Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-04-17
2003-07-01
Jastrzab, Jeffrey R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06587722
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a device for determining the effectiveness of stimulation in an electrical heart stimulator.
BACKGROUND OF THE INVENTION
In the practice of heart stimulation, one of the characteristic features of a stimulator is the length of its service life, that is the service life of the power source (typically a battery) which powers it. This length of time is directly linked to the power consumption of the stimulating system, a significant component of which is the energy released in the form of electrical stimulation applied to the heart muscle.
The significance of this aspect is particularly obvious in systems which are designed to be implanted in a patient's body. A stimulus is effective (and in this case it is said that it has “captured” the heart) if its energy exceeds a minimum value, the so-called “stimulation threshold” or “capture threshold”. This threshold value depends on the stimulating system and the characteristics of the heart muscle involved.
In particular, it cannot be assumed that the value of the stimulation threshold remains constant over time. Because in current practice the energy of the stimulus is decided upon and set by the cardiologist when the unit is checked, and cannot be altered until a subsequent check, the solution currently adopted is to set the energy of the stimulus at a value substantially higher than the stimulation threshold. This is in order to guarantee effective stimulation for different stimulation threshold conditions. A consequence of this is the fact that the energy delivered by the heart stimulator with every stimulus can be very much greater (even four times or more greater) than the minimum which is necessary and sufficient.
There is therefore in general a need to have systems such that energy can be saved when providing the stimulating action, while at the same time ensuring that the effectiveness of the stimulation is constant. This is in order to provide a significant advantage in the design of a heart stimulator, among other things providing a longer service life for the device.
As a rule, satisfaction of the requirement stated above requires that the stimulator be capable of establishing whether it has successfully induced contraction of the heart muscle when delivering a stimulus. With this information the system can establish the value of the stimulation threshold sufficiently frequently (and even for each individual stimulus) and adjust the energy of the stimulus to minimize the proportion of energy which is actually wasted.
In general terms, the stimulus delivery system can be regarded as an electrical circuit comprising the stimulator itself, the electrode which delivers the stimulus to the heart and the complex of physiological tissues which returns the stimulus current to the stimulator: the area of heart muscle in contact with the terminal of the stimulation electrode constitutes the “active” part of the electric circuit.
The behavior of this circuit has special features which are generally known and do not therefore need to be referred to in detail here. This is apart from one aspect, which is linked to the fact that once the stimulus—comprising a short electrical pulse of the magnitude of the order of a few volts and lasting of the order of a millisecond—has come to an end, part of its energy remains trapped in the circuit, giving rise to an appreciable potential difference which decreases over time as this energy is dissipated until the entire system returns to its initial conditions over a period of a few hundred milliseconds.
This tail electrical potential, usually known as the post-potential or stimulation artifact, or again the electrode polarization potential, may have a magnitude—measured immediately after stimulation—which is still of the order of a hundred millivolts. A typical profile for a post-potential signal of the type described is illustrated in profile a) of FIG.
1
.
On the other hand, in addition to mechanical contraction of the muscle, the heart's response to an effective stimulus is also manifested by an electrical response, known as the evoked potential, which is linked to the electrical activity of the cells during the contraction stage. This electrical potential (having the characteristics of a pulse of varying shape, lasting a few tens of milliseconds and of a magnitude of a few millivolts, which typically arises 10 to 50 milliseconds after the stimulus) can also be observed in the stimulator circuit, but superimposed on the stimulation post-potential. The magnitude of the latter may however be such as to render identification of the evoked response in the heart difficult.
A typical profile of an evoked response signal is shown in the bottom diagram, indicated by b) in FIG.
1
. It will be appreciated that the two diagrams a) and b) in
FIG. 1
are not to scale and that typically the peak for the post-potential signal may correspond to a value 10 to 100 times greater than the peak value for the evoked response signal. The waveform which can be observed after each effective stimulus is the result of the overlap (algebraic sum) of the two waveforms illustrated. If the stimulus is not effective, the component due to the evoked response (diagram b) will obviously be absent.
The complexity of problems described above has already been considered by the art through the adoption of a variety of solutions. There are systems in which detection of the evoked response is based on an analog filtering process with amplification of the potential measured on the stimulating electrode in comparison with a reference potential. Solutions of this type are described in for example documents EP-A-0 717 646, U.S. Pat. No. 5,561,529, U.S. Pat. No. 5,443,485, U.S. Pat. No. 5,718,720 and U.S. Pat. No. 5,873,898.
In substance these solutions provide for the greatest possible amplification of the evoked response and attempt to suppress the undesired part due to the stimulus post-potential as much as possible (typically through filtering).
This process has however proved difficult because, in the first place, as has been seen, the signal corresponding to the stimulus post-potential usually has a magnitude which is very much greater than the signal corresponding to the heart's evoked response, and the frequency spectra of the two signals in question largely overlap and therefore cannot be separated by filtering in the frequency field.
In particular an amplification and linear filtering system can easily be saturated by the post-potential signal, thus making it impossible to detect any evoked response by the heart.
The functioning of other systems is based on the presence or absence of events which are indirectly linked with capture, such as e.g., the occurrence of spontaneous heart contractions before and after the stimulus which are detected by methods which are well-known in the art of heart stimulation (see for example documents EP-A-0 850 662 and U.S. Pat. No. 5,861,012).
Of the methods based on knowledge of past events, some operate by comparing the profile of the potential after the stimulus with a sample signal in which only the post-potential is present without the evoked response. In order to establish that the heart muscle has been captured in a generic stimulus, the corresponding signal is compared with the sample signal, and capture is therefore stated to have occurred when the differences with respect to the sample are sufficiently large.
Solutions of this type are described in documents U.S. Pat. No. 4,674,508, , U.S. Pat. No. 4,686,988, U.S. Pat. No. 4,729,376, U.S. Pat. No. 4,817,605, U.S. Pat. No. 5,350,410, and U.S. Pat. No.5,417,718.
These systems have two main disadvantages. First, in order to obtain a sample signal, it is necessary to perform a specific operation comprising the release of a stimulus which is reliably ineffective (there are various techniques for achieving this result) followed by recording of the response generated. Second, the form and amplitude of the stimulation artifact can change, and in fact change in relation to the energ
Silvestri Luigi
Vitali Luca
Jastrzab Jeffrey R.
Popovich & Wiles, P.A.
Sorin Biomedica Cardio S.p.A.
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