A capture-detection cardiac pacemaker, and particularly a capture-detection cardiac pacemaker adapted to stimulate the heart based on variations in impedance in the myocardial impedance pattern.
This invention is generally directed to capture-detection cardiac pacemakers. Capture-detection cardiac pacemakers of the kind described herein are generally equipped with impedance measuring sensor means for checking the success of a pacemaker to induce stimulation of the heart. As is known, cardial impedance variations make it possible to determine the state of various cardiac functions, and in particular stimulation success. Accordingly, by monitoring stimulation success, it is possible to automatically adapt the stimulation pulses generated by the cardiac pacemaker to the stimulation stimulus threshold. Specifically, the stimulation stimulus threshold identifies the stimulation amplitude required to generate a stimulation success, i.e., a stimulated systole. As this stimulation stimulus threshold changes constantly as a consequence of hormone level, time of day, physical activity and so forth, the stimulation amplitude should be adapted to the variation of the stimulation stimulus threshold. If, for example, a sub-threshold stimulation is detected by virtue of an absence of stimulation success after the output of a stimulation pulse (as measured by the absence of a rise in impedance in the impedance pattern after output of the stimulation pulse), the stimulation amplitude should be increased. Such a dynamic variation of the stimulation amplitude ensures reliable operation of the cardiac pacemaker.
On the other hand, during the operation of the cardiac pacemaker the minimum energy required to provide stimulation should be used to guarantee a long service life for the battery, which is typically integrated in the cardiac pacemaker. Therefore, the stimulation amplitude should be increased only as far as necessary, i.e., only slightly above the stimulation stimulus threshold.
Besides checking the stimulation success on the basis of impedance measurement, methods are also generally known which measure the potential evoked by the stimulation pulse, or the QRS-complex. In this case, however, reliable detection of the stimulation success is only possible, at the earliest, between 100 and 130 milliseconds after output of the stimulation pulse by the cardiac pacemaker. Accordingly, there is a very severe time delay, and this causes difficulties in providing short-term corrective measures via stimulation pulse control.
In contrast, U.S. Pat. No. 5,766,230 discloses a cardiac pacemaker in which the stimulation success is checked while the stimulation pulse is still being outputted. In this pacemaker, at the end of the otherwise rectangular stimulation pulse, the stimulation amplitude falls away sharply due to a suddenly higher current flow signaling the stimulation success, and that produces a falling curve in the pulse shape. That falling curve in the pulse shape is used to detect the stimulation success. Although such a system provides very quick measurement, there is the disadvantage that the occurrence of a stimulation success can only be detected if the pulse shape changes before the end of the stimulation pulse. Otherwise, it is not possible with this prior art device to detect a stimulation success. As simulation procedures have shown, stimulation amplitudes near the stimulation stimulus threshold only result in a stimulation success after a delay of up to 3 milliseconds. However, the stimulation pulse is already completed at the end of this delay time and would, accordingly, not be detected by the prior art system. Stimulation success consequently can be reliably detected with such a system only if the stimulation pulse amplitudes are markedly above the stimulation stimulus threshold, because under such conditions the stimulation success delay occurs during the pulse output. Accordingly, detection of a stimulation success is not reliably implemented with this method at stimulation pulse amplitudes near the stimulation threshold. On the other hand, if stimulation pulses of over-high amplitude are always produced to ensure more reliable detectability, an excessively high energy output is required.
In another example of a prior art system, U.S. Pat. No. 5,843,137 discloses a cardiac pacemaker which makes use of an intracardial impedance measurement for checking stimulation success. The cardiac pacemaker essentially comprises a pulse generator for generating stimulation pulses, which, under the control of a control unit, are outputted by way of an electrode, and sensor means for measuring physiological parameters which are related to the cardiac function and with which stimulation success can be evaluated by way of the control unit. The sensor means sense the changes in impedance and other physiological values which are related to a stimulated systole. The control unit is capable of distinguishing between a natural and a stimulated systole. In that way, the control unit can inter alia implement adaptive adjustment of the stimulation amplitudes, in relation to the stimulation stimulus threshold, in dependence on the stimulation success. In such a system, measurement of the cardial impedance is effected intracardially. For that purpose the impedance between two electrodes associated with the cardiac pacemaker is measured (page 12, lines 7 through 9). In this prior art system the electrodes are the housing of the cardiac pacemaker itself, electrode lines introduced in the ventricle or atrium, and ring electrodes. Measurement is usually effected in a cyclic procedure, that is to say at equidistant time intervals, the one electrode is supplied with a measurement voltage and the current which flows between the two electrodes is measured. Impedance is determined in a known manner as the quotient between the measurement voltage and the current.
While a cardiac pacemaker of the kind described in the ""137 patent is capable of detecting stimulation success; there is the disadvantage that detection is only possible after the expiry of a relatively long period of time after the output of the stimulation pulse. This delay is caused by the requirement of an intracardial impedance measurement. The long period of time for measurement still prevents the adoption in good time of a corrective measurexe2x80x94such as a safety stimulation pulsexe2x80x94if stimulation success fails to appear. If, nonetheless, a safety stimulation pulse is outputted, the unnecessary stimulation pulse can disturb the cardiac cycle, i.e., the succession in respect of time of the diastolic and systolic phases of the heart.
The present invention is directed to a capture-detection cardiac pacemaker wherein a stimulation success can be reliably detected immediately after stimulation pulse output.
The cardiac pacemaker according to this invention generally comprises at least one stimulation electrode arranged in the region of the heart designed to output stimulation pulses to the heart, and a control unit connected to a pulse generator for controlling the stimulation pulse output. The control unit is connected to a sensor arranged at the myocardium for checking stimulation success at the heart by detecting the myocardial impedance pattern immediately after a stimulation pulse output and still within the diastolic phase of the heart, and controlling the stimulation pulse output based thereon. The appendant claims set forth advantageous developments of the invention.
The present invention includes the technical teaching that, for reliable detection of a stimulation success immediately after a stimulation pulse output, the sensor means should be arranged at the myocardium, and thus detect the myocardial impedance pattern directly after stimulation pulse output and still within the diastolic phase of the heart. The invention is also directed to a cardiac pacemaker in which the control unit controls the stimulation pulse output of the pulse generator based on the variations in impedance of the myocardial impedance pattern.
The invention relies on the fact that stimulation success is reflected in the pattern of myocardial impedance, more specifically during or in immediate succession to the stimulation pulse output by the cardiac pacemaker (approximately within between 2 and 10 milliseconds); and more precisely, in a phase in which the heart has not yet mechanically reacted to the stimulation pulse. The mechanical reaction time of the heart is between about 50 and 70 milliseconds. In contrast thereto, stimulation success based on the measurement of intracardial impedance is possible only at a markedly later time. The phenomenon of short-term detectability on the basis of myocardial impedance is based on electro-physiological events in the myocardium. The variations in the impedance pattern are caused predominantly by changes in the membrane properties and ion shifts between extracellular space and the interior of the cardiac muscle cell. In the event of a stimulation pulse output, the tissue impedance decreases greatly for a duration of between 1 and 2 milliseconds, due to the opening of the Na+-channels and the subsequent rise in conductivity of the cell membranes that this entails. The impedance fall occurs approximately by a factor of 3. This event, which can be clearly detected by way of the sensor means, identifies the occurrence of a stimulation success, that is to say the systole of the heart. Utilizing this method, it is possible for the occurrence or the failure of stimulation success to be reliably detected immediately after stimulation pulse output.
A particular advantage of ultra-fast detection of stimulation success is that, besides conventional adaptive adjustment of the stimulation amplitudes to the stimulation stimulus threshold in dependence on the stimulation success, it is also possible to provide for the generation of a safety stimulation pulse in the event of a stimulation failure. The safety stimulation pulse can thus be outputted without the patient noticing, and does not disturb the cardiac cycle. Accordingly, using the system according to the present invention, adaptive adjustment of stimulation amplitudes is possible without the person wearing the cardiac pacemaker suffering from xe2x80x9cskipped beatsxe2x80x9d as in the case of previously known methods for checking stimulation success. Furthermore, an otherwise necessary threshold test by the doctor can be eliminated.
In one embodiment, the safety stimulation pulse is preferably triggered by the control unit of the cardiac pacemaker in the event of a stimulation success of the heart failing to occur as a result of a sub-threshold stimulation. Alternatively, separate electronic means can be provided for that purpose.
In one embodiment, preferably the sensor means are integrated in the stimulation electrode. In such a system the sensor means and stimulation electrodes are disposed in an operatively integrated fashion. For example, the stimulation electrode can be formed in the manner of a fractal electrode. By means of a fractal electrode, it is possible to detect changes in the myocardial impedance pattern a few milliseconds after a stimulation pulse output. Only up to 2 milliseconds are required as a blanking time. In another set of embodiments, sensor means and stimulation electrodes of various configurations can be connected to the cardiac pacemaker.
In one embodiment, measurement of the myocardial impedance is effected on the basis of the current-voltage method. For that purpose, the sensor means are an electrode set preferably comprising at least two contacts which are arranged in spaced relationship on the myocardium. In such an embodiment, one contact is supplied with an alternating current by the pulse generator and the other contact, which serves as a measurement electrode, is connected to the control unit for ascertaining the myocardial impedance. To exclude any disturbing influence which may result from electrode polarization, which occurs beneath about 1 kilohertz, further mutually juxtaposed electrode sets can also be adopted.
In another embodiment of the invention, the measurement of the myocardial impedance is effected immediately prior to the stimulation pulse output in order to ascertain a variation in impedance in the myocardial impedance pattern. This pre-stimulation affords defined detection time windows which permit a comparison of variations in impedance at a minimum level of energy requirement. As a result, the energy requirement is low despite the measurement current, which is required by virtue of using the current-voltage method for measuring the myocardial impedance.
In accordance with a further embodiment of the invention, a system is provided to establish the extent by which the stimulation stimulus threshold is exceeded by ascertaining the time delay between the preceding supra-threshold stimulation pulse. Using such a system, the adaptive adjustment function of the stimulation pulses can be optimized in relation to the stimulation stimulus threshold.
In another embodiment of the invention, besides checking the stimulation success at the heart, the control unit also implements a check of natural heart actions. For the detection of natural heart actions, the control unit may continuously monitor the myocardial impedance pattern and ascertain a natural heart action by reference to a temporary fall in impedance in the myocardial impedance pattern without previous stimulation pulse output. This method is advantageously insensitive to, for example, myopotentials. In such an embodiment, in order to ensure reliable continuous monitoring of heart actions with a moderate energy requirement, the myocardial impedance pattern can be detected with a sampling rate of preferably about 500 Hz. This monitoring can be implemented over the entire period of time orxe2x80x94for further saving energyxe2x80x94in previously defined time windows.