Effect of hinge gap width on the microflow structures in 27-mm bileaflet mechanical heart valves

BACKGROUND AND AIM OF THE STUDY: Most bileaflet mechanical heart valves (BMHVs) incorporate some retrograde flow through their hinge mechanism to prevent flow stasis and inhibit microthrombus formation. This reverse flow is characterized by high velocities and shear stresses, thereby promoting platelet activation and hemolysis inside the hinge region. In the present study, the thromboembolic potential of three 27-mm BMHVs with varying hinge gap widths was assessed via in-vitro characterization of the hinge microflow structures. METHODS: Three 27-mm BMHV prototypes with different hinge gap widths (50, 100, and 200 microm) were provided by St. Jude Medical Inc. The valves were mounted in the mitral position of a left heart flow simulator, and two-dimensional laser Doppler velocimetry was used to measure the hinge velocity fields. RESULTS: All three valve prototypes revealed Reynolds shear stress (RSS) levels above 2000 dynes/cm2, which exceeded the threshold for platelet activation and hemolysis. The hinge flow fields were characterized by leakage jets during systole, and a strong vortical flow during diastole. The leakage jet size and corresponding RSS levels were found to increase with the hinge gap width. All three gap widths had RSS >4000 dynes/cm2 (range: 5640 to 13,315 dynes/cm2). The hinge with the smallest gap width registered the highest jet velocity magnitude (2.08 m/s) during systole. CONCLUSION: The study results showed that the hinge gap width influences washout and RSS levels inside the hinge recess. The 100-microm hinge gap width provided optimum fluid dynamic performance. In contrast, the two valves with large and small hinge gap widths may have higher thromboembolic potential.     

Comparison of Substance Use and Risky Sexual Behavior Among a Diverse Sample of Urban,HIV-Positive Men Who Have Sex with Men

ABSTRACT

The purpose of this study was to examine substance use among a racially and ethnically diverse group of HIV-positive men who have sex with men (MSM) living in six U.S. cities, model associations between drug use and serodiscordant unprotected anal intercourse (SDUAI), and characterize users of the substances strongly associated with risky sexual behavior. Baseline questionnaire data from 675 participants of the Positive Connections intervention trial were analyzed. Overall, substance use was common; however, the highest percentage of stimulant (30%), methamphetamine (27%), and popper (i.e., amyl nitrite) (46%) use was reported among white MSM and crack/cocaine (38%) use was highest among African American MSM. Popper use versus non-use (odds ratio = 2.46; 95% confidence interval = 1.55–3.94) and condom self-efficacy (1 standard deviation (sd) increase on scale; odds ratio = .58; 95% confidence interval = .46–.73) were significantly associated with SDUAI after adjusting for key demographic and psychosocial factors. These results highlight the importance of addressing drug use in the context of sex for possible HIV transmission risk.     

Characterization of Hemodynamic Forces Induced by Mechanical Heart Valves: Reynolds vs. Viscous Stresses

Bileaflet mechanical heart valves (BMHV) are widely used to replace diseased heart valves. Implantation of BMHV, however, has been linked with major complications, which are generally considered to be caused by mechanically induced damage of blood cells resulting from the non-physiological hemodynamics environment induced by BMHV, including regions of recirculating flow and elevated Reynolds (turbulence) shear stress levels. In this article, we analyze the results of 2D high-resolution velocity measurements and full 3D numerical simulation for pulsatile flow through a BMHV mounted in a model axisymmetric aorta to investigate the mechanical environment experienced by blood elements under physiologic conditions. We show that the so-called Reynolds shear stresses neither directly contribute to the mechanical load on blood cells nor is a proper measurement of the mechanical load experienced by blood cells. We also show that the overall levels of the viscous stresses, which comprise the actual flow environment experienced by cells, are apparently too low to induce damage to red blood cells, but could potentially damage platelets. The maximum instantaneous viscous shear stress observed throughout a cardiac cycle is <15 N/m². Our analysis is restricted to the flow downstream of the valve leaflets and thus does not address other areas within the BMHV where potentially hemodynamically hazardous levels of viscous stresses could still occur (such as in the hinge gaps and leakage jets).     

乳腺癌的不输血手术治疗

目的：探讨乳腺癌不输血手术治疗的可行性,方法：我院１９９４年１月至２０００年６月,３２１例乳腺癌患者施行乳腺极治性切除术,不预输血。其中根治术１９６例,改良根治术１２１例,双侧根治术４例,在创口合、体质状况恢复等方面进行分析。结果：３２１例乳腺癌患者行乳房根治术治疗,平均每例失血约１８５ｍｌ,仅有４３例用代血浆（血安定,贺期）,无１例输血和使用血制品,平均术后住院天数１１．８天,与围手术期输血者相比,在创口愈合,术后并发症,体质恢复等方面无明显差异,结论：对乳腺癌患者施行不输血乳房根治性切除术是可行的。