Extensions and improvements of the electrical conductance method

Continuous monitoring of cardiac output is important in patients who are undergoing intensive care, thoracic surgery or a catheterization for diagnostic reasons. In these patients arterial pressure is routinely determined. In the patients, who are undergoing a catheterization for diagnostic reasons, aortic pressure is detennined. During intensive care and thoracic surgery arterial pressure is determined in both the pulmonary artery and the artery femoralis or radialis. The radial or femoral catheter is a replacement of the pressure catheter in the aorta. To detennine cardiac output continuously from an arterial pressure signal, the aortic pressure was reconstructed from the peripheral pressure [Wesseling et al. 1976, Gratz et al. 1992]. For this continuous cardiac output monitoring from aortic pressure, a model of the circulation is used. A parameter of this model is the compliance of the arterial system, which is the change in volume per unit of length (i.e. segmental volume) over a change in pressure. The compliance is derived from in vitro measurements using a selected group of human aorta's [Langewouters 1984]. Cardiac output can also be detelmined from the pulmonary arterial pressure signal, which is directly measured in this artery. Thus, a reconstruction of this pressure signal is not needed. To calculate right ventricular output, i.e. cardiac output, according to a pulse contour method, we detennined the pulmonary arterial compliance. To determine arterial volume, which was needed to determine compliance we modified the conductance method. We studied the relationship between arterial volume and pressure at a large range of pulmonary arterial pressure. To outline the context in which the research presented in this thesis has been carried out, the function of blood vessels and of large arteries in particular will be described. Next, the anatomy of arteries will be considered. Subsequently, the terms concerning mechanics of blood vessels are explained and fmally the method to determine blood volume in large arteries; the conductance method, will be described.