Evaluating the utility of continuous thermodilution to better detect Coronary Microvascular Dysfunction among patients with chronic total occlusion.

All eligible study participants will undergo intracoronary physiology after completion of successful Chronic Total Occlusion (CTO) Percutaneous Coronary Intervention (PCI). Both intracoronary bolus and continuous thermodilution will be performed in the collateral vessel and CTO PCI vessel at baseline and at 3 months. Completion of intracoronary physiology will take approximately 20 minutes in total. Assessment of the skin vasculature through cutaneous optical coherence tomography (cOCT) will also be undertaken and this will be performed at baseline, 3 months and 12 months post index coronary angiogram. Bolus Thermodilution (BTh): Using a standard approach, coronary physiology will be measured using a PressureWire X (Abbott Vascular, California, USA) and Coroventis (Uppsala, Sweden). The coronary microcirculation will be assessed using the currently recommended BTh technique. In brief, a temperature and pressure sensor guidewire (PressureWire X) is positioned in the distal third of the artery. Time taken for a 3ml bolus of manually injected saline to transit along the coronary artery is measured. Using dedicated software (Coroventis) coronary artery blood flow may be calculated. Three injections of saline (and measurement of flow) are performed at rest and then during hyperaemic conditions, which are induced by infusion of peripheral adenosine (140mcg/kg/min) or an intra-coronary bolus. This technique then allows for the calculation of coronary flow reserve (CFR, (hyperaemic transit time / resting transit time)) and Index of Microcirculatory Resistance (IMR, (hyperaemic transit time x distal coronary pressure during hyperaemia)). Continuous Thermodilution: Following bolus thermodilution, continuous thermodilution shall be performed in the same two major epicardial arteries in which BTh has been measured. In brief, a temperature and pressure sensor guidewire (PressureWire X) is positioned in the distal third of the artery to be tested. The RayFlow infusion catheter (Hexacath, Paris, France) is advanced into the proximal 1-2cm of the artery. A continuous infusion of saline is then administered via the Rayflow catheter using a pressure injector, which is already available in the cath lab and usually used for the injection of contrast. Dedicated software (Coroventis) will be used to measure absolute coronary flow (Q) and resistance (R) a at two time points; resting (saline infusion 10ml/min (LAD) or 8ml/min (RCA)) and hyperaemic conditions (saline infusion 20ml/min (LAD) or 15ml/min (RCA)). Continuous thermodilution also allows microvascular reactivity to be captured (microvascular resistance reserve, MRR). Skin Assessment (cOCT): Assessment of cutaneous microvascular function will be performed during the index admission. Attempts will be made to perform cOCT on the day before or the day of invasive testing. Using a previously described technique, cOCT will be performed using a Telesto III imaging system (Thorlands Germany) with a detachable probe (LSMO3, Thorlabs). In brief, within a temperature and light controlled room high resolution digital photographs of the cutaneous microcirculation of the forearm will be acquired. Following acquisition of baseline images a blood pressure cuff shall be inflated (20mmHg above systolic pressure) on the upper arm to occlude blood flow. After 5 minutes of inflation the cuff shall be rapidly deflated to create reactive hyperaemia within the forearm. The microcirculation shall be re-imaged during this time to allow for an assessment of cutaneous microcirculatory reactivity. Speckle decorrelation analysis will be performed to ascertain information regarding blood flow, vessel characteristics and reactivity. This will be performed at baseline and repeated at 3 and 12-months.