Evaluating the utility of continuous thermodilution to detect Coronary Microvascular Dysfunction among patients with Acute Coronary Syndrome

All eligible study participants will undergo both intracoronary bolus and continuous thermodilution in both the infarct related artery (IRA) and non-IRA at baseline and at 3 months. 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. Each participant visit will take approximately one hour. Bolus Thermodilution: Using a standard approach, coronary physiology will be measured using a PressureWire X (Abbott Vascular, California, USA) and Coroventis (Uppsala, Sweden), in two major epicardial arteries; infarct-related artery (IRA) and non-infarct related artery (non-IRA), when identifiable. 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 CFR (hyperaemic transit time / resting transit time) and IMR (hyperaemic transit time x distal coronary pressure during hyperaemia). Continuous Thermodilution: Following bolus thermodilution, continuous thermodilution shall be performed in both the IRA and the non-IRA. In brief, a temperature and pressure sensor guidewire (PressureWire X) is positioned in the distal third of the artery to be tested. A 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) at three time points; baseline (no saline being infused), resting (saline infusion 10ml/min (LAD) or 8ml/min (RCA)) and hyperaemic conditions (saline infusion 20ml/min (LAD) or 15ml/min (RCA)). 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.