RECEIVINGANTENNA FOR AN IMPLANTABLE MEDICAL DEVICE

Cross-Reference to Related Applications

Not Applicable

Statement Regarding Federally Sponsored Research or Development

Not Applicable

Background of the Invention

1. Field of the Invention

[0001] The present invention relates to medical devices for implanting into an

animal, such as cardiac stimulation devices, and more particularly to such medical

devices that receive radio frequency signals via an antenna.

2. Description of the Related Art

[0002] A remedy for people with slowed or disrupted natural heart beats is to

implant a cardiac pacing device which is a small electronic apparatus that stimulates

the heart to beat at regular intervals. That device consists of a pulse generator,

implanted in the patient's chest, which produces electrical pulses that stimulate heart

contractions. Electrical wires extend from the pulse generator to several electrodes

placed nearby specific muscles of the heart, which when electrically stimulated

produce contraction of the adjacent heart chambers.

[0003] It is quite common that the wires extend through arteries or veins which

enter the heart so that the electrodes can be placed in the muscle of the heart chamber

requiring stimulation. The wires typically extend for some distance in the arteries or

veins and may pass, through one or two heart valves. In other patients, patch electrodes

are placed on the exterior heart surface with wires extending through tissue to the

pacing device. With either type of wire placement, it is important that the electrodes be

attached at proper positions on the heart to stimulate the muscles and produce

contractions. Thus, it is desirable to properly locate the electrodes for maximum heart

stimulation with minimal adverse impact to other physiological functions, such as blood

circulation.

[0004] More recently wireless pacing devices have been proposed, such as the one

described in U.S. Patent No. 6,445,953. With this type of device, a radio frequency (RF)

signal is transmitted from a conventional pacing circuit to stimulator devices placed on

the heart at locations where stimulation is to occur. For example, the stimulator device

can be implanted in a blood vessel of the heart. The radio frequency signal activates the

device which applies an electrical pulse to the heart tissue. Electrical power for

stimulating the heart is derived from the energy of the radio frequency signal.

[0005] One of the difficulties in such a wireless system is ensuring that the radio

frequency signal and a maximum amount of the RF energy is received by the stimulator

device. In the case of that prior patented device, the antenna was a coil wrapped around

a cylindrical structure that was embedded against the wall of a vein or artery. That type

of antenna received the greatest amount of energy from an electromagnetic field

oriented in a direction through the turns of the coil. However, since the antenna can be

implanted in different orientations in the patient's body depending on the location of the

vein or artery and the orientation of the transmitter antenna similarly varies, it is

difficult to ensure that the electromagnetic field from the RF signal will be properly

aligned with the antenna of the implanted device. Because the location at which the

medical device is implanted is based primarily on cardiac stimulation criteria, it is not

always possible to optimally orient its antenna for maximum energy reception.

[0006] A proposed transmitter, for sending signals to the implanted medical device,

employs an omnidirectional antenna which emits B fields that propagate along three

orthogonal axes. Thus one of those B fields or a vector sum of two or all of them will

be aligned with the antenna coil of the implanted medical device. Although this solves

the problem of misalignment of the transmitter and receiver antennas, the RF energy

from all the B fields is not received by the implanted device. Since that device is

powered by the RF energy, it is desirable to receive as much of the transmitted energy

as possible. That desire is especially acute when the medical device is an implanted

defibrillator as sucK apparatus requires a relatively large amount of power to defibrillate

a heart.

Summary of the Invention

[0007] An antenna assembly is provided by which a medical device, such as one

implanted in a patient, receives a radio frequency signal. The assembly has a first

antenna oriented on the medical device to receive a component of the radio frequency

signal that is oriented or propagates in a first direction. A second antenna is oriented on

the medical device to receive a component of the radio frequency signal that is oriented

or propagates in a second direction that is transverse to the first direction.

[0008] In one version of the antenna assembly the first and second antennas are

oriented along separate orthogonal axes. Preferably, each of the first and second

antennas comprises two conductive coils that are electrically connected together and

spaced apart along the respective axis. The preferred embodiment of the antenna

assembly further includes a third antenna oriented along a third axis that is orthogonal

to both the first and second axes.

[0009] Another aspect of the present antenna assembly provides a circuit that is

operably connected to combine electrical signals produced in all the antennas.

[0010] The present antenna assembly is particularly adapted for use with an

implanted medical device that has a cylindrical body. Here, a first antenna is oriented

on the cylindrical body to receive the radio frequency signal and a second antenna is

oriented on the cylindrical body at a location that is spaced 90° circumferentially from

the first antenna. Each of the first and second antennas may comprise two coils located

on opposite sides of the body and interconnected so that electrical signal produced in

the two coils additively combine. In this medical device, a third antenna may be wound

circumferentially around the body.

[0011] The various versions of the present antenna assembly provide multiple

antennas each for receiving the radio frequency signal that has a component oriented

in a given direction. This enhances reception of the radio frequency signal regardless

of the orientation of the medical device to the direction of that component. In other

cases, the radio frequency signal has a plurality of components, each oriented in a

different direction and each antenna of the assembly receives a different one of those

components. The individual electrical signals produced in each antenna are additively

combined into a single signal having greater strength than each of the individual

components.

Brief Description of the Drawings

[0012] FIGURE 1 depicts a cardiac pacing apparatus implanted in a patient;

[0013] FIGURE 2 is an isometric, cut-away view of a blood vessel with a cardiac

stimulation device implanted therein;

[0014] FIGURE 3 is a schematic block diagram of an electrical circuit on the

stimulation device; and

[0015] FIGURE 4 depicts one of the antennas of the stimulation device.

Detailed Description of the Invention

[0016] With initial reference to Figure 1, an apparatus 10 for applying electrical

stimulation to a heart 11 comprises a pacing device 12 and one or more stimulators 20

and 21 located in arteries or veins 14 through which blood flows within the heart muscles.

As will be described in greater detail, the pacing device 12 emits a radio frequency signal

16 which produces an electric voltage in the implanted vascular stimulators, thereby

stimulating the heart muscle to contract. The radio frequency signal 16 preferably

comprises a set of electromagnetic waves that propagate along three orthogonal axes,

however that signal alternatively may comprise a single electromagnetic wave or other

pluralities of waves. For example the transmitted signal may be circularly polarized with

multiple components having phase differences of 90 degrees. Although the present

invention is being described in the context of an apparatus for pacing the heart, the novel

antenna system can be used with defibrillators and other implantable medical devices.

[0017] Referring to Figure 2, a stimulator 20 is placed in the artery or vein 14 of the

heart 11. The body 23 of the stimulator 20 has a design similar to well-known vascular

stents and is in the form of a tube that initially is collapsed to a relatively small diameter

enabling it to pass freely through arteries or veins of a patient. The procedure for

implanting the stimulator 20 is similar to that used for vascular stents. For example, the

balloon at the end of a standard catheter is inserted into the annulus of the stimulator 20

in the collapsed, or reduced diameter, configuration. That assembly is inserted through

an incision in a vein or artery near the skin of a patient and threaded through the

vascular system to the appropriate location in the heart. Specifically, the stimulator 20

ultimately is positioned in a cardiac artery or vein 14 adjacent to a section of the heart

muscle where stimulation should be applied. The balloon of the catheter then is inflated

to expand the vascular stimulator 20 which, as seen in Figure 2, embeds the stimulator

against the wall of the blood vessel. The balloon is deflated, the catheter is removed

from the patient, and the incision is closed. Thereafter, the tubular configuration of the

stimulator allows blood to flow relatively unimpeded through the artery or vein.

[0018] With reference to Figures 2 and 3, a signal receiving circuit 22 is mounted on

the body 23 of the vascular stimulator 20. The signal receiving circuit 22 includes an

antenna array 28, a passive RF signal combiner 27, a radio frequency signal detector 26,

and first and second electrodes 24 and 25. The antenna array 28 preferably comprises

three antennas 31, 32 and 33 connected by a signal combiner 27 to inputs of the radio

frequency signal detector 26 that is tuned to the frequency (e.g. 27 MHz.) of the RF

signal 16 emitted by the pacing device 12. However, the detector does not necessarily

have to be a tuned circuit. Upon receiving the radio frequency signal 16, the detector 26

converts the energy of that signal into a differential voltage pulse that is applied across

the first and second electrodes 24 and 25. Those electrodes form an electric circuit path

with the patient's heart tissue, thereby stimulating that tissue. Thus, each time the

pacing device 12 emits a radio frequency signal 16, a pulse of electrical voltage is

produced in the vicinity of the stimulator 20 to stimulate the adjacent heart muscle.

[0019] The three separate antennas 31, 32 and 33 of the antenna array 28 are

oriented on the body 23 to receive radio frequency electromagnetic waves that

propagate along three orthogonal axes designated X, Y and Z, respectively. The first

antenna 31 includes first and second coils 34 and 35 mounted on opposite lateral sides

of the exterior surface of the body 23 (only one of those coils 34 is visible in Figure 2,

but see Figure 4). The first antenna 31 receives electromagnetic waves, or components

of such waves, that propagate in the X axis. The second antenna 32 is formed by third

and fourth coils 36 and 37 mounted on the top and bottom of the exterior surface of the

body 23 and receives electromagnetic waves, or components of such waves, that

propagate in the Y axis. It should be understood that the directional references herein to

lateral sides, top, and bottom are for understanding the physical relationship between

the first and second antennas 31 and 32 and relates only to the orientation of the

stimulator body 23 in Figure 2, which orientation may be rotated in a particular

installation of the stimulator in a patient. The third antenna 33 is a single fifth coil 38

that is wound circumferentially around the exterior surface of the body 23 to receive

electromagnetic waves, or components of such waves, that propagate in the Z axis. The

ends of the fifth coil 38 are connected to the RF signal combiner 27 which is a passive

network that combines the signals from the three antennas 31-33 jnto a single signal

that is fed to the detector 26.

[0020] The interconnection of the first and second coils 34 and 35 of the first

antenna 31 is depicted in Figure 4. It should be understood that the third and fourth

coils 36 and 37 of the second antenna 32 have the same construction as coils first and

second 34 and 35, respectively, with the only distinction being rotation 90° around the

circumference of the body 23. The first coil 34 has a first end 40 connected by a

conductor 42 to the signal combiner 27 of the signal receiving circuit 22. The first coil

34 is wound counterclockwise in an increasing radius spiral until reaching a second end

44. The number of turns of the spiral is chosen for optimum reception of the particular

frequency of the RF signal from the pacing device 12. The second end 44 of the first

coil 34 is connected by a linking conductor 46 to a first end 48 of the second coil 35.

[0021] The second coil 35 is centered diametrically opposite the center of the first

coil 34 along axis 47 and is wound from its first end 48 in a counterclockwise

decreasing radius spiral until reaching a second end 50. The second coil 35 preferably

has the same number of turns as the first coil 34. The second end 50 of the second coil

35 is connected by another conductor 46 to the signal combiner 27. It should be

understood that the spirals of the first and second coils 34 and 35 may be wound in the

opposite direction that those shown in Figure 4. However, electric current induced in

the first antenna by the RF signal from the pacing device 12 must flow in the same

direction, clockwise or counterclockwise in both coils. In other words, the two coils 34

and 35 are wound so that the electric voltage induced in each one combine in an

additive manner to produce a resultant voltage that is greater than the voltage induced

in either coil alone.

[0022] The foregoing description was primarily directed to a preferred embodiment

of the invention. Although some attention was given to various alternatives within the

scope of the invention, it is anticipated that one skilled in the art will likely realize

additional alternatives that are now apparent from disclosure of embodiments of the

invention. Accordingly, the scope of the invention should be determined from the

following claims and not limited by the above disclosure.