Aortic root dynamics and surgery: from craft to science

Abstract

Since the fifteenth century beginning with Leonardo da Vinci's studies, the precise structure and functional dynamics of the aortic root throughout the cardiac cycle continues to elude investigators. The last five decades of experimental work have contributed substantially to our current understanding of aortic root dynamics. In this article, we review and summarize the relevant structural analyses, using radiopaque markers and sonomicrometric crystals, concerning aortic root three-dimensional deformations and describe aortic root dynamics in detail throughout the cardiac cycle. We then compare data between different studies and discuss the mechanisms responsible for the modes of aortic root deformation, including the haemodynamics, anatomical and temporal determinants of those deformations. These modes of aortic root deformation are closely coupled to maximize ejection, optimize transvalvular ejection haemodynamics and-perhaps most importantly-reduce stress on the aortic valve cusps by optimal diastolic load sharing and minimizing transvalvular turbulence throughout the cardiac cycle. This more comprehensive understanding of aortic root mechanics and physiology will contribute to improved medical and surgical treatment methods, enhanced therapeutic decision making and better post-intervention care of patients. With a better understanding of aortic root physiology, future research on aortic valve repair and replacement should take into account the integrated structural and functional asymmetry of aortic root dynamics to minimize stress on the aortic cusps in order to prevent premature structural valve deterioration.

Figure 1

Aortic root radiopaque markers and sonomicrometry crystal placement location. Dagum et al. (1999) and Lansac et al. (2002) placed radiopaque markers and sonomicrometry crystals, respectively, at the nadirs of each aortic cusp (left, right and non-coronary) and on the base and top of each commissure. Lansac et al. also placed three sonomicrometry crystals at the STJ and three at the ascending aorta level. Dagum et al. placed markers at the tip of each aortic leaflet. RFT, right fibrous trigone; LFT, left fibrous trigone. Aortic root illustration adapted from Mihaljevic et al. (2003).

Figure 2

Four modes of aortic root deformation and the anatomy of the aortic base (annulus) and commissures. The aortic annulus is the coronet-like fibrous structure that supports the aortic leaflets, with the commissures attached to the cusps of the coronet. The coronet nadirs lie at the aortoventricular junction. Dagum et al. (1999) confirmed four distinct modes of aortic root deformation: (1) circumferential deformation at the base, (2) circumferential deformation at the level of the commissures, (3) longitudinal deformation and (4a and 4b) shear strain and torsional deformation.

Figure 3

Circumferential deformation of annular sectors ((a) left; (b) right; (c) NC annular sector) and the distance between corresponding pairs of aortic leaflet markers signalling leaflet opening and closure ((d) left and right leaflets; (e) NC and left leaflets; (f) NC and right leaflets) throughout the cardiac cycle. Data are expressed as group mean±s.e.m. Note that base circumferential expansion peaked at the end of IVC (when aortic valve opened) from which it thereafter contracted progressively throughout ejection and IVR (when the valve closed). Furthermore, we observed that the crossing point of the aortic pressure (AoP) and LV pressure tracings ((d–f) straight vertical line) preceded leaflet opening ((d–f) dashed vertical line) by approximately two frames or 33.3 ms.

Figure 4

Aortic root geometry at end-diastole. Data from Dagum et al. and expressed as group mean±s.d. per cent change of deformation (longitudinal, circumferential, shear and torsion) during end-diastole unless specified otherwise. The commissural circumference, computed as the sum of the three inter-marker lengths, and diameter, derived assuming a circular cross section, were 58.9±6.7 and 22.6±2.6 mm, respectively. The base circumference and diameter, similarly computed and derived, were 64.4±6.5 and 24.7±2.5 mm, respectively. The base-to-commissure diameter ratio was 1 : 0.92 (Dagum et al. 1999). Lansac et al. (2002) observed the following diameter ratio between the commissures, STJ and the ascending aorta with respect to the base: 0.68, 0.69 and 0.66, respectively.

Figure 5

Aortic root deformation during IVC. Data from Dagum et al. and expressed as group mean±s.d. per cent change of deformation (longitudinal, circumferential, shear and torsion) during IVC unless specified otherwise. Circumferential expansion of the base was not uniform during IVC (Dagum et al. 1999), being greatest in the left base and least in the non-coronary (NC) base. The circumferential expansion was paralleled by longitudinal elongation. In contrast to the non-uniform expansion of the base, both commissural expansion and longitudinal deformations were uniform during IVC in the left, right and NC regions. Aortic root shear or torsion deformation was not observed during IVC (Dagum et al. 1999). The per cent area change relative to total change over the entire cardiac cycle of the base and commissures during IVC were 40.3±6.1 and 52.7±9.7%, respectively. Lansac et al. observed the following per cent area change during IVC at different levels of the aortic root: base, 51±5%; commissures, 33±3%; STJ, 14±2%; ascending aorta, 6.6±1%.

Figure 6

Aortic root deformation during ejection. Data from Dagum et al. (1999) expressed as group mean±s.d. per cent change of deformation (longitudinal, circumferential, shear and torsion) during ejection unless specified otherwise. Longitudinal deformation was not observed during ejection (Dagum et al. 1999). Contraction of the base during ejection was not homogeneous: the left and right bases contracted significantly more than the base in the NC sector. In addition, the aortic root underwent non-uniform shearing during ejection that resulted in torsional deformation of the root. The left and NC commissures underwent anticlockwise torsion, while the right commissure underwent clockwise torsion (when looking from the aorta and towards the ventricle). The per cent area change of the base and commissures during ejection was −49±8 and 30±11%, respectively (Dagum et al. 1999). Lansac et al. (2002) observed the following per cent area change at each aortic root level during the last two-thirds of ejection: base, −54±2%; commissures, −67±1%; STJ, −68±3%; ascending aorta, −64±3%.

Figure 7

Three-dimensional computer-generated animation snapshots of the aortic root using the radiopaque marker data from Dagum et al.'s (1999) experiment at Stanford. Note that during ejection, the aortic root had a tendency to change from a clover-shaped cone to a more cylindrical shape. This computer simulation of the deformation of the annular fibroskeleton based on the marker data revealed that transformation from clover to cylindrical shape was explained by the geometric changes of the base relative to the commissures. The base underwent circumferential contraction, whereas the commissures continued to expand during ejection and at the point when both were of equal diameter, the aortic root achieved a cylindrical shape.

Figure 8

Shear (torsional deformation) of the aortic root. Data from Dagum et al. (1999) expressed as group mean±s.d. per cent change of deformation (longitudinal, circumferential, shear and torsion) during ejection. Shear deformation of the left, right and NC root was calculated from the triplet of markers (one commissures marker and two nadir markers) defining the aortic root regions. Torsion measured the degree of rotation of the commissures relative to the base caused by shear deformation at each region (when looking down from the aorta towards the ventricle). Black triangle, baseline configuration at end IVC; grey triangle, deformed configuration at the end of ejection.

Figure 9

Aortic root deformations during IVR. Data from Dagum et al. expressed as group mean±s.d. per cent change of deformation (longitudinal, circumferential, shear and torsion) during IVR unless specified otherwise. During IVR, the aortic root underwent further circumferential contraction (Dagum et al. 1999). The greatest circumferential contraction of the annulus occurs at the left base and the least at the NC base. In contrast to asymmetric annular circumferential contraction, the left, right and NC sinuses at the commissures contracted symmetrically. In addition, during IVR, the aortic root sheared and underwent torsional deformation and longitudinal compression. Longitudinal compression of the aortic root was symmetric among the left, right and NC regions of the aortic root (Dagum et al. 1999). The per cent area change relative to the total change over the entire cardiac cycle of the base and commissures during IVR was −41±6 and −65±11%, respectively (Dagum et al. 1999). Lansac et al. (2002) observed the following per cent area change during IVR at different levels of the aortic root: base, −44±4%; commissures, −29±1%; STJ, −14±2%; ascending aorta, −10.9±3.2%.

Figure 10

Aortic root deformations during diastole. Data from Dagum et al. expressed as group mean±s.d. per cent change of deformation (longitudinal, circumferential, shear and torsion) during diastole. During early diastole, the aortic root recoiled from its dynamically loaded configuration at the end of IVR (Dagum et al. 1999). The aortic root expanded circumferentially and elongated longitudinally. This rapid recoil can also be appreciated in figure 3, where circumferential deformations at both the base and the commissures showed a sharp inflection point at the end of IVR. Base expansion during early diastole was asymmetric, with the NC base having the least expansion. The per cent area change relative to total change over the entire cardiac cycle of the base and commissures was 50±5 and −17±3%, respectively. In addition, the aortic root untwisted and exhibited shearing and torsional deformation in a direction opposite to that seen during ejection and IVR (Dagum et al. 1999). Lansac et al. (2002) observed that the dynamics of re-expansion were different at each level during diastole. Although the basal (18±3%) and commissural (4.7±0.9%) areas re-expanded, the STJ (−0.2±0.6%) and ascending aorta (−6.4±2.4%) areas decreased during late diastole.

Figure 11

Annular circumferential per cent deformation with changes in LVP during IVC and IVR. Data are expressed as group mean±s.e.m. Annular circumferential stress–strain response for each of the three segments was nonlinear. Furthermore, analysis of stress–strain deformation showed marked hysteresis from IVC to IVR in the (a) left and (b) right annular sectors, but no hysteresis effect in the (c) NC annular sector.

Figure 12

Commissural circumferential per cent deformation with changes in LVP during IVC and IVR. Data are expressed as group mean±s.e.m. Commissural circumferential stress–strain response for each of the three segments was nonlinear. Furthermore, stress–strain deformation showed marked hysteresis from IVC to IVR in all the three commissural segments. (a) Left sinus, (b) right sinus and (c) NC sinus.