What defines a small annulus in aortic stenosis?

Definition of Small Aortic Annulus in Aortic Stenosis

A small aortic annulus in aortic stenosis is defined as a mean annular diameter less than 23 mm, though functional definitions also include indexed measurements below 1.0 cm/m² or valve size index (VSI) less than 12 mm/m². 1, 2

Anatomic Measurement Criteria

The definition of small annulus varies depending on the measurement approach used:

• Absolute diameter threshold: Mean annular diameter <23 mm is the most commonly applied cutoff in contemporary practice, particularly for procedural planning 1
• Indexed measurements: An annulus diameter indexed to body surface area <1.0 cm/m² represents a theoretically small aortic root 3
• Functional hemodynamic definition: VSI <12 mm/m², valve area index (VAI) <1.31 cm²/m², or prosthesis orifice diameter <19 mm may indicate the need for annular enlargement procedures 2

Clinical Context and Prevalence

Small aortic annulus is predominantly encountered in elderly women and poses considerable management challenges 4. The condition is particularly relevant because:

• Procedural eligibility: Extremely small annular dimensions can exclude patients from transcatheter aortic valve implantation (TAVI) because available valve sizes may not accommodate the anatomy 5, 6
• Surgical implications: Small annulus often necessitates surgical annular-enlargement techniques (root enlargement, supra-annular prostheses) to avoid patient-prosthesis mismatch 5, 6

Measurement Requirements and Pitfalls

Contrast-enhanced computed tomography angiography (CTA) is the gold-standard modality for annular assessment, providing precise quantification of annular area, perimeter, diameters, and calcium distribution 6. Critical measurement considerations include:

• Three-dimensional geometry: The aortic annulus is elliptical rather than circular in most patients, requiring multiple parameters (area, perimeter, minimum/maximum diameters) obtained through volumetric imaging 5, 6
• Echocardiographic limitations: Two-dimensional echocardiography alone is insufficient for accurate sizing; measurements can be affected by calcification protruding into the left ventricular outflow tract, yielding incorrectly small diameter readings 5
• Measurement timing: Left ventricular outflow tract diameter should ideally be measured in mid-systole at the same cardiac cycle point as maximum velocity, though the systolic frame yielding the largest diameter is a practical alternative when image quality is suboptimal 5

Impact on Treatment Selection

The presence of small annulus directly influences valve replacement strategy:

• TAVI considerations: Large annulus may exceed the size range of currently marketed transcatheter valves, while small annulus requires precise device sizing to reduce complications such as paravalvular leak and annular rupture 5, 6
• SAVR advantages: With small annulus, surgical approaches allow direct visualization and the option for annular enlargement procedures, whereas TAVI operators cannot directly visualize the annulus intra-procedurally 5, 6
• Equivalent outcomes: Recent randomized trial data in patients with severe aortic stenosis and small annulus (mean diameter <23 mm, predominantly women) showed no evidence of superiority between contemporary TAVI versus SAVR in valve hemodynamic results or clinical outcomes at median 2-year follow-up 1

Common pitfall: Overestimation of annular dimensions can lead to prosthesis oversizing, increasing the risk of annular rupture or coronary obstruction; therefore, precise multimodality imaging with CTA-based three-dimensional assessments is required 6.