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Medical Patent Abstract
An apparatus and method for enabling far-field radio-frequency communications
with an implantable medical device utilizing the device housing
as an antenna. Such radio-frequency communications can take place
over much greater distances than with inductively coupled antennas.
Medical Patent Claims
What is claimed is:
1. A method for operating an implantable medical device, comprising:
transmitting or receiving radio-frequency (RF signals using RF circuitry
connected to a dipole antenna formed by first and second conductive
portions of a housing of the implantable medical device; matching
an impedance of the dipole antenna to the RF circuitry at a specified
carrier frequency using an antenna tuning circuit; delivering electrostimulation
from therapy circuitry of the implantable medical device using a
therapy lead and the housing as electrodes; and, employing the antenna
tuning circuit as a high-pass filter to block low-frequency energy
generated by the electrostimulation from being received by the RF
circuitry.
2. The method of claim 1, wherein the antenna tuning circuit includes
a balun transformer.
3. The method of claim 1, further comprising converting between
a single-ended RF signal generated or received by the RF circuitry
and a differential signal generated or received by the dipole antenna
with the balun transformer.
4. The method of claim 2, wherein the antenna tuning circuit includes
a variable capacitor and further comprising matching the impedance
of the dipole antenna to the RF circuitry at a specified cater frequency
by adjusting the capacitance of the variable capacitor.
5. The method of claim 1, wherein the first and second conductive
portions of the housing are separated with an insulating material.
6. The method of claim 5, wherein the RF circuitry is disposed
in the first conductive portion of the housing and the therapy circuitry
is disposed in the second conductive portion of the housing.
7. The method of claim 6, further comprising supplying power to
the implantable medical device from a battery disposed in the first
conductive portion of the housing.
8. The method of claim 5, further comprising supplying power to
the implantable medical device from a battery disposed in the first
conductive portion of the housing and wherein the RE circuitry and
therapy circuitry are disposed in the second conductive portion
of the housing.
9. The method of claim 5, wherein RF signals are transmitted at
a specified carrier frequency such that a substantial amount of
far-field radiation is produced by the dipole antenna.
10. The method of claim 9, wherein the specified carrier frequency
is between 300 Mega-hertz (MHz) and 1 Giga-hertz (GHz).
11. The method of claim 9, wherein the specified carrier frequency
is approximately 2.2 Giga-hertz (GHz).
12. The method of claim 1, further comprising using a filter to
isolate the therapy lead from the RF signals.
13. The method of claim 12, wherein the filter is a low-pass filter.
14. The method of claim 12, wherein the filter is a notch filter.
15. The method of claim 1, wherein the first and second conductive
portions of the housing are separated by a header made of insulating
material and through with the therapy lead is routed.
Medical Patent Description
FIELD OF THE INVENTION
This invention pertains to implantable medical devices such as
cardiac pacemakers and implantable cardioverter/defibrillators.
In particular, the invention relates to an apparatus and method
for enabling radio-frequency telemetry in such devices.
BACKGROUND
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.
SUMMARY OF THE INVENTION
The present invention is an apparatus and method for enabling communications
with an implantable medical device utilizing far-field electromagnetic
radiation. Using far-field radiation allows communications over
much greater distances than with inductively coupled antennas. In
accordance with the invention, separate conductive portions of a
housing for the implantable device act as a dipole antenna for radiating
and receiving far-field radio-frequency radiation modulated with
telemetry data. The antenna is dimensioned such that a substantial
portion of the radio-frequency energy delivered to it at a specified
frequency by a transmitter in the implantable device is emitted
as far-field electromagnetic radiation. A tuning circuit may be
used to tune the antenna by optimizing its impedance.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates one embodiment of a split-can dipole antenna.
FIG. 2 illustrates an alternate embodiment of a split-can dipole
antenna with the device header separating the two housing portions.
FIG. 3 is a block diagram of the components of an exemplary cardiac
rhythm management device.
DETAILED DESCRIPTION
As noted above, conventional radio-frequency (RF) telemetry systems
used for implantable medical devices such as cardiac pacemakers
utilize inductive coupling between the antennas of the implantable
device and an external programmer in order to transmit and receive
RF signals. Because the induction field produced by a transmitting
antenna falls off rapidly with distance, such systems require close
proximity between the implantable device and a wand antenna of the
external programmer in order to work properly, usually on the order
of a few inches. The present invention, on the other hand, is an
apparatus and method for enabling telemetry with an implantable
medical device utilizing far-field radiation. Communication using
far-field radiation can take place over much greater distances which
makes it more convenient to use an external programmer. Also, the
increased communication range makes possible other applications
of the telemetry system such as remote monitoring of patients and
communication with other types of external devices.
A time-varying electrical current flowing in an antenna produces
a corresponding electromagnetic field configuration that propagates
through space in the form of electromagnetic waves. The total field
configuration produced by an antenna can be decomposed into a far-field
component, where the magnitudes of the electric and magnetic fields
vary inversely with distance from the antenna, and a near-field
component with field magnitudes varying inversely with higher powers
of the distance. The field configuration in the immediate vicinity
of the antenna is primarily due to the near-field component, also
known as the induction field, while the field configuration at greater
distances is due solely to the far-field component, also known as
the radiation field. The near-field is a reactive field in which
energy is stored and retrieved but results in no net energy outflow
from the antenna unless a load is present in the field, coupled
either inductively or capacitively to the antenna. The far-field,
on the other hand, is a radiating field that carries energy away
from the antenna regardless of the presence of a load in the field.
This energy loss appears to a circuit driving the antenna as a resistive
impedance which is known as the radiation resistance. If the frequency
of the RF energy used to drive an antenna is such that the wavelength
of electromagnetic waves propagating therein is much greater than
the length of the antenna, a negligible far-field component is produced.
In order for a substantial portion of the energy delivered to the
antenna to be emitted as far-field radiation, the wavelength of
the driving signal should not be very much larger than the length
of the antenna.
A dipole antenna is made of two lengths of metal, usually arranged
end to end with the cable from a transmitter/receiver feeding each
length of the dipole in the middle. An efficiently radiating resonant
structure is formed if each length of metal in the dipole is a quarter-wavelength
long, so that the combined length of the dipole from end to end
is a half-wavelength. A wire antenna for an implantable medical
device capable of emitting far-field radiation, however, may require
special implantation procedures and may also be broken or deformed
as a patient moves resulting in detuning. In accordance with the
present invention, a dipole antenna for an implantable medical device
is formed by separate conductive portions of the device housing
or can, referred to herein as a split-can dipole antenna. In one
embodiment, the conductive housing is split into two halves separated
by an insulating dielectric material, with each half connected to
transmitting/receiving circuitry contained within one of the housing
portions. Unlike wire antennas, a split-can dipole antenna does
not require any special implantation procedures and is a rigid structure
which is resistant to breakage or deformation.
An antenna most efficiently radiates energy if the length of the
antenna is an integral number of half-wavelengths of the driving
signal. A half-wave dipole antenna, for example, is a center-driven
conductor which has a length equal to half the wavelength of the
driving signal. The natural tuning of a split-can dipole antenna
depends, of course on the device size. For example, a typical lengthwise
dimension of an implantable cardiac rhythm management device may
be about 6.8 cm, which corresponds to a half wavelength of a 2.2
GHz carrier frequency. If each half of a split-can dipole antenna
is 3.4 cm, then the antenna is a half-wavelength dipole at that
carrier frequency. For medical device applications, carrier frequencies
between 300 MHz and 1 GHz are most desirable. As will be discussed
below, an antenna tuning circuit may be used to alter the effective
electrical length of an antenna by loading it with capacitance or
inductance. The split-can antenna is especially advantageous in
this respect as compared with conventional wire antennas because
it is physically wide and possesses a greater bandwidth. An antenna
with a greater bandwidth is easier to tune and is usable over a
greater range of frequencies once it is tuned. A larger antenna
bandwidth also allows a higher data rate and minimizes the risk
of losing communications due to frequency drift.
FIG. 1 shows an exemplary implantable medical device 100 with a
dipole antenna suitable for radiating and receiving far-field electromagnetic
radiation formed by respective halves of the device housing 101a
and 101b. The device housing is metallic and contains therapy circuitry
TC1 for providing particular functionality to the device such as
cardiac rhythm management, physiological monitoring, drug delivery,
or neuromuscular stimulation as well as circuitry RFC1 for providing
RF communications. One or more therapy leads 310 are connected to
the therapy circuitry contained within the housing by means of a
header 103 with feedthroughs located therein for routing the therapy
leads to the appropriate internal components. The two housing portions
101a and 101b are separated by a layer of insulating material 102.
FIG. 2 shows an alternate embodiment in which the header is made
of dielectric material and is interposed between the two housing
portions 101a and 101b, thus also serving to separate the two legs
of the dipole antenna. In either embodiment, the two housing portions
101a and 101b are hermetically sealed with a minimum number of feedthroughs
between them. A battery B1 is used to supply power to the electronic
circuitry within the housing. If the battery alone is contained
within one of the housing portions, then only two feedthroughs are
needed between the two housing portions, one for each battery terminal.
Alternatively, the battery and the RF circuitry can be placed in
one housing portion, with the rest of the device circuitry contained
in the other portion. This shields the sensitive therapy circuitry
from the very noisy RF circuitry.
FIG. 3 is a block diagram of an exemplary implantable cardiac rhythm
management device utilizing a split-can dipole antenna for radio-frequency
telemetry. In the figure, only one therapy lead 310 is shown but
it should be understood that a cardiac rhythm management device
may use two or more such leads. A microprocessor controller 302
controls the operation of the therapy circuitry 320 which includes
sensing and stimulus generation circuitry that are connected to
electrodes by the therapy leads for control of heart rhythm and
RF drive circuitry 330 for transmitting and receiving a carrier
signal at a specified frequency modulated with telemetry data. The
conductors of the therapy lead 310 connect to the therapy circuitry
320 through a filter 321 that serves to isolate the circuitry 320
from any RF signals that may be picked up by the lead. The filter
321 may be a low-pass filter or a notch filter such as a choke.
The RF drive circuitry 330 includes an RF transmitter and receiver
that are connected by a transmit/receive switch 333 to the dipole
antenna formed by the housing portions 101a and 101b. The microprocessor
302 outputs and receives the data contained in the modulated carrier
generated or received by the drive circuitry 330.
In this embodiment, the RF drive circuitry 330 is connected to
the dipole antenna through an antenna tuning circuit which loads
the antenna with a variable amount of inductance or capacitance
to thereby adjust the effective electrical length of the antenna
and match the antenna impedance to the impedance of the transmitter/receiver.
In this manner, the reactance of the antenna may be tuned out so
that the antenna forms a resonant structure at the specified carrier
frequency and efficiently transmits/receives far-field radiation.
The tuning circuit in this embodiment includes a balun transformer
400 and a variable capacitor 402 for loading the antenna with an
adjustable amount of reactance. The balun transformer drives the
two housing portions 180 degrees out of phase and thus also serves
to convert between the single-ended signal generated or received
by the transmitter/receiver circuitry and the differential signal
generated or received by the antenna. The balun transformer 400
also acts as a high-pass filter which blocks low frequency energy
from being passed to the RF circuitry such as may be generated when
the housing is used as an electrode in delivering electrostimulation
with a monopolar lead.
Although the invention has been described in conjunction with the
foregoing specific embodiment, many alternatives, variations, and
modifications will be apparent to those of ordinary skill in the
art. Such alternatives, variations, and modifications are intended
to fall within the scope of the following appended claims.
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