Replacement of the Proximal Arch

Bicuspid aortic valve (BAV) is the most common congenital heart anomaly: it occurs in 0.9% to 2% of the population, or approximately 4 million people in the United States. It is the most common cause of aortic valve disease in patients younger than age 70.

Aortopathy of Bicuspid Aortic Valve

Abnormalities of the aorta in patients with BAV were first reported in 1927, when the association between BAV and aortic dissection was noted. Bicuspid aortic valve increases the risk of aortic dissection 9-fold over that in the general population. Aortic dissection occurs at a younger age in BAV, and frequently in the presence of a hemodynamically normal BAV. Given the well-recognized association of BAV with proximal aneurysm, for many years a hemodynamic model was put forth to explain aneurysm formation. This line of thought postulated that so-called poststenotic dilation occurred because of a jet of flow through a stenotic bicuspid valve and its effect on the proximal aorta. Many lines of evidence in recent years have contradicted this view. Data from echocardiographic analysis of patients with BAV, when compared with those from subjects with tricuspid aortic valves (TAVs), revealed larger dimensions at the sinus of Valsalva, the sinotubular junction, and the ascending aorta, irrespective of the hemodynamic function of the valve. Serial echocardiographic studies have shown higher rates of growth at all levels of the proximal aorta above the annulus in patients with BAV in comparison with TAV. One study found significantly higher dimensions in the aortic arch in BAV patients than in TAV patients without significant valvular dysfunction. A study in a pediatric population found statistically higher dimensions at all levels of the proximal aorta from the annulus through the ascending aorta in all ages from 0 to 19 years. Furthermore, there was a statistically higher rate of growth at the aortic root, and most specifically in the ascending aorta, than in control subjects. Microscopic studies of aortic specimens taken at surgery from BAV patients, from Marfan patients, and from control subjects have shown that the media of the aorta above the BAV is consistently abnormal, irrespective of the hemodynamic function of the valve. The pathogenesis of this aortopathy currently suggests a genetic cause, wherein the extracellular matrix and microfibrillar proteins are deficient. In surgical samples that have been analyzed, fibrillin-1 is reduced in the aorta and pulmonary artery of BAV in comparison with TAV. Furthermore, matrix metalloproteinase-2 is increased more than 2-fold in BAV, and the increase correlates with the aortic diameter. Histologic studies of aortic samples from BAV and TAV patients have revealed more pronounced cystic medial necrosis, elastic fragmentation, and disorientation of smooth muscle fibers in BAV aortas. Investigators in one study have proposed that premature medial-layer smooth-muscle-cell apoptosis is the mechanism underlying aortic aneurysm formation in patients with aortic valve malformations. Increased apoptosis was present in BAV aortas irrespective of size, and in TAV aortas only with aortic dilation.

Surgical Implications

In consideration of this information regarding a congenital predisposition to aneurysm formation in BAV, what are the surgical implications of aortic disease in BAV? Indications for intervention in BAV for aortic stenosis (AS) or aortic insufficiency (AI) are well established. When should the aortic root, ascending aorta, or proximal aortic arch be treated surgically? Surgical studies have suggested that patients with BAV and moderate dilation of the ascending aorta (>4.5 cm) have an increased incidence of aortic complications (aneurysm, dissection, and sudden death), and that serious consideration should be given to replacing the ascending aorta in patients with BAV when it is 4.5 cm or greater in diameter. Data from the International Registry of Acute Aortic Dissection show that aortic diameter is not a good predictor of type A aortic dissection. The average maximal aortic diameter at dissection in nearly 600 patients in the registry over a 10-year period was 5.3 cm. Fully 60% of patients sustained dissection at a diameter of less than 5.5 cm, and 40% at a diameter of less than 5 cm. This has led many surgeons to recommend surgery for aneurysm of a BAV at 5 cm as a primary indication, and at 4 to 4.5 cm during surgery for AS or AI that is due to BAV.

Indications for aortic surgery, either at the time of surgery for AS or AI associated with BAV or primarily because of aortic enlargement, are emerging and becoming more standardized. Surgical approaches to aortic aneurysm in BAV must be tailored to the segment(s) of the aorta that are involved. Reduction aortoplasty and Dacron-graft aortic replacement have both been successfully applied to the ascending aortic segment. For aortic root involvement, composite-valve graft repair—and, more recently, valve-sparing aortic root replacement—have proved to be equally effective in capable hands.

Specifically with respect to the aortic arch, Miller's group at Stanford has presented data suggesting that the affected aorta in BAV includes the aortic arch. Conversely, Sundt's group, on the basis of follow-up of their patients at the Mayo Clinic, has recommended a selective approach to arch replacement in BAV: if the diameter of the proximal or mid-arch exceeds 4.5 cm, it should be replaced at the time of surgery on the more proximal aorta, the aortic valve, or both.

Summary

Multiple lines of evidence establish a relationship between BAV and proximal aortic aneurysms. Emerging indications and techniques are leading to a more standardized surgical approach to these patients. In the individual patient, however, one must use judgment to determine the best approach on the basis of several factors, including age, comorbidities, extent of concomitant surgery, and the expected risk of subsequent complications.