tortuosity (> 90°), especially when combined with wall calcifications, makes advancing the large sheath problematic, with a high risk of vascular complications that could potentially affect the final outcome. In addition, the existence of extensive circumferential calcifications limits the elastic dilation of the artery; thus, the minimum diameters referred to above are underestimated. Patients who do not meet the criteria of suitable peripheral arterial access may still be candidates for transapical implantation. For the assessment of aortic annulus diameter we should keep in mind that TTE underestimates its size by a mean of 1.4 mm compared to TEE [5, 10], while the latter method also underestimates the size by 1.2 mm compared with intraoperative measurement [10]. Therefore, in order to avoid undesirable and often catastrophic displacement of the prosthesis, there should be a margin of at least 1-2 mm between the diameter of the valve and the size of the aortic annulus estimated using TEE, so that the former may be successfully and safely anchored within the latter. Computed tomography scan aortography and angiography of the ascending aorta are the most appropriate examinations for investigating these aspects. Those examinations will also be used for the measurement of the dimensions of the ascending aorta and the aortic arch, which are essential for checking eligibility for the CoreValve (the most important being the diameter of the ascending aorta, which should be < 4.3 cm). The anatomy of the thoracic aorta (any chance of porcelain aorta) and the abdominal aorta should be studied by some imaging method for the existence of extensive atheromatosis, mural thrombi and aneurysm. Different transcatheter aortic valves On the basis of the first results from clinical trials, the CoreValve ReValving System and Edwards A B Percutaneous aortic valve implantation Lifesciences System obtained the CE mark of approval in 2007 with the specification that these valves are intended for patients with a high or prohibitive risk for surgical valve replacement or who cannot undergo AVR. The first generation balloon expandable valve was called the Cribier- Edwards valve (Edwards Lifesciences Inc), whereas at present the Edwards SAPIEN valve (Edwards Lifesciences Inc) is commercially available (Figure 2). The Edwards Lifesciences SAPIEN THV device is a balloon-expandable valve. It consists of bovine pericardium that is firmly mounted within a tubular, slotted, stainless steel balloon-expandable stent. Two valve sizes have been developed (23 mm and 26 mm). At present, available prosthesis sizes are 23 mm and 26 mm for aortic annulus diameters in the range 18-22 mm and 21-25 mm, respectively. The CoreValve ReValving device is a self-expanding frame-valve prosthesis (Figure 2). It consists of a porcine pericardial tissue valve that is mounted and sutured in a multilevel self-expanding nitinol frame. It is available in 26 mm and 29 mm sizes. The device has a broader upper segment (outflow aspect), which yields proper orientation to the blood flow. The first generation valve used bovine pericardial tissue and was constrained within a 25 French (F) delivery catheter. The second generation valve was built with porcine pericardial tissue within a 21 F catheter to allow access through smaller diameter vascular beds. The third generation of the device features a catheter with a valve delivery sheath size of 18 F and a follow-on shaft of 12 F. Newer devices that have first-in-man application include Paniagua (Endoluminal Technology Research, Miami, FL), Enable (ATS, Minneapolis, MN), AoTx (Hansen Medical, Mountain View, CA), Perceval (Sorin Group, Arvada, CO) Jena (JenaValve Technology, Wilmington, DE), Lotus Valve (Sadra Medical, Campbell, CA), and Direct Flow percutaneous aortic valve (Direct Flow Medical, Inc., Figure 2. Profile of the Edwards SAPIEN Transcatheter Heart Valve (A) and CoreValve Revalving System (B) Arch Med Sci 3, June / 2010 299