ATTENUATION OF INCREASED REGIONAL MYOCARDIAL OXYGEN-CONSUMPTION DURING EXERCISE AS A MAJOR CAUSE OF WARM-UP PHENOMENON

Objectives. The aim of this study was to test the hypothesis that the warm-up phenomenon is attributable to a reduction of increased myocardial oxygen consumption rather than to increased coronary blood flow during exercise. Background. The underlying mechanism of the warm-up phenomenon is not elucidated. Methods. Thirteen patients with effort angina were subjected to two consecutive supine ergometer exercise tests performed 15 min apart. All patients had severe proximal stenosis (> 90%) in the left anterior descending coronary artery. Great cardiac vein flow was measured before and during exercise. Both regional myocardial oxygen consumption and adenosine release were determined. Results. Exercise was continued for significantly longer before angina onset in the second than in the first exercise test (507 +/- 44 vs. 410 +/- 42 s, p < 0.01). The extent of ST segment depression in lead V5 of the electrocardiogram (ECG) was larger at the time of angina onset in the first (1.7 +/- 0.2 mm) than in the second (1.1 +/- 0.2 mm, p < 0.01) exercise test. Neither systemic hemodynamic variables nor great cardiac vein flow differed between the first and second exercise tests. In contrast, regional myocardial oxygen consumption assessed at 3 min of exercise was significantly (p < 0.01) less in the second than in the first test (8.0 +/- 0.8 vs. 8.7 +/- 0.9 ml/min). Adenosine release during the second test was higher (p < 0.05) than in the first test (2.5 +/- 0.5 vs. 3.9 +/- 0.5 nmol/min at 3 min of the first and second tests, p < 0.01). Conclusions. These results indicate that the warm-up phenomenon is not attributable to increased coronary flow but to attenuation of increased regional myocardial oxygen consumption, which may be mediated by adenosine A1 receptor activation.