Expert control of the arterial blood pressure during surgery

During and after many surgical procedures the patient's arterial blood pressure must be artificially decreased to a lower than normal level. Although there are alternatives, infusion of the drug sodium nitroprusside (SNP) is frequently the preferred technique to achieve this controlled hypotension. The fast action of the drug and the danger of a too low pressure make manual control of the SNP infusion flow rate, even if done by an expert, a difficult and demanding task. This is mainly due to an occasional large unpredictable variability over time of the patient's sensitivity to SNP, and to the fact that a multitude of other factors also influence the arterial pressure. Due to these and several other causes, current automatic controllers cannot handle all cases equally well. A new expert system based SNP controller was designed to perform well for all patients, regardless of their characteristics. It monitors and adjusts its own performance, employing a number of heuristics derived from a careful study of the properties of the arterial pressure signal, the effects of SNP and other clinical provocations on the arterial pressure, and the ways in which expert clinicians manually manage the SNP infusion. Expert systems technology allows the new controller to access and employ this type of expert medical knowledge, resulting in expert-level performance. The controller was tested on 30 patients undergoing cardiac surgery, both before, during and after bypass. It was safe, needed little attention, and performed well in all cases.