Apparatus and method for detecting and treating cardiac tachyarrhythmias

An arrhythmia control system and method having automatic determination of the tachycardia confirmation interval. The interval is dynamically set at the minimum of an absolute detection interval and a value calculated as a function of either average sinus interval or tachycardia cycle length or both. The continued presence or the reversion of a pathologic tachycardia can be reliably confirmed, particularly when the patient experiences an elevated heart rate due to, for example, exercise, prior to onset of the tachycardia.

BACKGROUND OF THE INVENTION 
This invention relates to implantable medical devices which monitor the 
cardiac state of a patient by sensing sinus rhythm, ventricular 
tachycardia and ventricular fibrillation and which deliver therapy in the 
form of electrical energy to cardiac tissue in an attempt to revert 
tachycardia and restore a normal sinus rhythm. 
As used herein, antitachycardia pacing will mean any pacing for the 
reversion of tachycardia. The term tachycardia refers to any fast, 
abnormal rhythm of the heart which may be amendable to treatment by 
electrical discharges and specifically includes ventricular tachycardia 
(VT), supraventricular tachycardia (SVT), ventricular flutter and/or 
ventricular fibrillation (VF). 
The term therapy as used herein includes the processes used between the 
detection and reversion of a tachyarrhythmia and includes the actions of 
antitachycardia pacing, cardioversion and/or defibrillation shocks. The 
term cardioversion refers to the discharge of electrical energy into the 
cardiac tissue in an attempt to terminate or revert a tachyarrhythmia. 
This may take the form of a high energy discharge (up to 40 Joules or 
more) or a low energy discharge (less than 1 Joule). Cardioversion shocks 
may or may not be synchronized to the rhythm of the heart. Defibrillation 
is a particular example of cardioversion. 
This invention applies equally to devices which deliver energy synchronized 
to an R-wave and to those that do not. It also applies to devices which 
use lower energy pulses (up to 1 Joule) as well as to devices which use 
higher energy pulses (up to 40 Joules or more). The invention applies to 
devices which deliver cardioverting shocks alone as well as to devices 
which deliver antitachycardia pacing pulses alone or in combination with 
cardioverting shocks. The invention will usually apply to ventricular 
implantable cardioverters, but is equally applicable to atrial 
cardioverters or multiple chamber cardioverters or defibrillators. The 
invention applies also to the delivery of any antitachycardia pacing pulse 
and post reversion pacing therapy. 
BACKGROUND ART 
U.S. Pat. No. 4,280,502 to Baker, Jr. et al. describes a method and 
apparatus for detecting and arresting a condition of tachycardia. In this 
device, the successive occurrence of a predetermined number of cardiac 
intervals which are below a preselected tachycardia threshold enables the 
system to initiate an output to arrest the tachycardia. 
Another antitachycardia pacer is disclosed in U.S. Pat. No. 4,561,442 to 
Vollmann et al. In this device, tachycardia detection is based on the 
number of times the rate threshold is exceeded before reverting to a 
normal cardiac cycle. An arrhythmia counter is incremented by one whenever 
it detects an intrinsic pulse-to-pulse interval shorter than the 
programmed tachycardia threshold rate and it is decremented by one 
whenever it detects an interval greater than the threshold interval. 
Because these devices use only a preselected tachycardia threshold as 
their means of detection, neither of them can differentiate between sinus 
tachycardias with pulse intervals shorter than the programmed threshold 
rate and pathologic tachycardias. 
A device which can recognize sinus tachycardia is described in the article 
"Clinical Evaluation of Automatic Tachycardia Diagnosis by an Implanted 
Device" by Warren et al. in E, Vol. 9, pp. 1079-1083, 1986. A sudden 
onset criterion is used in combination with a programmed tachycardia 
threshold to detect a tachycardia. Hence, for a tachycardia to be 
detected, the first interval to fall below the tachycardia threshold must 
be shorter than either of the two preceding cardiac intervals by a preset 
amount. The programmed tachycardia threshold is also used to confirm the 
continued presence or otherwise of the tachycardia. However, this device, 
while capable of differentiating between sinus tachycardia and pathologic 
tachycardia, is not able to detect a pathologic tachycardia which occurs 
after the heart rate is already elevated as, for example, in the situation 
of the patient gradually elevating the heart rate while exercising. 
Another device which can discriminate between sinus tachycardia and 
pathologic tachycardia is described in the article "Clinical Results with 
the Tachylog Antitachycardia Pacemaker" by Sowton in E, Vol. 7, pp. 
1313-1317 (Nov.-Dec. 1984). In this device, the tachycardia sudden onset 
detection algorithm becomes activated at some programmable cardiac 
interval threshold. This cardiac interval threshold is also used to 
confirm the continued presence or otherwise of the tachycardia. A problem 
with this device is that while being able to detect a pathologic 
tachycardia which occurs after the cardiac interval is already less than 
the cardiac interval threshold, as in an exercise situation, it would have 
difficulty confirming a successfully reversion of the tachycardia. This is 
because prior to onset of the pathologic tachycardia, the cardiac interval 
is shorter than the confirmation interval due to a gradual increase in 
heart rate in, for example, an exercise situation. It follows that after a 
successful reversion, the cardiac interval will return to a value near to 
that prior to onset, that is, to a point which is shorter than the 
confirmation interval. In this situation, the device does not recognize 
that the tachycardia has been reverted and it will continue to apply 
therapy. This can be a traumatic experience for the patient. [It is highly 
desirable from the viewpoint of patient safety to prevent the delivery of 
unnecessary therapy.] 
There is, therefore, a need for a device which is capable of not only 
reliably differentiating sinus tachycardias from pathologic tachycardias, 
but also of reliably confirming either the continued presence or the 
reversion of a pathologic tachycardia, particularly when the patient 
experiences an elevated heart rate prior to onset of the tachycardia. 
DISCLOSURE OF THE INVENTION 
It is an object of the invention to provide an antitachycardia therapy 
device which reliably detects pathologic tachycardias and distinguishes 
these from sinus tachycardias induced in a patient, for example, in an 
exercise situation where there is a gradual increase in heart rate. 
It is a further object of the invention to provide an antitachycardia 
therapy device which reliably detects pathologic tachycardias which occur 
at elevated heart rates and accurately confirms whether such tachycardias 
have been reverted by the antitachycardia therapy. 
It is a further object of the invention to reduce the likelihood of 
delivering unnecessary therapy to a patient after a tachycardia has 
reverted by the automatic calculation of the confirmation threshold based 
on the conditions prior to and after onset. 
According to the invention, there is provided an apparatus for treating 
cardiac arrhythmias comprising means for continuously measuring the 
average interval of a predetermined number of sinus intervals; detection 
means for detecting the presence of a tachycardia including sudden onset 
detection and an absolute detection interval; antitachycardia therapy 
means responsive to the detection means for delivering antitachycardia 
therapy to revert the tachycardia; tachycardia confirmation means for 
confirming the presence of the tachycardia, the tachycardia confirmation 
means including a threshold interval detector, means for determining a 
function of the average sinus interval measured immediately prior to the 
tachycardia; and means for setting the threshold interval to the lesser 
value of this function and the absolute detection interval. 
There is further provided an apparatus for treating cardiac arrhythmias 
comprising detection means for detecting the presence of a tachycardia 
including sudden onset detection and an absolute detection interval; means 
for measuring the tachycardia cycle length of the detected tachycardia; 
antitachycardia therapy means responsive to the detection means for 
delivering antitachycardia therapy to revert the tachycardia; tachycardia 
confirmation means for confirming the presence of the tachycardia, the 
tachycardia confirmation means including a threshold interval detector, 
means for determining a function of the tachycardia cycle length; and 
means for setting the threshold interval to the lesser value of this 
function and the absolute detection interval. 
In a preferred form of the invention, there is provided a medical device 
for the reversion of tachycardia. The device includes means for detecting 
the presence of a tachycardia and means for delivering antitachycardia 
therapy. The device also includes means for confirmation of the reversion 
of a tachycardia following the delivery of antitachycardia therapy. The 
confirmation interval is automatically determined as the minimum value of 
the absolute detection interval and a function of both the tachycardia 
cycle length and the average sinus interval prior to the onset of a 
pathologic tachycardia. 
The invention also includes a method of treating cardiac arrhythmias 
comprising the steps of continuously measuring the average interval of a 
predetermined number of sinus intervals; detecting the presence of a 
tachycardia by means of sudden onset detection and an absolute detection 
interval; delivering antitachycardia therapy responsive to the detecting 
to revert the tachycardia; confirming the presence of the tachycardia 
according to the level of a threshold interval; determining a function of 
the average sinus interval measured immediately prior to the tachycardia 
detecting; setting the threshold interval to the lesser value of this 
function and the absolute detection interval; and delivering further 
antitachycardia therapy responsive to the confirming. 
In accordance with the invention there is further provided a method of 
treating cardiac arrhythmias comprising of the steps of detecting the 
presence of a tachycardia by means of sudden onset detection and an 
absolute detection interval; measuring the tachycardia cycle length of the 
detected tachycardia; delivering antitachycardia therapy responsive to the 
detecting to revert the tachycardia; confirming the presence of the 
tachycardia according to the level of a threshold interval; determining a 
function of the tachycardia cycle length; setting the threshold interval 
to the lesser value of said function and the absolute detection interval; 
and delivering further antitachycardia therapy when the presence of the 
tachycardia is confirmed.

DEFINITION OF TERMS 
ABSOLUTE CHANGE OF INTERVAL DETECTION (ACID)--A tachycardia detection 
algorithm which requires a sudden and sustained decrease in sensed cardiac 
interval, where the required decrease (Delta) is an absolute amount. 
ABSOLUTE DETECTION INTERVAL (ADI)--A programmable parameter which 
determines the level of the cardiac interval above which a tachycardia is 
absent. 
ABSOLUTE INTERVAL DETECTION (AID)--A tachycardia detection algorithm which 
requires a sustained cardiac interval less than a threshold interval. 
AVERAGE SINUS INTERVAL (ASI)--A measure of the normal intrinsic heart 
interval of the patient. The number of intervals used to determine the 
average is small enough so that the average can track exercise induced 
interval changes, but large enough so that it is insensitive to ectopics. 
DELTA--A programmable parameter which determines the total interval 
reduction required for the sudden onset of a tachycardia to be detected 
when using an ACID or a PCID algorithm. 
PERCENTAGE CHANGE OF INTERVAL DETECTION (PCID)--An alternative tachycardia 
detection algorithm which requires a sudden and sustained decrease in 
sensed cardiac interval, where the required decrease (Delta) is a 
percentage of the average sinus interval, that is, for example, 
Delta=10%.times.ASI. 
TACHYCARDIA CYCLE LENGTH (TCL)--The period of time, or average period of 
time, between successive sense indications in a tachycardia. 
THRESHOLD INTERVAL (TI)--The level which the detected cardiac interval must 
be equal to or less than for the tachycardia to be confirmed. This level 
is automatically determined as the lesser value of the absolute detection 
interval and a function of the average sinus interval, or the tachycardia 
cycle length, or both. 
VERY SHORT INTERVAL DETECTION (VSID)--A tachycardia detection algorithm 
which detects fast obvious tachycardias, that is, tachycardias where 
cardiac intervals are less than that attainable by a sinus tachycardia. 
BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, there is depicted a block diagram of an arrhythmia 
control system 10. System 10 is designed to be implantable and includes a 
pulse module 11 and appropriate leads. More particularly, system 10 will 
generally include: a cardiac lead 12 connected to the patient's heart 14; 
a pacemaker 15 for the detection of analog signals representing cardiac 
electrical activity and for the delivery of pacing pulses to the heart; a 
microprocessor 16 which, in response to various inputs received from the 
pacemaker 15 as well as from a defibrillator 17, performs various 
operations so as to generate different control and data outputs to both 
pacemaker 15 and defibrillator 17; and a power supply 18 for the provision 
of a reliable voltage level to pacemaker 15, microprocessor 16 and 
defibrillator 17 by suitable electrical conductors (not shown). 
Defibrillator 17 produces a high voltage to charge its capacitors and then 
discharges them in response to control signals from microprocessor 16. A 
defibrillator electrode lead 19 transfers the energy of a defibrillator 
shock 20 from the implanted pulse module to the surface of the heart 14. 
Microprocessor 16 is connected to an external memory 21 by an address and 
data bus 22. An end-of-life (EOL) signal line 24 is used to provide, to 
microprocessor 16, a logic signal indicative of the approach of battery 
failure in power supply 18. 
As more fully described below, microprocessor 16 and pacemaker 15 are 
connected by a communication bus 25, a sense line 26, a pace control line 
27, a sensitivity control bus 28, and a pacing energy control 29. As also 
more fully described below, microprocessor 16 is connected to 
defibrillator 17 by a charge level line 30, a charge control bus 31, a 
shock control bus 32, and a dump control bus 34. 
Referring to FIG. 2, pacemaker 15 comprises pacing circuit 35 which 
includes a pacing pulse generator 36, sensing circuit 37, and telemetry 
circuit 38. In addition, there is a control block 39 which includes an 
interface to microprocessor 16. 
In operation, sensing circuit 37 detects analog signals 40 from the heart 
14 and converts the detected signals to digital signals. Furthermore, 
sensing circuit 37 receives an input sensing control signal (which 
determines the sensitivity of the detection circuits in sensing circuit 
37) by way of a sense control bus 41 from control block 39. A change in 
this sensitivity will affect the voltage deviation required at the sensing 
electrode for a sense to be registered. The operation of the logic which 
changes the sensitivity is described in more detail in U.S. Pat. No. 
4,940,054 entitled "Apparatus and Method for Controlling Multiple 
Sensitivities in Antitachyarrhythmia Device", of Richard Grevis and Norma 
Louise Gilli, assigned to the assignee of the present invention. 
Pacing circuit 35 also receives inputs from control block 39 including a 
pace control and a pacing energy control by way of pacing control bus 42 
which carries the signals on pace control line 27 and pacing energy 
control bus 29. The pace control determines the type of pacing to occur 
while the magnitude of the pulse energy is determined by the pacing energy 
control. Pacing circuit 35 causes pulse generator 36 to generate the 
pacing pulse 44 which is delivered to the patient's heart 14 by means of 
cardiac lead 12. 
Telemetry circuit 38 provides a bi-directional link between control block 
39 of pacemaker 15 and an external device such as a programmer. It allows 
data such as the operating parameters to be read from or altered in the 
implanted pulse module 11. 
Referring to FIG. 3, microprocessor 16 comprises two 16-bit timers 47 and 
48, CPU 49, vectored interrupt block 50, RAM 54, ROM 55, ports interface 
57 and an internal communications bus 58. RAM 54 acts as a scratch pad 
memory during execution of the various programs stored in ROM 55 and used 
by microprocessor 16. These programs include system supervisory programs, 
detection algorithms, and programming implementing the logic flow diagram 
of FIG. 4, as well as storage programs for storing, in external memory 21, 
data concerning the functioning of module 11 and the electrogram provided 
by cardiac lead 12. Timers 47 and 48 and associated control software 
implement some timing functions required by microprocessor 16 without 
resort entirely to software, thus reducing computational loads on and 
power dissipation by CPU 49. 
Signals received from telemetry circuit 38 permit an external programmer 
(not shown) to change the operating parameters of pacemaker 15 by 
supplying appropriate signals to control block 39. Communications bus 25 
serves to provide signals indicative of such control to microprocessor 16. 
Thus, it is also possible for an external programmer to control operation 
of defibrillator 17 by means of signals provided to microprocessor 16. 
Appropriate telemetry commands may cause telemetry circuit 38 to transmit 
data to the external programmer. Data which has been stored is read out, 
by microprocessor 16, on the communications bus 25, through control block 
39 in pacemaker 15, and into control block 38 for transmission to the 
external programmer by a transmitter in telemetry circuit 38. 
Microprocessor 16 receives various status and/or control inputs from 
pacemaker 15 and defibrillator 17. During normal paceing operations, the 
input signal to pacemaker 15 is a sense signal on sense line 26 which is 
used by microprocessor 16 to perform operations such as arrhythmia 
detection. Microprocessor 16 produces outputs such as the pace control on 
pace control line 27 which determines the type of pacing to take place. 
Other pacemaker control outputs generated by microprocessor 16 include a 
pacing energy control signal on pacing energy control bus 29 which 
determines the magnitude of the pulse energy, and sensitivity control 
signal on sensitivity control bus 28, which determines the sensitivity 
setting of the sensing circuit. 
Microprocessor 16 provides to defibrillator 17 a shock control signal on 
shock control line 32 which indicates that a shock is to be delivered to 
the patient, a dump control signal on dump control line 34 which indicates 
that a shock is to be dumped at an internal load within defibrillator 17, 
and a charge control signal on charge control bus 31 which determines the 
voltage level of the shock to be delivered. Charged voltage level line 30 
provides a digital signal representative of charge voltage from an analog 
to digital converter within defibrillator 17, thus providing a feedback 
loop which assures that a shock of proper energy level is delivered by 
defibrillator 17. 
Referring to FIG. 4, there is shown a logic diagram or flowchart of the 
microprocessor program used for the detection and confirmation of a 
tachycardia. The start is shown at step 70. At step 71 the ACID algorithm 
is initiated. The ACID criterion is met if the cardiac interval has 
decreased by at least an amount DELTA below the average sinus interval 
(ASI) within a period of time denoted as the count intervals and remains 
at that level or less for a programmable number of intervals, n, in a 
period of time denoted as the detection window, where, for example, n=4. 
At step 72, the cardiac interval is compared to an Absolute Detection 
Interval (ADI). If the cardiac interval is less than the ADI, a 
tachycardia has been detected. The confirmation Threshold Interval (TI) is 
then determined dynamically at 73 as the minimum of ADI and a value 
determined by a function of the tachycardia cycle length (TCL) and ASI, 
that is, TI=min [ADI,f(TCL,ASI)]. In a preferred embodiment, this function 
f is given by f(TCL,ASI)=TCL+K1.times.(ASI-TCL), where K1=0.7. If, for 
example, ADI is programmed at 400 ms and ASI is 385 ms when a tachycardia 
is detected with a TCL of 300 ms then 
f(TCL,ASI)=300+0.7.times.(385-300)=359.5 and TI will be set at TI=min 
[400,359.5]=359.5 ms. 
In another embodiment, the confirmation TI is set to the minimum of ADI and 
a value determined by a function of the TCL only, that is, TI=min 
[ADI,f(TCL)] where f(TCL)=K2.times.TCL and K2=1.2. If, for example, the 
ADI is programmed at 400 ms when a tachycardia is detected with a TCL of 
300 ms, then f(TCL)=1.2.times.300=360 and TI will be set at TI=min 
[400,360]=360 ms. 
In a further embodiment, the confirmation TI is set to the minimum of the 
ADI and a value determined by a function of the ASI only, that is, TI=min 
[ADI,f(ASI)], where f(ASI)=K3.times.ASI and K3=0.95. If, for example, ADI 
is programmed at 400 ms and ASI is 385 ms when a tachycardia is detected, 
then f(ASI)=0.95.times.385=366 and the TI will be set min[400,366]=366 ms. 
If the ACID criterion was not met at step 71, an optional test is made at 
step 74 for an obvious tachycardia; that is, a tachycardia having a 
cardiac interval less than that attainable by a sinus tachycardia, using a 
Very Short Interval Detection (VSID) criterion where at least x out of at 
most y sensed intervals must be smaller than a detection interval (TI). 
The parameters x and y are programmable and in a preferred embodiment x=15 
and y=18. If the VSID criterion is met, the confirmation interval TI is 
set to TI at 75 where TI is a programmable parameter and in a preferred 
embodiment TI=320 ms. 
The continued presence of the tachycardia is then confirmed at step 76 
using an absolute interval detection (AID) algorithm. The AID criterion is 
met if at least x out of at most y sensed intervals are smaller than the 
confirmation interval TI, where x and y are programmable parameters. In a 
preferred embodiment x=4 and y=4. Following the confirmation of the 
tachycardia, therapy is applied at step 77. Confirmation is repeated after 
the delivery of the therapy at step 78. If the tachycardia has not 
reverted, therapy is again applied. This cycle continues until the 
tachycardia reverts or until therapy has been applied K times, when 
therapy is stopped at step 79. The device then enters a passive state in 
which it remains until the tachycardia terminates, the device is 
reprogrammed or a magnet is applied. 
Referring to FIG. 5A, there is shown a diagram of cardiac interval as a 
function of time during which time the cardiac interval has decreased by 
at least DELTA from the ASI within the count intervals period. However, 
this decrease in interval is not sustained for a sufficient number of 
intervals and hence ACID is not triggered. 
In FIG. 5B, the cardiac interval falls below the ADI during the count 
intervals period. However, the change in interval is less than DELTA as 
may occur during exercise, and the ACID is not triggered. 
In FIG. 5C, the cardiac interval decreases by at least DELTA from ASI 
within the count intervals period. However, the cardiac interval never 
falls below the ADI and a tachycardia is not detected. 
FIG. 6 illustrates cardiac interval as a function of time during positive 
ACID. The sensed cardiac interval decreases by an amount at least as great 
as the parameter DELTA from an ASI within the count intervals period, 
where, for example, DELTA=80 ms and count=4. Because the cardiac interval 
then remains at that level, or less, for a programmable number of 
intervals, n, in the detection window and the cardiac interval is less 
than the ADI, the ACID and AID criteria are met, and the tachycardia is 
detected. 
The AID algorithm is then used to detect the continued presence of the 
tachycardia prior to the application of therapy. The tachycardia is 
confirmed as the cardiac interval is less than the confirmation TI, where 
TI=min [ADI,f(TCL,ASI)]=min[ADI,(TCL+0.7.times.(ASI-TCL))]=ADI. After the 
application of the therapy, confirmation is again performed and the 
tachycardia is found to have reverted. 
FIG. 7 illustrates the cardiac interval as a function of time during 
another positive ACID. The cardiac intervals need not decrease 
monotonically throughout the sudden onset window for a tachycardia to be 
detected. Confirmation of the presence of the tachycardia is again made 
prior to and after the application of therapy with the TI being determined 
as in FIG. 6. 
FIG. 8 illustrates the cardiac interval as a function of time during 
another positive ACID where the initial cardiac interval is less than the 
ADI. The sensed cardiac interval decreases by an amount at least as great 
as DELTA from the ASI within the count intervals period and remains at 
that level or less for a programmable number of intervals, n, in the 
detection window, thereby satisfying the ACID criteria. 
The AID algorithm is then used to detect the continued presence of the 
tachycardia prior to the application of therapy. In this instance, the 
onset of a tachycardia has occurred at an initially decreased cardiac 
interval such as that which may be reached during exercise. Because the 
ASI is less than the ADI, TI will be set to a value between the ASI and 
the TCL as determined by f(TCL,ASI)=(TCL+0.7.times.(ASI-TCL)). 
Confirmation is again performed following the application of therapy. If 
the ADI had been used for the confirmation threshold, as in prior art 
devices, successful reversion of the tachycardia would not have been 
confirmed as the cardiac interval is less than the ADI. However, because 
the TI has been set to a value between the ASI and the TCL, successful 
reversion of the tachycardia is recognized due to the cardiac interval 
being greater than the TI. 
Although the invention has been described with reference to a particular 
embodiment, it is to be understood that this embodiment is merely 
illustrative of the applications of the principles of the invention. For 
example, the embodiment described outlines the use of a combination of 
absolute interval detection and sudden onset detection for differentiating 
between sinus tachycardias and pathologic tachycardias where the sudden 
onset detection may be performed by ACID, PCID or other sudden onset 
algorithms. The invention is disclosed in a combined pacer and 
cardioverter device. However, it may be applied in devices which are 
limited in their operation to either antitachycardia pacing or 
cardioversion therapy.