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On the Utilization of Automatic Control Systems Within a Perfusion System

L.E. Lucke
Minnetronix, Inc., St. Paul, Minnesota, U.S.A

R. Griewski
Terumo Cardiovascular Systems, Ann Arbor, Michigan, U.S.A.

C. Koch
Terumo Cardiovascular Systems, Ann Arbor, Michigan, U.S.A.

Presented at: The World Congress on Medical Physics and Biomedical Engineering;
Sydney, Australia; 24-29 August 2003

Download a pdf version of this paper here.

Objective

Heart lung machines must be managed by a Perfusionist to maintain proper blood flow and blood pressure to the patient during a coronary bypass procedure (1). Much of the time, the Perfusionist makes small adjustments to the pumps in the heart lung machine to maintain flow and pressure while balancing the amount of blood outside the patient, in the perfusion circuit. Due to the nature of the procedure this process can be tedious and is prone to human error (2). Use of automatic control systems can mitigate human errors and provide support to the Perfusionist during the procedure. In this paper we present new control systems for automatically managing the blood pumps within a heart lung machine. These control systems have been introduced in the Terumo® Advanced Perfusion System 1. Note as of 10 March 2003, this system has not received 510K clearance from the FDA in the U.S.A.

Method

Heart lung machines are used to bypass a patient's heart during surgery (3). Such a system, with a simulated patient "load", is shown in Fig. 1.

Figure 1: Simulation of a Heart-Lung Bypass Operation

The simulator consists of a venous return line connected to a reservoir/oxygenator followed by a main pump which pumps the blood back into the patient through the arterial line. The main pump is responsible for circulating the blood during bypass surgeries. A secondary pump, the cardioplegia pump (CPG pump) may be used to deliver blood and drugs directly to the heart, or in the case of the simulator, back to the reservoir. In this circuit, a flow meter and/or pressure sensor is attached to the arterial line. A secondary flow sensor may be located at the outlet of the main pump. Other sensors, such as temperature or air bubble sensors (ABD) may be present.

Two types of pumps are commonly used within this system. A roller pump is a positive displacement pump and physically pushes the fluid through the system. Thus the speed of a roller pump is proportional to the flow. Another common type of pump is a centrifugal pump. In a centrifugal pump, the rotary motion of the pump is used to circulate the blood. Centrifugal pumps by nature of their design act as "afterload sensitive" pumps and respond to changes in pressure by decreasing flow when pressure increases, or increasing flow when the pressure decreases, although the pump speed remains constant.

During a surgical procedure, the Perfusionist maintains the arterial flow and pressure within the circulatory circuit. This is accomplished by adjusting the speed of the pump. With a roller pump, adjusting the speed directly affects the flow and pressure within the system. With a centrifugal pump, adjusting the speed directly affects the flow within the system.

The perfusionist watches the flow and pressure sensors and constantly makes small, incremental adjustments to the pump speed to maintain either a flow or a pressure that meets the metabolic demands of the patient. Historically, heart lung systems provide a way to set up a coarse automatic response within the system, such that when the flow or pressure crosses an unsafe threshold, the pump stops or slows down. While such coarse responses provide important safety protection, the perfusionist must still manage the pumps to provide fine control. Such a system is prone to human error (1). With an automatic digital control system (4) the pressure or flow within the circulation circuit is fed back to the pump and the pump controller automatically makes fine adjustments to the pump speed to maintain a fixed pressure or flow at the patient.

Arterial Pressure Control
A roller pump can be automatically controlled via feedback from either arterial line pressure or arterial pressure. Arterial line pressure is typically measured after the oxygenator and before the arterial filter in the arterial line of the perfusion circuit. Arterial pressure is typically measured in the patient's radial artery via a catheter. This type of control algorithm results in a roller pump varying its speed in response to changes in pressure, much like a centrifugal pump currently behaves, to maintain a specific pressure in the arterial line. In this case, the automatic control system resides in the pump and requires feedback from an external pressure sensor. The control loop is setup as a two-stage control system where the internal speed control loop is controlled by the external pressure feedback as shown in Fig. 2.

Figure 2: Two-stage Arterial Pressure Control System

Arterial Flow Control
A roller pump can be controlled via feedback from an arterial flow meter. Typically the arterial flow is measured distal to the arterial filter. This type of control algorithm results in a roller pump varying its speed in response to changes in flow. A potential benefit would be that the pump would be able to compensate for flow that is "stolen" from the arterial line due to opening up a shunt (purge line, hemoconcentrator line, recirculation line) or due to delivering a dose of cardioplegia. This shunting of flow to other locations means that the calculated flow displayed on the pump is not an accurate reflection of what is actually being delivered to the patient. . Figure 1 shows that flow meter, "F1" is in the optimum position on the perfusion circuit. During automatic control the pump would speed up during the shunting event and recover the original speed after the shunting stops.

A centrifugal arterial pump can be controlled to a given flow rate measured in the arterial line. The same benefits that are stated above for a roller pump to flow servo also apply here. In addition, a Perfusionist that does not want to see the centrifugal pump's "autoregulation" effect could use this control configuration. Automatic control will maintain a definitive flow rate, regardless of pressure or afterload changes.

In both cases the control system can be setup similarly to Fig. 2 by replacing the pressure feedback with flow feedback.

For either the arterial pressure control or the arterial flow control, it is necessary to safely bound the response of the system to prevent inappropriate behavior and to work with the Perfusionist to provide appropriate responses to widely varying pressures or flows. The control limits can be summarized as in Fig. 3. The system provides high and low flow or pressure alerts. The Perfusionist sets up these thresholds. In addition, during the automatic control of the pump, the pump speed also is bounded by the control algorithm so as not to stray too far from the original speed at the initiation of the flow or pressure control loop. The Perfusionist is alerted should any of the boundary conditions be reached.

Figure 3: Automatic Control Limits

Results

To simulate the effectiveness of the automatic control system, the patient simulator perfusion circuit shown in Fig. 1 can be used. The patient is simulated through a tubing load attached to an occluder (a computer controller clamp). By varying the occlusion at the patient load, it is possible to simulate pressure and flow changes at the patient. A typical system response to a large positive pressure change is shown in Fig. 4. Here the flow at the patient load is restricted while attempting to maintain a desired pressure to the patient. The roller pump automatically slows from 70 to 25 rpm and the average pressure at the patient, which was initially at 150mmHg, returns to that level after about four seconds.

Figure 4: Roller Pump Response to a Flow Change while Maintaining a Desired Pressure

Additional testing over a range of conditions has proven the safety and efficiency of the automatic control loop.

Conclusion

Automatic control systems are used in the Terumo System 1 to provide flow and pressure stability at the patient. This support reduces the need for the perfusionist to make minute adjustments to the system and thus helps to manage those portions of the procedure prone to human error. The safety and utility of these control loops has been demonstrated. These control systems can be extended to additional applications such as negative pressure control where a negative pressure is maintained in the venous return line to support active venous return.

References

1. K. Orihashi, Y. Matsuura, T. Sueda, H. Shikata, S. Morita, S. Hirai, M. Sueshiro, K. Okada, Flow velocity of central retinal artery and retrobulbar vessels during cardiovascular operations, The Journal of Thoracic and Cardiovascular Surgery, Vol. 114, No. 6, Dec. 1997.

2. M. de Leval, J. Carthey, D. Wright, V. Farewell, J. Reason, Human factors and cardiac surgery: A multicenter study, The Journal of Thoracic and Cardiovascular Surgery, Vol. 119, No. 4, April 2000, part 1.

3. C. Mora (Editor), R. Guyton, D. Finalyson, Cardiopulmonary Bypass: Principles and Techniques of Extracoporeal Circulation, Springer Verlag, March 1995.

4. M. Santina, A. Stubberud, G. Hostetter, Digital Control System Design, Int'l, Thomson Publishing, January 1994.