Significance of downsloping ST-segment depression induced by low-level exercise in severe coronary artery disease. Assessment with myocardial ischemia and collateral perfusion

Effect and mechanism of intermittent myocardial ischemia induced by exercise on coronary collateral formation

OBJECTIVE

To hypothesize that appropriate intermittent myocardial ischemia induced by exercise could safely promote coronary collateral formation in the ischemic area through the increased expression of vascular endothelial growth factor (VEGF) and its receptor fetal liver kinase-1 (Flk-1).

DESIGN

A balloon constrictor was surgically implanted in the first obtuse marginal coronary artery (OM1) of Guangxi BA-MA miniature pigs. The subjects were divided into three groups: sham-operated (SO), pure ischemia (PI), and exercise training (ET). Subjects in the ET group performed individualized treadmill programs with two episodes of exercise-induced ischemia for 8 wks; two preexercise episodes of pure ischemia induced by brief OM1 occlusion were also conducted. Only pure ischemia was induced in the PI group, and the SO group remained sedentary for the experimental period. VEGF and Flk-1 expression levels were measured by Western blot and real-time reverse transcription polymerase chain reaction analyses; capillary density by immunohistochemistry; relative myocardial blood flow by microspheres; and cardiac troponin I by enzyme-linked immunosorbent assay.

RESULTS

The relative myocardial blood flow, VEGF, Flk-1, and capillary density in the ET group were statistically higher than those in the PI and SO groups. All parameters in the PI group were statistically higher than those in the SO group. There was no myocardial damage in the ET or PI groups by cardiac troponin I.

CONCLUSIONS

Intermittent myocardial ischemia induced by exercise with optimal stimulation safely promotes coronary collateral formation through increased VEGF and Flk-1 expression in a porcine model.

Relation between exercise-induced myocardial ischemia as assessed by nitrogen-13 ammonia positron emission tomography and QT interval behavior in patients with right bundle branch block

Exercise-induced myocardial ischemia is difficult to detect with ST-T changes in patients with right bundle branch block (RBBB). We sought to predict exercise-induced myocardial ischemia with QT interval behavior during exercise in patients with RBBB. Twenty-two patients with angiographically proven coronary artery disease and RBBB and 9 healthy volunteers underwent nitrogen-13 ammonia positron emission tomography with bicycle ergometer exercise at a fixed workload of 25 W. Regional myocardial blood flow (RMBF) and electrocardiographic changes were measured both at rest and after 5 minutes of exercise. The QT interval was measured from the onset of the QRS complex to the offset of the T wave in lead V5. The deltaQT and deltaRMBF, which indicated values after 5 minutes of exercise minus values at rest, were negatively correlated (r = -0.74, p <0.001). Exercise-induced shortening of the QT interval (422 +/- 27 to 381 +/- 38 ms, p = 0.0020) was observed in 15 patients (group 1) and no change or prolongation (411 +/- 45 to 420 +/- 37 ms, p = NS) was observed in 7 patients (group 2). Multivessel disease was significantly more frequent but collateral circulation was significantly less in group 2 than in group 1 (p <0.01, p <0.05, respectively). Cardiac output at rest was significantly lower in groups 1 and 2 than in healthy volunteers (4.52 +/- 0.83 and 4.51 +/- 0.84 vs 6.20 +/- 0.83 L/min; p = 0.0014, p = 0.0003). Although RMBF at rest did not differ significantly among groups 1 and 2 and healthy volunteers (0.63 +/- 0.20 vs 0.69 +/- 0.13 and vs 0.77 +/- 0.14 ml/min/g), RMBF after 5 minutes of exercise was significantly lower in group 2 than in group 1 and healthy volunteers (0.78 +/- 0.11 vs 0.96 +/- 0.20 and vs 1.20 +/- 0.18 ml/min/g; p = 0.0289, p <0.0001). The number of regions of critical coronary artery disease was significantly greater in group 2 than in group 1 (4.0 +/- 1.2 vs 2.1 +/- 1.3, p = 0.0039). Our results suggest that the absence of QT interval shortening during exercise may indicate severe myocardial ischemia induced by exercise in patients with RBBB and coronary artery disease.