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CardioNerd Amit Goyal is joined by Dr. Erika Hutt (Cleveland Clinic general cardiology fellow), Dr. Aldo Schenone (Brigham and Women’s advanced cardiovascular imaging fellow), and Dr. Wael Jaber (Cleveland Clinic cardiovascular imaging staff and co-founder of Cardiac Imaging Agora) to discuss nuclear and complimentary multimodality cardiovascular imaging for the evaluation of abnormal coronary anatomy including anomalous coronary arteries and myocardial bridges. Show notes were created by Dr. Hussain Khalid (University of Florida general cardiology fellow and CardioNerds Academy fellow in House Thomas). To learn more about multimodality cardiovascular imaging, check out Cardiac Imaging Agora!

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Show Notes & Take Home Pearls

Five Take Home Pearls

1. Anomalous coronaries are present in 1-6% of the general population and predominantly involve origins of the right coronary artery (RCA). Anomalous origination of the left coronary artery from the right sinus, although less common, is consistently associated with sudden cardiac death, especially if there is an intramural course. Sudden cardiac death can occur due to several proposed mechanisms: (1) intramural segments pass between the aorta and pulmonary artery making them susceptible to compression as the great vessels dilate during strenuous exercise; (2) an acute angle takeoff of the anomalous coronary can create a “slit-like” ostium making it vulnerable to closure. Anomalous left circumflex arteries are virtually always benign because the path taken behind the great vessels to reach the lateral wall prevents vessel compression.
2. Myocardial bridging (MB) is a congenital anomaly in which a segment of the coronary artery (most commonly, the mid-left anterior descending artery [LAD]) takes an intramuscular course and is “tunneled” under a “bridge” of overlying myocardium. In the vast majority of cases, these are benign. However, a MB >2 mm in depth, >20 mm in length, and a vessel that is totally encased under the myocardium are more likely to be of clinical significance, especially if there is myocardial oxygen supply-demand mismatch such as with tachycardia (reduced diastolic filling time), decreased transmural perfusion gradient (e.g. in myocardial hypertrophy and/or diastolic dysfunction), and endothelial dysfunction resulting in vasospasm.
3. PET offers many benefits over SPECT in functional assessment of MB including the ability to acquire images at peak stress when using dobutamine stress-PET, enhanced spatial resolution, and quantification of absolute myocardial blood flow. For pharmacologic stress in evaluation of MB, we should preferentially use dobutamine over vasodilator stress. Its inotropic and chronotropic effects enhance systolic compression of the vessel, better targeting the pathological mechanisms in pearl 2 above that predispose a MB to being clinically significant.
4. CCTA can help better define the anatomy of MB as well as anomalous origination of the coronary artery from the opposite sinus (ACAOS), help with risk stratification, and assist with surgical planning.
5. Instantaneous wave-free ratio (iFR) measures intracoronary pressure of MB during the diastolic “wave-free” period – the period in the cardiac cycle when microvascular resistance is stable and minimized allowing the highest blood flow. This allows a more accurate assessment of a functionally significant dynamic stenosis than fractional flow reserve (FFR) – which can be falsely normal due to systolic overshooting.

Detailed Show Notes

1. What are some examples of abnormal coronary anatomies and how often do they lead to clinical events?
   • Abnormal coronary anatomy can relate to the origin (e.g. anomalous origination of coronary artery from the opposite sinus [ACAOS]), course (e.g. myocardial bridging [MB]), intrinsic properties (e.g. aneurysm or hypoplasia), or termination (e.g. fistula) of the coronary artery. In this episode and in these notes, we examine MB and ACAOS in more detail. For an excellent case discussion of anomalous left coronary artery from the pulmonary artery (ALCAPA) by the team from Massachusetts General Hospital, listen to CardioNerds Podcast Episode 81!
   • MB –Myocardial Bridging
     • MB is a congenital anomaly in which a segment of the coronary artery (most commonly, the mid-left anterior descending artery [LAD]) takes an intramuscular course and is “tunneled” under a “bridge” of overlying myocardium.
     • MB was originally identified at autopsy by Reyman in his dissertation, “Disertatio de vasis cordis propriis “ in 1737. In the largest subsequent autopsy study by Risse et al. involving 1056 patients, MB was demonstrated in 26% of patients.
     • Because it is so prevalent, it is difficult to determine its clinical significance. In most patients, MB is an incidental finding with an excellent survival rate (97% at 5 years); however, there are associations with myocardial ischemia, infarction, stress cardiomyopathy, arrhythmia, and sudden cardiac death (SCD).
     • MB can generally be classified into two subtypes: a “superficial” variant which represents 75% of cases and a “deep” variant in which the LAD deviates towards the right ventricle (RV) and dives into the intraventricular septum. The overlying muscle bundle in the deep variant is typically at an oblique or transverse angle resulting in twisting of the tunneled segment and more commonly compromised coronary flow.
     • One of the longest MB usually occurs in association with ACAOS! In this case, the left coronary artery comes off the right coronary cusp. The Left Main (LM) is around 3-4x longer in this instance and dives into the interventricular septum and takes a trans-septal course behind the pulmonary artery before emerging on the other side.
     • There is increased prevalence in certain patient populations: hypertrophic cardiomyopathy (HCM), patients with spontaneous coronary artery dissection (SCAD) +/- fibromuscular dysplasia (FMD), and heart transplant recipients
   • ACAOS – anomalous origination of coronary artery from the opposite sinus
     • Anomalous coronaries are present in 1-6% of the general population and predominantly involve the origin of the right coronary artery (RCA)
     • Anomalous origination of the left coronary artery from the right sinus, although less common, is consistently related to SCD. Separate studies have shown the incidence of SCD may be as high as 23% or 59% of cases in athletes under the age of 20 years.
     • In a large Armed Forces Institute of Pathology (AFIP) study of 6.3 million military recruits, the autopsies of recruits who suffered nontraumatic deaths over a 25-year period were reviewed and ACAOS was found to be the most common cause. It accounted for 33% (64 of 126) of nontraumatic deaths and all cases involved a left coronary artery with an interarterial course.

2. What features predispose MB or ACAOS to become clinically significant? What is the pathophysiology behind development of ischemia in those with clinically significant MB or ACAOS?
   • MB – myocardial bridging
     • Given the majority of MB is benign, correlating MB  as causative in myocardial ischemia and its consequences has been a diagnostic challenge.
     • In systole, the portion of the artery that is tunneled under the MB (bridge segment) is compressed. This can manifest clinically as angina, acute coronary syndrome, left ventricular (LV) dysfunction, arrhythmias, and SCD. However, the majority of myocardial perfusion occurs in diastole which is why MB is usually benign. Nonetheless, certain conditions in patients with MB can set up an oxygen supply-demand mismatch severe enough to lead to myocardial ischemia:
       • Exercise-related stress  leads to tachycardia which can decrease diastolic filling time for the coronary arteries and lead to more of the cardiac cycle to be spent in systole
       • Myocardial hypertrophy and diastolic dysfunction can affect the transmural perfusion gradient increasing supply-demand mismatch. Furthermore, LV hypertrophy can compress the microvasculature and reduce the microvascular reserve.
       • Endothelial dysfunction (driven by metabolic changes secondary to hypoxia) can contribute to coronary compression and lead to the development of accelerated atherosclerosis and/or coronary vasospasm (leading to compression of the epicardial coronary artery throughout the cardiac cycle, not just during systole)
       • There has been a recognized multiplier-effect described by Klues et al. in which the greater the degree of systolic narrowing of the MB, the greater the reduction in diastolic vessel diameter. This is also associated with increased retrograde flow in the coronary artery (which not only reduces perfusion but can introduce shear wall stress and predispose to plaque formation) and reduced flow reserve.
       • Myocardial ischemia can also occur due to “branch steal.” The LAD may have septal perforators that arise from the tunneled segment. When there is compression of the vessel under the MB, there can be “steal” from these septal branches due to the Venturi effect. The septal branches are essentially depressurized because as the vessel narrows, velocity increases but the fluid (coronary blood flow) exerts less pressure. Thus, mild to moderate MB severity typically demonstrates septal ischemia (due to branch steal) rather than distal ischemia downstream from the compression.
       • The vessel segment proximal to the bridge appears to develop atherosclerosis at increased rates approaching 90% — likely as the sequela of shear stress. In contrast, the tunneled segment of the artery is usually spared of atherosclerosis because:
         1. The intima is significantly thinner with a higher prevalence of contractile cells (thought to be negatively associated with development of atherosclerotic lesions)
         1. There is a lack of foam cells (lipid-laden macrophages that are important components of atherosclerosis)
         1. There is reduced expression of known vasoactive agents such as nitric oxide synthase, endothelin-1, and angiotensin-converting enzyme
   • ACAOS – anomalous origination of coronary artery from the opposite sinus
     • The mechanism of ischemia for ACAOS with an interarterial course (between the pulmonary artery and the aorta) and specifically an intramural course has not fully been determined. An intramural course refers to the proximal part of the epicardial coronary artery being contained within the aortic wall and sharing the aortic wall media without a separating adventitia. There are several proposed mechanisms for why this anatomic setup can lead to myocardial ischemia.
       • Compression of the vessel between the aorta and the pulmonary artery during intense exercise as the great vessels dilate
         1. The pulmonary artery likely needs to be enlarged secondary to concomitant pulmonary hypertension for this to occur
         1. An anomalous left circumflex is almost never clinically significant because the path it takes behind the great vessels to reach the lateral wall means the vessel is not exposed to compression!
       • Anacute angle takeoff of the coronary artery, which shares a common wall with the aorta, can result in a significantly narrowed coronary artery ostium (e.g., “slit-like”). This is even further narrowed during exercise when the great vessels expand.
       • Marked narrowing of the intramural segment due to hypoplasia of the intramural segment

3. What is the role of nuclear imaging in the evaluation of MB or ACAOS?

• MB – myocardial bridging
  • In general, nuclear imaging evaluation of MB has had mixed results, and available studies generally have smaller sample sizes and most are retrospective. Prior studies — predominantly involving exercise SPECT — have shown that reversible ischemia may be present in patients with MB and systolic compression of the vessel >50% or >75%. However, other studies in similar populations have shown that reversible ischemia was not inducible in patients with MB with similar degrees of systolic compression of the vessel. There are very few studies available assessing the utility of PET stress testing in patients with MB. One prior study demonstrated that PET stress testing revealed decreased myocardial perfusion reserve in patients with MB, although this was with adenosine rather than dobutamine stress (more on this below).
  • In patients who are symptomatic and have full encasement of the epicardial artery or a deep course seen on coronary CTA, it is reasonable to pursue functional testing, either noninvasive or invasive. Otherwise, we shouldn’t pursue functional testing as the overwhelming majority of MB are benign.
  • Rest/Stress myocardial perfusion imaging with PET has certain advantages over SPECT:
    • Improved spatial resolution
    • Ability to acquire stress images at peak stress if using dobutamine (versus lag time with SPECT)
    • Absolute myocardial blood flow quantification with PET (not available with SPECT where perfusion is relative)
    • With PET we can use either Rubidium-82 (half-life = 76 sec) or N-13 Ammonia (half-life = 10 min) as tracers to assess myocardial perfusion. If using dobutamine as the stress agent, either tracer can be injected at peak stress for the stress imaging. However, if we are using exercise as the stress agent, the patient must be transferred from the treadmill to the camera for stress image acquisition. Because of the time it takes to transfer, Rb-82 cannot be used for exercise PET; it’s half-life is so short (76 sec), it will be gone by the time images are acquired! Therefore, if using exercise PET, you must use N-13 Ammonia PET.
  • When picking a stress agent, remember the goal is to look for significant mechanical compression that leads to coronary ischemia. Therefore, a vasodilator (e.g., adenosine or regadenoson) would be inappropriate. Rather we need to increase chronotropy and inotropy to simulate mechanical compression and can use either dobutamine or exercise.
    • NOTE – If the patient has a resting significant mid ventricular or left ventricular outflow gradient (as occasionally seen in HCM) and/or is pacer-dependent, dobutamine and exercise may increase the LVOT gradient and compromise hemodynamics. Rather, consider rapid atrial pacing or other pharmacologic stress in these patients.
  • Dobutamine or exercise echocardiography are alternatives to nuclear imaging.
    • Since MB is often studied in younger patients and since younger patients generally have rapid heart rate recovery, if using exercise as a stressor, consider supine bicycle rather than treadmill so that stress TTE images may be acquired prior to HR recovery for increased sensitivity.
• ACAOS – anomalous origination of coronary artery from the opposite sinus
  • The 2018 AHA/ACC Guideline for the Management of Adults With Congenital Heart Disease state that in patients w/ ACAOS with either left coronary artery arising from the right sinus or right coronary artery arising from the left sinus, ischemic symptoms or ischemia during functional testing is a Class I indication for surgery.
  • After identification of ACAOS, it is reasonable to consider functional testing with nuclear imaging, however the sensitivity of this approach is not yet known. Also, the intense physical exertion that usually results in SCD in these patients is usually not achieve with standard stress tests, so the sensitivity of these tests are difficult to judge. The risks/benefits of functional nuclear imaging should be addressed with patients  in shared decision making with the patient.
  • A number of case reports have described inducible ischemia on myocardial perfusion imaging in patients with ACAOS. This may provide additional justification for surgical intervention.
    • Dr. Wael Jaber (our excellent podcast expert guest on this episode!) as part of the group with Cremer et al. demonstrated in a retrospective study of 27 patients with anomalous take-off of the RCA from the left coronary sinus (AAORCA) that patients with typical angina and exertional dyspnea had a significantly higher rate of demonstrable ischemia on an exercise N¹³-ammonia positron emission tomography (PET) protocol compared to patients without symptoms. The large majority (11/12) of the patients who underwent surgery had demonstrable ischemia on the above protocol. There were no deaths at 245 days in either the conservative management group or those who underwent surgery. Evidence of ischemia on exercise N¹³-ammonia positron emission tomography (PET) protocol in patients with may identify patients with AAORCA who would benefit from surgical vs. conservative management.

4. What is the role of coronary CTA (CCTA) in the evaluation of MB or ACAOS?

• Cross-sectional imaging with CCTA is crucial in the assessment of MB and ACAOS for both identifying the abnormality and risk stratification.
• MB – myocardial bridging
  • CCTA has increased the detection of MB from ~5% on invasive coronary angiography to ~21% – much closer to what has been identified on autopsy studies.
  • CCTA can help risk stratify by:
    • Quantifying the depth (>2mm considered clinically significant)
    • Quantifying the length (>20 mm considered clinically significant)
    • Observing the degree of encasement (more fully encased considered clinically significant)
    • Detect concurrent atherosclerosis (particularly proximal to the MB)
    • If there are high risk features and corrective surgery is planned, the anatomical information provided by the CCTA is useful for surgical planning.  NOTE – If depth >5 mm and/or length >25 mm, CABG is preferred over myotomy as risks of myotomy is considerable in these circumstances!
• ACAOS – anomalous origination of coronary artery from the opposite sinus
  • CCTA is endorsed by the European Society of Cardiology (ESC) as the first-line diagnostic imaging in known or suspected coronary artery anomalies. The American Heart Association (AHA) Committee on Cardiovascular Imaging provides a IIa recommendation for CCTA or MRI in the evaluation of anomalous coronary arteries.
  • CCTA can help risk stratify patients by identifying anatomic features that confer a higher risk of SCD:
    • Slit-like orifice of the coronary ostium
    • Acute angle of origin
    • Intramural segment: identification of an intramural segment is physiologically important (as discussed above), but also guides treatment. These patients can be potentially treated by coronary unroofing, unlikepatients with intraarterial course and no intramural segment
  • Some general considerations for CCTA evaluation:
    • Low heart rate is needed to optimize image quality. We commonly accomplish this by giving beta blocker and/or ivabridine.
    • Consider strategies to minimize radiation exposure as able (e.g., in ACAOS we can use prospective gating in which we choose only to image in a certain prespecified phase of the cardiac cycle rather than the whole cardiac cycle as done in retrospective gating)
  • To avoid radiation exposure and iodinated contrast administration with CCTA, cardiac MRI can be considered
    • Additionally, you can obtain concomitant assessment of ventricular size, function, shunt, perfusion, and viability
    • This comes at the expense of decreased spatial resolution, long examination time necessitating significant patient cooperation, and artifact and incompatibility (sometimes prohibitive) from pacemaker or other metallic implants

5. What is the role of left heart catheterization in the evaluation of MB and ACAOS?

• MB – myocardial bridging
  • Coronary angiography is useful for identifying MB, but is less sensitive than CCTA. Altogether, MB identified by coronary angiography may be more severe than those identified by CCTA, as the mechanical compression must be severe enough to be noticed angiographically. However, a majority of even these bridges have a benign natural history. Therefore, additional risk stratification may be obtained invasively to understand the functional significance of a MB. This can be obtained by combining invasive coronary angiography with tools such as instantaneous wave-free ratio (iFR) and intravascular ultrasound (IVUS).      
  • Fractional flow reserve (FFR) is the gold-standard for invasive assessment of intermediate fixed coronary stenoses and correlates with outcomes. iFR has been shown to be non-inferior to FFR.
  • However, FFR has not been validated in the assessment of dynamic compression, as with myocardial bridging. In dynamic compression, iFR offers some notable advantages over FFR:
    • In FFR we are checking the average of the blood pressure and flow over the whole cardiac cycle — systole and diastole. Because of systolic compression in MB, there is a spurious increase in the intracoronary systolic pressure that may yield a falsely normal FFR value — this is known as systolic pressure overshooting
    • In iFR, on the other hand, we are measuring intracoronary pressure during the diastolic “wave free” period — the period in the cardiac cycle when microvascular resistance is stable and minimized allowing the highest blood flow. This gives a more accurate assessment of functionally significant dynamic stenosis
    • iFR is considered positive if it is < 0.89. The grey zone in iFR studies is >0.86 and <0.93, so ideally, we want < 0.86 for a more definitive true positive.
    • Pre- and post-invasive intervention with dobutamine stress iFR testing in addition to relief of symptoms can help guide when to recommend to patients to return to exercise after an invasive intervention on the MB. IVUS imaging shows a highly specific “Half Moon” sign associated with MB—it is unclear why this happens. We can utilize assistance of provocative testing (dobutamine, acetylcholine, rapid A-pacing) to further assess the change in the vessel structure under stress
  • The Myocardial Bridge Study, led by Dr. Joanna Ghobrial, is an ongoing prospective study looking to correlate functional testing of MB (invasive and non-invasive) and long-term clinical outcomes.
• ACAOS – anomalous origination of coronary artery from the opposite sinus
  • Similar to in MB, iFR and IVUS are emerging tools utilized to help risk stratify patients with ACAOS
  • IVUS with concurrent dobutamine stress testing can allow for dynamic assessment of anomalous coronary arteries both at rest and under stress. The effects of physiologicor pharmacological stress on the morphology of the intramural segment of an interarterial coronary can provide additionaldata to guide which patients may warrant surgery – particularly patients with anomalous take-off of the right coronary artery from the left coronary sinus (in which surgical intervention is more controversial).

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