Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-12-09
2001-06-12
Jastrzab, Jeffrey R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06246909
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to implantable stimulator devices capable of providing electrical stimuli to the heart. The invention particularly relates to devices in which the spontaneous rate variability is measured.
DESCRIPTION OF THE PRIOR ART
A heartbeat can be divided into two phases: the filling phase and the ejection phase. These phases are shown diagrammatically in
FIGS. 1A-1E
in which the two atria are indicated by A and the two ventricles by B with the heart valves indicated by C. In a first phase (
FIG. 1A
) the atria and ventricles are empty and blood arrives from the veins into the atria and when these are full the blood passes via the one-way heart valves into the ventricles. The atria then contract (see
FIG. 1B
) to pump blood from the atria into the ventricles. The state is thus reached (see
FIG. 1C
) in which the atria are empty whilst the ventricles are full of blood. The ventricles then contract as shown in
FIG. 1D
to push the blood into the arteries. This is the heart beat. After the re-expansion of the ventricles the heart has arrived at the final stage (see
FIG. 1E
) has now returned to the same state as the initial stage of the heartbeat cycle.
As can be seen this cycle requires the heart to act in a coordinated sequential manner. In certain heart diseases this cycle is disturbed so that for instance the ventricle does not contract immediately after filling with blood but may contract too soon or too late. In order to treat this type of disorder, use is made of the fact that electrical pulses cause muscles to contract. Pacemakers were therefore developed to provide electrical pulses at appropriate times to a diseased heart so that it would function in the same manner as a healthy heart.
The basic function of a pacemaker is to provide pacing pulses for the muscles of the heart so that a diseased heart beats in a rhythm similar to that at which a healthy heart should beat. Where possible the pacemaker only provides a pacing pulse in a situation where the natural contraction of an atrium or ventricle does not take place at the expected appropriate point in time.
Although pacemakers have been used since about 1958 and are an undoubted success they are not always able to ensure that a diseased heart always functions in the same way as a healthy heart. One reason for this is that even for healthy individuals the heart does not always beat to the same rhythm, e.g. the natural heartbeat rate varies from individual to individual. Also for a single individual the rhythm may show variations from one beat to the next.
As mentioned above there may be variations in the time between heartbeats from one beat to the next. The time length for an unpaced heartbeat cycle, i.e. the length of time for the heart to complete a cycle, is known as the spontaneous interval. The time interval between the atrial contraction (
FIG. 1B
) and the ventricular contraction (
FIG. 1D
) within a cycle is known as the AV-delay interval.
During clinical studies performed on animals, a variation in the spontaneous interval has been observed. This spontaneous interval variation (SIV) is not a result of workload, psychological or other influence. In a stable situation with no change of influence of environmental factors there may be a long spontaneous interval. This long interval is then followed by a short spontaneous interval and vice-versa. Corresponding variations in the AV-delay have been observed.
The range of variation is found to be different from one species to another, but it is always present. The interval variation in dogs has been found to be extreme compared to that of, for instance, sheep or pigs. Maximum variability is observed when the dog is awake but at rest and unstressed. On some occasions spontaneous intervals are found to vary from 412 ms (milliseconds) to 1505 ms between heartbeats under the same conditions. A corresponding variation in AV-delay is also observed (e.g. from 82 to 125 ms).
FIG. 2
shows an IEGM (intracardiac electrogram) for a dog at rest with a pacemaker operating in DDD mode at a rate of 30 per minute and an A-V delay of 250 ms. The variation in the spontaneous rate interval is clearly visible here.
It is known from literature that a low spontaneous interval variation may result from ischaemia, age and heart insufficiency. It is also known that spontaneous interval variation is the result of Vagus (Parasympathetic) nerve activity whereas heart rate increase results from Sympathetic nerve system activity.
It appears that the extreme variation in dogs is in some way connected with dogs' ability to react instantaneously, i.e. to go from rest to maximum alert with minimal delay. Also the fact that interval variation is known to be reduced in humans by disease, age and disabilities leads to the conclusion that a high variability in the spontaneous interval indicates a high reserve of available capacity. High spontaneous interval variation in humans appears to be a sign of health whereas a low spontaneous interval variation is an indicator of heart insufficiency.
The variation in the spontaneous interval causes a problem with Bradycardia stimulation. In this case the interval before a pacing pulse is given is prolonged so as to allow for a spontaneous pulse (hysteresis). This time is difficult to determine due to the spontaneous interval variation and if the hysteresis time is set too long for an unhealthy heart it can lead to a dangerously low heartbeat rate. Also, with a response rate pacemaker the maximum rate allowed may be too high for an unhealthy heart. The present invention aims to overcome these problems.
According to the abstract of U.S. Pat. No. 5,265,617 there is disclosed a device in which heart rate variability, i.e. the variability or the time between successive R-waves, together with T-wave alternans are measured and are used together for diagnosing cardiac vulnerability to ventricular fibrillation.
EP-A-647 454 states that the changes in the difference between the sensor indicated pacing rate and the actual pacing rate can be used as a measure of the time rate of change of the sensor rate. Variations in the intrinsic heart rate are not apparently measured.
According to the abstract of U.S. Pat. No. 5,466,245, in a DDD-type pacemaker the A-V delay is optimised by finding its value associated with the minimum heart rate variability index. The natural heartbeat is not used.
WO-93/16756 discloses a pacemaker in which the differences between two successive spontaneous intervals are measured. An average is taken of only the positive values of these differences.
EP-358 303 discloses taking the mean value of a number of successive spontaneous intervals. The mean value is then itself used to control a parameter.
SUMMARY OF THE INVENTION
it is an object of the present invention to provide a cardiac stimulator wherein the aforementioned problems associated with known cardiac stimulators are avoided.
The object is achieved in accordance with the principles of the present invention in an implantable stimulating device for electrical stimulation of the heart, having a stimulus generator which generates electrical pacing stimuli and at least one connection lead for delivering the stimuli from the generator to the heart, a detector for identifying spontaneous intervals between successive intrinsic heartbeats and for measuring a variation with respect to the average of at least five of these spontaneous intervals. A signal representing this variation is supplied to a control unit which controls a pacing control parameter of the stimulus generator dependent on the value of the variation.
This invention is based on the recognition that the spontaneous interval variation in the intrinsic heartbeat can be used to control parameters of a pacemaker or other device generating electrical stimuli or to monitor the heart.
These spontaneous intervals are present for almost all pacemaker patients, e.g. patients having so-called AV II-block have up to 75% spontaneous heartbeats.
Intrinsic heartbeat means the natural heartbeat without external in
Jastrzab Jeffrey R.
Pacesetter AB
Schiff Harlin & Waite
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