[Collateral circulation and the coronary reserve]

The mechanisms regulating coronary collateral circulation are largely unknown owing to both the complex and variable nature of clinical models and the difficulty to obtain quantitative and differentiated flow measurements within the various coronary tree portions. With the aim of assessing collateral flow reserve, we studied 19 patients with effort angina, without myocardial infarction and with isolated occlusion of either the left anterior descending coronary artery (n = 14) or the circumflex coronary artery (n = 5). Flow values were measured basally, during atrial pacing induced tachycardia and following ev dipyridamole infusion (0.56 mg/kg of body weight in 4 min), by means of positron emission tomography and nitrogen-13 ammonia as flow tracer, within both regions depending on collateral circulation and the remote ones. Results have been compared with those obtained in 13 normal subjects. Basal flow values in regions depending on collateral circulation and in the remote regions (0.61 +/- 0.11 vs 0.63 +/- 0.17 ml/min/g) were found to be similar, but lower than in normal subjects (1.00 +/- 0.20 ml/min/g, p0.01). During atrial pacing, flow increased to 0.83 +/- 0.25 and to 1.11 +/- 0.39 ml/min/g, in the regions depending on collateral circulation and in the remote regions, respectively (p0.05 as compared to baseline); again, values were lower than in normal subjects (1.86 +/- 0.61 ml/min/g, p0.01). Dipyridamole infusion further increased flow in the remote regions (1.36 +/- 0.57 ml/min/g, p0.01 as compared to atrial pacing) but it did not in the regions depending on collateral circulation (0.94 +/- 0.37 ml/min/g, NS as compared to atrial pacing); both values were lower than in normal subjects (3.46 +/- 0.78 ml/min/g, p0.01). Flow reserve in the regions depending on collateral circulation was found to have a direct linear correlation with the one in the remote regions (r = 0.83; p0.01). In conclusion, in spite of basal hypoperfusion, collateral circulation maintains a flow reserve which, even if reduced, is able to cope with moderate increments in oxygen consumption. An analogous flow reduction can be observed in the remote regions, suggesting that the entire coronary tree is involved, beyond the obstructive lesions of the main arterial branches.