Ascending aorta flow derangement is a marker of outflow obstruction in hypertrophic cardiomyopathy
Journal of Cardiovascular Magnetic Resonance volume 16, Article number: P293 (2014)
Alterations in systolic blood flow are a characteristic finding in patients with obstructive hypertrophic cardiomyopathy (HCM). Echocardiography is the gold standard for hemodynamic assessment in HCM, but this technique is generally limited to measurement of the left ventricular outflow tract (LVOT) pressure gradient and may not provide a complete description of the hemodynamic impact of the disease. In this study, we sought to employ time-resolved, three-dimensional phase contrast (4D flow) MRI to visualize and quantify 3D blood flow patterns in the LVOT and ascending aorta (AAo) in patients with obstructive and non-obstructive HCM.
Obstructive (n = 12) and non-obstructive (n = 18) HCM patients as well as 10 normal volunteers were included in this IRB-approved study. Obstruction was defined as LVOT pressure gradient > 30 mmHg on patients' most recent echocardiography study (ΔPecho). Septal thickness, LVOT diameter and septum/free wall ratio were measured on SSFP cine MRI. 4D flow MRI data analysis included correction for eddy currents and velocity aliasing, followed by flow visualization and quantification in dedicated software (EnSight, CEI, Apex, NC). 3D blood flow patterns within the LVOT and AAo were graded for the presence of helical flow (absent = 0, mild/moderate = 1, severe = 2) by two observers blinded to diagnosis, and the results were averaged. MRI-measured pressure gradient (ΔPMRI) was calculated from the peak systolic 3D blood velocity profile within the LVOT using the simplified Bernoulli equation. (Figure 1) The Mann-Whitney U test was used to compare groups and Spearman's (rS) or Pearson's (r) correlations were used as appropriate.
There was higher grade helical flow in obstructive patients (1.6 ± 0.4) compared to both non-obstructive patients (1.1 ± 0.64, p = 0.04) and controls (0.1 ± 0.31, p < 0.001). Similarly, obstructive patients had higher ΔPMRI(53.8 ± 29.2 mmHg) than non-obstructive patients (33.6 ± 28.6 mmHg, p = 0.048) and controls (10.5 ± 5.6 mmHg, p < 0.001) Non-obstructive patients had higher grade helical flow (p < 0.001) and ΔPMRI(p = 0.004) than controls (Figure 2). In the cohort of patients, helical flow correlated with ΔPMRI(rS = 0.58, p = 0.001) and ΔPecho (rS = 0.46, p = 0.01). A significant correlations was also found between ΔPecho and ΔPMRI(r = 0.41, p = 0.03). Interestingly, ΔPMRI tended to be greater than ΔPecho (mean difference: 10.6 ± 35.3 mmHg). There were no correlations observed between helix grade or ΔPMRIwith septal thickness, average outflow diameter, or septum/free wall ratio.
Our results demonstrate that AAo flow derangement assessed using 4D flow MRI is more severe in obstructive HCM than non-obstructive HCM, and is strongly correlated with LVOT pressure gradient. This finding suggests that flow derangement is a unique marker of disease severity in this population. Further studies are required to evaluate how helical flow correlates with patient symptoms and outcomes in HCM.
NIH NCI 5R25CA132822-04, NIH NHLBI R01HL115828; AHA13SDG14360004.
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Allen, B.D., Choudhury, L., Barker, A.J. et al. Ascending aorta flow derangement is a marker of outflow obstruction in hypertrophic cardiomyopathy. J Cardiovasc Magn Reson 16 (Suppl 1), P293 (2014). https://doi.org/10.1186/1532-429X-16-S1-P293