Self-calibrating rate-adaptive cardiac pacemaker

Details Self-calibrating rate-adaptive cardiac pacemaker Self-calibrating rate-adaptive cardiac pacemaker

1999-12-15

2002-10-08

Evanisko, George R. (Department: 3762)

Surgery: light, thermal, and electrical application

Light, thermal, and electrical application

Electrical therapeutic systems

Reexamination Certificate

active

06463325

ABSTRACT:

FIELD OF THE INVENTION
The invention concerns a cardiac pacemaker and more particularly to a self-calibrating rate-adaptive cardiac pacemaker.
BACKGROUND OF INVENTION
Rate-adaptive cardiac pacemakers wherein the stimulation rate is set in dependence on signals received from the body of the patient, and which reflect the physiological demand of the patient with regard to cardiac activity, have long been known and used in a clinical context. Various proposals have also been put forward for self-adaptation (auto-calibration) of such rate-adaptive cardiac pacemakers.
For example, WO 93/20889 proposes a dual-sensor arrangement with one circuit for detecting the minute volume, and an additional activity sensor where the stimulation rate is determined based on target rates, which can be derived for the individual sensors. In another example, U.S. Pat. No. 5,065,759 proposes a dual-sensor arrangement in which the QT-interval is detected and evaluated as a, ‘physiologically exact’, but slowly responding parameter, and wherein physical activity is detected and evaluated as a rapidly responding parameter.
In yet another example, EP 0 147 820 also discloses a rate-adaptive pacemaker in which one of the two sensors is used as a so-called closed-loop sensor, which detects signals from the heart-circulation regulating circuit for rate adaptation purposes, whereas a further sensor only provides a monitoring function. In this system, it is only when errors in rate adaptation are detected by way of the monitoring sensor that the closed-loop sensor is temporarily replaced by the monitoring sensor or the configuration of the sensor characteristic is re-calibrated.
Meanwhile, EP 0 498 533 A1 proposes a rate-adaptive pacemaker operating with two sensors, in which the upper rate limit is set utilizing hemodynamical monitoring. Various sensors are known for this function, including a sensor designed to detect changes in the impedance of the right ventricle.
Finally, in an unpublished German patent application No. P 198 04 843.2, a self-calibrating rate-adaptive pacemaker was proposed in which a closed-loop rate adaptation algorithm based on intraventricularly detected impedance signals was calibrated by means of an acceleration sensor.
Signals emanating from the sympathetic nerve in the context of the autonomous system for heart-circulation control, referred to as ‘sympathetic’ signals, primarily indicate the need for cardiac minute volume, and in the neural equilibrium of the sound heart-circulation system, find their antagonist in signals emanating from the vagus nerve, referred to as ‘vagal’ or ‘parasympathetic’ signals, which indicate the attainment of upper limits in terms of the efficiency of the cardiac minute volume. In contrast to sympathetic signals, vagal signals therefore have an inhibiting effect.
In the known rate-adaptive pacemakers the change in inotropy is measured and used as a measurement of the required cardiac minute volume, and thus the optimum stimulation rate. In this respect, a linear relationship is assumed between inotropy and heart rate, i.e., a rise in inotropy is immediately answered by a proportional rise in the stimulation rate. Measurement of the inotropy by way of the ventricular contraction dynamics (by means of unipolar impedance measurement) predominantly detects the sympathetic component of autonomous regulation. Vagal components, and their (generally inhibiting) effect on the heart rate, are in practice not detected and taken into consideration. This ‘purely sympathetic’ pacemaker consequently functions—at least in relation to heart rate—in an analogous fashion to a patient with a low level of baroreceptor reflex sensitivity; the vagal tone is artificially reduced to zero or set to a constant value and the sympathetic tone alone has a controlling action. This sympathetic dominant system results in two disadvantages: 1) rapid heart rate adaptations, as are possible with vagal involvement in relation to the functioning heart, cannot be implemented by the pacemaker; and 2) the function of the vagus nerve for controlling the heart rate dynamics also does not have any effect. This means that long-term effects (for example general physical fatigue—so-called ‘burn-out’—, a harbinger of incipient heart insufficiency, etc.) remain substantially disregarded.
This means that the actual advantage of autonomous monitoring, namely sparing the inotropic reserves and thus protecting against primary cardiomyopathies or arrhythmias, are not utilized in an optimal fashion. As a consequence of disregarding the vagal contribution, the calculated stimulation rate is not physiologically correct in terms of its absolute level, even if fall and rise times may be correct. The result is that the myocardium may, under some circumstances, be overloaded or not adequately loaded.
Accordingly, a need exists for a cardiac pacemaker of the general kind set forth above, which is optimized from the physiological point of view, which operates in a reliably self-calibrating fashion, and which can be implemented without problems.
SUMMARY OF THE INVENTION
The present invention is directed to a self-calibrating rate-adaptive cardiac pacemaker, and more particularly to a self-calibrating rate-adaptive cardiac pacemaker comprising a first measuring and processing device for detecting a first, predominantly sympathetically influenced physiological parameter (Z(tm)) and for obtaining a rate control parameter (RCPp), which has a control input for controlling the functional dependency of the rate control parameter on the first physiological parameter, in particular a response factor and/or an upper limit rate; and a stimulator unit for producing and outputting stimulation pulses at a stimulation rate which is determined by the rate control parameter, characterized by a second measuring and processing device for detecting and evaluating a second, predominantly vagally influenced physiological parameter (AVI) and for outputting a calibration signal (Cal) which is dependent on the evaluation result, to the control input of the first measuring and processing device.
In one embodiment, the invention is directed to a pacemaker which operates with a suitable vagal control contribution, and which uses signals from two closed-loop sensors for rate control purposes and more specifically a sensor which hereinafter is referred to for the sake of brevity as the ‘sympathetic’ sensor and a sensor which is referred to as the ‘vagal’ sensor. The influence of the vagus is primarily felt in the electrical and mechanical activity of the atria, such as, for example, the atrial evoked stimulation response {AER—to be measured by a unipolar procedure), the atrial monophase action potential {MAP—to be measured by a bipolar procedure), the atrial refractory time, the intra-atrial impedance, and also the AV-transition time. Meanwhile, the influence of the sympathetic nerve is expressed primarily in the activity of the ventricle, and more particularly to the ventricularly evoked stimulation response {VER), the ventricular monophase action potential {MAP), the ventricular refractory time, the intra-ventricular impedance, and the QT-interval. In such an embodiment, the sensor for signals which are dominated by the sympathetic nerve is referred to as the ‘sympathetic’ sensor, and the sensor for signals dominated by the vagus nerve is referred to as the ‘vagal’ sensor. In such a system the sympathetic signals react to changes in the heart-circulation system more slowly (with a time constant of about cardiac cycles) than vagal signals (with a time constant of about one cardiac cycle) and therefore the sensor time characteristics are also correspondingly different.
In one alternative embodiment, the invention includes the ability to current control the stimulation frequency in a closed-loop stimulation (CLS) with reference to the signals from the ‘sympathetic’ sensor.
In another alternative embodiment, the relative changes in the sensor signal are converted, in accordance with the response amplification effect or the res

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