| Summary: | Revascularization of Chronic Total Occlusions (CTO) by percutaneous coronary interventions is limited by low success rates, primarily due to difficulty in guidewire crossing. There are a number of contributing factors that make guidewire crossing challenging. Two of the most significant impediments are: a) inability to adequately visualize the CTO to appropriately plan a pathway to the distal lumen, and b) difficulty in physically crossing the rigid endcap at the proximal end of CTO without using stiff wires. Moreover, there is a significant knowledge gap in the composition of CTOs, and the consequent impact of that composition on crossability.
This thesis presents tools and techniques to help mitigate the current shortcomings, while shedding new light on CTO composition and maturation. The tools and techniques presented herein are based upon ultrasound approaches with the intent of eventually developing these strategies into catheter based solutions.
Recent studies have suggested that the presence of microvessels in CTO may provide a preferred pathway for guidewire crossing. However, due to limited resolution and a lack of soft tissue contrast in angiography, microvessels within CTO cannot generally be detected by in-vivo angiographic techniques, and when they are visualized, it is unknown whether or not they are intraluminal. In this thesis, high frequency ultrasound with Power Doppler overlays is shown to be capable of detecting and tracking transluminal recanalization channels using an in vivo porcine model of CTO. It is also shown that ultrasound is a more sensitive technique to detect and map these channels than MRI. Furthermore, features of microvasculature in CTOs that had not previously been seen are presented.
A technique was then developed to facilitate guidewire crossing through the proximal endcap, also known as the proximal fibrous cap (PFC). In order to assess the ease with which a probe is able to
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perforate the PFC, a system was designed and to measure the force required for PFC puncture. This system was validated by examining the required puncture forces for CTOs of different ages. It was shown that CTOs less than 6 weeks in age are significantly easier to puncture than those greater than 12 weeks. This coincides with differences in composition, with the presence of softer materials at the earlier time point, such as thrombus and proteoglycans compared to stiffer fibrotic materials which predominate at late timepoints.
After development and validation of a reliable technique to measure ease of PFC puncture, the efficacy of therapies designed to modify PFC compliance could be assessed. The use of ultrasound mediated microbubble (UMM) disruption to act as an adjuvant to accelerate collagenase therapy in CTO was examined. A significant reduction in puncture force and an increase in the amount of collagen degraded was achieved using a combined UMM + collagenase treatment compared with collagenase therapy alone and UMM treatment alone.
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