Medical devices are implanted in human bodies to perform tasks including, for example, monitoring physiological conditions, diagnosing diseases, treating diseases, or restoring functions of organs or tissues. Examples of such implantable medical devices include cardiac rhythm management systems, neurological stimulators, neuromuscular stimulators, and drug delivery systems. Because such a device may be implanted in a patient and typically remain therein for a long time, even up to the patient's life expectancy, the size and power consumption of the device are inherently constrained. Consequently, an implantable device may depend on an external system to perform certain functions. A function of a device providing communication between the implantable device and the external system is referred to as telemetry. Examples of specific telemetry functions include programming the implantable device to perform certain monitoring or therapeutic tasks, extracting an operational status of the implantable device, transmitting real-time physiological data acquired by the implantable device, and extracting physiological data acquired by and stored in the implantable device.
In certain instances, the patient's health condition may determine the amount of telemetry activity between the implantable device and the external system. For example, an implantable device stabilizing a body function of an already stable patient may need infrequent telemetry during follow-ups. However, an implantable device worn by a very ill patient to treat an unstable, life-threatening condition may need frequent telemetry for monitoring and/or device-reprogramming. The amount of telemetry activity also depends on the type of the implantable device. A self-contained device performing relatively simple tasks may require only infrequent check-ups. A device performing complicated tasks, such as frequent real-time data processing, may require access to an external system having computing capabilities required for the task. Such a device may require frequent or even continuous telemetry.
One particular example of implantable medical devices is a cardiac rhythm management device implanted in a patient to treat irregular or other abnormal cardiac rhythms by delivering electrical pulses to the patient's heart. Such rhythms result in diminished blood circulation. Implantable cardiac rhythm management devices include, among other things, pacemakers, also referred to as pacers. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly or irregularly. Such pacers may coordinate atrial and ventricular contractions to improve the heart's pumping efficiency. Implantable cardiac rhythm management devices also include devices providing cardiac resynchronization therapy (CRT), such as for patients with congestive heart failure (CHF). CHF patients have deteriorated heart muscles that display less contractility and cause unsynchronized heart contraction patterns. By pacing multiple heart chambers or sites, CRT device restores a more synchronized contraction of the weakened heart muscle, thus increasing the heart's efficiency as a pump. Implantable cardiac management devices also include defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Such defibrillators may also include cardioverters, which synchronize the delivery of such stimuli to portions of sensed intrinsic heart activity signals. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. In addition to pacers, CRT devices, and defibrillators, implantable cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other implantable systems or devices for diagnosing or treating cardiac arrhythmias.
Typically, an implantable cardiac rhythm management device communicates, via telemetry, with an external device referred to as a programmer. One type of telemetry is based on inductive coupling between two closely-placed coils using the mutual inductance between these coils. This type of telemetry is referred to as inductive telemetry or near-field telemetry because the coils must typically be closely situated for obtaining inductively coupled communication.
In one example, an implantable device includes a first coil and a telemetry circuit, both sealed in a metal housing (referred to as a “can”). An external programmer provides a second coil in a wand that is coupled to the programmer. During device implantation, a physician evaluates the patient's condition, sometimes by using the implanted device to acquire real-time physiological data from the patient and communicating the physiological data in real-time to the external programmer for processing and/or display. The physician may also program the implantable device, including selecting a pacing or defibrillation therapy mode and parameters required by that mode based on the patient's condition and needs. The data acquisition and device programming are both performed via the inductive telemetry. If the patient's condition is stable after implantation, he or she needs no attention from the physician or other care provider until a scheduled routine follow-up. During a typical routine follow-up, the physician reviews the patient's history with the implantable device, re-evaluate the patient's condition, and re-program the implantable device if necessary.
The inductive telemetry requires the two coils to be closely placed, typically by placing the wand on the body surface over the implantable device. Because the wand is coupled to the programmer using a cable, the inductive telemetry limits the patient's mobility. This limitation is tolerable for patients requiring infrequent routine follow-ups. However, some patients may be very ill or unstable to such an extent that the device is incapable of adjusting itself to provide adequate therapy in a timely manner. Where the patient's condition is life-threatening, telemetry must be active constantly to immediately alert a care provider. Using inductive telemetry would constantly restrain the patient who may otherwise enjoy a more active life.
Alternatively, a far-field radio-frequency (RF) telemetry may substitute for, or supplement to, the inductive telemetry. An RF transceiver in the implantable device is used to communicate with an RF transceiver in the external programmer. With a far-field RF telemetry, the patient is typically free of any body surface attachment that limits mobility. However, RF telemetry typically consumes more energy and requires a larger circuit and battery than inductive telemetry.
Therefore, the present inventors have recognized that there is a need for a method and apparatus to provide an adequate telemetry to an implantable device to satisfy each individual patient's needs without increasing the size and/or the cost of the implantable device.