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.

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.

Inclusion Criteria:

• Chronic coronary syndrome (including patients with anginal equivalents).
• Reversible ischemia on non-invasive testing, indication to cardiac catheterization;
• Indication to the assessment of microvascular function (note: patients will be asked for participation and consented prior to the first measurement of microvascular function, which will be conducted as per clinical indication; only those with evidence of pure or mixed microvascular disease (index of microvascular resistances (IMR)>25 will proceed into the study);
• Willingness to participate and ability to understand, read and sign the informed consent;
• Age>18 years

Exclusion Criteria:

• Previous CABG with patent grafts to the left anterior descending coronary
• Epicardial coronary disease (FFR <0.80 with evidence of a focal stenosis) in the left anterior descending territory
• Severe valvular heart disease
• Any cardiomyopathy; pulmonary or renal disease
• Inability to provide informed consent
• Any disease reducing life expectancy

Investigators performing the analysis of the data (post-hoc) and patients are blinded to the sequence of the interventions.