Augmentation of vessel squeezing at coronary-myocardial bridge by nitroglycerin: Study by quantitative coronary angiography and intravascular ultrasound

Background Nitroglycerin is known to augment vessel wall squeezing ai the site with coronary-myocardial bridging (CMB). This study was designed td define the mechanism of nitroglycerin-induced augmentation of CMB in clinical settings. Methods We analyzed nitroglycerin reactivity at the site with CMB in 39 patients. Maximal and minimal diameters of CMB during a cardiac cycle were measured by quantitative angiography before and after intracoronary administration of 250 mu g nitroglycerin. In 15 patients, CMB sites were observed by intravascular ultrasound to determine the intimal thickness and the time-serial change in vessel area. Results Before nitroglycerin, CMB was demonstrated with angiography in 25 patients, and the remaining 14 patients showed CMB after nitroglycerin. The maximal diameter during diastole increased from 1.4 +/- 0.4 mm to 1.9 +/- 0.4 mm after nitroglycerin, whereas the minimal diameter during systole decreased from 1.0 +/- 0.4 mm to 0.7 +/- 0.4 mm (P <.01). Thus nitroglycerin augmented the percent vessel narrowing during systole from 24% +/- 21% to 65% +/- 16% (P <.01). Under these conditions, intravascular ultrasound showed the reduction of the cross-sectional area of the sites with CMB by -38% +/- 16% (P <.01) during systole, and this phenomenon continued to early diastole (-30% +/- 16%). The intimal thickness was 0.32 +/- 0.10 mm, which suggests the absence of atherosclerotic disease at CMB sites. Conclusions These results indicate that nitroglycerin-induced augmentation of the percent narrowing of CMB can be derived from further systolic compression of the vessel lumen as well as diastolic expansion, probably because of the increase in vessel compliance after nitroglycerin. We suggest that the delayed dilation of coronary lumen during the early diastole may contribute to the occurrence of myocardial ischemia.