Automatic arterial pressure regulation using isoflurane: comparison with manual control

A self-tuning, closed-loop controller, based on the algorithm of Clarke and Gawthrop, was used to regulate the inspired concentration of isoflurane to reduce arterial pressure electively in 33 patients undergoing ENT surgery. The patients were allocated randomly to one of four groups and received differing doses of fentanyl and labetalol to vary the range of sensitivities to the hypotensive action of isoflurane. The performance of the controller was evaluated at two target arterial pressures (AP), by its response to simulated changes in AP and by a comparison with a further group of eight patients with manual control of AP. The controller's undershoot of AP (range 2.8 +/- 0.5-4.5 +/- 1.3 mm Hg) and % time spent within +/- 5 mm Hg of the target AP (range 83 +/- 3.4-89 +/- 2.2%) were acceptable and equalled the manual performance figures (range 3.3 +/- 0.8 mm Hg; 90 +/- 5%). The regulation of induced hypotension in all four groups was rapid, accurate, stable and reproducible.