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Regarding “Thrombus within an aortic aneurysm does not reduce pressure on the aneurysm wall”

2001; Elsevier BV; Volume: 33; Issue: 3 Linguagem: Inglês

10.1067/mva.2001.111750

1097-6809

William R. Mower, William Quinones,

Vascular Procedures and Complications

We have carefully reviewed the recent article by Schurink et al, examining the ability of thrombus to reduce pressure on aneurysm walls, and are concerned about the methodology used in the study as well as the authors' conclusions.1Schurink GWH van Baalen JM Visser MJT van Bockel JH Thrombus within an aortic aneurysm does not reduce pressure on the aneurysm wall.J Vasc Surg. 2000; 31: 501-506Abstract Full Text Full Text PDF PubMed Scopus (150) Google Scholar In the article, the authors use Pearson's correlation coefficient and Wilcoxon signed rank test to compare blood pressure measurements obtained from radial arteries against pressures measured within an aneurysm thrombus. The authors conclude that their high correlation coefficients and large P value indicate that pressure within a thrombus is unchanged from aortic pressure, and that thrombus therefore cannot reduce wall stress and reduce risk of rupture. Our first concern is that the statistics used in this analysis are inappropriate and interpreted incorrectly. Correlation coefficients assess relatedness and are not useful in detecting differences.2Dawson-Saunders B Trapp RG Basic and clinical biostatistics.2nd ed. Appleton and Lange, Norwalk (CT)1994Google Scholar The high correlation coefficients found in this study merely indicate that patients with high radial artery pressures are also likely to have high thrombus pressures. These indices provide no information on pressure differences or the ability of thrombus to reduce pressure. While the Wilcoxon signed rank test is appropriate for examining differences, it is unsuitable for assessing the absence of a difference. Furthermore, a P value of.46 does not signify a lack of difference, as implied by the authors, but indicates that their sample size (n = 9 cases) is too small to make meaningful conclusions. The authors also use radial artery pressures as surrogate measures for aortic lumen pressures. This is inappropriate because peak pressure, mean pressure, and the pressure pulse are all known to change significantly during arterial transmission.3Latham RD Westerhof N Sipkema P et al.Regional wave travel and reflections along the human aorta: a study with six simultaneous micromanometric pressures.Circulation. 1985; 72: 1257-1269Crossref PubMed Scopus (347) Google Scholar The investigators would need to directly compare pressure in the abdominal aortic lumen with pressure in the thrombus if they wish to assess whether any differences exist. The authors also incorrectly assert that “wall stress is defined by blood pressure, the diameter of the vessel, and the wall thickness.” Their assertion is a restatement of the law of Laplace and only applies to perfect cylinders or spheres with infinitely thin walls. This simple relationship fails when applied to the complex geometries that characterize most aneurysms.4Elger DF Blackketer DM Budwig RS Johansen KH The influence of shape on the stress in model abdominal aortic aneurysms.J Biomech Eng. 1996; 118: 326-332Crossref PubMed Scopus (109) Google Scholar Furthermore, the ability of thrombus to reduce stress relates to its ability to sustain tensile loads and absorb stresses from the aneurysm wall, and not from its ability to shield the aneurysm wall from intraluminal pressures.5Mower WR Quinones WJ Gambhir SS The effect of intraluminal thrombus on abdominal aortic wall stress.J Vasc Surg. 1997; 26: 602-608Abstract Full Text Full Text PDF PubMed Scopus (168) Google Scholar Finally, the protective effects of thrombus, if any, are still unclear. It appears that thrombus provides some degree of mechanical stress reduction (a protective effect), but may still be associated with an increased propensity for rupture. The increased risk of rupture may be due to nonstructural effects of the thrombus (for example, blocking the flow of blood, oxygen, and nutrients to the aneurysm wall). Alternatively, thrombus formation may preferentially occur in aneurysms that have abnormal walls that not only trigger thrombosis, but are also mechanically weaker than nonthrombogenic walls. The ultimate relevance of intraluminal thrombus clearly requires more study. We would like to stress the need for carefully designed investigations that employ valid methods, appropriate statistical analyses, and thoughtful conclusions. 24/41/111750