Servo fluid injection streamlines cardiac catherterization

To effectively treat coronary disease, physicians need to see how blood flows through the arteries and heart. To accomplish this, an intervential cardiologist injects radiographic material (contrast) into the patient, then views images of the coronary anatomy on overhead monitors. Acist Medical Systems (Eden Prairie, MN) claims to have developed the first angiographic-fluid-delivery system to combine programmable computer and motion-control technology with physician-interactive control for precise fluid delivery.

While the last decade has seen many breakthroughs in the devices used to treat coronary artery disease, angiographic-fluid-delivery systems have lagged behind. Most intervential cardiologists today use a confusing array of stopcocks, a maze of tubing, and an awkward hand syringe to inject contrast. Motorized injectors emerged several years ago for use in ventriculargrams that demand higher contrast volumes and rates. But they don't let the physician control and vary the flow rate during these injections.

With more than 2 million cardiac angiograms performed in the U.S. every year (Source: The Cowen Group), the opportunity to find a better method presented itself to visionary Robert Wilson. A noted University of Minnesota cardiologist, Dr. Wilson envisioned this new system would overcome the limitations of current angiography. "My colleagues and I joked about how we would like to perform angiography as simply as we pump gas in our cars--just squeeze a hand controller; the more you squeeze, the more you get."

Setting out to use as much off-the-shelf computer technology to semi-automate the process, Wilson teamed up medical and technical experts and founded Acist Medical Systems with the goal of providing the cardiologist with total control over every element of the procedure. "Competitive contrast systems are simply an on/off switch that delivers the entire amount of contrast programmed, with no variable control over the injection whatsoever," says Acist's Vice President of Marketing Mark Gillick.

"Additionally, manual systems have complicated setup requirements that take longer than our method. Our design simplifies the physician's work using an interactive, power-assisted control handpiece. The physician controls all aspects of the procedure with the AngioTouch^TM controller while viewing injection data on the system's control monitor. The system's preset parameters help facilitate reproducible injection of coronary arteries. And allow the physician to choose more or less aggressive parameters with the touch of a finger."

The Acist system injects either contrast or saline fluids at the discretion of the physician during the cardiac procedure. It automatically aspirates and purges. "Because the injection process is more precise, shorter imaging times are possible, which means less radiation exposure for the patient and physician. And because it is a simpler system to set up and operate, fewer personnel may be required. It also conserves contrast media, and most important, new built-in safeguards maximize patient safety," says Gillick.

Sensors for safety. The automatic syringe is equipped with sensors to monitor the amount of available contrast, and the entire refilling process--including automatic purging of air--to ensure a continuous, uninterrupted supply of contrast media. The system automatically detects air as contrast is dispensed. Other safety features include a backlit automatic syringe and high-pressure tubing for enhanced visibility in the low light of the cath lab, along with a hand controller lockout to prevent unintentional actuation of the hand controller trigger.

Audible indicators alert the user of button depressions, system messages, and touch screen commands. The ACIST Injection System also provides the ability to automatically switch between high- and low-pressure ports so that the patient's blood pressure is monitored any time fluid is not being dispensed.

"Aside from a custom rod-type actuator from Tol-O-Matic (Hamel, MN), we use mostly off-the-shelf components," explains Acist's Vice President of New Product Development Doug Duchon. Leveraging technologies already proven in the field cut development time. "We designed custom electronics, consisting primarily of a dual-processor logic board and an analog power board, to support the motion-control hardware." Advanced Motion Controls supplies the AMC BE25A20 servo amplifier that runs in encoder-velocity-feedback mode. And a 2,000-line, incremental, digital, quadrature encoder system from U.S. Digital supplies feedback.

After receiving U.S. Food and Drug Administration clearance to market in December 1997, it was introduced at the March 1998 American College of Cardiology conference in Atlanta. And most recently, at the Tenth Annual Symposium on Transcatheter Cardiovascular Therapeutics, an estimated ten thousand physicians eagerly observed live-case demonstrations via satellite from hospitals in Washington D.C. and New York.

Injection system overview. The ACIST Injection System increases the efficiency and control of the cardiac catheterization procedure. First to combine fully programmable computer technology with physician-interactive control of fluid delivery into one complete system, it provides precise control of contrast flow into the coronary arteries while replacing all of the stopcocks and maze of tubing with a single line and the patented AngioTouch Hand Controller.

AngioTouch Hand Controller.The hand controller replaces the manual syringe, and is the only system to offer singlehanded, variable flow-rate control of all contrast and saline injections. No compromise in visualization under fluoroscopy is seen when injecting around PTCA balloons or intracoronary stents, allowing the use of smaller catheters for diagnostic (4 French) or interventional (6 French) procedures.

Touch screen monitor.Designed to provide precise injection and procedure information, the touch screen monitor provides a continuously updated display of total contrast delivered, the actuals of the last injection, and automatically refills the contrast syringe when needed. Moreover, it offers default parameters for the left coronary artery, the right coronary artery, or the left ventricle/aorta, based upon physician preference and patient weight.

Automated manifold. The system's automated manifold eliminates the need for a completely separate system for injecting coronary arteries or the left ventricle. Aspiration of contrast injection and saline flushing is controlled automatically and the system deposits waste material in a disposable bag. The system automatically switches the pressure transducer off/on, both before and after each injection, to support patient safety and physician control over the injections.

The pressure waveform signal instantly reappears once the injection is completed. This gives the physician total control of the procedure, and provides immediate access to the patient's vital signs without having to look down at the table to manipulate manifolds, stopcocks, etc.

"The physicians who have used the ACIST Injection System have been extremely impressed with the control they have over the entire angiographic procedure," explains Acist Vice President Mark Gillick. "Rather than looking down at the table to manipulate manifolds and stopcocks, physicians can now focus their full attention on the diagnosis and treatment, allowing them to deliver contrast at the touch of a button and accurately place a balloon or stent without ever taking their eyes off the monitor."

"ACIST moves beyond the limitations of both manual and motorized devices to set new standards for catheterization performance," says Gillick. The system's modular design easily fits various cath-lab configurations. It simplifies delivery of contrast media into the patient's arteries, while increasing the procedure's efficiency, productivity, and patient management. ACIST offers single-hand control, touch screen convenience, complete procedure monitoring, and continuous optimal/variable fluid-injection control.

That was then, this is now...

Manual and Power Injections

Continuous turning of stopcocks for injecting, refilling, or monitoring pressures

ACIST Injection System

Total physician control over the entire procedure

Rod-type actuator frees physician's hands

Key to Acist's fluid-injection control is Tol-O-Matic's electrically powered, screw-drive actuator. "While the actuator's design is straightforward," explains Acist's VP of R&D Doug Duchon, "Tol-O-Matic's ability to prototype rapidly and give us exactly what we needed made the choice easy. Moreover, because we are a small company, many of the firms we contacted gave us a "lukewarm" reception, while Tol-O-Matic engineers expressed genuine interest in the task at hand. In fact, Tol-O-Matic actually built a custom extrusion for this actuator."

It was evident when Acist design engineers started developing early prototypes of the system that a custom actuator was needed. Enter Tol-O-Matic engineers, with new and exclusive electric linear-actuator products and a willingness to provide customization as needed on challenging projects. "We looked at using a standard Tol-O-Matic BC3 cylinder body and a standard lead screw and ball nut for the actuator," said Derek Wise, Tol-O-Matic design engineer, "but because of Acist's special requirements, we needed to work off a clean sheet of paper to build a prototype with all of the special features they wanted.

Acist's engineers defined the requirements of the actuator and the design evolved as the system developed. Design targets included:

A rugged and reliable design

The cylinder body is a custom aluminum extrusion which allows component mounting from all four sides. This configuration accommodated design changes along the way, plus it furnished a versatile design platform for future product models which Acist is planning for radiology and ultrasound applications.

Unlike many actuator applications where carrying and moving a load are the primary requirements, the Tol-O-Matic rod-type actuator carries no load, but is designed to provide movement and thrust force to a syringe in the injection system. As a result, the actuator was more compact because it uses no radial bearings. A custom-designed, stainless-steel rod end transmits the actuator's motion and force to the syringe plunger and provides consistently smooth straight-line motion.

To power the actuator, Acist specifies Electrocraft (Eden Prairie, MN) brushless-dc servo motors. The NEMA-34 frame (E3629) servo motor ships directly from Electrocraft to Tol-O-Matic, where it's mounted onto the backing plate of the actuator. It provides a peak torque of 575 inch-oz.

Tol-O-Matic integrates another ten-turn, boron-wound, precision potentiometer supplied by Riverside, CA-based Bourns Inc., for extra safety. Mounted in the drive area, it provides position feedback data, and shuts the system down in the event of problems such as a broken belt. "To insure high accuracy, Tol-O-Matic calibrates each unit individually prior to shipment," explains Wise.

Two gears drive the potentiometer system. Originally designed of aluminum, one gear was redesigned with an aluminum/acetal composite to reduce the possibility of system contamination due to wear and to operate with less noise. And hard-coat, anodized belt pulleys reduce system contamination due to belt wear.

The actuator lead nut incorporates special Delrin AF antirotation keys. A new design feature for Tol-O-Matic actuators, these keys prevent the thrust rod from rotating, and they also actuate micro-switches which provide position data to the system controller. These switches shut the system down to prevent the unit from travel past end of stroke. The actuator motor mounting plate has a special access port which allows easy drive-system removal and adjustment when calibrating the actuator and performing maintenance. Finally, actuator components include special finishes such as nickel plating some steel components, and anodizing all aluminum components.

In addition to assembling the actuator and motor components for Acist, Tol-O-Matic provides three separate operating tests of the final assembly. A runout test verifies system tolerances. A second test with a dynamometer confirms current stability so there is no potential for electrical surges. A final test checks the potentiometer gears for system backlash.

"Even with all of the modifications, the final actuator design is relatively simplistic," says Wise, "but the best designs are straightforward and easy to operate. We liked it so much that we just released the RSA series of rod screw actuators. With six new sizes, we've got all kinds of new applications lined up including: third axis in gantry systems, indexers, platform tables, and others."

Other Applications

Servo-fluid injection