Implantable medical devices (IMDs) include implantable cardiac function management devices, such as, for example, implantable pacers, implantable cardioverters, implantable defibrillators, implantable cardiac resynchronization devices, or any other implantable device for monitoring or influencing cardiac function.
Certain IMDs perform impedance measurements. For example, certain IMDs can measure a “lead impedance,” such as exhibited by implanted electrodes located in association with the subject's tissue. Such electrodes may be located on an intravascular leadwire, for example, on a housing of an electronics unit, or elsewhere.
In other examples, certain IMDs can measure a time varying impedance that represents a physiological signal of interest. One example would be an intracardiac impedance, such as between two electrodes located within a heart. Such information can be used to determine a cardiac stroke volume, a pre-ejection interval, or other measurable physiological parameter of interest. This, in turn, can be used to determine or control a therapy, such as a pacing rate, for example. Another example of an impedance-derived time-varying physiological signal of interest would be a thoracic impedance signal observed across at least a portion of a subject's thorax. Physiological information provided by a thoracic impedance signal can include, for example, a cardiac stroke component, which modulates the thoracic impedance signal according to heart contractions, a respiration component, which modulates the thoracic impedance signal according to the subject's breathing, or fluid status information, which modulates the thoracic impedance signal according to pulmonary edema or other changes in the subject's thoracic fluid status.