The Initial Systolic Time Interval (ISTI), obtained from the electrocardiogram (ECG) and impedance cardiogram (ICG), is considered to be a measure for the time delay between the electrical and mechanical activity of the heart and reflects an early active period of the cardiac cycle. The clinical relevance of this time interval is subject of study. This paper introduces a method using ISTI to evaluate and predict the circulatory response to fluid administration in patients after coronary artery bypass graft surgery and presents preliminary results of a pilot study by comparing ISTI with cardiac output (CO) responsiveness. Also the use of the pulse transit time (PTT), earlier recommended for this purpose, was investigated. The results showed an inverse relationship between ISTI and CO at all moments of fluid administration and also an inverse relationship between the changes ΔISTI and ΔCO before and after full fluid administration. No relationships between PTT and CO or ΔPTT and ΔCO were found. It is concluded that ISTI is dependent upon preload, and that ISTI has the potential to be used as a clinical parameter assessing preload.