Method and apparatus for detecting changes in electrocardiogram signals

Method and apparatus for detecting changes in electrocardiogram signals. A first cardiac electrogram signal is sensed, where the first cardiac electrogram signal has a voltage and includes a first cardiac complex. A ventricular activation is detected in the first cardiac complex and a first timer is started, where the first timer times a first specified time. A first voltage value is measured from the first cardiac electrogram signal at the first specified time after the ventricular activation. A second voltage value is also measured from a defined portion of the first cardiac electrogram signal. A comparison value is then calculated from the first and second voltage values measured from the first cardiac electrogram signal. The first voltage value is recorded when the comparison value is greater than or equal to a predetermined value.

FIELD OF THE INVENTION 
The present subject matter relates generally to medical devices and more 
particularly to a method and apparatus for detecting and recording changes 
in electrocardiogram signals. 
BACKGROUND OF THE ART 
Implantable cardiac rhythm management devices (ICRMDs) sense the heart's 
electrical activity and, if an arrhythmia is detected, deliver therapy to 
terminate it. Many patients with ICRMDs are at particular risk for 
myocardial ischemia, which has been reported to trigger arrhythmias 
(20-50% of ventricular tachyarrythmias). 
Presently, to assess the presence or absence of ischemnia in patients, 
cardiac electrophysiologists (EPs) perform 12-lead body-surface 
electrocardiograms (ECGs) or 24-hour Holter monitoring. Myocardial 
ischemia has been known for years by EPs to be manifested on ECGs as 
ST-segment deviation (STD). Unfortunately, 12-lead ECGs are obtained over 
a short duration (&lt;1 hour), and generally detect only chronic levels of 
ischemia. Holter recordings are more apt to detect transient ischemic 
events, but the 24-hour recording periods are still only snapshots of the 
patient's ischemic condition. 
Recently, investigations have shown that similar STD changes occur in the 
electrograms from intra- and extracardiac signals. Other morphological 
features of electrocardiograms have also been reported to be 
representative of ischemia, including ST-segment-duration decrease, 
R-wave-duration increase, R-wave-amplitude decrease, T-wave 
duration/amplitude change, change in the rate of rise/fall of the T-wave, 
decrease in T-wave uniformity, and change in position of the J point. 
Implantable medical devices have been suggested as being useful for signal 
analysis of the electrical potential curve sensed from the heart. One 
suggested approach has been to sense cardiac signals with an implantable 
medical device and then compare them to a template signal stored in the 
medical device. Based on this comparison, an assessment as to whether 
myocardial ischemia is present can be made. Using the stored template 
signal, however, may not yield the most accurate and reliable way of 
detecting the presence of myocardial ischemia. Once the template signal 
has been programmed, it remains constant. In contrast, the cardiac signals 
being sensed by an implantable device changes over time as a result of 
changes to the tissue that surrounds the implanted electrodes. As the 
tissues change, so do the signals the electrodes are able to sense. So, 
the cardiac signal sensed by the implantable medical device begins to 
change relative the stored template signal. As this change takes place, 
the template signal becomes less and less useful in identifying myocardial 
ischemia. 
Additionally, the entire baseline value of the sensed cardiac signal may 
change over time. Gradual changes in the baseline value of a cardiac 
signal can indicate a change in cardiac condition which a physician may 
wish to investigate more closely. Thus, a need still exists for 
improvements in acquiring measurements and comparing those measurement in 
an effort to identify myocardial ischemia. 
SUMMARY OF THE INVENTION 
The present subject matter provides a method and an apparatus for detecting 
and recording myocardial ischemia. One unique aspect of the present 
subject matter is that the sensed electrocardiogram signal from which the 
assessment of myocardial ischemia is made is also used as the basis from 
which the comparison is made. So, the present system does not rely on a 
static template in order to identify the occurrence of myocardial ischemia 
in a sensed electrocardiogram signal. 
According to the present subject matter, a first cardiac electrogram signal 
is sensed, where the first cardiac electrogram signal has a voltage and 
includes at least a first cardiac complex. As the first cardiac complex is 
sensed, a ventricular activation is detected in the first cardiac complex. 
A first voltage value is then measured from the first cardiac electrogram 
signal at a first specified time after the ventricular activation. 
In addition to measuring the first voltage value, a second voltage value is 
also measured from the same first cardiac electrogram signal. The second 
voltage value is measured at a defined portion along the first cardiac 
electrogram signal. In one embodiment, the defined portion of the first 
cardiac electrogram signal is a portion within the first cardiac complex, 
Alternatively, the defined portion is in a second cardiac complex sensed 
in the first cardiac electrogram signal. A comparison value is then 
calculated from the first and second voltage values. The first voltage 
value is then recorded when the comparison value is greater than or equal 
to a predetermined value. 
In one embodiment, the second voltage value is measured from a PQ or TQ 
segment of the first cardiac complex. The comparison value is calculated 
by subtracting the second voltage value from the first voltage value. This 
results in a voltage value which is then compared to a voltage value for 
the predetermined value. 
Alternatively, a second cardiac complex is sensed in the first cardiac 
electrogram signal. The occurrence of the ventricular activation is 
detected in the second cardiac complex, and the second voltage value is 
measured at the first specified time after the ventricular activation. A 
voltage rate of change is then calculated for the comparison value from 
the first and second voltage values. The voltage rate of change is then 
compared to the predetermined value having the units volts/time. 
In one embodiment, cardiac complexes in the cardiac electrogram signal are 
sensed and analyzed according to the present subject matter after a first 
predetermined time period. When a cardiac complex in the cardiac 
electrogram signal has a first voltage value, relative the second voltage 
value, that is greater than or equal to the predetermined value, then a 
second time period is used to analyze the sensed cardiac complexes. This 
second time period has a shorter interval than the first time period and 
overrides the use of the first time period. Cardiac signals are sensed and 
analyzed according to the present subject matter as the second time 
expires. 
In one embodiment, when the comparison value for a cardiac complex is 
greater than or equal to the predetermined value the second time is 
restarted to allow for another cardiac complex to be analyzed after the 
second timer expires. Alternatively, when the comparison value for a 
cardiac complex is less than the predetermined value the second timer is 
stopped and the first timer is restarted to allow for another cardiac 
complex to be analyzed after the first timer expires.

DETAILED DESCRIPTION 
In the following detailed description, reference is made to the 
accompanying drawings which form a part hereof and in which is shown by 
way of illustration specific embodiments in which the invention can be 
practiced. These embodiments are described in sufficient detail to enable 
those skilled in the art to practice and use the invention, and it is to 
be understood that other embodiments may be utilized and that electrical, 
logical, and structural changes may be made without departing from the 
spirit and scope of the present invention. The following detailed 
description is, therefore, not to be taken in a limiting sense and the 
scope of the present invention is defined by the appended claims and their 
equivalents. 
Referring now to FIG. 1, there is shown a flowchart illustrating one 
embodiment of the present subject matter. At 100, at least a first cardiac 
electrogram signal is sensed by a diagnostic device, where the diagnostic 
device includes two or more electrodes for sensing cardiac electrogram 
signals. Moreover, the diagnostic device can either be an external or an 
internal device. An example of external devices include Holter monitors. 
An additional example includes an implantable cardiac rhythm management 
device (ICRMD), such as an implantable cardiac pacemaker or implantable 
cardioverter-defibrillator. 
Two or more electrodes are used in conjunction with the diagnostic device 
to sense at least the first cardiac electrogram signal. In one embodiment, 
the first cardiac electrogram signal is sensed from a left ventricular 
location of a heart. Sensing electrogram signals from the left ventricular 
region of the heart is important as the left ventricle is responsible for 
pumping arterial blood and knowledge of ischemic events in this region can 
be used to provide better patient therapy. 
In one embodiment, the first cardiac electrogram signal is a far-field 
electrogram signal, where the far-field signal has a voltage and includes 
cardiac complexes representative of at least a portion of the cardiac 
cycle. In an alternative embodiment, the first cardiac electrogram signal 
is a near-field electrogram signal, where the near-field signal also has a 
voltage and includes cardiac complexes representative of at least a 
portion of the cardiac cycle. 
Electrogram signals can be sensed between any number of electrodes 
implanted or positioned in any number of locations in, on and around the 
heart. In one embodiment, the first cardiac electrogram signal is sensed 
between a first electrode positioned in the region of the left ventricle 
of a heart and a second electrode positioned away from the first 
electrode. For example, the first cardiac electrogram signal is sensed 
between a first electrode implanted within a coronary vein of the heart 
and the housing of the ICRMD. Alternatively, a second electrode, 
positioned outside the heart (e.g., positioned on the epicardial surface 
of the heart) is used in conjunction with the first electrode and the 
housing of the ICRMD to sense far-field signals across the left 
ventricular region of the heart. 
In addition to sensing the first cardiac electrogram signal from the left 
ventricular region of the heart for use with the present subject matter, 
is it also possible to sense additional cardiac electrogram signals (e.g., 
a second cardiac electrogram signal, a third cardiac electrogram signal, 
etc.) between additional electrodes implanted in, on and around the heart. 
Therefore, additional electrocardiogram signals, including far-field and 
near-field cardiac signals, sensed across additional left ventricular 
locations and far-field and near-field cardiac signals, sensed across 
right ventricular locations, can be used with the present subject matter. 
At 120, ventricular activation is detected from the first cardiac 
electrogram signal. In one embodiment, ventricular activation is detected 
when a predetermined portion of a QRS-complex is sensed in an electrogram 
signal. In one embodiment, the predetermined portion of the QRS-complex is 
a ventricular R-wave detected in the sensed cardiac complex. In one 
embodiment, the R-wave is identified by detecting a maximum deviation of 
the cardiac signal from a baseline signal during the QRS-complex. Other 
portions of the QRS-complex can also be detected to indicate ventricular 
activation, including the Q-wave (i.e.,the start of the QRS-complex as 
noted by a sustained deflection from baseline signal). Additionally, 
different positions within a QRS-complex, including the R-wave, can be 
used to identify a ventricular activation. 
A first voltage value is then measured from the first cardiac electrogram 
signal at a first specified time after the detection of the ventricular 
activation. The first specified time is a programmable value and can be 
selected to allow for a voltage measurement to be made at any portion 
along the cardiac signal. For example, a first specified time can be 
selected to allow for the cardiac signal voltage measurement, or 
measurements, to be made during the occurrence of an ST-segment of the 
cardiac cycle. 
FIG. 2 shows one embodiment of a cardiac electrogram signal with a cardiac 
cycle 200. The cardiac cycle 200 is a composite electrogram signal sensed 
across a region of cardiac tissue. The cardiac cycle 200 includes a P-wave 
204, representing a trial depolarization, a Q-wave 206, a QRS-complex 208, 
representing ventricular depolarization, and a T-wave 212, representing 
ventricular repolarization. An ST-segment 216 lies between the occurrence 
of the S-wave, approximately at 220, and the beginning of the T-wave 212, 
approximately at 224. 
In one embodiment, measuring voltages of the cardiac signal during the 
ST-segment 216 is useful in detecting the presence of myocardial ischemia. 
When myocardial ischemia is present, the ST-segment 216 will deviate from 
a baseline value. In one embodiment, deviating from the baseline value can 
either be an ST-segment voltage value that is either elevated or depressed 
relative to the baseline value. In the example shown in FIG. 2, a baseline 
value is shown at 230 and has a value of approximately 0 volts. In an 
alternative embodiment, different portions of the cardiac electrogram 
signal can measured and used as baseline signals in the assessment of the 
ST-segment 216. For example, the voltage value of the electrogram signal 
in a PQ-segment 240 can be used to determine the change in ST-segment 
value for assessing the presence of myocardia ischemia. Additionally, the 
TQ-segment or the TP segment could also be used to determine the change in 
ST segment values. 
In detecting a change in the ST-segment 216, a predetermined region of the 
ST-segment is defined from which to take a voltage measurement. In one 
embodiment, the predetermined region is taken as the voltage of the 
cardiac signal in the ST-segment at the first specified time after the 
ventricular activation. In one embodiment, the first specified time is 
selected in the range of 20 to 500 milliseconds, where 80 milliseconds is 
one of many appropriate values. In the present embodiment, however, it is 
understood that the first specified time will depend upon the portion of 
the cardiac complex that is taken to represent the ventricular activation, 
where an earlier portion of cardiac complex may require a longer first 
specified time as compared to a later portion of the cardiac complex which 
may require a shorter first specified time. The voltage of the cardiac 
signal sensed at the first specified time is then recorded at 140 and is 
retrievably stored for review and analysis. 
Referring now to FIG. 3, there is shown an additional embodiment of the 
present subject matter. At 300, at least one cardiac electrogram signal is 
sensed by an ICRMD. In one embodiment, a first cardiac electrogram signal 
is sensed, where the signal has a voltage and includes at least a first 
cardiac complex. In addition to sensing cardiac electrogram signals, a 
first timer is also started. The first timer counts off a first 
predetermined time, after which voltage values are measured from the 
cardiac electrogram signals. In one embodiment, the first predetermined 
time is an interval over which the ICRMD waits, or delays, before 
measuring and analyzing the cardiac electrogram signals. 
At 310, the first predetermined time is analyzed to determine whether the 
first time has expired. When the first time has not expired, a path is 
taken back to 310 to make another inquire whether the first predetermined 
time has expired. In one embodiment, the first predetermined time is a 
programmable time period of between 1 second and 7 days. Alternatively, 
the programmable time period is between 1 minute and 24 hours. In one 
embodiment, a first time period of 5 minutes is an acceptable value. 
When the predetermined time expires, a path is taken to 320. At 320, a 
ventricular activation is detected in the first cardiac complex in the 
first cardiac electrogram signal. In one embodiment, ventricular 
activation is detected from a far-field signal as previously described. In 
an alternative embodiment, a near-field electrogram signal sensed across a 
ventricular location is used to detect the occurrence of a ventricular 
activation. The near-field electrogram signal can detect the occurrence of 
ventricular R-waves, which are used to indicate the occurrence of 
ventricular activation. For example, determining the occurrence of 
ventricular activation from the near-field signal can include determining 
a maximum deflection point of the near-field electrogram signal sensed for 
the cardiac complex. The maximum deflection point of the near-field signal 
is then taken as the occurrence of the ventricular activation. 
Alternatively, the start (or beginning) of the deflection in the cardiac 
signal indicating ventricular activation is used as the occurrence of 
ventricular activation. Other portions of the near-field signal could also 
be taken as indicating ventricular activation, where one factor in 
determining an appropriate choice would be the repeatable nature of the 
portion of the near-field signal. 
In one embodiment, a near-field electrogram signal is sensed from a right 
ventricular region of the heart between a pacing/sensing electrode and an 
additional electrode. For example, sensing the near-field electrogram 
signal can include sensing the signal between a pacing/sensing electrode 
and a defibrillation coil electrode both positioned along a transvenous 
catheter. Alternatively, the near-field signal is sensed between the 
pacing/sensing electrode and the housing of the ICRMD. 
Once the ventricular activation is sensed, at 330 a first voltage value of 
the first cardiac electrogram signal is measured at a first specified time 
after the ventricular activation determined from the first cardiac 
complex. In one embodiment, the first specified time is determined by the 
attending physician. The first specified time after the ventricular 
activation is a predetermined value in the range of between 20 to 500 
milliseconds, where 80 milliseconds is an appropriate value. 
In an alternative embodiment, the value of the first specified time depends 
on the sensed ventricular activation rate. One aspect of the present 
subject matter is to make voltage measurements along cardiac signals 
within a particular portion of the cardiac cycle. For example, voltage 
measurements taken from cardiac signals along the ST-segment of a cardiac 
complex are useful in determining the occurrence of myocardial ischemia as 
previously discussed. As the ventricular rate increases, the time in which 
the cycle occurs decreases. This decrease in the cycle results in a 
decrease in the relative time between the portions of the QRS-complex and 
the ST-segment (e.g., the time between the R-wave and T-wave begins to 
decrease as the heart rate increases). Thus, as the ventricular rate 
changes, so does the location of the ST-segment relative the ventricular 
activation. 
In the present embodiment, the first specified time is calculated by taking 
a percentage of the time of a ventricular interval. In one embodiment, the 
ventricular interval is calculated from consecutively sensed ventricular 
activations. For example, a second cardiac complex is sensed in the first 
cardiac signal, where the second cardiac complex precedes the first 
cardiac complex. From these two cardiac complexes a ventricular activation 
interval is calculated. The first specified time is then calculated by 
taking a first predetermined percentage of the ventricular activation 
interval. So, as each ventricular activation interval is calculated, the 
first specified time is calculated by multiplying the time of the 
ventricular interval by the first predetermined percentage. In one 
embodiment, the first predetermined percentage is a programmable value in 
the range of approximately 2 to 50 percent of the ventricular activation 
interval. Alternatively, the range is approximately 10 to 20 percent of 
the ventricular activation interval. 
In an additional embodiment, when two consecutive ventricular activations 
are sensed, the rate of ventricular activations will be assessed only on 
ventricular activations that are, in the case of patients in sinus rhythm, 
beats originating from the atrium (atrially paced or sinus beats). In the 
case of patients being paced in the ventricle, the implantable pulse 
generator would have the programmed rate. Furthermore, in the case of a 
patient in an atrial tachyarrhythmia, the entire basing of the ST (and PQ 
or TQ) regions on the ventricular rate would not be valid. In one 
embodiment, the mechanisms by which beats are analyzable or not could be 
based on the morphology of the R wave or, more simply, by the consistency 
of the rate (or rather, the lack of a high standard deviation of RR 
intervals). 
In an alternative embodiment, the first voltage value is calculated from 
two or more voltage values measured, or acquired, from the first cardiac 
electrogram signal. For example, two or more voltage values measured from 
a far-field electrogram signal are used to calculate the first voltage 
value. One way of acquiring these two or more voltage values is to make 
voltage measurements at a predetermined sampling frequency in the interval 
between the first specified time and a second specified time after the 
occurrence of the ventricular activation. In one embodiment, the second 
specified time is timed by the voltage acquisition timer. In one 
embodiment, the sampling frequency is a programmed value in the range of 
between 10 Hz and 10 kdHz, where 100 Hz is an appropriate value. The 
second specified time is also a programmable value, where the second 
specified time is programed in the range of between 5 milliseconds to 600 
milliseconds. 
Alternatively, the second specified time is calculated by multiplying the 
ventricular activation interval (as previously described) by a second 
predetermined percentage. In one embodiment, the second predetermined 
percentage has a value greater than the first predetermined percentage 
used to calculate the first specified time such that two or more voltage 
measurements can be made in the interval between the first and second 
specified times. In one embodiment, the second predetermined percentage is 
a programmable value in the range of 2 percent to 50 percent of the 
ventricular activation interval. 
Once the multiple voltage measurements have been acquired for each sensed 
electrogram, the first voltage value is calculated from the two or more 
voltage values measured during interval between the first and second 
specified time for each sensed electrogram. In one embodiment, the first 
voltage value for each sensed electrogram is an average voltage value of 
the voltage values measured during the interval. Alternatively, the first 
voltage value for each sensed electrogram is a median voltage value of the 
voltage values measured during the interval. 
The voltage of the cardiac signal is then recorded at 340 and path 350 is 
taken back to 300 where the first timer begins to time the first 
predetermined time again. In addition to recording the voltage of the 
cardiac electrogram signal, the time at which the cardiac complex occurred 
is also recorded. Additional information related to the cardiac state can 
also be recorded and stored for retrieval and/or analysis at a later time. 
Such information can include, but is not limited to, ST slope, R wave 
amplitude and duration, PR interval, T wave duration, and T wave 
amplitude. 
Referring now to FIG. 4, there is shown an additional embodiment of the 
present subject matter. In FIG. 4, an encircled number one connects to the 
encircled number 1 shown in FIG. 3. After measuring the cardiac signal 
voltage at 330, a path, connected by the encircled number ones, is taken 
to 400. At 400, a second voltage value is measured at a defined portion of 
the first cardiac electrogram signal. In one embodiment, the second 
voltage value is measured from at least one point between the P-wave and 
the Q-wave of the first cardiac complex. In one embodiment, this value is 
measured at a predetermined time after the occurrence of the P-wave, but 
before the beginning of the Q-wave. In an alternative embodiment, the 
second voltage value could be measured in a region between the T-wave and 
the Q-wave of the first cardiac complex. Selection of either the PQ-region 
of the TQ-region will be dependent upon where, and how many, electrodes 
are positioned in order to sense the cardiac signal. In an additional 
embodiment, is it possible to take multiple measurements along these 
regions and determine the value by taking the average or median value. 
The first voltage value and the second voltage value measured from the 
first cardiac complex are then used to calculate a comparison value. In 
one embodiment, the comparison value is calculated by subtracting the 
second voltage value from the first voltage value. An absolute value of 
the difference is then taken, and the comparison value is subsequently 
compared to a predetermined value. In one embodiment, the predetermined 
value is a programmable value between approximately 0.01 and 10 
millivolts. In an alternative embodiment, the predetermined value is a 
programmable value between approximately 0.2 and 0.4 millivolts. 
In an alternative embodiment, a rate of voltage change is calculated 
between the first and second voltage values. For example, a second cardiac 
complex is sensed in the first cardiac electrogram signal. In one 
embodiment, the second cardiac complex precedes the first cardiac complex. 
The occurrence of the ventricular activation is determined from the second 
cardiac complex, as previously described. The second voltage value is then 
measured from the cardiac signal at the first specified time after the 
ventricular activation determined from the second cardiac complex. The 
comparison value is then calculated from the first and second voltage 
values by calculating a rate of voltage change between the first and 
second voltage values. The comparison value is then compared to the 
predetermined value. In the present embodiment, the predetermined value is 
a rate of change threshold value. In one embodiment, the rate of change 
threshold value is a programmable value in the range of 0.001 to 50 
millivolts/minute. 
At 410, the comparison values compared to the predetermined value to 
determine whether the comparison value is greater than or equal to the 
predetermined value. When the comparison value is less than the 
predetermined value, the path is taken, via the encircled two, from 410 to 
340 on FIG. 3 where the first voltage value is recorded. Alternatively, 
when the comparison value is greater than or equal to the predetermined 
value, the path is taken to 415. At 415, the first voltage is recorded. In 
addition to recording the first voltage, additional information related 
the first cardiac complex, and the second cardiac complex, are recorded. 
As previously discussed, information related to the cardiac complex can 
include the voltage of the cardiac signal at the first specified time, the 
time the cardiac complex occurred, the heart rate, raw electrocardiogram 
signals, and other cardiac information as is known. 
In addition to recording the voltage of the sensed cardiac signal at the 
first specified time, a second timer is started at 420. The second timer 
times out a second predetermined time and overrides the first timer. In 
one embodiment, the second predetermined time is of a shorter duration 
than the first predetermined time period used at 310. This allows for 
cardiac complexes to be analyzed and recorded at a more rapid rate once 
the first cardiac complex is detected as having a first voltage value that 
exceeds the first predetermined value as previously described. In one 
embodiment, the second predetermined time is a programmable value in the 
range of 0.5 seconds to one hour, where 30 seconds is an appropriate 
value. 
At 430, the second predetermined time is examined to determine if it has 
expired. When the time has not expired, a path is taken back to 430 to 
inquire again if the time has expired. When the time expires, a path is 
taken to 440. At 440, the first voltage value and the second voltage value 
from the electrogram signal are measured as previously described. The 
comparison value is then calculated and compared to the predetermined 
value at 450 as previously described. When the comparison value is greater 
than or equal to the predetermined value, path 460 is taken back to 410 
where the first voltage value of the sensed cardiac signal is recorded in 
a manner previously described. The second predetermined time is then 
restarted at 420 to allow for additional cardiac complexes to be sensed 
and analyzed. 
Alternatively, when the comparison value is less than the predetermined 
value, path 470 is take to encircled number 3. Encircled number 3 connects 
to 340 in FIG. 3. At 340, the first voltage value is then recorded as 
previously described. 
After recording the first voltage value at 340, the first predetermined 
time is started and tested at 310 as previously described. 
In an alternative embodiment, at 450 the comparison value is the rate of 
voltage change between a first voltage value for a first cardiac complex 
and a second voltage value for a second cardiac complex is used to 
determine whether to return to 415 or to take path 470 to 340. In the 
present embodiment, assume a first voltage value has been recorded at 410 
after it is determined the comparison value is greater than or equal to 
the predetermined value. After the second predetermined time expires, the 
second voltage value is measured from the second cardiac complex at 440. 
At 450, the rate of voltage change is then calculated for the first and 
second voltage values. The rate of voltage change (i.e., the comparison 
value) is then compared to the predetermined value to determine whether to 
proceed back to 415 or to340. When returning to 415, the rate of voltage 
change calculated at 450 will be determined from the second voltage value 
(measured after the second predetermined time) and the subsequent voltage 
value (e.g., a third voltage value measured from a third cardiac complex) 
measured from a cardiac complex sensed after the second predetermined 
time. In one embodiment, the rate of change threshold (the predetermined 
value) is a programmable value in the range of between 0.10 and 1.0 
millivolts/minute. 
In an additional embodiment, a rate of voltage change is calculated between 
a comparison value calculated for the first cardiac complex and a 
comparison value calculated for a second cardiac complex sensed in the 
first cardiac signal. For example, the second cardiac complex is sensed in 
the first cardiac electrogram signal, and the occurrence of the 
ventricular activation from the second cardiac complex is detected. The 
first voltage value of the first cardiac electrogram signal is then 
measured at the first specified time after the ventricular activation 
determined from the second cardiac complex. The second voltage value is 
measured at the defined portion of the first cardiac electrogram signal 
from the first cardiac complex. The second voltage value is also measured 
at the defined portion of the first cardiac electrogram signal from the 
second cardiac complex, where the second voltage value measured for the 
first cardiac complex is a separate second voltage value than the second 
value measured for the second cardiac complex. A comparison value is then 
calculated for the first cardiac complex from the first and second voltage 
values measured from the first cardiac complex. Likewise, a comparison 
value is calculated for the second cardiac complex from the first and 
second voltage values measured from the second cardiac complex. A rate of 
voltage change is then calculated from the comparison value for the first 
cardiac complex and the comparison value for the second cardiac complex. 
The rate of voltage change is then compared to the predetermined value (in 
millivolts/minute) and the first voltage value of both the first cardiac 
complex and the second cardiac complex are recorded when the rate of 
voltage change is greater than or equal to the predetermined value. 
Additionally, the present embodiment can be used to assess which path to 
take after 410 and/or 450. 
In an additional embodiment, additional algorithms for determining when to 
proceed from using the first timer to using the second timer (i.e., moving 
from 400 to 420 and from 450 to 410 via path 460) are also within the 
scope of the present subject matter. For example, additional algorithms 
can be based on best-fitting trended (past) values of the difference 
between voltage values from ST segments and TQ or PQ segments over certain 
durations. The determination to begin the second timer would then be based 
on parameters from these best fits (linear or higher order fits). 
Alternatively, a threshold could alternatively be set based on the natural 
variation of the relevant parameter. So, for example, if a patient has a 
period of ST segment levels in which the standard deviation of all of the 
values are, for example, 0.02 mV, then the threshold for beginning the 
second timer would be a value proportional to this standard deviation. And 
if a patient has highly fluctuating ST levels, this patient may have a 
proportionately higher threshold for starting the second timer. 
In an additional embodiment, once 415 is reached a duration timer is 
started. In one embodiment, the duration timer times an interval over 
which the second time period overrides the first time period. In other 
words, the duration timer times an interval over which the path is taken 
from 450 to 415, regardless of the result at 450, during the interval. 
Thus, when 410 is satisfied (i.e., the path is taken to 415) the duration 
timer is started. Running concurrently with the duration timer is the 
second timer. The second timer times out the second predetermined time 
after which the voltage values are measured from the cardiac signals at 
440. At 450, the duration timer is checked to see if it has expired. When 
the duration timer has not expired, path 460 is taken to 415 where the 
voltage value is recorded and the second timer is started again at 420. 
When the duration timer expires, the comparison value is compared to the 
predetermined value at 450 as previously described. The path taken from 
450 then depends upon the result of the comparison at 450. In one 
embodiment, the interval of the duration timer is a programmable value in 
the range of 1 to 60 minutes, where 10 minutes is an acceptable value. 
Electrodes for sensing cardiac signals can be placed at any number of 
positions within and outside of the heart. For example, electrodes can be 
placed in the left ventricular region of the heart for sensing cardiac 
signals. In one embodiment, a transvenous lead (or catheter) having at 
least a first electrode positioned along the peripheral surface of the 
catheter is positioned within the coronary vein, with the second electrode 
positioned in the thoracic cavity. In one embodiment, the first electrode 
is positioned in the coronary sinus. In an alternative embodiment, the 
first electrode is positioned within a coronary vein and extending toward 
the heart's apex along one of the primary branches from the coronary 
sinus, such as the anterior interventricular branch, a lateral branch, or 
the posterior interventricular branch to position the first electrode 
adjacent the left ventricle. In one embodiment, the first at least one 
coil electrode is situated near the catheter's distal end, such that it 
resides along the ventricular epicardium, and not along the 
atrioventricular groove. 
Alternatively, the first and/or second electrodes are positioned in an 
intrathoracic space. In an alternative embodiment, the first electrode is 
positioned in the intrapericardial space, including directly on the 
epicardium, which involves accessing the pericardial space through the 
pericardium sack. The electrode is then positioned within the 
intrapericardial region between the pericardium sack and the epicardial 
surface of the heart. The electrode is then used to sense the first 
cardiac signal between a second electrode positioned in a thoracic cavity. 
Alternatively, the first cardiac signal can be sensed between the first 
electrode and one or more other implanted electrodes. 
In addition to sensing cardiac complexes from the left ventricular 
locations, additional cardiac complexes can be sensed from other location 
within and on the heart. For example, cardiac signals can be sensed from 
the right ventricular region along with the sensed cardiac signals from 
the left ventricular region. In one embodiment, sensing a cardiac signal, 
or signals, from the right ventricular region is accomplished through the 
use of an Endotak transvenous electrode catheter (CPI/Guidant, St. Paul, 
Minn.). The Endotak catheter includes a sensing/pacing tip electrode for 
sensing near-field ventricular signals, and a first and second 
defibrillation coil electrode used in sensing far-field cardiac signals. 
Cardiac signals sensed from both the right and left ventricular regions 
can then be analyzed as previously described. In one embodiment, sensing 
and analyzing both far-field signals allows the ischemic state of the 
heart to be more accurately assessed than the situation where only the far 
field signal sensed from the right side of the heart is analyzed. 
Referring now to FIG. 5, there is shown one embodiment of a system 500 
according to the present subject matter. The system 500 includes a first 
catheter 506 physically and electrically coupled to an implantable cardiac 
rhythm management device (ICRMD) 516. Examples of ICRMDs 516 include 
implantable cardioverter defibrillators (ICDs), ICDs with cardiac pacing 
capabilities, and implantable cardiac pacemakers. 
In one embodiment, the first catheter 506 includes a first electrode 508 
positioned along the peripheral surface of the first catheter 506. In one 
embodiment, the first electrode 508 is positioned adjacent the distal end 
of the first catheter 506. In an additional embodiment, the housing of the 
ICRMD 516 is a second electrode 510 and is used to sense far-field signals 
between the first electrode 508 and the second electrode 510. 
In addition to having the first catheter 506, it is possible to have 
additional catheters physically and electrically coupled to the ICRMD. For 
example, a second catheter 512 can be included in the present system, 
where the second catheter 512 includes a third electrode 516, a fourth 
electrode 524 and a sensing/pacing electrode 530. The electrodes 516, 524 
and 530 are positioned on the peripheral surface of the second catheter 
512 and electrically coupled to electronic control circuitry contained 
within the ICRMD 516. In one embodiment, the second catheter 512 is an 
Endotak lead.TM. (CPI/Guidant, St. Paul, Minn). 
The ICRMD 516 contains control circuitry which receives cardiac signals 
sensed between predetermined combinations of electrodes and the housing of 
the ICRMD 516. The control circuitry housed within the ICRMD 516 amplifies 
the cardiac signals being sensed between the electrodes and the housing 
and analyzes and records cardiac data on a plurality of cardiac complexes 
as previously described. 
In addition to measuring and recording the cardiac signals during the 
cardiac complexes, the ICRMD 516 includes telemetry circuitry which allow 
for communication with a medical device programmer 550. In one embodiment, 
medical device programmer 550 is used to receive and transmit programming 
and operating instructions, including those for carrying out the present 
subject matter, to the ICRMD 516. Additionally, the medical device 
programmer 550 is used to download data relating to the recorded voltages 
and cardiac complexes analyzed using the control circuitry housed within 
the ICRMD 516. In one embodiment, the commands, instructions and data are 
transmitted over a radio frequency telemetric link 560 established between 
the ICRMD 516 and the medical device programmer 550. 
Upon downloading data relating to the plurality of cardiac complexes, the 
medical device programmer 550 may be used to plot the voltage values 
recorded for the plurality of cardiac complexes as a function of time when 
the cardiac complex occurred. In one embodiment, this information is 
plotted on a display screen 570 of the medical device programmer 550 for 
review and analysis. This information can then be used in identifying the 
occurrence of myocardial ischemia. For example, upon plotting the voltage 
levels of the cardiac complexes as a function of time, regions along the 
plot where the values of voltages deviate from approximately zero (little 
to no deviation from the first standard) can be located and identified as 
regions indicating the occurrence of an ischemic event. In addition, 
knowledge of the ischemic history of a patient could be useful in 
selecting/optimizing device-based therapy for the prevention of arrhythmia 
onset. Additional information related to each of the plotted points can 
also be recalled and displayed on the display screen 570 by using a 
pointing device, such as a computer mouse, to highlight and select a 
cardiac complex of interest. 
Referring now to FIG. 6, there is shown an embodiment of a block diagram of 
an implantable cardiac rhythm management device (ICRMD) 600. The ICRMD 600 
includes control circuitry 602 which receives one or more cardiac signals 
and determines and records the voltage of the cardiac signals during 
predetermined portions of sensed cardiac complexes. In one embodiment, the 
control circuitry 602 is a programmable microprocessor-based system, with 
a microprocessor 604 and a memory circuit 606, which contains parameters 
for various pacing and sensing modes and stores data indicative of cardiac 
signals received by the control circuitry 602. The control circuitry 602 
further includes terminals labeled with reference numbers 608, 610, 612 
and 614 for connection to the electrodes attached to the surface of a 
first catheter and a second catheter as previously described. 
The control circuitry 602 is encased and hermetically sealed in a housing 
620. In one embodiment, the housing 620 is suitable for implanting in a 
human body. In one embodiment, the housing 620 is made of titanium, 
however, other biocompatible housing materials as are known. A connector 
block 624 is additionally attached to the housing 620 to allow for the 
physical and the electrical attachment of catheters and electrodes to the 
ICRMD 600 and the encased control circuitry 602. In one embodiment, the 
connector block 624 includes at least a first input/output socket (not 
shown) for allowing a lead connector of the first catheter to be coupled 
to the ICRMD 600. In the present embodiment, a first and second 
input/output sockets are provided to allow for the lead connectors of the 
first and second catheters to be coupled to the ICRMD 600. 
The ICRMD 600 includes terminal 608 which receives electrical signals from 
the first electrode 508. Terminal 608 and the housing 620 are coupled to 
sense amplifier 626 to allow for a first cardiac signal (a far-field 
signal) to be sensed between the first electrode 508 and the housing 620. 
The ICRMD 600 also includes terminals 610 and 612 which are coupled to the 
third electrode 516 and the fourth electrode 524, respectively. In 
addition, the housing 620 of the ICRMD 600 is coupled in common with the 
fourth electrode 524. Terminals 610 and 612 are coupled to sense amplifier 
628 to allow for a second cardiac signal (a far-field signal) to be sensed 
between the third electrode 516 and the fourth electrode 524. 
Alternatively, the second cardiac signal is sensed between the third 
electrode 516 and the fourth electrode 524/housing 620, which are in 
common. The ICRMD 600 further includes terminal 614 which receives 
electrical signals from the pacing/sensing electrode 530. The terminals 
610 and 614 are coupled to sense amplifier 630 to allow for a third 
cardiac signal (a near-field signal) to be sensed between the 
pacing/sensing electrode 530 and the third electrode 516. Other catheter 
and electrode combinations can also be used with the present embodiment. 
Additionally, changes to the number and types of catheters, electrodes and 
sensing electronics can be made to the present system without departing 
from the present subject matter. 
The ventricular activation detector circuit 636 is coupled to the first 
input/output socket, where the ventricular activation detector circuit 
receives a first cardiac signal through the first input/output socket and 
analyzes the first cardiac signal to detect the occurrence of a 
ventricular activation in a first cardiac complex. In one embodiment, the 
first cardiac signal is received from sense amplifier 630, which is 
coupled to the ventricular activation detector 636. In one embodiment, the 
first cardiac signal is a near-field signal which the ventricular 
activation detector circuit 636 analyzes to detect the occurrence of 
ventricular activations. 
Alternatively, the first cardiac signal is received from sense amplifier 
626, which is coupled to the ventricular activation detector 636. In one 
embodiment, the first cardiac signal is a far-field signal sensed between 
the first electrode and the housing 620 for which the ventricular 
activation detector circuit 636 analyzes to detect the occurrence of 
ventricular activations. 
The cardiac morphology detector circuit 640 is also coupled to the first 
input/output socket. In one embodiment, the output of sense amplifiers 626 
and 628 are shown coupled to cardiac morphology detector 640 so that the 
cardiac morphology detector circuit 640 receives the first cardiac signal 
through the first input/output socket. The cardiac morphology detector 
circuit 640 is also coupled to the ventricular activation detector circuit 
636. The cardiac morphology detector circuit 640 analyzes the first 
cardiac signal to detect the occurrence of cardiac complexes. 
As a cardiac complex is sensed by both the ventricular activation detector 
circuit 636 and the cardiac morphology detector circuit 640, the 
ventricular activation detector 636 detects the occurrence of the 
ventricular activation and provides a signal to the cardiac morphology 
detector circuit 640. When the cardiac morphology detector circuit 640 
receives the signal, it measures the first voltage value from the cardiac 
signal at the first specified time after the ventricular activation in the 
cardiac complex. In one embodiment, the first specified time after the 
ventricular activation is a programmable value which is stored in the 
memory 606 of the ICRMD 600. 
In an additional embodiment, the cardiac morphology detector circuit 640 
measures two or more voltages of the first cardiac electrogram signal at a 
predetermined sampling frequency between the first specified time and a 
second specified time after the occurrence of the ventricular activation. 
The cardiac morphology detector circuit 640 then calculates the first 
voltage value of the first cardiac electrogram signal from the two or more 
voltages as previously described. In one embodiment, the predetermined 
sampling frequency is programmed into the ICRMD 600 and used by the 
cardiac morphology detector 640 to time the voltage measurements. 
In an alternative embodiment, the first specified time is calculated from 
consecutively sensed pairs of ventricular activations (ventricular 
intervals) sensed from cardiac complexes. For example, the ventricular 
activation detector circuit 636 analyzes the first cardiac signal to 
detect the occurrence of ventricular activation in a second cardiac 
complex, wherein the second cardiac complex precedes the first cardiac 
complex. The cardiac morphology detector circuit 640 then calculates a 
ventricular activation interval from the ventricular activation of the 
first cardiac complex and the ventricular activation of the second cardiac 
complex. The cardiac morphology detector circuit 640 then calculates the 
first specified time by taking a predetermined percentage of the 
ventricular activation interval as previously discussed. 
The cardiac morphology detector circuit 640 also measures the second 
voltage value of the first cardiac electrogram signal at the defined 
portion of the first cardiac complex as previously discussed. In one 
embodiment, the cardiac morphology detector circuit 640 measures the 
second voltage value between the P-wave and the Q-wave of the first 
cardiac complex. A cardiac data processing circuit 644 is also included in 
the control circuitry 602, and is coupled to the ventricular activation 
detector circuit 636 and the cardiac morphology detector circuit 640. The 
cardiac data processing circuit 644 calculates the comparison value from 
the first and second voltage values. 
The microcontroller 604 is coupled to the cardiac data processing circuit 
644 the ventricular activation detector circuit 636 and the cardiac 
morphology detector circuit 640. The microcontroller 604 receives the 
comparison value from the cardiac data processing circuit 644. The 
microcontroller 644 then compares the comparison value to the 
predetermined values stored in the memory 606 to determine whether the 
comparison value is greater than or equal to the predetermined value. The 
microprocessor 604 then records the first voltage value in the memory 606 
when the comparison value is greater than or equal to a predetermined 
value. 
As previously discussed, a first timer is used to time a first 
predetermined time over which the ICRMD 600 waits before carrying out the 
described subject matter. The microcontroller 640 contains the first timer 
which counts off the first predetermined time. When the first time expires 
the microcontroller 640 controls the ventricular activation detector 
circuit 636 and the cardiac morphology detector circuit 640 to analyze the 
cardiac signals, such as the first cardiac signal, to detect the 
occurrence of the ventricular activation in the first cardiac complex, and 
to preform the previously described operations. 
In an additional embodiment, the microcontroller 605 contains the second 
timer. The second timer overrides the first timer and counts off the 
second predetermined time, which is shorter in duration than the first 
predetermined time. The microcontroller 604 starts the second timer when 
the comparison value is greater than or equal to the predetermined value. 
After the second time expires, the microcontroller 604 signals the 
ventricular activation detector circuit 636 and the cardiac morphology 
detector circuit 640 to analyze the first cardiac signal for the 
occurrence of the ventricular activation in the first cardiac complex and 
to measure the first and second voltage values after the second 
predetermined time expires. The microprocessor 604 then restarts the 
second timer when the comparison value is greater than or equal to the 
predetermined value. The second predetermined time is a programmable value 
which is stored in the memory 606 of the ICRMD 600. 
In addition to the second timer, the microcontroller 604 can also contain a 
duration timer which counts off a duration interval time. In one 
embodiment, the microcontroller 604 starts the duration timer and the 
second timer when the comparison value is greater than or equal to the 
predetermined value. The microcontroller 604 then signals the ventricular 
activation detector circuit 636 and the cardiac morphology detector 
circuit 640 to analyze the first cardiac signal for the occurrence of the 
ventricular activation in the first cardiac complex after the second 
predetermined time expires. The cardiac morphology detector circuit 640 
then measures the first and second voltage values. The microprocessor 604 
then restarts the second timer again, and continues to use the second 
timer while the interval of the duration timer is being timed. 
As previously discussed, the rate of voltage change between portions of 
cardiac complexes can be used to determine whether to begin measuring 
voltage values at the second predetermined time period. In one embodiment, 
the ventricular activation detector circuit 636 analyzes the first cardiac 
signal for the occurrence of ventricular activation in a second cardiac 
complex. The cardiac morphology detector circuit 640 also analyzes the 
first cardiac signal for the occurrence of the second cardiac complex. The 
cardiac morphology detector circuit 640 then measures the second voltage 
value from the first cardiac signal at the first specified time after the 
ventricular activation in the second cardiac complex. The cardiac data 
processing circuit 644 then calculates the comparison value as a rate of 
voltage change between the first and second voltage values. Additionally, 
the cardiac data processing circuit 644 can be used to calculate the rate 
at which voltage values change between consecutively measured cardiac 
complexes once the use of the second specified time period has begun. 
In an additional embodiment, the ventricular activation detector circuit 
636 analyzes the first cardiac signal for the occurrence of ventricular 
activation in a second cardiac complex. In addition to the ventricular 
activation detector circuit 636 sensing the second cardiac complex, the 
cardiac morphology detector circuit 640 also senses and analyzes the 
second cardiac complex in the first cardiac signal. The cardiac morphology 
detector circuit 640 then measures the first voltage value of the first 
cardiac signal at the first specified time after the ventricular 
activation in the second cardiac complex. In addition, the cardiac 
morphology detector circuit 640 measures the second voltage value at the 
defined portion of the first cardiac signal from the first cardiac 
complex. Also, the cardiac morphology detector circuit 640 measures the 
second voltage value at the defined portion of the first cardiac signal 
from the second cardiac complex. 
As each of the first and second voltage values are measured from the first 
and the second cardiac complexes, then are supplied to the cardiac data 
processing circuit 644. As the first and second voltage values measured 
from the first cardiac complex are measured, the cardiac data processing 
circuit 644 calculates the comparison value. Likewise, as the first and 
second voltage values are measured from the second cardiac complex, the 
cardiac data processing circuit 644 calculates the comparison value for 
the second cardiac complex. The cardiac data processing circuit 644 then 
calculates the rate of voltage change between the comparison value 
calculated from the first cardiac complex and the comparison value 
calculated from the second cardiac complex. The microcontroller 604 then 
records the first voltage value of both the first cardiac complex and the 
second cardiac complex in the memory 606 when the rate of voltage change 
is greater than or equal to the predetermined value. 
Power for the ICRMD 600 is supplied by battery 650. In addition, the ICRMD 
600 includes a transmitter/receiver 654 for transmitting and receiving 
programming instructions, parameter values, cardiac data, and other 
information related to the functioning of the ICRMD 600 between the 
medical device programmer 660. In one embodiment, transmitting and 
receiving of information is accomplished over a radio frequency telemetric 
link established between the ICRMD 600 and the medical device programmer 
660. 
In addition to measuring the voltage value of cardiac signals along the 
ST-segment, voltages values and duration of other portions of the cardiac 
complex can be measured using the present subject matter. For example, 
R-wave amplitude and duration, ST-segment duration, TQ-segment amplitude 
and duration, and T-wave amplitude, duration, and uniformity can be sensed 
and evaluated according to the present subject matter.