Diagnosis of Pulmonary Hypertension Using Cardiac Magnetic Resonance Images

1. Pulmonary hypertension Pulmonary hypertension (PH) is manifested as an increase in mean pulmonary artery pressure (i.e., mPAP ≥ 25 mmHg) at rest on right heart catheterization (RHC) (Badesch 2009). It is a complex and multidisciplinary disorder causing restricted flow through the pulmonary arterial circulation due to increased pulmonary vascular resistance (PVR). It can be classified into five groups (Simonneau et al 2004): Group I - Idiopathic PAH, Familial PAH, and PAH associated with collagen vascular disease et al; Group II - PH with left heart disease; Group III - PH associated with lung diseases and/or hypoxemia; Group IV - PH due to chronic thrombotic and/or embolic disease; and Group V - miscellaneous causes of PH. The prognosis of PH is poor. The National Institutes of Health (NIH) Registry followed 194 patients with IPAH enrolled at 32 clinical centers from 1981 to 1985 (D'Alonzo et al 1991). The reported median survival rates of 68%, 48% and 34%. Similar results have been reported in Japan, India and Mexico.

Right heart catheterization (RHC) is the current reference standard for diagnosing PH (Galie et al 2009; McLaughlin et al 2009), according to ACCF/AHA expert consensus (McLaughlin et al 2009) and ESC/ERS guidelines (Galie et al 2009). Three hemodynamic measurements are essential from RHC: right ventricular systolic pressure (RVSP), mean pulmonary artery pressure (mPAP) and pulmonary vascular resistance (PVR). Further, patients with PH undergo RHC to assess clinical response to vasodilator challenge in order to guide therapy. It helps identify patients with better prognosis and patients who could potentially benefit from treatment with calcium channel blockers. A positive acute response is defined as a reduction of pulmonary artery pressure ≥10mmHg and ≤40mmHg. Although RHC plays a pivotal role in PH diagnosis, it is invasive and not without its own inherent risks (Hoeper et al 2006). For these reasons, development of a noninvasive alternative to RHC for diagnosis of PH is paramount.

Noninvasive diagnosis of PH and prediction of response to vasodilator challenge is clinically needed.

Several noninvasive methods for diagnosis of PH have been propounded, the most common being Doppler echocardiography. While routinely used to estimate pulmonary arterial pressure, Doppler measurement of tricuspid regurgitation jet peak velocity has inherent limitations: (i) the reliance on the visualization of the tricuspid regurgitant jet which is not always detectable, (ii) the fact that peak velocity of the jet may be difficult to measure in the presence of severe tricuspid regurgitation, and (iii) the need for adequate acoustic windows (Hinderliter et al 2003; Hachulla et al 2005). Furthermore, as recently emphasized in the ERS/ESC guideline (Galie et al 2009), estimation of mPAP or PVR must be an integral part of a complete cardiac echocardiographic examination.

It has long been recognized that systolic flattening and abnormal motion of the inter-ventricular septum (IVS), from either echocardiography (King et al 1983; Reisner et al 1994; Ricciardi et al 1999) or cine CMR images (Roeleveld et al 2005; Dellegrottaglie et al 2007; Alunni et al 2010), are signs of increased pulmonary arterial pressures. However, these studies are either qualitative or limited to 2D assessment, relying for analysis on subjective selection of the imaging plane and cardiac cycle phase. In the last 10 years, despite advances in biomedical engineering and computational approaches, no rigorous studies have been undertaken to study the relationship between quantitative 3D IVS shape and PH.

It is well documented that RV dysfunction and the 6-min walk test (6MWT) are associated with increased risk of mortality in PH (van Wolfersen 2007). Study team has recently developed a new method for assessment of RV function in terms of area strain, integrating radial, circumferential and longitudinal deformation (Zhong et al 2012). Therefore, we will also investigate the relationship between RV area strain and exercise capacity in PH.