How Heart Valves Evolve to Adapt to an Extreme-Pressure System: Morphologic and Biomechanical Properties of Giraffe Heart Valves.

BACKGROUND: Heart valves which exist naturally in an extreme-pressure system must have evolved in a way to resist the stresses of high pressure. Giraffes are interesting as they naturally have a blood pressure twice that of humans. Thus, knowledge regarding giraffe heart valves may aid in developing techniques to design improved pressure-resistant biological heart valves. METHODS: Heart valves from 12 giraffes and 10 calves were explanted and subjected to either biomechanical or morphological examinations. Strips from the heart valves were subjected to cyclic loading tests, followed by failure tests. Thickness measurements and analyses of elastin and collagen content were also made. Valve specimens were stained with hematoxylin and eosin, elastic van Gieson stain, Masson's trichrome and Fraser-Lendrum stain, as well as immunohistochemical reactions for morphological examinations. RESULTS: The aortic valve was shown to be 70% (95% CI 42-103%) stronger in the giraffe than in its bovine counterpart (p <0.001). No significant difference was found between mitral or pulmonary valves. After normalization for collagen, no significant differences were found in strength between species. The giraffe aortic valve was found to be significantly stiffer than the bovine aortic valve (p <0.001), with no significant difference between mitral and pulmonary valves. On a dry weight basis, the aortic (10.9%), pulmonary (4.3%), and mitral valves (9.6%) of giraffes contained significantly more collagen than those of calves. The elastin contents of the pulmonary valves (2.5%) and aortic valves (1.5%) were also higher in giraffes. CONCLUSIONS: The greater strength of the giraffe aortic valve is most likely due to a compact collagen construction. Both, collagen and elastin contents were higher in giraffes than in calves, which would make giraffe valves more resistant to the high-pressure forces. However, collagen also stiffens and thickens the valves. The mitral leaflets showed similar (but mostly insignificant) trends in strength, stiffness, and collagen content.

Subcoronary Stentless Aortic Valves are Not Superior to Supra-Annular Stented Valves Regarding Turbulent Stress.

BACKGROUND: Regions of turbulence downstream of bioprosthetic heart valves may cause damage to blood components, vessel walls, and also to aortic valve leaflets. Stentless aortic heart valves are known to possess several hemodynamic benefits such as a larger effective orifice area and a lower aortic transvalvular pressure difference compared to their stented counterparts. To date, turbulence analysis downstream of a stentless valve prosthesis has been investigated exclusively indirectly, using magnetic resonance imaging or in animal settings only. The study aim was to investigate turbulence using direct Doppler ultrasonography measurements in subcoronary stentless and stented valves in human subjects. METHODS: Either stented pericardial valve prostheses (Mitroflow) or stentless valve prostheses (Solo) were implanted in 15 patients in a randomized fashion. Following surgery, blood velocity was measured in the cross-sectional area downstream of the valves using 10 MHz ultrasonic probes connected to dedicated pulsed Doppler equipment. As a measure of turbulence, Reynolds normal stress (RNS) values were calculated, as well as two-dimensional maps of the turbulence distribution. Preoperative and perioperative data were collected prospectively, and postoperative data retrospectively, from hospital records. RESULTS: The median follow up was 1,624 days. No differences were found in perioperative or postoperative clinical data. Implantation of the Mitroflow valve was significantly faster than that of the Solo valve (p <0.05). Neither was any difference found in the mean or max RNS between the two valve groups. However, the turbulence profiles showed a large variation in the Solo valve compared to the more uniform profiles of the Mitroflow valve. CONCLUSIONS: Comparable turbulent flow values were found between the two valve types, although the Solo group exhibited a large variation in turbulence profiles. As no clear clinical advantages were shown to exist for stentless valves, a normal stented valve should be the first choice in most cases.