Learning anatomy is foundational first step in the pathway to become a great clinician. This post is written to address a potential nomenclature issue in cardiac anatomy .
What constitutes Aortic annulus.?
“Aortic annulus” by default conveys a discrete, planar, circular ring. However, from a macroscopic and histological perspective, a distinct, single-plane collagenous ring does not exist at the base of the aortic valve. Rather, aortic annulus is a complex, three-dimensional , Morpho-anatomical functional zone spanning the ventriculo-aortic junction. This brings the aortic leaflets to be arranged in a crown-like formation that crosses multiple spatial planes. The lowest points of these attachments (the nadirs) dip into the left ventricular outflow tract, while the highest points (the commissures) extend superiorly to anchor at the sinotubular junction.
Can anyone see a single plane of Aortic annulus in the above Image ? Note the leaflets , pan whole area from the virtual ring to the ST junction making the annulus a well secluded zone with from ST junction to the virtual lower ring.
Why the misconception ?
The misconception of a flat aortic annulus came from extrapolation of AV valve anatomy to the semilunar valves. In the mitral and tricuspid positions, the leaflets anchor to a continuous, planar fibrous skeleton. Consequently, their hinge lines and leaflet planes remain the same in a reatively single plane throughout the cardiac cycle.
Systolic and diastolic disconnect
This complex anatomy of aortic annulus also results in a clear disconnect between systole and diastole. During systole, the aortic leaflets open vertically like flushed doors, against the sinus walls , while the basal plane vanish.Conversely, during diastole, the leaflets fall medially and down to meet at the center, creating a horizontal plane of coaptation.
Timing of measurement of aortic annlus
We are asked to measure the aortic annulus in peak systole. Ironically at this point of time , every measurement becomes virtual in the aortic annular zone. At least in diastole, the so called virtual lower end becomes real with leaflets hanging down forming a point of reference.
TAVI and Aortic annulus measuerment
The measurement of the aortic annulus has become essential due to the rise of TAVI, as CT scans accurately define the annulus compared to echocardiography, revealing the distinct height of the aortic annulus that was previously unrecognized. read below, for all the relevant measurements before doing a TAVI.
Image source : Jun-Hyok Oh IJC Heart & Vasculature Volume 37, December 2021, 100917
Final message
The perception of aortic annulus as a single anatomical plane in our minds is very firm and it is due to the flaw in the way we learned cardiac anatomy. To make things right, we need to change aortic annulus as the “aortic annular zone” in anatomical literature. This change must be made at the earliest, right from the first clinical years of undergraduate education. Recognizing this is vital for understanding surgical or percutaneous repair or replacement of prosthetic valves.
Postamble (For the fellows)
When someone asks you what is aortic annular size, please ask them to confirm which dimension they are talking about. If they are surprised teach them, there is distinct multiple horizontal (Maximal, minimal) and vertical distance for each annulus.(Which can correlate to a certain extent to the coronary height, the TAVI guys talk about)
References
- Piazzese ED, de Jager SE, van Straten AH, Elenbaas TW, Soliman-Hamad MA. Surgical anatomy of the aortic valve and root—implications for valve repair. Ann Cardiothorac Surg. 2019;8(3):324-331. doi:10.21037/acs.2019.04.09
- Loukas M, Bilinsky S, Bilinsky E, El-Sedfy A, Anderson RH. The anatomy of the aortic root. Clin Anat. 2014;27(5):748-756. doi:10.1002/ca.22295
- de Kerchove L, El Khoury G. Anatomy and pathophysiology of the ventriculo-aortic junction. Ann Cardiothorac Surg. 2013;2(1):87-92. doi:10.3978/j.issn.2225-319X.2012.11.15
Dr.S.Venkatesan MD 