Objectives.This study was conducted to determine the response timeof the Opti-Q continuous cardiac output (CCO) device to a step change incardiac. Design.Prospective study. Setting.University hospitalanimal lab. Model.Female sheep. Interventions.In ten animals,cardiac output was altered suddenly by opening and closing a peripheralarteriovenous shunt to test the response time of the CCO system.Measurements and main results.Cardiac output was measured continuouslyby thermodilution and ultrasonic techniques while an arteriovenous shunt wasopened and closed. A total of 53 dynamic observations were made (5–6 peranimal). The mean response time of the continuous cardiac output device was86 seconds and was unaffected by the magnitude or direction of the change incardiac output. It was also unaffected by the animal's weight. CCOvalues were not statistically different from standard thermodilutionmeasurement (p= 0.895). Shunt flow ranged from 430 to 1730 ml/minand averaged 812 ml/min. The mean CCO with the shunt closed was 4.62 L/min.There was 1.5 to 2 minutes under or overshoot in cardiac output in 11% of themeasurements. Conclusions.Continuous cardiac output measurement wasas accurate as those made by standard bolus thermodilution. The averageresponse time to acute changes in cardiac output was approximately 1.5 minutesor ten times faster than previously reported systems. Response time isindependent of animal mass, shunt volume and the direction of cardiac outputperturbations.