A myocardial bridge is a congenital vascular malformation. The coronary arteries and their branches usually travel in the subepicardial fat on the surface of the heart or in the deep epicardial surface. When a segment of the coronary artery is surrounded by myocardium, the segment is called a myocardial bridge and the segment is called a wall coronary artery. Myocardial bridges may be associated with local factors in the development of coronary heart disease and may also cause myocardial ischemia. This segment of coronary artery covered by the myocardial bridge is compressed during cardiac systole, and systolic stenosis occurs, while coronary artery compression is relieved during cardiac diastole and coronary stenosis is also relieved.
I. Etiology
The cause of the stenosis is that the myocardial segment of the coronary artery, especially the myocardial segment of the left anterior descending branch, can be compressed during systole, and the symptoms of myocardial ischemia mostly appear after middle age.
Clinical manifestations
The clinical manifestations of myocardial bridges are closely related to the typing.
1.Superficial type
Because the myocardial bridge is thin and short, it has less influence on coronary blood flow, and most of them have no myocardial ischemic symptoms and corresponding electrocardiogram changes.
2.Deep type
Because the myocardial bridge is thick and long, it has great influence on the coronary blood flow, and angina pectoris and ST-T changes of myocardial ischemia appear on the ECG. If myocardial bridge is complicated by coronary atherosclerosis secondary to thrombosis or plaque dislodgement, clinical symptoms of myocardial infarction and corresponding ECG changes may occur. Myocardial ischemia is more likely to occur when myocardial bridges are combined with tachyarrhythmias.
III. Examination
1.Coronary angiography
If coronary artery systolic stenosis or combined with delayed diastolic relaxation is found, it indicates the presence of myocardial bridges. However, coronary angiography can only detect myocardial bridges that have a significant effect on coronary blood flow. The detection of myocardial bridges is related to their length, the direction of travel of the bridging fibers, and the organization between the myocardial bridge and the associated artery. Some myocardial bridges are difficult to detect on angiography because of almost complete occlusion of the proximal coronary artery or because fixed stenosis from atherosclerosis limits coronary perfusion and obscures the systolic stenosis, or because of the presence of vasospasm. Coronary angiography often fails to detect atherosclerotic stenosis at myocardial bridges.
2.Intracoronary Doppler examination
The coronary blood flow velocity in the myocardial bridge part is found to rise significantly in early diastole as a peak, then fall soon after, followed by a plateau until it falls again in systole. The peak is due to the presence of the maximum perfusion pressure in the coronary artery and the corresponding continuous reduction of the vascular area, resulting in a significant pressure step difference between the two ends of the myocardial bridge. When the myocardial bridge relaxes in diastole, the pressure step difference between the two ends disappears, the vascular area expands rapidly, and the blood flow velocity decreases quickly.
3.Intravascular ultrasound
Atherosclerosis formation is often found in the coronary arteries proximal to the myocardial bridge. Reduced intracoronary flow reserve is detected by intracoronary Doppler.
IV. Diagnosis
The diagnosis of myocardial bridges still depends on coronary angiography and intracoronary Doppler and ultrasound, in addition to clinical symptoms and corresponding electrocardiographic changes. Superficial types are difficult to diagnose because of asymptomatic or mild symptoms, and even coronary angiography can only detect those with longitudinal myocardial bridges. There is such a big difference in the detection rate of myocardial bridges in autopsy and coronary angiography.
V. Treatment
Symptomatic myocardial bridges and myocardial bridges with atherosclerotic plaques can be treated with medication or surgery.
1.Drug treatment
Angina caused by systolic wall coronary artery compression may be effective for β-blockers and calcium antagonists such as verapamil and diltiazem.
2.Surgical treatment
Surgery should be performed if the angina is difficult to be controlled by medication. There are two types of surgery, namely myocardial bridge resection and coronary artery bypass grafting.
(1) Myocardial bridge resection is suitable for superficial type, and the myocardial bridge is found under general anesthesia at room temperature to completely remove the compression on the coronary artery and restore the distal blood flow. Myocardial bridge resection is rarely performed in conjunction with coronary artery bypass grafting.
(2) Coronary artery bypass grafting is indicated for longitudinal type or combined with atherosclerotic stenosis. Coronary artery bypass grafting can be performed under normothermic general anesthesia, normothermic extracorporeal circulation or hypothermic extracorporeal circulation. The graft material can be autologous saphenous vein or internal mammary artery.