Endothelial release of nitric oxide (NO) is a defining characteristic of non-diseased vascular tissue. Healthy coronary arteries respond to endothelial-dependent stressors with vasodilatation but those with endothelial dysfunction respond with paradoxical vasoconstriction. The combination of new non-invasive 3T coronary MRI methods and isometric handgrip exercise (IHE), has been suggested as a means to noninvasively quantify coronary endothelial function (CEF). However, IHE may trigger neural, neuro-hormonal and other vasoreactive responses; thus, it is unknown whether the IHE-induced coronary response is, in fact, primarily mediated by NO. Furthermore, it is not known whether the MRI-IHE test is reproducible over time.To test the hypothesis that the IHE-induced coronary response is NO-mediated, we performed MRI-IHE studies before and during the infusion of monomethyl-L-arginine (L-NMMA, 0.3mg/kg/min), a NO synthase inhibitor in 8 healthy subjects. To study reproducibility we performed 2 MRI-IHE studies ~8 weeks apart in 8 coronary artery disease (CAD) patients and 9 healthy subjects. Changes from rest to IHE in coronary cross-sectional area (%CSA) and coronary blood flow (%CBF) were measured with cine MRI. L-NMMA completely blocked coronary vasodilation during IHE (%CSA change 15.4%±2.8% with placebo vs -1.6%±1.3% with L-NMMA; p<0.0001) and the normal increase in coronary blood flow (%CBF change 50.2%±6.7% with placebo vs -2.1%±6.7% with L-NMMA; p< 0.0001). Moreover, there was a strong correlation between repeated measures for %CSA change with IHE at the two exams (R=0.91, p<0.0001) and %CBF change with IHE (R=0.80; p<0.001). Bland-Altman analysis and intra-class correlation coefficients for %CSA and %CBF change with IHE (0.90 and 0.80, respectively) indicated good agreement and little variability between repeated measures. In summary the coronary response to IHE is largely mediated by endothelial-derived NO and is reproducible over several weeks. Thus MRI-IHE is a noninvasive, reproducible tool to assess CEF, arguably the first to noninvasively measure macro- and micro-vascular NO-mediated coronary responses. This noninvasive tool may be useful in future studies of the impact of interventions on CAD pathogenesis.
Noninvasive evaluation of coronary endothelial function by using 3 T phase contrast cine MRI
