Ultrasonographic Vascular Mechanics: Feasibility, Usefulness and Clinical Applications

• Main Author: Cardoso Teixeira, Rogério Paiva
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2016
• Subjects: Medical imaging; Physiology
• ISBN: 9798607325114

The development of accurate non-invasive methods of early diagnosis of vascular degenerative changes is of considerable clinical interest, given that cardiovascular disease remains the leading cause of death worldwide and large artery damage is a major contributor to cardiovascular disease. Ultrasound delivers dynamic images of the heart and central arteries. Two-dimensional speckle tracking echocardiography (2D-STE) is a semi automated analysis based on frame-by-frame tracking of tiny echo-dense speckles within the myocardium, from which deformation variables such as strain, strain rate, velocity and displacement can be studied. Initial attempts to study cardiac mechanics were focused on the left ventricular chamber, but its usage has been expanded and validated for the right ventricle, as well as the thin-walled atrial chambers. Later, direct vessel-wall tracking has been achievable through 2D-STE. The focus on previous vascular mechanics studies was the circumferential expansion and recoil of the vessel wall, which enabled the assessment of a positive systolic strain plus a positive and negative strain rate. Vascular mechanics assessment with 2D-STE has been validated with sonomicrometry studies and an association with vascular mechanics and the collagen content of vascular wall has also demonstrated, promoting vascular mechanics with 2D-STE as a new imaging surrogate of vascular stiffening. We used 2D-STE to study aortic mechanics in patients with aortic stenosis (AS), with hypertension, and atrial fibrillation (AF), in order to assess i) the methodology feasibility and reproducibility; ii) to study the variability of vascular mechanics; iii) to assess the association of vascular mechanics and vascular stiffness. In the first part of our research we studied 45 patients with moderate to severe AS (aortic valve area ≤ 0.85 cm2/m2) with 2D-STE at the level of the thoracic ascending aorta. We demonstrated that the left ventricular stroke volume index was the most important variable to explain aortic strain variability. Moreover, the vascular rigidity assessed with the aortic β1 stiffness index was useful to explain the aortic strain rate variability. As an exploratory results, we have showed that aortic mechanics were associated with mortality. Subsequently we used 2D-STE to study vascular mechanics at the level of the aortic arch. We enrolled a cohort of 61 apparently healthy participants, and we reported normal values. In this study we have also included a group of 46 hypertensive patients that had lower values of aortic mechanics than the healthy group (strain: 6.3±2.0% vs 11.2±3.2% and strain rate: 1.0±0.3 vs 1.5±0.4 s-1, respectively, both P<0.01). We have demonstrated that aortic arch mechanics correlated with the gold standard method used to study vascular stiffness (pulse wave velocity, with the Complior® method) and finally we have also identified that parameters of vascular mechanics were associated with left ventricular relaxation. After adjustments for age and pulse pressure, aortic arch strain was significantly lower in hypertensive patients, when compared to healthy subjects. Finally, we studied aortic mechanics at the level of the descending aorta in a cohort of 44 patients with non-valvular AF who needed cardioversion and were referred for transesophageal echocardiography (TEE). We concluded for a positive association of vascular mechanics and the left atrial appendage function. Moreover, as the CHA2DS2VASc score increased both the vascular strain (r=-0.38, P=0.01) and the vascular strain rate (r=-0.42, P<0.01) decreased. Aortic strain remained independently associated with a past history of stroke after adjustment for the CHA2DS2VASc score. The feasibility values for vascular mechanics with 2D-STE ranged from 85% to 95% for the selected patients included in the three studies. Of the total 1176 segments included in the studies, we extracted 2D-STE data for 1075 aortic wall segments. Regarding reproducibility, data was considered adequate, in particular for the assessment of global strain and strain rate. In conclusion, it was possible to study vascular mechanics with 2D-STE at three different aortic levels. Our worked contributed to promote vascular mechanics as an imaging vascular risk marker. The usefulness of aortic strain and strain rate was established to identify higher risk subgroups of patients with degenerative AS and non-valvular AF. Aortic arch strain remained significantly lower for hypertensive patients, when compared to healthy subjects.