Exercise-induced ST-segment depression: imbalance between myocardial oxygen demand and myocardial blood flow

Objective - ST-segment depression is believed as a common electrocardiograp hic sign of myocardial ischemia during exercise testing. Ischemia is genera lly defined as oxygen deprivation due to reduced perfusion. However, the ex act relationship of the ischemic definition to ST-segment depression remain s unclear. This study was conducted to evaluate the correlation between myo cardial oxygen demand and myocardial blood flow (MBF) when ischemic (horizo ntal or downsloping) ST-segment depression of greater than or equal to 0.1 mV 80 ms after the J point developed during low-level exercise. Methods and results - Seventy-two patients with angiographically proven cor onary artery disease (CAD) and 9 healthy volunteers underwent exercise posi tron emission tomography (PET), Myocardial oxygen demand was defined as a r ate-pressure product (RPP, heart rate x systolic blood pressure) during exe rcise and MBF was quantified by nitrogen-13 ammonia perfusion PET. The myoc ardial demand-supply balance (MDSB) index was calculated from the MBF ratio (values during exercise/values at rest) against the RPP ratio (values duri ng exercise/values at rest). The MDSB index was significantly lower in pati ents with ischemic ST-segment depression than in patients with non-ischemic ST depression and healthy volunteers (0.82 +/- 0.16 vs. 1.02 +/- 0.17, p < 0.0001 and vs. 0.99 +/- 0.14, p = 0.0109). Further, the presence of inadeq uate increase in MBF of less than or equal to 10% (2 SD below the mean % va lue of healthy volunteers) during exercise in regional myocardium perfused by stenotic CAD significantly correlated with exercise-induced ischemic ST- segment depression (p = 0.0105), Conclusions - Our study could demonstrate that exercise-induced ischemic ST -segment depression is associated with myocardial ischemia due to exercise- induced imbalance between myocardial oxygen demand and global and regional MBF supply in patients with proven CAD.