Note: Descriptions are shown in the official language in which they were submitted.
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ATHERECTOMY POWER CONTROL SYSTEM
Field of the Invention
The present invention relates to medical devices in general, and in particular
to atherectomy devices for removing deposits from a patient's vessel.
Background of the Invention
Vascular disease is one of the leading causes of death in the United States.
One form of vascular disease occurs when a patient's artery walls become
occluded
with growth or deposits. Left untreated, these blockages can cause heart
attacks, high
blood pressure, strokes, or even death.
One promising technique used to treat vascular occlusions is described in
U.S. Patent No. 4,990,134 issued to Auth. With this procedure, an abrasive bun
is
inserted into a vessel and rotated at high speed. When rotated at speeds of
between
140,000 and 200,000 rpm, the burr exhibits a behavior called "differential
cutting,"
whereby soft tissue is unaffected by the burr but the denser occluding
material is
ablated. The ablated particles from the occlusion are small enough to prevent
reembolization downstream and are removed from the body as waste.
One danger with all ablation devices involves thermal damage to the vessel
caused by frictional heating of the occlusion during the ablation procedure.
As the
burr contacts an occlusion, the torque on a motor that rotates the burr
increases. The
increased torque increases the power that is dissipated by the bun, thereby
increasing
the likelihood that thermal damage to the surrounding tissue may occur.
In the past, most ablation devices had a tachometer that displayed the speed
of
the burr. The physician was instructed to monitor the rotational speed as it
engaged
an occlusion and not to let the speed decrease by more than some predefined
amount,
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e.g., by more than 5.000 rpm. In practice, this was difficult to do because of
the
rapidly changing loads encountered as the burr ablates a new lumen in a
vessel. In
addition, the manual process is distracting because the physician has to
monitor the
tachometer as well as to concentrate on advancing the high-speed burr through
the
patient's vessel.
Given the difficulty in controlling the power dissipated at an ablation site,
there is a need for a system that can automatically control the speed of a
prime mover
that rotates a burr such that power dissipated into a patient's vessel does
not exceed a
predetermined threshold.
Summary of the Invention
To limit the power dissipated into a patient during an ablation procedure, the
present invention is an atherectomy system that includes a power controller
which
monitors the speed and torque characteristics of a prime mover that rotates an
ablating burr. From the speed and torque applied by the prime mover, the power
controller determines the power dissipated under no-load conditions when the
burr
has not engaged an occlusion. During ablation, the torque present in the no-
Ioad
condition is subtracted from the torque measured during ablation and a power
signal
equal to the power dissipated at the burr is produced. This power is compared
with a
maximum ablation power. The power controller then adjusts the speed of the
prime
mover such that the total power dissipated does not exceed the maximum
ablation
power.
In one embodiment of the invention, the prime mover comprises an electric
motor. The power controller receives signals indicative of the speed of the
electric
motor as well as the electric current supplied to the motor in order to
determine its
torque. From the torque and speed, the power dissipated at the bun is
calculated and
the power controller adjusts the speed of the motor to maintain the power
dissipated
at a level that is at or below the maximum ablation power.
In another embodiment of the invention, the prime mover comprises a gas
turbine. The power controller determines the speed of the turbine as well as
the
pressure of gas used to drive the turbine in order to determine the torque
provided.
From the speed and torque, the power dissipated is calculated. The power
controller
adjusts the pressure to the turbine to maintain the power dissipated at a
level that is at
or below the maximum ablation power.
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Brief Description of the Drawings
The foregoing aspects and many of the attendant advantages of this invention
will become more readily appreciated as the same become better understood by
reference to the following detailed description, when taken in conjunction
with the
S accompanying drawings. wherein:
FIGURE 1 illustrates a constant power atherectomy device that includes an
electric motor according to a first embodiment of the present invention;
FIGURE 2 is a block diagram of a power controller that controls the speed of
the electric motor shown in FIGURE 1 such that the power dissipated during
ablation
remains below a predetermined threshold;
FIGURE 3 illustrates a constant power atherectomy device that includes a
turbine according to a second embodiment of the present invention; and
FIGURE 4 illustrates a flow chart showing the steps performed by the
constant power atherectomy devices according to the present invention.
Detailed Description of the Preferred Embodiment
The present invention is a constant power atherectomy system that maintains
the power dissipated in a patient during an ablation procedure at or below a
predefined level.
FIGURE 1 illustrates a constant power atherectomy system according to a
first aspect of the present invention. As with conventional atherectomy
systems, the
present invention includes an ablation burr 12 that is secured to the distal
end of a
drive shaft 14. The ablation burr 12 is positioned within a patient's vessel
15 and
advanced to a site that is just proximal to an occlusion 16. The burr is then
rotated at
high speed and an abrasive surface 18 on the bun engages the occlusion 16 and
removes particles that are sufficiently small such that they will not
reembolize
downstream of the burr. Passing the rotating burr over the occlusion increases
the
size of and/or creates a new lumen in the vessel, thereby restoring blood
flow.
As discussed above, one potential problem associated with all atherectomy
devices occurs due to the frictional heating of the occlusion caused by the
rotating
burr. If the heat dissipated becomes too great, thermal damage of the vessel 1
S,
including cell death, can occur. High thermal energy has also been associated
with
platelet aggregation and restenosis. To reduce the likelihood of thermal
damage at
the ablation site during the atherectomy procedure, the present invention
utilizes a
power control system that limits the amount of power dissipated in the vessel.
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Rotating the drive shaft 14 is a prime mover 20 which, in accordance with a
presently preferred embodiment of the invention, comprises an electric motor.
In
general, most electric motors are not capable of rotating at the high speeds
required to
cause the differential cutting at the ablation burr. To provide the speeds
required, the
present invention utilizes a D.C. brushless motor capable of producing speeds
up to
100,000 rpm. The motor drives a friction wheel 21 A, which engages a smaller
drive
wheel 21 B that is coupled to the drive shaft 14. The size ratio between the
friction
wheel 21 A and the drive wheel 21 B was selected to be 4.24:1. Therefore, the
motor
can be driven at speeds of 37,000 to 38,000 rpm in order to produce a bun
speed of
160,000 rpm. The larger friction wheel and drive wheel are spring loaded to
ensure
the two wheels remain engaged and to compensate for wear. With the larger
friction
wheel and smaller drive wheel, the motor is capable of maintaining rotational
burr
speeds of between 100,000-200,000 rpm. Controlling the speed of the motor is a
conventional motor controller 22. As with traditional motors, the motor
controller 22
adjusts the current delivered to the motor 20 to maintain its speed at some
desired
level.
To maintain the power dissipated at the ablation site at a level that is at,
or
below, a predetermined threshold, the present invention includes a power
controller 24 that adjusts the operating characteristics of the motor 20 such
that the
power delivered by the burr at the ablation site does not exceed the
predetermined
threshold. The power controller 24 monitors the torque provided by the motor
20 as
well as its speed of rotation in order to calculate the power dissipated by
the drive
shaft and the ablation burr. The speed of rotation may be received from an
external
tachometer 26 or may be determined from the commutation signals provided by
the
motor controller 22. In addition, the torque provided by the motor, which is a
function of the current delivered to the motor, may be determined from an
external
ammeter 28 disposed in line with the leads that provide current to the motor
or may
be determined from the motor controller 22.
Under no-load conditions, when the bun 12 has not yet engaged the
occlusion 16, a certain amount of power will be dissipated due to frictional
losses in
the drive shaft and accompanying catheters that route the burr to the
occlusion. Any
additional power that is dissipated above the no-load value is assumed to be
delivered
at the point of ablation. In operation, the bun is placed adjacent the
occlusion, but
not engaging it. An operator presses a "platform" switch 30, which causes the
power
controller 24 to compensate for the power dissipated under the no-load
condition.
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During ablation, the power controller 2a determines the additional power that
is
dissipated at the ablation site and compares this with the predetermined
threshold.
The power controller 24 then adjusts the speed of the electric motor 20 such
that the
power dissipated at the ablation site does not exceed the threshold.
FIGURE 2 illustrates a block diagram of the power controller 24 shown in
FIGURE 1. The power controller includes a resistor 40 that is positioned in
line with
a lead that delivers the driving current to the motor 20. A differential
amplifier 42
measures a voltage across the resistor 40 in order to produce a signal having
a
magnitude that is proportional to the electrical current that drives the
motor. The
current signal from the differential amplifier 42 is applied to a low pass
filter 44
having a 3dB frequency of 20 Hz that removes high frequency components from
the
current signal. As will be appreciated by those skilled in the art, the
particular 3dB
frequency used may be adjusted to alter the response time of the control
system and
its stability. The output of the low pass filter 44 is applied to a sample and
hold
circuit 46. Upon activation of the platform switch 30 shown in FIGURE 1, the
sample and hold circuit maintains a sample of the filtered current signal and
applies it
to a first input 48a of a differential amplifier 48. Applied to a second input
48b of the
differential amplifier 48 is the filtered current signal produced at the
output of the
low pass filter 44.
A frequency to voltage converter 50 receives the commutation signals that are
applied to the motor and converts these signals to a corresponding voltage
that is
proportional to the speed of the motor. The output of the frequency-to-voltage
converter 50 is applied to a iow pass filter 52 having a 3dB frequency of 7
Hz. The
output of the low pass filter 52 is applied to a first input 54a of a
multiplier circuit 54.
Applied to another input 54b of the multiplier 54 is the output of the
differential
amplifier 48. Between the differential amplifier 48 and the second input 54b
of the
multiplier 54 is a switch 60 that is opened or closed in order to selectively
connect
the output of the differential amplifier 48 to the input 54b of the multiplier
54. Prior
to calibration by pressing the platform switch, the switch 60 remains open to
avoid
erroneous signals produced by the multiplier 54.
The multiplier 54 produces a signal that is proportional to the power
dissipated at the burr in watts. The power dissipated is a function of the
torque
expended by the motor (which is related to the current delivered to the motor)
and its
speed.
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In the presently preferred embodiment of the invention, the multiplier 54
calculates the power dissipated at the burr according to the equation:
~z
P~~~atts =
1000
where w is the rotational speed of the bun in the radians/sec and ~ is the
torque in newton-millimeters. The relationship between the current delivered
to the
motor and the torque produced is generally obtained from a motor's
specifications or
may be determined experimentally. The output of the differential amplifier 42
is
therefore calibrated to produce a signal that is proportional to the torque of
the motor
in the appropriate units to calculate the power of the burr in watts.
After the platform button has been pressed, the sample and hold circuit 46
causes the differential amplifier 48 to subtract the torque expended by the
motor
during no-load conditions such that the value of the power signal produced by
the
multiplier 54 is only proportional to the power dissipated at the burr.
The output of the multiplier 54 is applied to a power meter 62 that provides a
1 S visual indication of the power dissipated at the ablation site. The output
of the
multiplier 54 is also fed through a switch 64 to one input 66a of a
differential
amplifier 66. The switch 64 has two positions such that the output of the
multiplier 54 can be selectively connected to the input 66a of the
differential
amplifier 66. When the switch 64 is closed, the power controller operates to
maintain the power dissipated at the bun below a predetermined threshold. With
the
switch open, no feedback control is present.
Applied to a second input 66b of the differential amplifier 66 is a voltage
that
is proportional to a threshold or maximum ablation power to be delivered by
the burr
during ablation. When the voltage produced by the multiplier circuit 54
exceeds the
threshold, the differential amplifier 66 produces a positive going signal,
which is
applied to a summation circuit 68. The output of the differential amplif er 66
is
prevented from going negative by a diode 70 such that the differential
amplifier 66
only contributes to the output of the summation circuit 68 when the power
dissipated
by the atherectomy burr exceeds the power threshold.
The summation circuit 68 adds the output of the differential amplifier 66 and
the output of the low pass filter 52. The output of the summation circuit 68
is applied
to the first input 72a of an error amplifier 72. Applied to a second input 72b
of the
error amplifier is a voltage that is proportional to a desired RPM of the
motor. The
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output of the error amplifier 72 drives a speed control input of the motor
controller 22. shown in FIGURE 1. to adjust the speed of the motor. When the
output of the differential amplifier 66 is greater than zero, meaning that the
power
dissipated at the bun is greater than the predetermined threshold. the output
of the
summation circuit increases. This causes the output of the error amplifier 72
to
decrease and reduce the speed of the motor such that the power dissipated at
the burr
decreases.
Although the power controller shown in FIGURE 2 is an analog circuit, it will
be appreciated that a microprocessor could be also be used to control the
power
dissipated by the atherectomy burr. FIGURE 4 is a flow chart showing the steps
to
be performed by a digital controller/microprocessor in accordance with the
present
invention to maintain the power dissipated during ablation at or below a
predefined
maximum level. Beginning at a step 120, the prime mover, typically an electric
motor as shown in FIGURE 1 or a turbine shown in FIGURE 3, is brought up to a
desired ablation speed. For most atherectomy devices, differential cutting of
the bun
occurs most readily when the burr is rotated at speeds between 140,000 and
200,000 rpm. At a step 122, the power controller determines whether the
platform
switch was pressed. As indicated above, the platform switch causes the power
controller to compensate for the power dissipated by the drive shaft and burr
under
no-load conditions. If the platform switch has not been pressed, processing
returns to
step 122 until the switch is pressed.
Once the platform switch has been pressed, processing proceeds to step 124,
wherein the power controller determines the torque expended by the prime
mover.
From the speed of the burr and the torque, the no-load power is determined at
step 126 using Equation 1 described above. At a step 128, the physician begins
ablating an occlusion in a patient's vessel. At a step 129, the power
controller
determines the total power being dissipated by the atherectomy device. The no-
load
power is subtracted from the total power at a step 130 to compute the power
dissipated at the bun.
At step 132, it is determined if the power being dissipated at the bun during
ablation exceeds the predetermined threshold. If so, the power controller
reduces the
speed of the prime mover at step 134. The processing then returns to step 129
and
the monitoring of the dissipated power continues until the ablation procedure
is
complete. In the presently preferred embodiment of the invention, the power
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threshold is set at approximately one watt. However, this threshold may vary
depending on the location of the occlusion in the body. blood flow, etc.
Although the presently preferred embodiment of the invention utilizes an
electric motor, a second embodiment of the invention uses a turbine 150 to
rotate the
drive shaft. A pressure control unit I 52 controls the speed and power of the
turbine.
A power controller 154 interfaces with the pressure control unit 152 in order
to
control the maximum power delivered at the ablation site. The power controller
154
may interface with an external tachometer 156 that monitors the speed of
rotation of
the drive shaft. In addition, the power controller may receive an indication
of the
pressure used and/or the flow rate of the gas to drive the turbine from a
pressure
sensor 158. A platform switch 160 that causes the power controller to
compensate
for the power dissipated by the burr and drive shaft under no-load conditions.
In contrast to the electric motor in which the applied torque can be readily
determined from the current delivered to the motor, in the turbine, the torque
is not as
straightforward to obtain. One method is to characterize the performance of
the
turbine by employing a dynamometer to generate a family of torque-speed
curves. At
each pressure input to the turbine, a unique torque-speed relationship (curve)
exists.
Once this family of curves is experimentally generated (one curve for each
input
pressure selected), the information can be charted in a "look-up" table.
During
operation, speed can be determined with a tachometer, pressure can be
determined
with a pressure transducer, and with these two values know, torque can be
found in
the "look-up" table. With speed and torque now known, power can be calculated.
In operation, the power controller 154 adjusts the pressure and/or flow rate
of
the gas applied to the turbine through a high speed solenoid valve (not shown)
to
slow its speed and maintain the level of power dissipated at the burr at or
below a
predetermined threshold.
As can be seen from the above, the present invention therefore operates to
automatically control the level of power dissipated in a patient's vessel
during an
ablation procedure. Because the speed of the prime mover is automatically
adjusted,
the physician does not have to monitor the operating characteristics of the
device and
is free to focus his or her attention on positioning and advancing the burr
within a
vessel.
