Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-03-21
2002-12-31
Layno, Carl (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06501989
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an evoked response detector for a heart stimulator for determining evoked response in the presence of polarization, the heart stimulator having a pulse generator and a control unit for controlling the pulse generator to produce stimulation pulses of varying amplitudes, and a lead being intended to be introduced into the heart of a patient and connected to the pulse generator for delivering stimulation pulses to the heart, and wherein the evoked response detector has a measuring and memory unit for measuring and storing the electrode signal picked up by the lead in response to delivered stimulation pulses.
2. Description of the Prior Art
To reduce the energy consumption of heart stimulators an automatic threshold search function, a so called AUTOCAPTURE™ function, is provided to maintain the energy of the stimulation pulses at a level just above that which is needed to effectuate capture, cf. e.g. U.S. Pat. No. 5,458,623. A reliable detection of the evoked response, which then is necessary, is, however, not a simple matter, especially when it is desired to sense the evoked response with the same electrode as the one delivering the stimulation pulse. This is because of the fact that the evoked response potential is small in amplitude compared to the residual polarization charge. The residual charge decays exponentially but tends to dominate the evoked potential for several hundreds of milliseconds after the stimulation. If the polarization is too high, it could be wrongly interpreted by the evoked response detector as a capture, i.e. contraction of the heart. The AUTOCAPTURE™ algorithm could then by mistake adjust the output amplitude of the stimulation pulse to a value below the actual capture level, which will result in no capture. If the used pacing lead has significant polarization this could consequently disturb the AUTOCAPTURE™ function and result in loss of capture.
Several attempts have been made to solve the lead polarization problems in connection with evoked response detection. One way of reducing these problems is to use special low polarizing leads.
Another method is described in U.S. Pat. No. 5,417,718, which discloses a system for maintaining capture wherein electrical post-stimulus signal of the heart, following delivery of a stimulation pulse, is compared to a polarization template, determined during a capture verification test. A prescribed difference between the polarization template and the post-stimulus signal indicates capture. Otherwise loss of capture is presumed and the stimulation energy is increased a predetermined amount to obtain capture.
In U.S. Pat. No. 5,697,957 a method and an apparatus are described for extracting an evoked response component from a sensed cardiac signal by suppressing electrode polarization components. An autocorrelation function is then calculated according to an autocorrelation algorithm and applied to the sensed cardiac signal. The autocorrelated signal thus obtained and the sensed cardiac signal are normalized and the difference between these two normalized signals is formed to thereby extract the evoked response component if it is present.
In U.S. Pat. No. 5,741,312 a method and an apparatus are described to determine stimulating threshold through delivery pulse pairs consisting of a first lower amplitude search pulse with variable amplitude and a second regular pacing pulse within 50-100 ms. Threshold search is executed by incrementing the amplitude of the search pulse until an evoked response is detected. Alternatively the period from regular pacing pulse to the T-wave is measured and capture on the search pulse is determined as a sudden shortening of this interval. U.S. Pat. No. 5,741,312 further discusses methods to minimize polarization by optimizing pulse parameters of a two- or triphasic pacing pulse. The system disclosed in U.S. Pat. No. 5,741,312 however, makes no attempt to determine the polarization free evoked response signal by subtracting polarization determined by the actual stimulation amplitude from the signal picked up by the electrode in response to a stimulation pulse.
There is mostly at least one significant slope in the bipolar measured IEGM signal, which makes it possible to discriminate the evoked response signal from slowly varying signals such as polarization signals. Thus in U.S. Pat. No. 5,431,693 a method of verifying capture of the heart by a cardiac pacemaker is described. Observing that the non-capture potential is exponential in form and the evoked capture potential, while generally exponential in form, has one or more small-amplitude perturbations superimposed on the exponential wave form, these perturbations are enhanced for ease of detection by processing the wave form signal by differentiation to form the second derivative of the evoked response signal for analysis for the evoked response detection.
Unipolar detection of evoked response signals is however not possible by this technique. Abrupt slope changes or superimposed small-amplitude perturbations are leveled out if the measurements are made over the longer distance from the electrode to the stimulator casing.
The unipolar sensed evoked response signal thus differs from the bipolar sensed evoked response signal both in duration and amplitude, see Baig et al, “Comparison of Unipolar and Bipolar Ventricular Based Evoked Responses”, Br Heart J. 1992, 68: 398-402. The duration of the evoked QRS complex is a measure of total ventricular bipolarization time in the area of the heart subtended by a sensing bipole, and it depends on the extension of the bipole. This means that the unipolar evoked response signal measured between the electrode tip and the casing of the heart stimulator has a longer duration than the bipolar evoked response measured between tip and ring electrodes. This is illustrated in
FIG. 1
herein in which the upper curve shows an unfiltered cardiac signal measured by a unipolar lead and the lower curve the cardiac signal sensed by a bipolar electrode. Known; evoked response detectors, the function of which is based on the detection of a slope of the evoked response signal, typically in a detection window of 15-60 msesc after the stimulation pulse, are therefore not suited for detection of evoked response by unipolar electrodes.
It has now appeared that the evoked response signal amplitude is, fairly constant, independent of the stimulation pulse amplitude, i.e. the evoked response signal amplitude does not vary with the amplitude of the stimulation pulse (provided that the stimulation amplitude is above the capture threshold). Further, it has been found that the electrode polarization is approximately linearly dependent on the stimulation pulse amplitude for a constant pulse duration, as disclosed in European Application 0906768.
SUMMARY OF THE INVENTION
An object purpose of the present invention is to provide a heart stimulator having an evoked response detector detector for determining evoked response based on the above circumstances which does not depend on any slope measurements of the sensed signal, which can be used with both low polarizing and high polarizing unipolar sensing electrode leads.
The above object is achieved in accordance with the principles of the present invention in a heart stimulator having a pulse generator which emits stimulation pulses and a lead connected to the pulse generator adapted for introduction into the heart of a patient for delivering the stimulation pulses to the heart, a control unit for varying the respective amplitudes of the stimulation pulses, and an evoked response detector for determining evoked response in the presence of polarization, the evoked response detector including a measuring and storage unit for measuring the electrode signal picked up by the lead in response to respective stimulation pulses and for storing the measurement. For determining the magnitude of the polarization for different stimulation amplitudes, in a first embodiment the control unit controls the pulse generato
Andersson Peter
Budgifvars Göran
Larsson Berit
Shojael Feresteh
Uhrenius Åsa
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