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Objectives: To assess mechanisms of myocardial perfusion impairment in patients with hypertrophic cardiomyopathy (HCM)., Methods: Fourteen patients with obstructive HCM (mean (SD) age 53 (10) years, 11 men) underwent intravenous adenosine myocardial contrast echocardiography (MCE), positron emission tomography (PET) and cardiac catheterisation. Fourteen healthy volunteers (mean age 31 (4) years, 11 men) served as controls. Relative myocardial blood volume (rBV), exchange flow velocity (beta), myocardial blood flow (MBF), MBF reserve (MFR) and endocardial-to-subepicardial (endo-to-epi) MBF ratio were measured from the steady state and contrast replenishment time-intensity curves., Results: Patients with HCM had lower rest MBF (for LVRPP-corrected)--mean (SD) (0.92 (0.12) vs 1.13 (0.25) ml/min/g, p<0.01)--and hyperaemic MBF--(2.56 (0.49) vs 4.34 (0.78) ml/min/g, p<0.01) than controls. Resting rBV was lower in patients with HCM (0.094 (0.016) vs 0.138 (0.014) ml/ml), and during hyperaemia (0.104 (0.018) ml/ml vs 0.185 (0.024) ml/ml) (all p<0.001) than in controls. beta tended to be higher in HCM at rest (9.4 (4.6) vs 7.7 (4.2) ml/min) and during hyperaemia (25.8 (6.4) vs 23.1 (6.2) ml/min) than in controls. Septal endo-to-epi MBF decreased during hyperaemia (0.86 (0.15) to 0.64 (0.18), p<0.01). rBV was inversely correlated with left ventricular (LV) mass index (p<0.05). Both hyperaemic and endo-to-epi MBF were inversely correlated with LV end-diastolic pressure, LV mass index, and LV outflow tract pressure gradient (all p<0.05). MCE-derived MBF correlated well with PET at rest (r = 0.84) and hyperaemia (r = 0.87) (all p<0.001)., Conclusions: In patients with HCM, LV end-diastolic pressure, LV outflow tract pressure gradient, and LV mass index are independent predictors of rBV and hyperaemic MBF.