Page 117 - Assessing right ventricular function and the pulmonary circulation in pulmonary hypertension Onno Anthonius Spruijt

tissue might contribute to RV failure is unknown. In addition, using MRI perfusion techniques it was shown that pulmonary perfusion reserve is decreased in PAH [37].
Although the direct clinical value of these techniques is presently still uncertain, application of these techniques have advanced our understanding of RV function and failure and offers tools to increase our understanding in the near future.
Assessment of Pulmonary artery Flow and distensibility
Velocity encoded cine MRI provides an accurate and reproducible tool for quantification of pulmonary artery flow [38-40]. Kondo et al. described in 1992 for the first time the application of this technique in PAH patients [41]. It was found that in comparison to controls, flow acceleration time and distensibility of the pulmonary artery were decreased in patients with primary PAH; a finding similar to that observed in studies using Doppler echocardiography for measuring flow velocity. Several attempts have been made to estimate pulmonary artery pressure from the flow signal [42-44]. However, a validation study performed by Roeleveld et al. showed that all these approaches fail to estimate pulmonary arterial pressure accurately [18]. This can be explained by the fact that acceleration time is not only a parameter of the pulmonary vascular bed but is also influenced by the pump function characteristics of the RV.
The major advantage of flow measurements, however, is not the estimation of pulmonary artery pressure but the accurate assessment of effective stroke volume of the RV from MRI flow measurement in the pulmonary artery [38-41]. Stroke volume can be considered as one of the most important hemodynamic parameters in PAH, since it is a direct reflection of RV function in relation to its increased afterload. Earlier studies showed that stroke volume is decreased in PH and that exercise does not lead to an increase in stroke volume [45]. In addition, it was shown that baseline stroke volume accurately predicts 6-minutes walking distance in PAH and that improvements in stroke volume in patients under treatment are directly related to an improvement in the six minute walking distance [24, 46]. Furthermore, it was shown that stroke volume derived from the pulmonary artery flow measured by MRI is a strong predictor of mortality in PAH [18].
An additional feature of the abnormal flow in PH is the so called vortical flow which is flow rotating around an ax. It was found that vertical flow in the main pulmonary artery is a typical characteristic of the presence of PH [47] and that the duration of the vortex is related to the main pulmonary artery pressure [48].
Chapter 7
115
7