Implantable medical devices, including cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators, typically have the capability to communicate data with a device called an external programmer via a radio-frequency telemetry link.  A clinician may use such an external programmer to program the operating parameters of an implanted medical device.  For example, the pacing mode and other operating characteristics of a pacemaker are typically modified after implantation in this manner.  Modern implantable devices also include the capability for bidirectional communication so that information can be transmitted to the programmer from the implanted device.  Among the data which may typically be telemetered from an implantable device are various operating parameters and physiological data, the latter either collected in real-time or stored from previous monitoring operations. 
Telemetry systems for implantable medical devices utilize radio-frequency energy to enable bidirectional communication between the implantable device and an external programmer.  An exemplary telemetry system for an external programmer and a cardiac pacemaker is described in U.S. Pat.  No.  4,562,841, issued to Brockway et al.  and assigned to Cardiac Pacemakers, Inc., the disclosure of which is incorporated herein by reference.  A radio-frequency carrier is modulated with digital information, typically by amplitude shift keying where the presence or absence of pulses in the signal constitute binary symbols or bits.  The external programmer transmits and receives the radio signal with an antenna incorporated into a wand which can be positioned in proximity to the implanted device.  The implantable device also generates and receives the radio signal by means of an antenna, typically formed by a wire coil wrapped around the periphery of the inside of the device casing. 
In previous telemetry systems, the implantable device and the external programmer communicate by generating and sensing a modulated electromagnetic field in the near-field region with the antennas of the respective devices inductively coupled together.  The wand must therefore be in close proximity to the implantable device, typically within a few inches, in order for communications to take place.  This requirement is an inconvenience for a clinician and limits the situations in which telemetry can take place.