Page 118 - Assessing right ventricular function and the pulmonary circulation in pulmonary hypertension Onno Anthonius Spruijt

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Since the aorta flow can be measured in the same plane as the pulmonary artery flow, the ratio of right to left flow and vice versa can be used in the assessment of intra-cardiac shunts. Beerbaum et al. validated this MRI based shunt measurement approach, showing that right to left and left to right intra-cardiac shunts in children with congenital heart disease can be measured accurately in less than 60 seconds [38, 49]. A limitation of this technique is that in dilated pulmonary arteries turbulent flow can occur, causing an inaccuracy of the pulmonary blood flow estimates by MRI [49]. Another parameter that can be assessed from the flow measurements is the diameter and distensibility of the pulmonary artery. It is known that a progressive dilatation of the pulmonary artery harbours a poor prognosis [50]. In addition, from invasive studies it is known that compliance of the pulmonary vascular bed is decreased in advanced disease, and that this decrease is related to a poor outcome [51, 52]. Since compliance is quantitative measure of the elasticity of the large pulmonary vessels, this gives an important functional characterization of the vascular tree. Indeed, it was shown that loss of compliance of the large pulmonary vessels measured by MRI is related to a poor outcome [53].
Combining MRI flow measurements with invasive measurements
Measurement of the pulmonary vascular resistance requires an accurate assessment of pulmonary blood flow and pressure. Certain conditions, such as congenital heart disease might preclude a reliable invasive estimation of pulmonary blood flow. In these cases, combined MR measured flow with invasive measured pulmonary artery pressure, provides accurate estimations of pulmonary vascular resistance. In a group of 24 children with either suspected PAH or congenital heart disease it was shown that this approach provides more reliable data than invasive measurements [54]. Another application of the combined measurements of pressure and flow is the assessment of arterial input impedance [55, 56], as a comprehensive representation of RV load. Input impedance takes into account the pulsatile nature of blood flow and pressure and not only consists of pulmonary vascular resistance but also of compliance and characteristic impedance. The assessment of input impedance can only be obtained from spectral analysis of pressure and flow curves. Although the relevance of such measurements has been acknowledged for a long period of time, clinical application has been hampered by the requirement of simultaneous and instantaneous pulmonary pressure and flow, and sophisticated analysis techniques. With the advent of MRI scanners in the catheterization laboratory [57-60], pressure and flow can be measured