System for detecting pacer mediated tachycardia

A system is provided for detecting pacer mediated tachycardia. Atrial electrical signals (P waves) are sensed and ventricular electrical signals (R waves) are sensed. When a P wave is sensed, after a predetermined AV delay, a stimulating pulse is provided to the ventricle if no R wave is sensed during the AV delay. A threshold rate is selected for the sensed PP interval. If the ventrical has been stimulated and the threshold rate is exceeded for a selected number of PP intervals, the AV delay is increased by a predetermined time (delta). A determination is made whether the next sensed PP interval has increased by delta. If the next sensed PP interval has increased by delta, then pacer mediated tachycardia has occurred. In another embodiment, if the threshold rate is exceeded for a selected number of PP intervals, and the ventricle has been stimulated, then the sensed PP interval is measured and the AV delay is increased to a desired minimum PP interval minus the actual measured PP interval. If the rate continues to be greater than the threshold on the next heartbeat, then pacer mediated tachycardia is not present, and the initial AV delay is reestablished.

BACKGROUND OF THE INVENTION 
The present invention concerns a novel system for detecting pacer mediated 
tachycardia. 
In cardiac pacers in which electrical signals from the atrium are sensed 
and the ventricle is stimulated after a selected AV delay, the possibility 
of a pacer mediated tachycardia exists where there is retrograde 
conduction of the ventricular stimuli. To understand this phenomenon more 
clearly, reference is made to FIG. 1. For purposes of this discussion, it 
will be assumed that the pacer is operating in a demand mode, such as a 
DVI mode or a DDD mode. For simplicity, only the P waves and R waves of 
the heart's electrical waveform are illustrated. The upwardly-pointing 
arrows signify sensing and the downwardly-pointing arrows signify a 
stimulating pulse. 
In FIG. 1 there is a normal heartbeat. The P wave is sensed at time 10 in 
the atrial channel and after an AV delay 14, the R wave is sensed at time 
16 in the ventricular channel 18. Under these conditions, no pacing pulses 
are generated. In FIG. 2, however, there is AV block. Thus, the P wave is 
sensed in the atrial channel, but no R wave is sensed in the ventricular 
channel during a selected AV delay 14. Therefore, a stimulus is provided 
to the ventricle at time 16 which is the end of the selected AV delay. 
FIG. 3 also illustrates an AV block situation. However, in FIG. 3 there is 
retrograde conduction 19 whereby the ventricular stimulus 20 conducts back 
to the atrium and, at time 22, the electrode in the atrium senses the 
retrograde P wave that has been caused by the pacer stimulus. Since no R 
wave is detected during the next AV delay 14a, at the end of the AV delay 
14a another stimulus 24 is provided to the ventricle. Again, this other 
stimulus 24, if there is retrograde conduction, conducts back to the 
atrium causing the atrial electrode to sense another retrograde P wave at 
time 26. The cycle will repeat, resulting in tachycardia that is pacer 
mediated. 
Referring now to FIG. 4, a heartbeat rate that is equal to the heartbeat 
rate of FIG. 3 is illustrated, although the FIG. 4 heartbeat is sinus 
tachycardia, not pacer mediated tachycardia. 
It is, therefore, an object of the present invention to determine whether a 
detected tachycardia is pacer mediated. 
Another object of one embodiment of the present invention is to modify the 
pacer operation in the event that the tachycardia is pacer mediated. 
Other objects and advantages of the present invention will become apparent 
as the description proceeds. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a system is provided for 
detecting pacer mediated tachycardia. The system comprises the steps of 
sensing electrical signals from the atrium and sensing electrical signals 
from the ventricle. A stimulating pulse is provided to the ventricle, at a 
first predetermined time after an electrical signal from the atrium is 
sensed, if no electrical signal from the ventricle is sensed within the 
first predetermined time. A threshold rate for the interval between sensed 
electrical signals from the atrium is selected. A detection is made 
whether the threshold rate is exceeded for a selected number of electrical 
signals from the atrium. If the threshold rate is exceeded for the 
selected number of electrical signals from the atrium, then the first 
predetermined time is increased by a second predetermined time. A 
determination is then made whether the interval between sensed electrical 
signals from the atrium has increased by the second predetermined time. If 
the interval between sensed electrical signals from the atrium has 
increased by the second predetermined time, this indicates that pacer 
mediated tachycardia has occurred. 
In the illustrative embodiment, the system also includes the step of 
determining if a stimulating pulse has been provided to the ventricle 
prior to the step of increasing the first predetermined time by the second 
predetermined time. 
In another embodiment of the invention, if the threshold rate is exceeded 
for the selected number of electrical signals from the atrium and it is 
determined that a stimulating pulse has been provided to the ventricle, 
then the first predetermined time is increased by a minimum desired time 
interval between sensed electrical signals from the atrium minus the 
actual sensed time interval between electrical signals from the atrium. It 
is then detected whether the threshold rate is exceeded for the next 
heartbeat. If the threshold rate is exceeded for the next heartbeat, this 
indicates a pacer mediated tachycardia is not present, and then the first 
predetermined time interval is reestablished. On the other hand, if the 
rate on the next heartbeat is less than threshold, this indicates pacer 
mediated tachycardia but the pacer has been modified to prevent the rate 
from exceeding a desired rate. 
A more detailed explanation of the invention is provided in the following 
description and claims, and is illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT 
The system of the present invention will be discussed using the operation 
of a DDD mode cardiac pacer. An example of a prior art DDD mode cardiac 
pacer capable of being programmed to operate in the manner described in 
the present specification is disclosed in Vollmann U.S. Pat. No. 
4,467,810. In the DDD mode, a sensing and pacing electrode is connected to 
the atrium and a pacing and sensing electrode is connected to the 
ventricle. When a P wave is sensed, no atrial stimulus is issued and an 
atrial refractory period is provided during which sensed P waves are 
ignored. When the P wave is sensed, however, after a selected AV delay the 
ventricle is stimulated. However, if an R wave is sensed during the AV 
delay, the stimulating pulse to the ventricle is inhibited. When an R wave 
is sensed or when a stimulating pulse is issued to the ventricle, the 
timing cycle is reset. 
In order to determine whether there is pacer mediated tachycardia, 
referring to FIGS. 5 and 6, a threshold rate for sensed P waves is 
selected, for example, 130 beats per minute. A determination is then made 
whether this threshold rate is exceeded for a selected number of beats, 
for example, four beats. If the threshold rate is exceeded for four beats, 
after the next P wave is sensed a determination is made whether the 
ventricle has been stimulated at the end of the AV delay. If the ventricle 
has been stimulated, the AV delay is then changed by a selected amount 
(delta), for example, 150 milliseconds. Thereafter, the interval between P 
waves is measured. If the interval between P waves is now greater by 150 
milliseconds than the previous interval between P waves, as shown in FIG. 
5, this will indicate that the tachycardia is pacer mediated. On the other 
hand, if the interval between P waves remains substantially the same as 
the interval between P waves prior to the increase of the AV delay, this 
indicates that there is no pacer mediated tachycardia. 
If there is pacer mediated tachycardia, one method of modifying the problem 
is to provide an extended atrial refractory period on the next cycle for a 
substantial time, for example, 500 milliseconds, beyond the ventricular 
stimulus, to mask the detection of the retrograde P wave. 
Another method of correcting for pacer mediated tachycardia is to stimulate 
the atrium shortly after the detection of the retrograde P wave, to cause 
the depolarization of the reentry path and to make retrograde conduction 
impossible following the next ventricular stimulus. 
Referring to FIG. 7, another method of detecting pacer mediated tachycardia 
is illustrated. In the FIG. 7 embodiment, if the rate is detected to be 
greater than the threshold for four beats, then a determination is made 
whether the ventricle has been stimulated. If the ventricle has been 
stimulated, the sensed PP interval is measured. The AV delay is then 
increased to a desired minimum PP interval minus the actual measured PP 
interval. For example, assume the minimum desired PP interval is 1,000 
milliseconds, the original AV delay is 200 milliseconds, and the actual 
measured PP interval is 500 milliseconds (the 200 millisecond AV delay 
plus a 300 millisecond retrograde false P wave). The 500 milliseconds is 
subtracted from the 1,000 milliseconds and the original AV delay is thus 
increased by 500 milliseconds resulting in a new AV delay of 700 
milliseconds. A determination is then made whether the rate is less than 
the threshold on the next beat. If the rate is less than the threshold on 
the next beat, then there is pacer mediated tachycardia. However, the 
increase to the AV delay (which in this embodiment resulted in a 700 
millisecond AV delay) effectively modified the operation of the pacer to 
prevent the rate from exceeding a selected maximum rate. 
On the other hand, if the rate is greater than threshold on the next beat, 
the tachycardia was not pacer mediated and the AV delay is reset to its 
original value of, for example, 200 milliseconds. 
Although illustrative embodiments of the invention have been shown and 
described, it is to be understood that various modifications and 
substitutions may be made without departing from the novel spirit and 
scope of the present invention.