The aim of this study is to test whether an increase in coronary sinus pressure leads to a change in coronary microvascular resistances in patients with angina pectoris and with an indication to measurement of microvascular function as per clinical guidelines.~All patients with a clinical indication for the assessment of microvascular function will be invited to participate and will sign the informed consent at least 24 hours before the study. Patients with an index of microvascular resistances >25 (i.e. evidence of microvascular dysfunction) will undergo study-specific procedures in the same session (i.e. no additional invasive procedure is required for the study).~Study-specific procedures include the repetition of the assessment of microvascular resistances at rest and during coronary sinus occlusion. Coronary sinus occlusion will be achieved by inflating a balloon sized to ~70% of the diameter of the coronary sinus. Since the implantation of a coronary sinus reducer is a therapeutic option for this type of patients, also this procedure (inflation of an undersized balloon in the coronary sinus) provides clinically relevant information (sizing of the vessel, effect on microvascular resistances and feasibility of the intervention).~The primary goal of the study is to study the effect of coronary sinus occlusion on microvascular resistances at rest and during hyperemia.
MACCHUS is a randomized, single-center, controlled, cross-over interventional study in which coronary artery resistances will be measured at baseline and during temporary occlusion of the coronary sinus.