On the other hand, MB can be clinically significant and may cause various clinical manifestations such as malignant arrhythmias, myocardial infarction and sudden death in rare instances by systolic compression of the tunneled segment persisting into diastole during stress and changing regional hemodynamics that contribute to atherosclerotic plaque formation proximal to the bridge in a higher frequency (40).
MBs are usually diagnosed incidentally during CCA (20) by the typical findings of 'milking' effect and a 'step down-step up' phenomenon induced by systolic compression of the tunneled segment (40).
The clinical significance of MB is most likely due to the anatomic narrowing of the tunneled segment caused by the overlying myocardium.
The current gold standard for diagnosing MB is CAG with the typical "milking effect" and a "step down-step up" phenomenon induced by systolic compression of the tunneled segment .
The myocardial tissue covering the artery is called a myocardial bridge, and the artery itself is called a tunneled segment
. (13) The bridging can be superficial or deep and has been most commonly described in the mid LAD--80% (Figure 12).
Therefore, a possible compression of myocardium in tunneled segment could not be evaluated.
Since it is an extremely rare anomaly, clinical significance of this variation whether this variation compresses the tunneled segment or not are not known.
There is an evidence from pathophysiologic and intravascular ultrasound studies that the arterial segment proximal to the myocardial bridge (MB) has a higher frequency of atherosclerosis, whereas the tunneled segment
is relatively spared.
Muscle overlying an epicardial segment of a coronary artery, first mentioned by Reyman in 1737 (1), is termed a myocardial bridge, and the artery coursing within the myocardium is called a tunneled artery, It is characterized by systolic compression of the tunneled segment, which remains clinically silent in the vast majority of cases.
While previously considered a clinically insignificant variant, ICD recordings demonstrated an increased flow velocity in the tunneled segment. Frame-by-frame analysis on IVUS revealed a delayed relaxation after systolic compression, which may extent significantly into diastole (13).
Most new pipe installed on the projects utilizes ductile iron except for tunneled segments
where pipe diameters were less than 42 inches; then, vitrified clay was used.