a signal is 
generated on R-OUT line 202, which in turn is provided to the timing and 
control circuitry 212. The lower or base sensing threshold "S" is defined 
by the DC offset of the comparator circuit 414 in conjunction with the 
programmed amplification of the signal by amplifier 356. This sensing 
threshold is increased from the lower or base threshold following sensed R 
waves, delivered ventricular pacing pulses and delivered atrial pulses as 
follows. 
A signal from absolute value circuit 364 corresponding to an R-wave passes 
through amplifier 400 which is configured to operate as an non-inverting 
peak voltage follower, and is applied via switch 1 (402) which is normally 
closed, to capacitor C2. This voltage reflects the amplitude of the sensed 
R-wave and is applied to comparator 414 via amplifier 410 and inverter 
412, in conjunction with associated resistors R4 and R3 in such a fashion 
that the voltage stored on capacitor C2 is effectively increases the 
sensing threshold. The voltage on capacitor C2 decays over a relatively 
long time constant T1, determined by the values of R2 and C2 to provide a 
variable sensing threshold "V.sub.s (t)". T1 may, for example, be an RC 
time constant of 450 milliseconds, providing for a gradual decay of the 
sensing threshold following a sensed ventricular depolarization in 
precisely the fashion described in conjunction with U.S. Pat. No. 
5,117,824 issued to Keimel, incorporated by reference above. The variable 
sensing threshold V.sub.s (t) defines the effective sensing threshold 
until the variable sensing threshold V.sub.s (t) falls below the base or 
lower sensing threshold. The lower or base sensing threshold S is 
effective thereafter. 
Following a delivered ventricular pacing pulse, on expiration of a blanking 
period defined by a blanking signal on line VHP2BLK (FIG. 3), a reference 
voltage is placed on line VPRFR (FIG. 3) and input into amplifier 400. In 
the same fashion as a sensed R-wave, this voltage signal also serves to 
recharge capacitor C2, providing an increased voltage threshold which 
decays from the value of the signal on line VPRFR, to define a variable 
sensing threshold V.sub.s (t) in the fashion described above following an 
R wave. The value of the reference signal on line VPRFR may be fixed or 
may vary as a function of the programmed base sensing threshold S. 
Following a delivered atrial pacing pulse, the signal on line VBDET (FIG. 
3) operates to open switch 402 and switch 408 which are normally closed. 
Opening switch 402 prevents adjustment of the variable sensing threshold 
V.sub.s (t) defined by capacitor C2 and resistor R2. This variable sensing 
threshold continues to decay, and is applied to the summing node 403 of 
amplifier 410 and then via inverter 412 to comparator 414. Concurrent with 
the opening of switches 402 and 408, switch 406 is closed in response to a 
signal on line VTCBLK, which passes a reference voltage on line VREF 
through to charge capacitor C1. The voltage on capacitor C1 is also 
applied to the summing node 403 of amplifier 410 and then via inverter 412 
to comparator 414. The sum of the voltages on capacitors C1 an C2 thus 
serves to define the effective sensing threshold while the signal on line 
VREF is applied to capacitor C1. The signal on line VREF is applied to 
capacitor C1 for a preset interval, which may be, for example, 30 
milliseconds, so that an increased sensing threshold is defined for this 
interval. The amplitude of the reference signal on line VREF may be fixed 
or may vary as a function of the value of the lower sensing threshold S. 
When the signal on line VTCBLK terminates, the voltage on capacitor C1 is 
discharged via R1 to provide an exponentially decreasing sensing threshold 
V.sub.ap (t), in the same fashion as provided by resistor R2 and capacitor 
C2. However, the values of capacitors C1 and R1 are preferably chosen to 
define a much shorter time constant T2, which may be for example, 50 
milliseconds. Allowing the sensing threshold to decay in this fashion 
assists in preventing incorrect detection of R waves as a result of an 
abrupt decrease in sensing threshold. After opening of switch 406, the 
signal on line VBDET terminates, closing switches 402 and 408. This in 
turn completes the discharge of capacitor C1, so that the effective 
sensing threshold is now once again the greater of the variable sensing 
threshold V.sub.s (t) and the base sensing threshold S. 
The operation of the amplifier according to the present invention produces 
several benefits. By providing for an increased ventricular sensing 
threshold following a delivered atrial pacing pulse as opposed to simply 
blanking the ventricular amplifier, inappropriate sensing of the pacing 
pulse itself and of any post pacing polarization of the ventricular 
electrodes is prevented, while sensing of R-waves closely spaced to the 
delivered atrial pacing pulse is facilitated. By providing for an 
exponential decay of the increased threshold value following an atrial 
pacing pulse, inappropriate ventricular oversensing associated with an 
abrupt change in sensing threshold is avoided. These basic benefits of the 
present invention are disclosed in the context of an amplifier which also 
adjusts its sensing threshold following sensed R-waves and delivered 
ventricular pacing pulses. However, it is believed that the beneficial 
aspects discussed above may also be usefully employed in amplifiers which 
do not automatically adjust ventricular sensing thresholds following 
sensed R-waves or after delivered ventricular pacing pulses. 
The description of the operation of ventricular amplifier 200 applies as 
well to the atrial amplifier 204, with the exception that the amplifier is 
coupled to electrodes 317 and 321 (FIG. 2) and that signals corresponding 
to those on lines VINB, VHP2BLK, VBDET, VTCBLK and VREF are instead 
generated at corresponding times following delivery of a ventricular 
pacing pulse, to provide an automatically adjusted effective sensing 
threshold. One additional difference between the implementation of the 
device in the context of the atrial sense amplifier 204 and the 
ventricular sense amplifier 200 may be that following a delivered atrial 
pacing pulse, the effective sensitivity is not adjusted, accomplished by 
simply omitting a signal corresponding to that on line VPRFR, described in 
conjunction with FIG. 3. In such case, the operation of the amplifier 
following a sensed atrial event does not correspond to that described in 
the above-cited Keimel et al patent. 
FIG. 5 is a timing diagram illustrating the operation of the detector 366 
(FIG.3) to define the various variable sensing thresholds described above. 
The upper portion of FIG. 5 is a diagram illustrating the variable sensing 
thresholds V.sub.3 (t) and V.sub.ap (t), discussed above. The variable 
sensing threshold V.sub.s (t) is, shown decaying below the base or lower 
sensing threshold S at 500. At 502, a ventricular pacing pulse is 
generated, which initiates a blanking signal on line VHP2BLK at 504. On 
expiration of the blanking signal on line VHP2BLK at 506, a reference 
voltage signal on line VPREFR is applied at 508 to the input of the 
detector 366 (FIG. 3) to increase the effective ventricular sensing 
threshold to a multiple A*S of the base sensing threshold. As discussed 
above, the value of A*S may be fixed or may vary as a function of the 
value of the base or lower sensing threshold S. The variable sensing 
threshold V.sub.s (t) decays thereafter according to time constant T1 as 
discussed above. 
At 510, an atrial pacing pulse is delivered. Concurrent with delivery of 
atrial pacing pulse 510, a blanking signal on line VINB is initiated at 
512, which disconnects the input of the amplifier from the ventricular 
electrodes via blanking circuit 350 (FIG. 3). Concurrent with initiation 
of this blanking signal, a threshold increase signal is generated on line 
VTCBLK at 514 which may persist, for example, for 30 milliseconds and 
which applies the voltage on line VREF to capacitor C1 (FIG. 4) to define 
an increased effective sensing threshold V.sub.ap (t) at 520. A signal on 
line VBDET is initiated at 516 which prevents modification of the variable 
sensing threshold V.sub.s (t) as discussed above. Also illustrated, a 
signal on line VHP2BLK provides for a blanking period corresponding to he 
duration of the signal on line VINB. During the time interval between the 
expiration of the input blanking signal on line VINB at 522 and the 
expiration of the threshold increase signal on line VTCBLK at 524, the 
amplifier operates to sense signals at an increased effective sensing 
threshold B*S, defined by the amplitude of the line of VREF (FIG. 4) added 
to the threshold defined by the voltage on capacitor C2. The value of this 
increase in sensing threshold, as discussed above, may be fixed, or may be 
varied as a function of the base or lower sensing threshold S. The value 
of B*S may be less than the value of increased threshold A*S. Following 
expiration of the increase threshold signal on line VTCBLK at 524, the 
effective sensing threshold V.sub.ap (t) (decays according to time 
constant T2 until expiration of the signal on line VBDET at 526, as 
discussed above. Consideration of the signal on line VBDET may be, for 
example, 60 milliseconds, to allow for substantial decay of the variable 
sensing threshold V.sub.ap (t), prior to discharge of capacitor C1 (FIG. 
4). 
While the above invention is described in the context of a dual chamber 
arrhythmia treatment device, in which it is believed to be particularly 
valuable, the present invention may also be applicable in other devices 
which sense in one chamber and pace in another chamber of the heart, 
including cardiac pacemakers which operate in pacing modes such as DDI 
mode, VVD mode, DDD mode, and the like. Alternatively, the invention may 
be usefully practiced in the context of pacemakers which pace both 
ventricles and/or both atria, for example as disclosed in U.S. Pat. No. 
4,928,688, issued to Mower et al, U.S. Pat. No. 5,540,727, issued to 
Tockman et al., U.S. Pat. No. 5,403,356, issued to Hill et al. or U.S. 
Pat. No. 5,720,768 issued to Verboven-Nelisson, all of which are 
incorporated herein by reference in their entireties. In addition, while 
the invention is described above in the context of a device which employs 
sense amplifiers which provide for an automatically adjusted sensing 
threshold following sensed events and following pacing pulses delivered to 
the chamber to which the amplifier is coupled, the present invention may 
also be usefully employed in the context of devices which do not adjust 
the effective sensing thresholds following either or both of such events. 
Further, while the disclosed embodiment of the invention takes the form of 
a microprocessor controlled device, the invention is of course equally 
useful in the context of a device in which the various time intervals 
employed to control the sensing thresholds are determined by hardware, for 
example by a digital circuit employing dedicated logic, or by analog 
timers. The specific mechanism by which the time intervals associated with 
the operation of the adjustable threshold function are defined is not 
critical to successful use and enjoyment of the present invention. As 
such, the above disclosure should be taken as exemplary, rather than 
limiting, with regard to the claims which follow.