Models of venous return and their application to estimate the mean systemic filling pressure

Mean systemic filling pressure is the equilibrium pressure in the systemic circulation when the heart is arrested and there is no flow. This pressure is a measure of the stressed volume of the systemic circulation and regarded as the driving pressure for the venous return during steady states [1-3]. In this thesis the terms stationary level and steady state refer to a state in which the mean pressure and flow, averaged over a heart cycle, do not change. Dynamic conditions refer to changes in averaged pressure and flow between heart beats. The property of mean systemic filling pressure as driving pressure fat venous return has been explained by Guyton [1] and others [2,3]. Most methods that are used to determine mean systemic filling pressure require a total stop of the circulation by inflating a balloon in the right atrium [4] or arresting the heart by fibrillation [5-9]. In 1985 Versprille & Jansen developed a method to determine mean systemic filling pressure in the intact circulation [10]. Their method required seven different steady states of central venous pressure and cardiac output. The total procedure lasted approximately 45 min. If circulatory conditions are nor stable for such a time span, this method is not useful. A main purpose of the research described in this thesis was to develop faster methods to determine the mean systemic filling pressure in the intact circulation.