FLOW CAPACITY OF INTERNAL MAMMARY ARTERY GRAFTS - EARLY RESTRICTION AND LATER IMPROVEMENT ASSESSED BY DOPPLER GUIDE-WIRE - COMPARISON WITH SAPHENOUS-VEIN GRAFTS

Objectives. The purpose of this study was to assess flow dynamics and Row capacities of internal mammary artery and saphenous vein grafts to the left anterior descending coronary artery. Background. The postope rative bow capacity of internal mammary artery grafts to the left ante rior descending coronary artery has been reported to be restricted com pared with that of saphenous vein grafts in studies using radionuclide angiography. A recently developed Doppler guide wire has been used to analyze the Row dynamics of bypass grafts and to clarify the mechanis m of this limited Row capacity. Methods. Phasic Row velocity recording s were obtained in the midportion of the bypass graft and within the n ative left anterior descending artery, using a 0.018-in. (0.046-cm) 12 -MHz Doppler guide wire, in 53 patients: 27 patients with an internal mammary artery graft (16 with a new graft assessed 1 month postoperati vely and Il with an old graft assessed at 1 year) and 26 patients with a saphenous vein graft (13 with a new graft assessed 1 month postoper atively and 13 with an old graft assessed at 1 year). All patients wer e studied at baseline rest and during hyperemia induced by intravenous infusion of dipyridamole, 0.56 mg/kg body weight, over 4 min. Results . In the left anterior descending artery itself, systolic and diastoli c peak velocities, the time average of the instantaneous spectral peak velocity (time averaged peak velocity), vessel diameter and the calcu lated flow volume did not differ significantly among the four graft gr oups. The time averaged peak velocity was significantly greater for ne w than for old arterial grafts or for new or old vein grafts (mean +/- SD 27 +/- 9 vs. 19 +/- 6, 11 +/- 5 and 12 +/- 6 cm/s, respectively, p < 0.01). However, because the diameter of new arterial grafts was sig nificantly smaller than that of the other three grafts (2.4 +/- 0.1 vs . 2.9 +/- 0.2 [p < 0.05], 3.6 +/- 0.6 [p < 0.01] and 3.4 +/- 0.5 mm [p < 0.01], respectively), there was no difference in calculated flow vo lumes at rest (62 +/- 17 vs. 58 +/- 15, 61 +/- 18 and 58 +/- 19 ml/min , respectively, p = NS) between new arterial grafts and the other graf ts. Although the maximal time-averaged peak velocity during hyperemia was significantly greater in new than in old arterial grafts or new or old vein grafts (47 +/- 17 vs. 40 +/- 7, 31 +/- 8 and 34 +/- 12 cm/s, respectively, p < 0.01), the flow reserve of new arterial grafts was significantly smaller than that of the other three groups (1.8 +/- 0.3 vs. 2.6 +/- 0.3, 2.8 +/- 0.5 and 3.0 +/- 0.6, respectively, p < 0.01) because the baseline time-averaged peak velocity of these new grafts was far greater than that of the other groups. Conclusions. Internal m ammary artery graft how early after operation is characterized by a hi gher rest velocity than that of vein graft flow. This high velocity ma intains Row volume at baseline condition in compensation for the small er diameter. Although Row reserve does not differ significantly betwee n new and old vein grafts, that for internal mammary artery grafts is significantly reduced soon after bypass surgery. This restricted flow capacity improves late postoperatively because of an increase in diame ter and a decrease in Row velocity from baseline levels.