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

Determination of the mean cross-sectional area of the thoracic aorta using a double indicator dilution technique

A double indicator dilution technique for determining the mean cross-sectional area (CSA) of a blood vessel in vivo is presented. Analogous to the thermodilution method, dilution of hypertonic saline was measured by an electrical conductance technique. Because the change in conductance rather than absolute conductance was used to calculate CSA, pulsatile changes in shear rate of blood and conductance of surrounding tissues had no effect on the data. To calculate CSA from an ion mass balance, cardiac output was needed and estimated from the thermodilution curve using the same 'cold' (hypertonic) saline injection. The mean CSA, obtained from this double indicator dilution method (CSA(GD)), was compared with the CSA obtained from the intravascular ultrasound method (IVUS) in 44 paired observations in six piglets. The regression line is close to the line of identity (CSA(GD) = -1.83 + 1.06 · CSA(IVUS), r = 0.96). The difference between both CSAs was independent of the diameter of the vessel, on average -0.99 mm2 ± 2.64 mm2 (mean CSA(GD) = 46.84 ± 8.21 mm2, mean CSA(IVUS) = 47.82 ± 9.08 mm2) and not significant. The results show that the double indicator dilution method is a reliable technique for estimating the CSA of blood vessels in vivo