Implantable medical devices can be used to provide pacing therapy to patients who have cardiac rhythm problems. For example, an implanted medical device may provide pacing therapy to a patient with sinus node dysfunction, where the heart fails to properly initiate depolarization waves, or an atrio-ventricular conduction disturbance, where the conduction of depolarization waves through the heart tissue is impaired. Implantable medical devices with pacing functionality can include pacemakers, cardiac resynchronization therapy (CRT) devices, remodeling control therapy (RCT) devices, and implantable cardioverter defibrillators (ICD).
Implanted medical devices with pacing functionality, such as a pacemaker, typically deliver a pacing pulse of electricity to the heart in order to produce a heartbeat at the correct time. The implanted medical device includes electronic circuitry that is contained within an enclosure, often called a can. The can and associated electronics are implanted in the patient's chest and one or more leads are routed from the can, through the patient's vasculature, and to the patient's heart tissue. Electrical pulses are delivered through the leads to the heart tissue, initiating contraction of the heart.
One issue associated with cardiac pacing therapy is the need to adapt the pacing rate in response to the changing metabolic demands of the patient. For example, while a patient is sitting, sleeping, or otherwise being sedentary, the patient's cardiac output requirements are relatively low. However, when engaged in physical activity, a patient's cardiac output requirements increase in order to transport more oxygen to, and carbon dioxide from, various body tissues. The greater the intensity of the physical activity, the greater the cardiac output required to sustain the activity.
Strategies have been devised for adapting the pacing of the heart in response to exercise or exertion, referred to as “adaptive rate pacing” or “rate adaptive pacing”. These strategies generally depend on measuring a parameter that serves as an index of exertion and then adjusting the pacing rate in response to changes in the measured parameter. However, these strategies frequently modulate the pacing rate inappropriately because the parameters used do not always correlate well with exertion. For example, in the case of accelerometer data, it is difficult for the device to determine whether the body motion sensed is the result of the patient's exertion or whether it is attributable to other conditions such as riding in a car on a bumpy road or in an airplane that is accelerating rapidly.
With many known rate adaptive pacing strategies, it is necessary to apply a pacing gain rate, that is, the rate at which the pacing rate increases as a function of exertion, and it is also necessary to set a maximum pacing rate. Values for the pacing gain rate and maximum pacing rate are generally determined based on statistical averages derived from physiological studies. Frequently, these statistical average values are used across all patients. However, even among individuals of the same age group, there can be a wide variation in physiology that is related to the degree and scope of cardiac disease and other individual differences. Thus, where a statistical average physiological characteristic is used, it will not be ideal for all patients, and in fact may be significantly inappropriate for some patients. For at least these reasons, a need exists for improved rate adaptive cardiac pacing systems and methods.