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Ann Thorac Surg 2007;83:954-957
© 2007 The Society of Thoracic Surgeons
Department of Cardiac Surgery, University Clinic of Schleswig-Holstein, Lübeck, Germany
Accepted for publication October 23, 2006.
* Address correspondence to Dr Sievers, Department of Cardiac Surgery, University Clinic of Schleswig-Holstein, Campus Lübeck, Ratzeburger Allee 160, 23538 Lübeck, Germany (Email: h.sievers{at}herzchirurgie-luebeck.de).
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Abstract |
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Methods: Fresh porcine thoracic aortas, including the root, were set up in a mock circulation before and after prosthetic replacement of the ascending aorta. Cyclic changes in aortic dimensions were measured by ultrasonic micrometers at defined positions at the proximal part of the descending aorta. At the same positions, aortic pressures were recorded simultaneously using Millar tip manometers. Wall thickness was measured after pulsatile testing, and the resulting wall tension was calculated from the Laplace law.
Results: After prosthetic replacement of the ascending aorta, peak systolic pressure in the proximal descending aorta increased from 117.6 ± 6.1 mm Hg to 129.2 ± 6.3 mm Hg, resulting in a rise of wall tension by 12.4% ± 4.2% (p = 0.001). The maximum rate of pressure rise (dp/dtmax) increased by 42.6% ± 16.4% (p < 0.001).
Conclusions: Replacement of the ascending aorta with noncompliant prosthetic material significantly increases wall tension and rate of pressure rise in the residual aorta. This may have clinical impact with respect to a sudden and sustained rise of mechanical load, especially at the vulnerable proximal descending aorta.
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Introduction |
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Although the local problem usually is solved after prosthetic ascending aorta replacement, remote aortic surgery is frequently necessary [5, 6]. Reoperations are performed because of recurrent dissections of the ascending aorta and extension of dissections into the aortic arch and new aneurysm formation, or dissection of the descending thoracic aorta [6, 7]. Marfan syndrome, the type of dissection and its distal extent and acuity, and whether the tear was resected, are predictors for subsequent aortic surgery [5, 8].
We hypothesized that the prosthetic graft used for the replacement of the ascending aorta induces an additional pathophysiologic aspect promoting the development of postprocedural abnormalities in the remote aorta. The noncompliant prosthetic material causes a local loss of Windkessel function that may change the aortic hemodynamics; that is, augmentation of pressure amplitude and thereby induction of additional mechanical load to the residual aorta that may be of clinical relevance, especially in the vulnerable proximal part of the descending thoracic aorta. We therefore investigated wall tension in the proximal descending aorta after prosthetic replacement of the ascending aorta in an in vitro model.
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Material and Methods |
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Replacement of the ascending aorta was performed using the reimplantation technique [2]. In brief, the aorta was cut at the height of the first supraaortic branch and resected down to the three sinuses of Valsalva, leaving a rim of sinus tissue of 2 mm to the crown-shaped aortic annulus. Then the aortic valve was implanted into a Dacron (DuPont, Wilmington, DE) prosthesis (Boston Scientific Corp, Wayne, NJ) using interrupted U-shaped 4-0 Prolene suture (Ethicon, Norderstedt, Germany) at the annulus in a planar fashion and a continuous 4-0 Prolene suture for attachment of the valve from inside the prosthesis. Finally, the graft was distally connected to the aortic arch by a continuous 4-0 Prolene suture.
Data Acquisition and Analysis
Aortic diameters were measured by ultrasonic micrometric crystals (Sonometrics Corp, London, Ontario, Canada) placed around the proximal descending aorta at distances of 1 cm and 3 cm from the left subclavian artery. At the same positions, microtip catheter pressure transducers (Millar Instruments Inc, Houston, TX) were inserted into the central lumen of the vessel to determine aortic pressures (Fig 1). After pulsatile testing, wall thickness was measured from excised circumferential pieces of the aorta by a stereomicroscope with eyepiece micrometer (Carl Zeiss GmbH, Göttingen, Germany). The resulting wall tension was calculated from the Laplace law [T = (p x d)/(2 x w)], where T is wall tension; p is pressure; d is diameter, and w is wall thickness.
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Results |
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Notable changes were found for the maximum rate of pressure rise (dp/dtmax), which increased by more than 42% compared with the native aortic condition (632.3 ± 83.4 mm Hg/s versus 446.2 ± 70.1 mm Hg/s, p < 0.001). Representative changes in descending aortic pressure are depicted in Figure 2.
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Comment |
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After prosthetic replacement of the ascending aorta, the Windkessel function of this dominant part of the aorta disappears because of the minimal distensibility of common prosthetic material [10]. Effects of this reduced aortic compliance on left ventricular performance [11] and on aortic root hemodynamics [12, 13] have been described before; however, little is known about the implications on the proximal descending aorta.
As Simon-Kupilik and colleagues [12] have demonstrated for the aortic root, pressure amplitude and diameter increased markedly and calculated wall stress index increased in vitro and in vivo by 22% and 16%, respectively. This can also be confirmed by our experiments for the proximal descending aorta. Furthermore, a significant increase of the pressure-time differential dp/dt could be observed. These findings indicate that in case of prosthetic replacement of the ascending aorta, an acute and sustained additional load is applied to the residual aorta that may promote the development of further aortic pathologies such as aneurysms and type B dissections.
Whether this observation reaches clinical significance will need further investigations. In this context some recent studies reported a higher incidence of descending aortic aneurysm formation in patients who received operations on the ascending aorta for type A dissections [14]. Engelfriet and colleagues [15] described a fourfold increased risk for dilatation of the distal aorta in Marfan patients with previous elective aortic surgery [15].
Typically, the site of tears in acute type B aortic dissection is near the distal circumference of the left subclavian artery. It can be speculated that one weak point of the descending aorta is located in this area, where the aortic arch turns into the descending aorta and the continuity of the aortic arch is interrupted by the left subclavian orifice. The pressure at this site in the convexity of the aortic arch is also somewhat increased compared with that at the concavity owing to centrifugal forces and momentum. During measurements only minor pressure gradients were observed between the inner and outer curvature of the arch.
We used fresh and healthy aortic tissue in our experiments, and wall tensions were calculated from these variables. Diameters are increased in aneurysms, however, and the Laplace wall tension will tend to have even higher values. In patients with risk factors for developing type B dissection, especially in patients with Marfan syndrome or preexisting descending aortic aneurysm, the incidence of rupture of the intimal layer is furthermore increased owing to the structural weakness of the aortic wall [4]. On the other hand, the compliance in the atherosclerotic aorta is decreased compared with our experimental setting. This may attenuate the observed effect of increased pulse pressure amplitude; however, it must be considered that pulse pressure is increased in the stiffer atherosclerotic aorta anyway.
Clinical Implications
It is known that patients undergoing replacement of the ascending aorta need regular follow-up, not only for evaluation of valve function in cases of valve-sparing procedures but also to asses the fate of the remaining aorta. This is especially mandatory in patients with residual pathologies of the aorta, such as Marfan patients or those with a descending aortic aneurysm or remaining dissection [5, 16–18]. Because there is a nonnegligible risk of reoperation in these patients, some authors favor total arch replacement in patients with dissection of the ascending aorta [16]. Others recommend total arch replacement, leaving an "elephant trunk" in patients with Marfan syndrome and aortic dissection [19]. However, the prolonged operation time with these strategies increases the overall operative risk.
The strategy in our department in cases of acute type A aortic dissection is to treat the dissection predominantly with valve-sparing techniques, perform a close follow-up, and electively treat emerging complications. Although the data presented in this study may support the concept of prophylactic arch replacement and leaving an elephant trunk to cover the zone of risk in the proximal descending aorta also in acute cases, we believe the operative procedure in these situations should be kept as short as possible, with close follow-up of patients. However, in patients with elective surgery of the ascending aorta and residual pathologies of the aortic arch or descending aorta, or both, a more liberal approach to a complete surgical treatment of the aortic arch and especially of the proximal descending aorta, may be in favor of improved long-term outcome.
Further studies are necessary to substantiate these considerations. In view of conservative treatment, there is evidence that regulation of blood pressure in Marfan patients, especially the use of a ß-blocker, is of importance to reduce the risk of postprocedural aortic pathologies after replacement of the ascending aorta [20]. Close follow-up of these patients is mandatory, preferably under the surveillance of specialized centers.
Conclusion
The loss of compliance of the ascending aorta abruptly increases wall tension of the proximal descending aorta. Especially in patients with residual pathologies of the descending aorta, these findings may have important implications on the strategy of their follow-up and probably also for replacement of the thoracic aorta.
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This article has been cited by other articles:
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  E. Girdauskas, T. Kuntze, M. A. Borger, V. Falk, and F. W. Mohr Distal Aortic Reinterventions After Root Surgery in Marfan Patients Ann. Thorac. Surg., December 1, 2008; 86(6): 1815 - 1819. [Abstract] [Full Text] [PDF]
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