M12.5 Blood Pressure Control in Post-Operative Patients

Health care is an important area that can benefit from control systems technology. For example, after undergoing open-chest surgery (such as cardiac bypass or lung surgery), many patients have a higher than desired blood pressure; one reason that a lower blood pressure is needed is to reduce bleeding from sutures. Typically, a critical care nurse will set up a continuous infusion of the drug sodium nitroprusside to reduce the patient's blood pressure. The nurse must make frequent checks of blood pressure and change the drug infusion rate to maintain the desired blood pressure. In a sense, the nurse is serving as a feedback controller. There is a clear motivation to develop an automated feedback control system, where a blood pressure sensor sends a signal to a blood pressure controller, which adjusts the speed of a drug infusion pump. In addition to providing tighter regulation of blood pressure, this strategy frees up the nurse to spend more time monitoring patients for other problems.

A 70-kg patient exhibits the following input-output behavior for the effect of sodium nitroprusside (SNP) on mean arterial pressure (MAP):

where the pressure unit is mm Hg, the drug flow units are ml/hr and the time unit is minutes. The initial MAP (with no SNP infused) is 175 mm Hg. The desired MAP setpoint is 100 ± 15 mm Hg, and the maximum allowable (short-term) SNP infusion rate is 150 ml/hr. The desired settling time (time for the MAP to remain within limits) is 5 minutes.