What are the tests that Stanford uses to diagnose myocardial bridges?
1. CT scan
2. Stress Echocardiogram with Ultrasound
3. Cardiac catheterization including either dFFR, iFR, or RFR - this is the final test, providing the green light for surgery
Important note: FFR is “useless” for MBs according to Stanford
What does Stanford look for in the tests?
Signs in the CT scan:
1) black fat disappears - coronary arteries are supposed to sit in a buffer of fat around them, which shows up as black/dark bordering the artery, as opposed to the gray of the heart muscle itself. So one sign of an MB is if you follow the artery down and see the dark "fat border" between the artery and the heart disappear somewhere along the way, indicating that in that section the band of muscle, not fat, is up against the artery. This can be tricky to find because the CT scan has to be viewed at the right angle since the muscle band is only on one side, which is easier on better CT machines that take images at more angles aka more “slices.”
2) artery becomes narrowed (if the MB does not extend to the end of the artery, the artery usually widens again farther down after the MB)
3) artery becomes smudged/blurry/choppy
In the stress ultrasound, Stanford looks specifically for “focal end-systolic to early-diastolic buckling in the septum with apical sparing.” Their 2013 article A Novel Stress Echocardiography Pattern for Myocardial Bridge With Invasive Structural and Hemodynamic Correlation by Lin, Tremmel, et al. explains this pattern that they discovered.
In the cardiac catheterization, Stanford looks for many things:
1) the milking effect, which is simply the squeezing of the artery in the MB area, which compresses and expands just like if you squeezed on a straw.
2) The % compression on the artery in the MB, at rest and at elevated heart rate
3) The length of the MB in mm – anything over 2 cm is typically considered significant, although MBs under 2 cm can and do sometimes cause major symptoms warranting surgery
4) The location of the MB, which they map precisely relative to junctions with septal arteries branching off and other landmarks
5) Spasticity (i.e. spasms) in the MB i.e. vasospasms (mild, moderate, severe)
6) dFFR – diastolic Fractionated Flow Ratio, or iFR – instantaneous wave-free ratio. This is the ratio of the rates of blood flow measured at two different points in the artery, before and after the MB, including during diastole ie while the heart is relaxed. So for example, if the dFFR is .7 or 7/10, it means that the blood flow rate after the MB is 7/10 of the rate before the MB, proving that the MB caused a reduction in flow. In general in cardiology, dFFR of .75 or below, or iFR of .89 or below are considered to be significant evidence of ischemia i.e. lack of blood supply causing shortage of oxygen to the body, providing justification for surgery.
Important notes on dFFR/iFR/RFR:
1) The cardiac catheterization must include a dobutamine challenge. Dobutamine speeds up the heart rate (induces tachycardia) so that the effects of the MB at high heart rates can be measured. This is essential because the faster the heart beats, the less time the artery has to reopen after being squashed each beat by the MB, and thus the loss of blood flow through the MB can become much higher. Many patients whose dFFR or iFR does not measure below the cutoff at normal/resting heart rate measure far below the cutoff during the dobutamine challenge.
2) Any drugs that slow the heart rate can result in a misleading dFFR or iFFR at resting heart rate.
(1) the new artery can sometimes get blocked and blood still goes down the original MB artery(2) After bypass, there is still some local ischemia in whatever areas of the heart are still getting blood from the original MB artery that's compressed.
Stress echocardiagrams: If the test did not find an MB, does it mean I don't have an MB?
No. It is common for docs to miss the signs of MBs on a stress echocardiogram. Stanford wrote this 2013 paper, and then followed it up with this 2020 paper, on a "novel" i.e. new pattern to look for to spot MBs on stress echo tests, but the % of docs and technicians who have read this paper and know what to look for is probably well under 1%. The name of the pattern is focal septal buckling with apical sparing. The 2020 paper concluded that this pattern was an "accurate predictor" of an MB. So when your doc tells you that you don't have an MB based on a stress echo, take it with multiple grains of salt. You could send your stress echocardiogram images to Stanford's 2nd opinion program to look for this pattern.