Aortic stenosis: pathophysiological effects on the myocardium and predictors of clinical events :

Thesis

The management of the asymptomatic patients with severe aortic stenosis (AS) is challenging; clinicians have to balance the risks of early surgery against the risk that irreversible myocardial damage may occur with a conservative management strategy.

It has become increasingly apparent that prognosis in asymptomatic AS depends not only on the degree of valvular stenosis, but also on the myocardial response to pressure overload and understanding the mechanisms of myocardial decompensation may help to guide management in the future. The degree of myocardial fibrosis, microvascular dysfunction, hypertrophy and left ventricular (LV) geometry may all play important roles. However, current guidelines for management of asymptomatic AS limit assessment of the myocardium to the measurement of ejection fraction with echocardiography. More advanced techniques may provide greater information that could be clinically useful.

This thesis seeks to further our understanding of the mechanisms of the myocardial response to AS, using Cardiac Magnetic Resonance (CMR) in patients with moderate and severe AS. Myocardial perfusion in AS is examined in chapter 3. The results show that CMR first pass perfusion can be carried out safely and is well tolerated by AS patients. Microvascular dysfunction in these patients was associated with age, exercise time and markers of diastolic dysfunction. Myocardial strain is examined in chapter 4, utilizing a new software tool to look at strain throughout the left ventricle, and also to explore the relationship between strain and myocardial fibrosis. The results show that there are significant variations in circumferential strain measurements, depending on slice position in the LV, and also that there was no relationship found between strain and the degree of LV fibrosis.

In chapter 5, the potential of CMR T1 mapping to identify fibrosis is examined using a new shortened non-contrast sequence (ShMOLLI - Shortened Modified Look-Locker Inversion) developed in our unit. CMR T1 values were validated against histological quantification of myocardial fibrosis in a large group of moderate and asymptomatic AS. A good correlation was found between ShMOLLI derived T1 values, with T1 values increasing with the severity of AS.

The clinical value of measuring myocardial perfusion and LV global strain is examined in chapter 6 by linking these to prognosis. Measurement of circumferential strain could predict prognosis in asymptomatic AS, but myocardial perfusion showed poor ability to predict events.

In conclusion, this thesis offers further insights into the changes that occur in the myocardium of patients with asymptomatic moderate and severe AS, using established and new CMR techniques. The clinical value of measuring these CMR parameters to aid risk stratification is shown, and the future potential for monitoring new therapies in these patients is discussed in the final chapter.

Authors: Bull, SC

• Alternative title: physiology of the myocardium in aortic stenosis
• Publication date: 2012
• DOI: 10.5287/ora-kzr1bdjy2
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: myocardium; Aortic stenosis; physiology
• Subjects: Cardiovascular disease