Myocardial supply and demand

Calcium channel blockade with felodipine does not affect metabolic coronary vasodilation in patients with coronary artery disease

The effect of calcium channel blockers may affect the feedback mechanism between myocardial metabolic activity and coronary blood flow. To test this hypothesis the effect of calcium channel blockade on metabolic coronary flow regulation was studied. In 10 patients with stable coronary artery disease, coronary sinus blood flow and myocardial oxygen supply and consumption (MVO2) were measured both at sinus rhythm and during atrial pacing (30 beats/min above sinus rate), at control and during infusion of felodipine, a vasoselective dihydropyridine. The myocardial oxygen supply-consumption ratio at control (i.e., the slope of the regression line characterizing normal metabolic flow regulation) was 1.58 (95% CI, 1.38-1.80). Following infusion of felodipine, systemic and coronary vascular resistance during sinus rhythm decreased by 20 +/- 11% and 23 +/- 15%, respectively, and coronary venous oxygen saturation increased from 36 +/- 6% at control to 42 +/- 7% (p = 0.047) during infusion of felodipine. The myocardial oxygen supply-consumption ratio, characterizing metabolic flow regulation during felodipine, was 1.52 (95% CI, 1.26-1.78) and thus not different from control. Metabolic coronary flow regulation was not affected by administration of felodipine, although the setpoint of this regulation mechanism might have been offset by the initial drug-induced coronary vasodilation, which persisted during pacing.

Metabolic coronary-flow regulation and exogenous nitric oxide in human coronary artery disease: assessment by intravenous administration of nitroglycerin

We sought to evaluate the effect of intravenous administration of the nitric oxide--donor substance nitroglycerin (NTG) on metabolic coronary-flow regulation in patients with coronary artery disease (CAD). In 12 patients with stable CAD, we measured coronary sinus blood flow and myocardial oxygen supply and consumption (MVO2) at sinus rhythm and during atrial pacing (30 beats/min above sinus rate), both at control and during infusion of NTG, 1 microg/kg/min, and NTG, 2 microg/kg/min. To study metabolic coronary vasodilation, changes in myocardial oxygen supply were related to pacing-induced changes in MVO2, by using standard regression analysis. The myocardial oxygen supply/consumption ratio (i.e., the slope of the regression line at control, characterizing physiological metabolic coronary flow regulation) was compared with the ratios obtained during infusion of NTG. Compared with control measurements, NTG, 1 microg/kg/min, and NTG, 2 microg/kg/min, attenuated pacing-induced increases in MVO2 by 29 and 60%, respectively, whereas coronary blood flow during pacing remained unchanged. At control, normal metabolic coronary-flow regulation resulted in a myocardial oxygen supply/demand ratio of 1.39 (95% CI, 1.29-1.49). This ratio did not change during NTG, 1 microg/kg/min: 1.44 (95% CI, 1.33-1.56). However, during NTG, 2 microg/kg/min, this ratio significantly increased to 1.84 (95% CI, 1.63-2.05; p<0.01). Intravenous administration of high-dose NTG, a donor of exogenous NO, blunts pacing-induced increases in MVO2 and may increase metabolic coronary vasodilation in patients with CAD.

The effect of nitroglycerin on pacing-induced changes in myocardial oxygen consumption and metabolic coronary vasodilation in patients with coronary artery disease

In the present study, we assessed the potential effect of nitroglycerin IV (NTG), a donor of exogenous nitric oxide, on metabolic coronary flow control in patients with coronary artery disease. In 12 patients scheduled for coronary artery surgery, arterial blood pressure, pulmonary capillary wedge pressure, coronary sinus blood flow (continuous thermodilution), myocardial oxygen supply (DVO2), and myocardial oxygen consumption (MVO2) were measured at sinus rhythm and in response to atrial pacing at 30 bpm greater than baseline sinus rate. These measurements were repeated during infusion of NTG 1 and 2 microg x kg(-1) x min(-1). At control, in the absence of NTG, MVO2 increased from 13.7 +/- 3.4 mL O2/min during sinus rhythm to 19.3 +/- 5.5 mL O2/min during pacing. NTG 1 and 2 microg x kg(-1) x min(-1) blunted the pacing-induced increase in MVO2 dose-dependently. During NTG 1 microg x kg(-1) x min(-1), MVO2 increased from 12.9 +/- 3.3 mL O2/min at sinus rhythm to 17.3 +/- 4.7 mL O2/min during pacing (P = 0.01 versus control pacing); during NTG 2 microg x kg(-1) x min(-1), MVO2 increased from 13.4 +/- 3.3 mL O2/min to 15.9 +/- 3.7 mL O2/min (P = 0.008 versus control pacing). However, the pacing-induced increase in DVO2 per mL O2/min increase in MVO2 (delta DVO2/delta MVO2), was significantly greater during the infusion of NTG 2 microg x kg(-1) x min(-1) (1.85 +/- 0.56; P = 0.023) compared with control (1.51 +/- 0.22). This was associated with an increase in coronary sinus hemoglobin oxygen saturation (30% +/- 5% at control pacing and 34% +/- 6% during pacing with NTG 2 microg x kg(-1) x min(-1); P = 0.018), which indicates that during the infusion of NTG, there was more metabolic coronary vasodilation than achievable solely on the basis of the metabolic stimulus.

IMPLICATIONS

Our findings suggest that nitroglycerin, a donor of exogenous nitric oxide, reduces pacing-induced increases in myocardial oxygen consumption and enhances metabolic coronary vasodilation in patients with coronary artery disease, in whom endogenous nitric oxide activity may be reduced.