Multimodality Imaging of the Bicuspid Aortic Valve: Echo and Beyond

By Milind Desai, MD, and Lars Svensson, MD, PhD

Although echocardiography is still the mainstay imaging method for evaluating a bicuspid aortic valve (BAV), the vast majority of patients can benefit from multimodality imaging. This article outlines the evolution and key roles of various methods in the context of BAV, one of the most common valvular lesions evaluated in the developed world.

Imaging to assess valve morphology

Understanding valve morphology is important, but precise characterization may not be feasible when assessing a patient with heavily calcified BAV. The Sievers classification is often used to characterize BAV: type 0 for no raphe, type 1 for one raphe, and type 2 for two raphes. True BAVs (type 0) are rare; the most common configuration is a type 1 BAV with left-to-right cusp fusion. Mostly, we describe the site of fusion, and if there are two raphes, then that is usually a unicuspid valve.

Heavily calcified valves may be difficult to image using echocardiography and transesophageal echocardiography. Tomographic imaging — with either CT or MRI — is excellent for evaluating valve morphology and distinguishing between true and functional BAVs, although functional BAV is uncommon in the post-rheumatic fever era.

Quantifying aortic stenosis and measurement of aortic annulus

When quantifying the severity of aortic stenosis (AS), it is important to ensure that pressure gradients across the aortic valve are measured from multiple windows, including apical, right sternal border and suprasternal notch. The highest gradient needs to be recorded. Additionally, it is important to determine stroke volume index, as it provides incremental prognostic value and aids in risk stratification.

Also crucial to assessment of AS is measurement of the aortic annulus and left ventricular (LV) outflow tract (LVOT). It is important to recognize that neither the annulus nor the LVOT is perfectly circular (which the diameter measurement assumes), especially in BAV. 3D reconstruction using either tomographic imaging or transesophageal echo is better for ascertaining the LVOT and annular areas.

Quantifying aortic regurgitation

Although LV size is a critical parameter for determining the severity of chronic aortic regurgitation (AR), other pertinent quantification measurements include vena contracta width, jet width and LVOT height, diastolic flow reversal, pressure half-time, jet density, regurgitant volume and fraction, and effective regurgitant orifice area. LVOT and annular diameter are important measurements for determining the likelihood of BAV repair (Ann Thorac Surg. 2014;97:1539-1547).

Most recent guidelines to determine AR severity are based primarily on echocardiography, and a comprehensive assessment incorporating LV dimensions and AR quantification is essential. It needs to be kept in mind that we should typically not diagnose chronic severe AR in the setting of a normal LV size.

A hierarchical grading system can help determine chronic AR severity, using the following parameters:

• LV size and volume index
• Vena contracta width
• Holodiastolic flow reversal
• Ratio of jet width to LVOT
• Ejection fraction

Severity can then be graded as:

• Chronic severe AR — dilated LV, with one or more parameters in the severe range
• Chronic moderate AR — normal LV index with one or more parameters in at least moderate range
• Chronic mild AR — normal LV index with one or more parameters in the mild range

For acute AR, LV size is not a distinguishing factor, because even severe disease can occur in the setting of normal LV size.

Cardiovascular MR for assessing AR

AR is typically evaluated with phase-contrast cardiovascular MR (PC-CMR) studies, which measure forward volume, regurgitant volume, quantification of stroke volume and regurgitant fraction. One must be aware of this technology’s limitations to use it effectively.

Knowing the plane of imaging prescribed (e.g., sinotubular junction or mid-ascending aorta) is essential, as errors can occur, especially if the plane is close to the annulus. Translational valve motion can produce undersampling of flow, and arrhythmias and prominent LV trabeculations can also affect results. PC-CMR is not valid if the patient has multiple lesions or intracardiac shunts.

Tomographic imaging for assessing aortopathy

The Z-score — derived from transthoracic echocardiography using diameters from leading edge to leading edge during diastole — is traditionally used to evaluate the aorta. But there are several limitations with this method:

• It does not account for eccentric dilation of the aorta.
• For adults, Z-scores are available only for the aortic root and not the ascending aorta.
• Body surface area is significantly affected by obesity.

Because of these issues, tomographic imaging is critical for assessing aortopathy, and we have suggested indexing aortic root area to height. This was validated by our group in a study of nearly 1,000 patients in which we found that a root-area-to-height ratio ≥10 cm2/m was more strongly associated with long-term cardiovascular mortality than was aortic diameter (Circ Cardiovasc Imaging. 2017;10:e006249).

Professional guidelines differ somewhat regarding size thresholds for surgery. Recommendations from the American College of Cardiology/American Heart Association call for a threshold diameter of 5.5 cm in patients with BAV without risk factors (5.0 cm with risk factors), but in patients with a short stature and intermediate aortic root/ascending aortic diameters, we also incorporate the cross-sectional area-to-height ratio in our decision-making process. Centers of excellence also operate at a size greater than 5.0 cm (Circulation. 2016;133:680-686).

Global longitudinal strain for AS and AR

There is emerging data on use of LV global longitudinal strain (LV-GLS) in prognostication in the setting of chronic severe AR. Cleveland Clinic has conducted a series of studies evaluating the role of LV global longitudinal strain for AS and AR outcomes:

• LV strain was found to be independently associated with mortality and to have incremental prognostic value in patients with moderate-severe or severe AS and preserved ejection fraction (EF) (Circ Cardiovasc Imaging. 2014;7:938-945). In a follow-up multicenter meta-analysis, of which Cleveland Clinic was a part, an LV-GLS worse than approximately –14% was associated with significantly reduced longer-term survival (JACC Cardiovasc Imaging. 2019;12:84-92).
• Our experience with 208 patients who underwent aortic valve replacement (AVR) for severe symptomatic AS (and baseline LVEF ≥ 50%) revealed that most cases resulted in preserved EF and better strain values. But patients with continued abnormal strain postoperatively had poorer outcomes, suggesting that strain may be a more sensitive prognostic marker than LVEF (JAMA Cardiol. 2016;1:494-496).
• In recently published data on more than 1,000 asymptomatic patients with severe chronic AR and preserved LVEF, we have demonstrated that an LV-GLS threshold of –19% provided incremental prognostic utility for longer-term survival (JACC Cardiovasc Imaging. 2018;11:673-682).
• In patients with preserved LVEF who underwent aortic valve surgery for asymptomatic chronic AR, a baseline strain value of less than –19% was associated with reduced survival, and those with persistently impaired strain at 3- to 12-month follow-up had increased mortality (JACC Cardiovasc Imaging. 2019 Feb 11 [Epub ahead of print]).

Stress echocardiography for asymptomatic AS and AR

Stress echocardiography can help stratify risk in asymptomatic patients with severe AS and preserved LVEF. Our experience in 533 such patients has shown that a lower percentage of age- and sex-predicted metabolic equivalents and slower heart rate recovery are associated with longer-term mortality, whereas AVR is associated with improved survival (Circ Cardiovasc Imaging. 2016;9:pii:e004689).

Similarly, stress echocardiography has been demonstrated to have incremental utility in asymptomatic patients with chronic severe AR (Circ Cardiovasc Imaging. 2014;7:352-362).

4D MRI flow patterns: Not ready for prime time

Different 4D MRI flow patterns have been associated with various BAV characteristics, such as right-noncoronary fusion vs. right-left fusion. So far, however, no association between different flow patterns and outcomes has been found.

The bottom line for BAV management

Knowing when to operate for BAV is critical, and the techniques discussed above can help with that decision. Cleveland Clinic’s experience with 1,890 consecutive patients with BAV from 2003 to 2007 found that concomitant aortopathy was common. Undergoing AVR or repair (with or without concomitant aortic surgery) was associated with a significantly lower incidence of death and dissection (J Thorac Cardiovasc Surg. 2016;151:1650-1659).

Our data have shown that, in patients with BAV needing valve surgery, the aorta should be replaced if more than 4.5 cm in diameter (J Thorac Cardiovasc Surg. 2011;142:622-629). For patients with BAV undergoing surgery primarily for an enlarged aorta, we have reported a 0.25% mortality risk (Ann Thorac Surg. 2015;100:1666-1673) and shown that the aorta should be replaced when the aortic root is larger than 5.0 cm or the ascending aorta is larger than approximately 5.2 cm. In young patients with BAV and regurgitation without calcification, the BAV can be repaired in about 90% of patients with better than 91% freedom from reoperation at 10 years (Ann Thorac Surg. 2014;97:1539-1547).

An additional study of 1,417 Cleveland Clinic patients with grade III or greater chronic AR and preserved LVEF showed that long-term survival significantly improved following aortic valve surgery, with outcomes much better than those for patients not operated on. Patients who underwent surgery fared as well as the normal age- and gender-matched U.S. population (J Am Coll Cardiol. 2016;68:2144-2153).

Bottom line? Patients with BAV have the best long-term outcomes when their imaging evaluation is focused on (1) accurate assessment of the degree of AS and/or AR, along with assessment of concomitant aortopathy, and (2) appropriate referral for surgery at an experienced center.

Dr. Desai is Professor of Medicine at Cleveland Clinic Lerner College of Medicine and Medical Director of Cleveland Clinic’s Aorta Center. Dr. Svensson is a cardiothoracic surgeon and Chair of Cleveland Clinic’s Miller Family Heart & Vascular Institute.