1. Field of the Invention
The invention relates to the field of implantable medical devices and to systems and methods of detecting the orientation of a patient having an implantable medical device.
2. Description of the Related Art
A variety of implantable medical devices are known to automatically monitor a patient's physiologic condition and to selectively provide therapy when needed. Pacemakers and/or cardioverter defibrillators (ICDs) are implantable medical devices which are configured to monitor a patient's cardiac activity and selectively provide therapy for a variety of cardiac arrhythmias. Cardiac arrhythmia can be defined as a group of conditions in which the muscle contraction of the heart is irregular, e.g., faster or slower than normal. Implantable pacemakers and/or ICDs typically include a stimulation pulse generator which generates therapeutic stimulation for delivery to patient tissue and a microprocessor-based controller which regulates the delivery of that therapy. Implantable pacemakers and/or ICDs are also typically configured to monitor the patient's cardiac activity, to detect any possible abnormalities or cardiac arrhythmias based on the monitored cardiac activity, and to respond accordingly when an abnormality is detected. For example, therapeutic stimulation may be provided when the implantable pacemaker and/or ICD detects a cardiac arrhythmia.
The cardiac signals that are monitored for detecting cardiac arrhythmias may also be useful in detecting other potential abnormalities that may be indicative of future heart problems. For example, a consistently low amplitude of the monitored cardiac signal may be indicative of a heart problem which can lead to future heart failure, if not detected early. Thus, the cardiac signals that are monitored can be stored and analyzed for early detection of other potential problems.
However, changes in the patient's cardiac activity are not always indicative of abnormalities or cardiac arrhythmias. Changes in the orientation of the patient, for example, can also result in changes in the monitored cardiac activity. Generally, abnormalities detected in the intracardiac electrogram (IEGM) signals that exceed predetermined thresholds may result in changes in the performance of the ICD. Thus, a change in the IEGM signal that is caused by factors other than heart abnormalities or cardiac arrhythmias, if not properly detected, may result in changes in the performance of the ICD and/or improper diagnosis of a heart problem. Further, the metabolic needs of the patient may vary depending upon the patient's posture. As such, being able to distinguish between various patient postures such as, patient lying down, lying on the right side, lying on the left side, sitting, or standing up can be helpful in providing input to the implanted device to ensure that the patient's heart is being stimulated appropriately based upon the patient's need.
Accordingly, there is a need for implantable pacemakers and/or ICDs to detect changes in the orientation of the patient and to properly relate any changes in the cardiac activity to changes in the orientation.