M12.6 Critical Care Patients

Critical care patients have often suffered a "disturbance" to the normal operation of their physiological system; this disturbance could have been generated by surgery or some sort of trauma (e.g., a heart attack). A responsibility of the critical care physician is to main tain certain patient outputs within an acceptable operating range. Two important outputs to be maintained are mean arterial pressure (MAP) and cardiac output (CO). During surgery the anesthesiologist will infuse several drugs into the patient in order to control these states close to the desired values. A conceptual diagram is shown in Figure M12-2.

The goal of this control system design is to manipulate the flow rate of two drugs, dopamine (DPM) and sodium nitroprusside (SNP), to maintain the two outputs at their desired setpoints. A successful implementation of such a strategy allows the anesthesiologist to spend more time monitoring other patient states, such as "depth of anesthesia."

A simplified model representing the input-output behavior for a particular patient is

Equation M12.14

where inputs 1 and 2 are SNP and DPM (ml/hr), and outputs 1 and 2 are MAP (mmHg) and CO (liters/min). The time constants have units of minutes.