ATHERECTOMY POWER CONTROL SYSTEM

Abstract: A power control system maintains the power dissipated by an atherectomy burr during an ablation procedure at or below a predetermined threshold. A power control circuit monitors the speed and torque provided by a prime mover that rotates the ablation burr in order to calculate the power dissipated under no-load conditions. During ablation, the power dissipated is determined and the no-load power is subtracted to calculate the ablation power. The control circuit adjusts the speed of the prime mover to maintain the ablation power at or below the predetermined threshold.

Description: 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 burr 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 burr, 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, 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 bun- has not engaged an occlusion. During ablation, the torque present in the no-load 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 burr 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. 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 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 burr 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 15, 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. 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 21B was selected to be 4.24: 1. Therefore, the motor can be driven at speeds of 37,000 to 38,000 m in order to produce a burr speed of 160,000 φm. 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 φm. 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 burr 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 burr 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. During ablation, the power controller 24 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 low 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. -o- In the presently preferred embodiment of the invention, the multiplier 54 calculates the power dissipated at the burr according to the equation: where ω is the rotational speed of the burr in the radians/sec and τ 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 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 burr 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 amplifier 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 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 burr 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 bun- 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 bun. 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 bun is rotated at speeds between 140,000 and 200,000 φm. 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 bun 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 burr 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 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 152 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 cunent 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 bun within a vessel.

Claims: The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 1. An atherectomy power control system comprising: a drive shaft; an ablation burr disposed at the distal end of the drive shaft; a prime mover coupled to the drive shaft that rotates the drive shaft and the ablation bun at an adjustable speed; and a power control circuit that monitors the speed and torque of the prime mover as the ablation bun engages an occlusion in a patient's vessel and adjusts the speed of the prime mover such that a power dissipated at a site of the occlusion is less than or equal to a predetermined maximum. 2. The atherectomy power control system of Claim 1, wherein the power control circuit compensates for a power dissipation that occurs when the ablation bun is rotated in the patient's vessel but is not engaged with the occlusion. 3. The atherectomy power control system of Claim 2, wherein the prime mover comprises a variable speed electric motor and the power control circuit adjusts the speed of the motor to maintain the power dissipated during ablation at a level less than or equal to the predetermined maximum. 4. The atherectomy power control system of Claim 2, wherein the prime mover is a turbine and the power control circuit adjusts a pressure and/or flow rate at which the turbine is operated to maintain the power dissipated during ablation at a level less than the predetermined maximum. 5. A method of ablating an occlusion from a patient's blood vessel, comprising: advancing an atherectomy device including a drive shaft and a burr disposed at the distal end of the drive shaft into the patient's vessel; rotating the drive shaft and the burr at a high speed; engaging the occlusion with the rotating bun; determining a power dissipated at the bun as the burr engages the occlusion; and adjusting the rate of rotation of the drive shaft and burr such that the power dissipated at the bun is at or below a predetermined threshold. 6. The method of Claim 5, wherein the step of determining the power dissipated at the bun further comprises: compensating for a power dissipation that occurs when the drive shaft and burr are in the patient's vessel but not engaging the occlusion. 7. An atherectomy system, comprising: a drive shaft; an ablation bun disposed at a distal end of the drive shaft; an electric motor that rotates the drive shaft at a high speed to induce differential cutting properties of the bun, the motor including a friction wheel that engages a smaller drive wheel to which the drive shaft is coupled, wherein the size of the friction wheel is greater than the drive wheel such that the speed at which the drive shaft is rotated is greater than the speed of the motor. 8. The atherectomy system of Claim 7, wherein the friction wheel and driven wheel are spring biased together.

Inventors: HIBLAR TOM, HIRST PAUL A, WENTZEL DAVID, HIBLAR, TOM, HIRST, PAUL, A., WENTZEL, DAVID
Applicants: SCIMED LIFE SYSTEMS INC, SCIMED LIFE SYSTEMS, INC.
Application year: 2000
Main classification: A61B17
IPC codes: A61B17, A61B19