Vasopressor Preload Modulation During General Anaesthesia

Is is debated whether administration of pure vasopressor may augment cardiac output (CO) by means of preload modulation. We wish to test this in 20 patients under general anesthesia, with preload dependency induced by head-up-tilt and preload modulation with infusion of phenylephrine. To document any changes in CO the patients will be monitored by Lithium dilution method. Furthermore we will monitor changes in peripheral perfusion index and test the ability of this non-invasive modality to track changes in CO.