811-3 Beneficial Effect of β-Blockade on Myocardial Blood Flow and Vasodilator Capacity in Humans

β1-receptor blockade reduces cardiac work and might thereby lower myocardial blood flow (MBF) at rest. However, the effect of β1-blockade on hyperemic MBF is unknown. To evaluate the net effect of β1-receptor blockade (Metoprolol, 50mg p.o. 12 hours and 1 hour prior to the study) on MBF at rest and during dipyridamole (DIP) induced hyperemia 10 healthy volunteers (mean age: 24±5 years; 8 males, 2 females) were studied with serial N-13 ammonia PET (2 compartment model) at control and again after β1-receptor blockade. The resting rate pressure product (RPP) declined from 6621±555 at control to 5168±975 (p<0.05) during Metoprolol. Resting heart rate (HR) decreased from 63±6 to 54±5 beats/min (p<0.05). Similarly, the RPP declined from 10193±1330 during baseline DIP to 8575±1770 (p<0.05) during DIP+Metoprolol. Resting MBF declined in proportion to cardiac work by 20% from 0.61±0.10 to 0.51 ±0.11ml/g/min (p<0.05). Overall, resting MBF was linearly related to the RPP (y=0.00003x+0.134; r=0.67; P<0.005). Thus, resting MBF normalized to the RPP *(105) did not change from baseline to β1-blockade (0.91±0.1 vs 0.99±0.19; p=NS). Hyperemic MBF increased from 1.86±0.28 to 2.40±0.70ml/g/min during (p<0.05). The decrease in resting MBF together with the increase in hyperemic MBF during β-blockade resulted in a significant increase in the myocardial flow reserve (MFR) from 3.16±0.80 to 4.74±0.73; p<0.01) Coronary vascular resistance (mean arterial blood pressure/MBF) did not differ at rest (140±27 vs 127±22mmHg/ml/g/min) but declined during DIP from 42±7 to 32±7mmHg/ml/g/min during DIP + Metoprolol (p<0.05). Thus, resting MBF decreases with β1-blockade in proportion to the decrease in cardiac work which contributes to the improvement in flow reserve. At the same time, β1-blockade modulates hyperemic MBF most likely because of a) β1-blocker induced negative inotropy which reduces extravascular compressive forces, and/or b) decreases in heart rate which increases the duration of the diastolic flow phas