Dependency of contractile reserve on myocardial blood flow - Implications for the assessment of myocardial viability with dobutamine stress echocardiography

Background Contractile reserve, improvement in contractile function during inotropic stimulation, is a proposed marker of viable myocardium. This study was designed to address, in patients with left ventricular dysfunction due to chronic coronary artery disease, whether contractile reserve depends on myocardial blood flow. Methods and Results We studied 19 patients, at rest and during dobutamine, with 2D echocardiography for regional mechanical function and PET for regional myocardial blood flow ([O-15]water) and oxygen consumption ([C-11]acetate). Of 166 myocardial segments, 21 had normal systolic function, 56 were dysfunctional but contractile reserve-positive, and 89 were dysfunctional and contractile reserve-negative. Myocardial blood flow at rest was lower in contractile reserve-negative (0.41+/-0.18 mL.g(-1).min(-1)) than in contractile reserve-positive (0.50+/-0.22 mL.g(-1).min(-1)) and normal segments (0.55+/-0.20 mL.g(-1) min(-1), P<.009). After dobutamine infusion, blood flow increased less in contractile reserve-negative 60.63+/-0.38 mL.g(-1).min(-1)) than in contractile reserve-positive (1.28+/-0.65 mL.g(-1).min(-1)) and normal segments (1.93+/-0.83 mL.g(-1).min(-1), P<.0001). Likewise, myocardial oxygen consumption was lower at rest in contractile reserve-negative (clearance rate of [C-11]acetate, 0.043+/-0.012 min(-1)) than in contractile reserve-positive (0.048+/-0.01 min(-1)) and normal segments (0.058+/-0.008 min(-1), P<.02). Myocardial oxygen consumption with dobutamine increased less in contractile reserve-negative (0.060+/-0.013 min(-1)) than in contractile reserve-positive (0.077+/-0.016 min(-1)) and normal segments (0.092+/-0.021 min(-1), P<.0001). Of segments defined as viable by PET, 54% were contractile reserve-negative and exhibited lower blood flow with dobutamine (0.72+/-0.36 mL.g(-1).min(-1)) than with viable, contractile reserve-positive segments (1.29+/-0.70 mL.g(-1).min(-1), P<.0001). Conclusions Contractile reserve depends, in part, on the level of myocardial blood flow at rest and during inotropic stimulation.