Abstract— Using an Adaptive Identification Algorithm, the variations of Arterial Compliance were monitored during the cardiac cycle. A computer-simulated experiment demonstrated the contribution of a time-varying Compliance in reducing the Energetic Load on the Heart. Index Terms— Arterial Compliance, Cardiac Load, Computer Simulation, Energy, Model. I. INTRODUCTION Several electrical models of the Arterial System exist [1], [2], [3]. They are grouped into two categories, the lumped- parameter and the distributed-parameter ones. We chose a well-established lumped-parameter model: the 3-element Windkessel, first proposed by Westerhof [4]. A circuit representation of the model is shown in Fig. 1. The decision to use this particular model was influenced by the study of Burkhoff et al. [5] who examined the validity of this model, and found it to be a reasonable representation of cardiac load for purposes of predicting stroke volume, stroke work, oxygen consumption, and systolic and diastolic aortic pressures. In addition, the model is fairly simple, with only three passive elements representing the arterial system as the cardiac load. Fig. 1. The 3-element Windkessel model Manuscript received May 23, 2007. A. Vrettos is an Associate Professor at Waubonsee College, Technology, Mathematics & Physical Sciences Department, Sugar Grove, IL 60554 USA (phone: 630-466-2934; fax: 903-223-3189; e-mail: avrettos@waubonsee.edu). S. T. Karamouzis is the Truman & Anita Arnold Chair of Computer & Information Sciences at Texas A&M University - Texarkana., Texarkana, TX 75505 USA (e-mail: stamos.karamouzis@tamut.edu). In this model the Compliance C represents the compliance of the Aorta and the large arteries. Rp is represents the resistance of the deep circulation, and responds to the metabolic needs of the body and to the Mean Arterial Pressure. Rc is the characteristic Impedance, i.e. the impedance theoretically measured when the animal is totally vasodilated. It is unlikely that either one of the two resistive elements change within one beat, during steady-state function. However there is evidence the Compliance C changes during a cardiac cycle [6], [7], [8], [9], [10] even in steady-state operation of the heart. In this study we attempt to monitor the instantaneous changes in Compliance during a cycle, and the contribution of these changes in the reduction of the energetic load on the Heart. II. MATERIALS AND METHODS A. Physiological Data Acquisition Nine female Yorkshire pigs, 40-47 Kg, were preanesthetized with ketamine HCl(20 mg/kg) and xylazine (0.05 mg/kg). The pigs were then anesthtized with isoflurane. A midline tracheostomy was performed and the trachea was intubated. End-tidal CO2 was monitored, and arterial blood gases were kept within normal limits. A carotid artery was cannulated just long enough to reach the aortic root. Aortic Pressure was measured using a Spectramed model P23-XL pressure transducer. A midline sternotomy was performed and, and the aortic root was dissected free. An ultrasonic flowmeter transducer (Transonic Systems) was placed around the aortic root. It was connected to a Transonics Systems Model T201 flowmeter. Pressure and Flow were recorded on analog tape and then digitized. B. Identification Technique Resistance Rc was calculated as the ratio of the maximum derivative of Aortic Pressure P A over the maximum derivative of Aortic Flow Q, according to a technique verified by Lucas [11]: MAX MAX A dt dQ dt dP Rc ) / ( ) / ( = (1) Resistance Rp was calculated as the difference between the ratio of Mean Aortic Pressure and Mean Aortic Flow, and Rc: Identification of Time-Varying Compliance and its Role in Cardiac Energetics using Computer Simulation Andreas Vrettos and Stamos T. Karamouzis Proceedings of the World Congress on Engineering and Computer Science 2007 WCECS 2007, October 24-26, 2007, San Francisco, USA ISBN:978-988-98671-6-4 WCECS 2007