1. Field of the Invention
The present invention is directed to a medical device implantable in the body of a patient which includes means for stimulating a physiological event in the patient with an adjustable stimulation intensity, and wherein the sensitivity which is used to set the stimulation intensity, with reference to a signal from a sensor, can be reduced, and to a method for operating such a device.
2. Description of the Prior Art
Implantable devices for stimulating a physiological event in a patient with a stimulation intensity which is automatically adapted to the physical activity of the patient are known in the art. Information regarding the physical activity of the patient is obtained from a sensor, usually also implanted in the patient for generating a physical activity signal. The selected parameter of the physical activity signal is then monitored and variations in this parameter cause corresponding adjustments in the stimulation activity. It is also known to lower, under certain circumstances, the sensitivity which is used to set the stimulation intensity with reference to the signal from the sensor.
As used herein, the term "stimulation intensity" means any parameter, or combination of parameters, of the signal which is supplied from the device to the patient to stimulate the desired physiological event. This can include the duration, the frequency, the repetition rate, the amplitude, and combinations thereof, with which the means for stimulating in the device take effect to bring about the stimulation of the event. The term "sensitivity" as used herein indicates the value which the selected parameter of the physical activity signal, which is used as an indicator of the physical activity, must have in order to cause the setting of a specific stimulation intensity. A reduction in the sensitivity thus means that a higher value of the signal parameter, such as amplitude, must be present at a current time than was previously necessary in order to cause a setting of the same stimulation intensity.
Such devices are intended to permit the patient in whom the device is implanted to lead as normal a life as possible. Dependent on the physical activity of the patient, the physiological event should be stimulated with a stimulation intensity which corresponds as closely as possible to that which would be present in a healthy patient, given the same physical activity. This should be guaranteed both for conditions of high physical activity and for resting conditions.
A device of this type is disclosed in European application 0 080 348, corresponding to U.S. Pat. No. 4,428,378. This patent discloses an implantable heart pacemaker having a housing in which a piezoelectric pressure sensor is disposed, the sensor monitoring mechanical oscillations in the patient which arise during physical activity of the patient in whom the pacemaker is implanted due to movements of the muscles and connecting structure which propogate as pressure waves. These mechanical oscillations are then converted by the sensor into a corresponding electrical signal. The stimulation intensity, i.e. the stimulation rate with which the pacemaker stimulates the heart in the absence of natural heart beats, is then set dependent on the physical activity of the patient with reference to this signal. It is possible to reduce the sensitivity with which the adaptation of the stimulation frequency ensues with reference to the signal from the piezoelectric pressure sensor, proceeding from a maximum value. This permits the stimulation intensity to be matched to individual conditions. Adaptation of the stimulation intensity ensues between upper and lower limit values, the upper limit value being reached during conditions of extremely high physical activity, and the lower limit value taking effect in the resting condition, for example, during sleep. A problem in this known device is that during sleep the patient in whom the pacemaker is implanted assumes a position causing pressure to be externally exerted onto the piezoelectric pressure sensor. This causes the sensor to generate a signal without the presence of physical activity on the part of the patient. This in turn causes an increase in the stimulation repetition rate, which is uncomfortable for the sleeping patient.