Microvascular no-reflow occurs in greater than 50% of patients following primary percutaneous coronary intervention (PPCI) for ST-elevation myocardial infarction (STEMI) and, although it adversely affects outcomes, it is poorly understood. The aim of this thesis was to study the physiology of the microcirculation in patients following STEM I. We studied the microcirculatory physiology using thermodilution and Doppler flow wire techniques. The infarct-related artery was studied at PPCI and 24-hours postPPCI. Zero flow pressure (pzfL hyperaemic myocardial resistance (hMR), index of microcirculatory resistance (IMR) and coronary flow reserve (CFR) were calculated. Furthermore, a novel parameter 'hyperaemic backward expansion wave ratio' (hBEW ratio) was computed using WIA as the ratio of hyperaemic and resting BEW wave intensity. The extent of myocardial injury was determined by contrast cardiac magnetic resonance (CMR) at day two and 6 months post-PPCI. pzf was found to be superior to IMR and hMR in predicting infarction size following PPCI. Furthermore, pzf correlated significantly with transmurality of infarction, salvage index and 6-month ejection fraction. Using a cut of pzf ~42mmHg allowed the identification of a cohort of patients with adverse clinical, angiographic and CMR features of infarction. Secondly, hBEW ratio was significantly smaller at PPCI compared to 24h post-PPCI and in a control cohort of patients undergoing stable PCI. This was driven by a similar resting BEW between the cohorts, but a significantly smaller hyperaemic response was observed following PPCI and, to a lesser extent, at 24h post-PPCI compared to patients having stable PCI. Furthermore, there was a significant relationship between hBEW ratio and CFR. pzf may provide important prognostic information at the time of PPCI. Secondly, the affect of hyperaemia on BEW intensity is attenuated following STEMI suggesting muted microvascular function and hBEW ratio offers mechanistic insight based on computational phasic fluid dynamics regarding poor flow reserve.