Could increased axial wall stress be responsible for the development of atheroma in the proximal segment of myocardial bridges?

[Coronary CT Angiography-Based Mechanomics Predicts Atherosclerotic Plaque Formation in Regions Proximal to Myocardial Bridging]

Objective

To assess with machine learning the predictive value of mechanomics derived from coronary CT angiography (CCTA) for atherosclerotic plaque formation in regions proximal to myocardial bridging (MB) in the left anterior descending coronary artery (LAD).

Methods

This retrospective study included a cohort of patients with MB in LAD and no atherosclerotic plaque formation in LAD as confirmed by two CCTA conducted between January 2007 and April 2021 at our hospital. The interval between the two CCTA examinations was more than 3 months. The primary endpoint was the formation of atherosclerotic plaques in regions proximal to the myocardial bridging. Patient demographic characteristics and clinical risk factors were documented. Then, the patients were matched by age and sex in a 1-to-1 ratio and divided into two groups, those with plaque formation and those without plaque formation. Computational fluid dynamics analysis was performed based on CCTA. Key anatomical parameters of MB, including location, length, depth, and systolic compression index, were meticulously measured on the CCTA images. Mechanomic data were extracted from the region proximal to the MB. A multivariate Cox regression analysis was performed to identify significant features. A random forest algorithm was used to select mechanomic features for subsequent modeling and to assign scores for each patient's mechanomic features. The log-rank test and Kaplan-Meier curves were used to investigate the mechanomic model's predictive performance concerning plaque formation. Additionally, the operator characteristic curves were applied to evaluate how well the model could predict plaque formation across various myocardial bridge subgroups.

Results

A total of 104 patients with LAD MB were recruited. The mean age of the subjects were (54.56±10.56) years and 75.00% (78/104) of them were male. Among them, 52 developed plaque formation over a median follow-up period of 3.0 years. Apart from a smoking history, which was more prevalent in the group with plaque formation than that in the group without plaque formation (21.15% vs. 5.77%, P=0.04), no significant differences between the groups were observed in terms of the other clinical or anatomical characteristics (all P≤0.05). The participants were divided into a training set (n=74) and a validation set (n=30) at a 7∶3 ratio. With the mechanomics model constructed using the random forest algorithm, the patients were classified into a high-score group (≥0.46) and a low-score group (<0.46) based on a cutoff score of 0.46. The mechanomics model achieved a sensitivity of 0.87 (0.58-0.98) and an accuracy of 0.63 (0.44-0.79) in the validation set. The multivariate Cox regression model revealed a strong positive association between mechanomics and plaque formation (hazards ratio [HR]: 10.58; 95% confidence interval [CI]: 3.23-34.64, P<0.001). The log-rank test showed that the high-score group in the mechanomics model was more likely to develop plaques at the proximal regions of the myocardial bridge compared to the low-score group (P<0.001). The area under the curve (AUC) for plaque formation, as predicted by the model, was 0.88 (95% CI: 0.82-0.95) for the entire population, 0.89 (95% CI: 0.82-0.96) for the training set, 0.86 (95% CI: 0.74-0.99) for the validation set, 0.92 (95% CI: 0.86-0.97) for the superficial MB group, 0.86 (95% CI: 0.74-0.98) for the long MB group, and 0.91 (95% CI: 0.83-0.98) for the short MB group.

Conclusion

The mechanomic assessment holds substantial potential as a predictive tool for atherosclerotic plaque formation in regions proximal to MB in LAD.

Prevalence of myocardial bridges in the Mexican population: A morphometric and histological analysis

BACKGROUND

Myocardial bridge (MB) is described as an abnormal band of myocardium covering a variable portion of any coronary artery.

METHODS

The current study explores the presence of MB throughout the coronary arterial system and provides a morphometric description through instrumented dissection of a sample of 100 human hearts. The study shows a higher prevalence of MB in the Mexican population than in previous reports.

RESULTS

In the total sample (n=100), MB was identified in 96% of it. A total of 421 MBs were observed, with a mean of 4.38mm (±0.28) per dissected heart. The most frequently affected vessel is the anterior interventricular artery where a total of 52 MBs were found, of the total sample studied.

DISCUSSION

The high prevalence of MB among Mexican patients could be the result of a genetic association for this population or the neoformation of MB after birth due to lifestyle-associated factors. Further studies are required to better understand the high prevalence of MB among Mexican subjects.

Relationship between Different Degrees of Compression and Clinical Symptoms in Patients with Myocardial Bridge and the Risk Factors of Proximal Atherosclerosis

OBJECTIVE

To explore the relationship between different degrees of compression and clinical symptoms in patients with the myocardial bridge and the risk factors of proximal atherosclerosis.

METHODS

The clinical data of 156 patients with the myocardial bridge who underwent selective coronary angiography in our hospital from December 2010 to December 2015 were collected. The patients were divided into Noble grade I group (102 cases) and Noble grades II-III group (54 cases) according to the degree of mural coronary artery systolic stenosis. According to the results of coronary angiography, 156 patients with the myocardial bridge were divided into an atherosclerosis group (the myocardial bridge combined with atherosclerosis at the proximal end of the myocardial bridge of simple wall coronary artery), 91 cases, and a control group (isolated myocardial bridge), 65 cases. The relationship between different degrees of compression and clinical symptoms in patients with the myocardial bridge was observed, and the logistic regression model was used to analyze the risk factors of proximal atherosclerosis in patients with the myocardial bridge.

RESULTS

The incidence of atherosclerotic stenosis, angina pectoris, and myocardial infarction in the proximal part of the myocardial bridge in the Noble grades II-III group was higher than that in the Noble grade I group (P < 0.05). The differences in age, hypertension, and Noble classification between the two groups were statistically significant (P < 0.05). The differences of total cholesterol (TC) and C-reactive protein (CRP) between the two groups were statistically significant (P < 0.05). Multivariate analysis showed that age, hypertension, Noble grade, and CRP were all risk factors for proximal atherosclerosis in patients with the myocardial bridge (P < 0.05).

CONCLUSION

The more severe the compression of the myocardial bridge, the greater the risk of cardiovascular events for patients and the higher the incidence of atherosclerotic stenosis in the proximal part of the myocardial bridge. In addition, the occurrence of atherosclerosis in the proximal coronary artery of the myocardial bridge may be affected by age, hypertension, Noble grade, and CRP level.

Abnormal shear stress and residence time are associated with proximal coronary atheroma in the presence of myocardial bridging

BACKGROUND

Atheromatous plaques tend to form in the coronary segments proximal to a myocardial bridge (MB), but the mechanism of this occurrence remains unclear. This study evaluates the relationship between blood flow perturbations and plaque formation in patients with an MB.

METHODS AND RESULTS

A total of 92 patients with an MB in the mid left anterior descending artery (LAD) and 20 patients without an MB were included. Coronary angiography, intravascular ultrasound, and coronary physiology measurements were performed. A moving-boundary computational fluid dynamics algorithm was used to derive wall shear stress (WSS) and peak residence time (PRT). Patients with an MB had lower WSS (0.46 ± 0.21 vs. 0.96 ± 0.33 Pa, p < 0.001) and higher maximal plaque burden (33.6 ± 15.0 vs. 14.2 ± 5.8%, p < 0.001) within the proximal LAD compared to those without. Plaque burden in the proximal LAD correlated significantly with proximal WSS (r = -0.51, p < 0.001) and PRT (r = 0.60, p < 0.001). In patients with an MB, the site of maximal plaque burden occurred 23.4 ± 13.3 mm proximal to the entrance of the MB, corresponding to the site of PRT.

CONCLUSIONS

Regions of low WSS and high PRT occur in arterial segments proximal to an MB, and this is associated with the degree and location of coronary atheroma formation.

Haemodynamic impacts of myocardial bridge length: A congenital heart disease

OBJECTIVES

There is an association between long and thick myocardial bridging (MB), haemodynamic perturbations and increased risk of myocardial infarction. This study aims to investigate the alteration in coronary haemodynamics with increasing the length of MB.

METHODS

Angiography and intravascular ultrasound were performed in 10 patients with varying length of MB in the left anterior descending (LAD) artery. In silico models of MB were developed based on the reconstructed three-dimensional model of the LAD. The entire LAD was divided into 3 segments, proximal (pre-bridge), bridge and distal (post-bridge). Transient computational fluid dynamics simulations were performed to derive distribution of blood residence time and wall shear stress (WSS) over entire vessel including proximal, bridge and distal segments.

RESULTS

With increasing the length of MB, a decreasing trend was observed in the WSS over proximal segment whereas an increasing trend was found in the WSS over bridge segment. When patients were divided into 2 groups based on the average length of MB in the whole cohort (L_ave = 23.92 mm), patients with bridges longer than L_ave had smaller WSS and higher residence time in the proximal segment compared to those with bridges shorter than L_ave (0.59 ± 0.31 vs 0.21 ± 0.14 Pa and 0.0021 ± 0.0015 vs 0.0045 ± 0.0021 s). In contrast, patients with bridges longer than L_ave had greater WSS in the bridge segment compared to those with bridges shorter than L_ave (1.37 ± 1.66 vs 2.53 ± 3.14 Pa). No significant difference was found in the distal WSS of patients with short and long bridges.

CONCLUSION

Our findings revealed a direct relationship between the length of MB and haemodynamic perturbations in the proximal segment such that the increased length of MB is associated with decreased WSS and increased residence time.

Development of a Computational Fluid Dynamics Model for Myocardial Bridging

Computational fluid dynamics (CFD) modeling of myocardial bridging (MB) remains challenging due to its dynamic and phasic nature. This study aims to develop a patient-specific CFD model of MB. There were two parts to this study. The first part consisted of developing an in silico model of the left anterior descending (LAD) coronary artery of a patient with MB. In this regard, a moving-boundary CFD algorithm was developed to simulate the patient-specific muscle compression caused by MB. A second simulation was also performed with the bridge artificially removed to determine the hemodynamics in the same vessel in the absence of MB. The second part of the study consisted of hemodynamic analysis of three patients with mild and moderate and severe MB in their LAD by means of the developed in silico model in the first part. The average shear stress in the proximal and bridge segments for model with MB were significantly different from those for model without MB (proximal segment: 0.32 ± 0.14 Pa (with MB) versus 0.97 ± 0.39 Pa (without MB), P < 0.0001 — bridge segment: 2.60 ± 0.94 Pa (with MB) versus 1.50 ± 0.64 Pa (without MB), P < 0.0001). When all three patients were evaluated, increasing the degree of vessel compression shear stress in the proximal segment decreased, whereas the shear stress in the bridge segment increased. The presence of MB resulted in hemodynamic abnormalities in the proximal segment, whereas segments within the bridge exhibited hemodynamic patterns which tend to discourage atheroma development.

Myocardial bridges: A review

Much has been written regarding the potential clinical significance of myocardial bridges. As such bridging is often seen in normal individuals, it is clear that not all arteries bridged by myocardial segments produce clinical symptoms thereby suggesting that this feature may simply be an anatomical variant. However, some authors who have considered these bridges as the cause of cardiac ischemia have suggested two potential mechanisms for their pathophysiology. The first is a phasic systolic compression of the bridged segment with persistent mid-to-late diastolic reduction in arterial diameter and the second proposes a reduction in arterial flow. Both mechanisms may contribute to a reduced reserve in coronary blood flow. In this review, we discuss the evidence that exists regarding myocardial bridging and the potential for bridging to cause myocardial ischemia.

A haemodynamic model for heart-mural coronary artery-myocardial bridge

An experimental model for heart-mural coronary artery-myocardial bridge was established based on the theory of haemodynamics. The application of the model demonstrated that it can repeat to great extent the phenomenon of the myocardial bridge compressing the mural coronary artery, which results in abnormal haemodynamic characteristics. The results of simulation experiments are mostly consistent with clinical research.