Implantable medical devices are implanted in a patient's body to diagnose and treat a wide variety of medical conditions. One type of implantable medical devices is implantable cardiac stimulating devices, which detect and treat cardiac arrhythmias by applying electrical stimulation to the heart.
There are several types of implantable cardiac stimulating devices, each designed to treat a specific type of arrhythmia. One such device is a cardiac "pacemaker," which is used to treat an abnormally slow heart rate, a condition known as bradycardia. A cardiac pacemaker administers electrical "pacing pulses" to the heart in order to regulate the beating of the heart so that it will properly perform the physiological function of the heart.
While early models of pacemakers delivered pacing pulses at a consistent, predetermined rate, some types of modern pacemakers pace at a controlled variable or intermittent rate depending on the needs of the patient. For example, "demand pacemakers" can sense the patient's intrinsic heart rate and will deliver pacing pulses only when the intrinsic heart rate falls below a predetermined threshold rate. This threshold rate typically is a programmable parameter that is set by a physician in accordance with the physiological requirements of the patient, often determined by testing and monitoring of the patient at the physician's office.
Another type of pacemaker, the "rate-responsive" pacemaker, has the capability of varying the rate of pacing pulses, and therefore varying the patient's heart rate, by sensing physiological characteristics of the patient in order to determine the appropriate heart rate at any point in time. For example, when the patient performs activities demanding physical exertion, motion sensors, such as, for example, accelerometers associated with the pacemaker, sense increased motion and cause the gradual increase of the rate of delivery of pacing pulses accordingly, in order to increase heart rate. Other parameters that may be sensed by the pacemaker include body temperature, body impedance, and sound. By allowing the heart rate to depend on physiological variables such as level of physical activity, rate-responsive pacemakers permit bradycardia patients to engage in physical activities that their illness would normally preclude.
Other types of implantable cardiac stimulating devices are used to detect and treat arrhythmias known as "tachycardia" and "fibrillation." Some of these devices can also treat bradycardia. In contrast to the abnormally slow heart rate symptomatic of bradycardia, tachycardia is a condition in which the patient's heart beats at a faster than normal rate. Tachycardia prevents the chambers of the heart from filling sufficiently with blood between beats, thereby adversely effecting the heart's performance. Devices called "cardioverters" are used to treat tachycardia by employing pacing pulses in special sequences known to be effective in treating tachycardia. Fibrillation is a severe and rapid beating of the heart, rendering it essentially ineffective as a pump, and is considered the most severe form of cardiac arrhythmia. Implantable devices used to treat fibrillation are known as "defibrillators," and deliver high-energy shocks to the heart.
Devices called "implantable cardioverter-defibrillators," or "ICDs," treat both tachycardia and fibrillation. ICDs employ a variety of electrical means to alter the pacing of the heart based on the severity of the detected arrhythmia. A commonly used approach is "tiered therapy," whereby the aggressiveness of the electrical stimulation is increased as the severity of an arrhythmia episode increases and as less aggressive therapies fail.
Implantable cardiac stimulating devices acquire and store various types of data in order to administer therapy properly to the patient. For example, sensory data relating to physiological conditions is collected. Sensory data can include information pertaining to the patient's activity level or cardiopulmonary needs, to the mechanical activity of the patient's heart such as cardiac wall motion data, or to an intracardiac electrogram (IEGM) sensed through leads to the patient's heart. For example, in a "dual-chamber" device, leads are connected to the atrium for sensing P waves and to the ventricle for sensing R waves, and one or both of these waves may be recorded by the implantable device. A related type of data represents the occurrence of cardiac events and the response of the device thereto ("marker" data). Yet another type of data indicates the status of operational parameters of the implantable device, including battery voltage, battery impedance, and lead integrity data.
Performance of the implantable device is controlled by programmable parameters stored in the device. The programming and analysis of an implantable cardiac stimulating device is typically an ongoing process involving periodic supervision by a physician, in which the physician obtains the sensory, marker, and/or operational data and, based on an analysis of this data, evaluates the performance of the implantable device. Based on the performance of the device and the patient's medical condition as indicated by the data, the physician may reprogram parameters stored in the implantable device, thereby optimizing performance of the device.
A data processing device such as a "programmer/analyzer" is used by the physician for analyzing the data obtained from the implantable medical device, and/or for computing appropriate values for programmable parameters for the device. In prior art systems, the programmer/analyzer typically communicates with the implantable device using wireless telemetry. Wireless telemetry allows noninvasive communication of stored data from the implanted device to the programmer/analyzer subsequent to implantation of the device, such as during patient follow-up visits. The programmer/analyzer conducts an "interrogation" of the implantable device by causing a message to be sent to the implantable device using wireless telemetry, directing the implantable device to transmit data back to the programmer/analyzer or to collect data, or both. Typically, a telemetry head included on a "telemetry wand" is used for the interrogation. The telemetry wand is held close to the patient's chest during interrogation and is wired to the programmer/analyzer.
The implantable device responds to the interrogation by assembling the requested data, formatting it for telemetric transmission, and transmitting it back to the programmer/analyzer using wireless telemetry. The programmer/analyzer receives the data and uses it to analyze the performance of the device and, if necessary, compute appropriate adjustments to the device's programmable parameters. The programmer/analyzer can then re-program the implantable device by transmitting a message adjusting its programmable parameter(s). Thus, by analyzing data obtained by interrogation of the implantable device, the physician can noninvasively monitor and optimize the performance of the device on a regular basis.