Introduction. The internal mammary artery (IMA) is a living conduit subjected to the influence of vasoactive agents. Both nitroglycerin (TNG) and milrinone (M) are known to dilate the IMA. [1,2] Alpha agonists are frequently used to treat excessive vasodilatation after CPB and may decrease IMA flow despite TNG or M, thus risking myocardial ischemia. [3] The purpose of this study was to measure changes in grafted IMA flow after the administration of phenylephrine in patients receiving either TNG, milrinone, or both drugs concomitantly. [3] Methods. After approval from the institutional review board and informed consent, 30 adult patients undergoing coronary artery bypass grafting (CABG) with IMA placement were studied. IMA flow was measured with a calibrated laser Doppler probe (Transonic Inc, Ithaca NY). The IMA was anastomosed to the left anterior descending artery with a continuous 8-0 prolene suture in all cases. After separation from CPB, patients were then randomized to receive either a loading dose of milrinone (M, 50 [micro sign]g/kg), an infusion of nitroglycerin (TNG, 2 [micro sign]g/kg/min), or a loading dose of milrinone followed by TNG (M+TNG). Phenylephrine was then administered by infusion and titrated to raise the mean arterial pressure (MAP) by 20% or to 60 mm Hg. IMA flow was measured prior to the administration of the drugs, 10 minutes following either the beginning of the TNG infusion or the conclusion of the M bolus, and after the target MAP was reached. Statistical analysis was performed with ANOVA of repeated measures. A p < 0.05 was considered significant. Results. The administration of phenylephrine in the presence of TNG increased mean IMA flow in 6/10 patients (25 +/- 10 mL/min to 52 +/- 12 mL/min; p < 0.001); however, IMA flow decreased in 4/10 patients (33 +/- 16 mL/min to 24 +/- 17 mL/min; p < 0.04). Phenylephrine increased mean IMA flow from 55 +/- 15 mL/min to 87 +/- 13 mL/min in the M group (p < 0.01) and from 48 +/- 10 mL/min to 67 +/- 12 mL/min in the TNG+M group (p<0.02). No patients receiving M or TNG+M exhibited a reduction in IMA flow. Discussion. In patients receiving milrinone, alone or in combination with TNG, the use of alpha adrenergic agonists after CPB appears safe since IMA flow is preserved or enhanced. In the presence of TNG, stimulation of alpha receptors may actually decrease IMA flow in some patients and may be a risk factor for post-CPB myocardial ischemia. (Figure 1)Figure 1