Noninvasive imaging in the assessment of the cardiopulmonary vascular unit

We predict that cardiac MRI and PET will significantly contribute to a better understanding of the pathophysiological processes that lead to the development of chronic RV failure in PAH. Imaging studies have demonstrated that, in the setting of chronic pressure overload, the RV compensates enduringly to sustain CO by an increase in wall mass, dilatation, and contractility and marked changes in the RV shape. With the passage of time, these compensatory mechanisms fail, resulting in increased wall stress and impaired global RV function. Other factors that might contribute to disturbed RV function are a reduced wall deformation and an inefficient RV contraction pattern. The resulting interventricular asynchrony is associated with leftward septum bowing, impaired LV filling, and decreased stroke volume. Furthermore, the RV becomes mechanically insufficient: More oxygen is required for a comparable power output. At the same time, RV oxygen delivery is impaired and tissue oxygenation is reduced. Alterations in myocardial metabolism have been observed in PAH, but their overall relevance and whether they represent cause or consequence of RV failure remain unclear. With the current evidence, it can confidently be stated that RV imaging parameters measured at baseline correlate with exercise capacity and functional class and predict survival.^69,136 Moreover, RV imaging parameters have been shown to respond to treatment,^35,137 and changes in these parameters after treatment reflect altered exercise capacity¹³⁸ and predict subsequent survival.¹² What is lacking at this point, however, is the demonstration of reliable monitoring and improved overall clinical outcome when a treatment strategy based on specific imaging parameters is used. In the near future, it can be expected that the importance of changes in cellular functions and signaling pathways will become clearer and the changes will be "imageable." This might allow a regional and quantifiable analysis of processes such as angiogenesis, apoptosis, and neurohormonal factors. Table 5 provides an overview of currently available clinical imaging tracers that could be relevant for the assessment of molecular processes of RV diseases in patients. In addition, recent developments in (hybrid) PET and MRI might allow an integrated RV assessment in vivo. They will likely provide an important basis for simultaneous measurements of multiple myocardial disease processes.