1. Field of the Invention
The present invention generally relates to the field of implantable medical devices. More specifically, the present invention relates to an implantable lead for sensing mechanical activity of a human heart, the lead comprising a first non-conductive polymeric tube extending from a proximal end to a distal end of said lead, a first conductor provided in a lumen of said first polymeric tube, and a sensor.
2. Description of the Prior Art
Within the field of implantable medical devices, such as cardiac stimulators, cardioverters and defibrillators, different types of sensors have long been used for sensing and monitoring mechanical and electrical characteristics of the human heart. For instance, piezo-resistive, piezocapacitive or piezoelectric sensors could be used for sensing mechanical properties, such as pressure changes in the heart or in other blood vessels.
One example of a piezoelectric sensor suitable for use as a pressure sensor is shown in WO 02/34130. By using a pressure sensor in the heart, tachycardias, such as fibrillation, can be detected and also distinguished. Atrial fibrillation can be separated from ventricular fibrillation and electrical activity can be separated from mechanical activity. This is of importance for controlling the type of stimulation therapy to be delivered by a heart stimulator.
Furthermore, ever since the introduction of rate responsive implanted cardiac stimulators, a number of different parameters have been used for determining the activity level of the patient, which in turn is used for controlling the rate at which the heart of a patient is to be stimulated by the pacemaker. One of the most common sensors used is the piezoelectric accelerometer.
Unlike piezoresistive and piezocapacitive sensors, piezoelectric sensors are advantageous in that they are not energy consuming. Piezoelectric sensors are arranged to alter the mechanical stress of the piezoelectric material in response to a change of loads emanating from for instance an acceleration of a seismic mass or from a change in pressure acting on the sensor. This results in a transport of electrons or electrical charges within the material, which provides a change in voltage across the piezoelectric sensor. This voltage corresponds to the load to which the sensor is subjected. Thus, by monitoring the voltage changes across the piezoelectric sensor, changes for instance in pressure acting on the sensor can be monitored.
However, when using a piezoelectric sensor, such as a pressure sensor or an accelerometer, within a chamber of the heart or a blood vessel, measurements have shown that the voltage signal produced by the sensor is affected by motion artifacts. Such motion artifacts can be due to movements of the patient, i.e. jumping, running, etc., or the motions of the heart and the thorax during their regular cardiovascular operation. The motion artifacts could be large enough to disturb a sensor signal, especially when using a piezoelectric pressure sensor since the voltage changes produced by the piezoelectric sensor from pressure changes are very small. Furthermore, it could be difficult to separate the contribution of the motion artifacts from the useful signal contribution from pressure changes. Thus, depending on the contribution from the motion artifacts and the magnitude of the useful contribution to the pressure signal, the interpretation of the sensor signal could be rendered difficult or even impossible.