A high-resolution radiation hybrid map of the human genome draft sequence.

We have constructed a physical map of the human genome by using a panel of 90 whole-genome radiation hybrids (the TNG panel) in conjunction with 40,322 sequence-tagged sites (STSs) derived from random genomic sequences as well as expressed sequences. Of 36,678 STSs on the TNG radiation hybrid map, only 3604 (9.8%) were absent from the unassembled draft sequence of the human genome. Of 20,030 STSs ordered on the TNG map as well as the assembled human genome draft sequence and the Celera assembled human genome sequence, 36% of the STSs had a discrepant order between the working draft sequence and the Celera sequence. The TNG map order was identical to one of the two sequence orders in 60% of these discrepant cases.

Late strut fracture of the Beall model 105 disc valve prosthesis.

Strut fracture, with embolization of the disc occluder, caused the death of a 64-year-old man who had a Beall model 105 heart valve prosthesis in the mitral position for 13 years. Scanning electron microscopy of the fractured surface revealed evidence of a fatigue failure mechanism in the metal wire. The case is unique in that strut fractures affecting this prosthesis had only been observed previously in the range of 141 to 342 days after implantation. Morphological changes in this valve prompted reexamination of three other model 105 prostheses that had been recovered from the mitral area at necropsy or surgery 9 to 10 years after insertion in 1972 or 1973. All had been kept in dry storage. The three prostheses and the valve described above showed previously unrecognized cracks in the pyrolytic carbon coating of the struts, which form the cage that limits occluder movement. The defects were located at or near the base of struts, where they entered the sewing ring and were bent to pass into the strut supporting ring. We believe that the cracks in the carbon coating precede total strut fracture and postulate that they are needed for the metal wire to be subject to a fatigue failure mechanism.

A numerical analysis of the backflow between the leaflets of a St Jude Medical cardiac valve prosthesis.

Clinically significant, unexplained hemolysis has been reported to occur in St Jude Medical (SJM) cardiac valve prostheses. The leakage phase of backflow is identified as having the most hemolytic potential, when compared with the other phases of the cardiac cycle. A two-dimensional, laminar, constant fluid property finite element analysis (FEA) is used to calculate the peak shear stress obtained in the flow through the narrow slit formed by the closed leaflets of an idealized size 29 mm (tissue annulus diameter) SJM cardiac valve prosthesis, during the leakage phase of backflow. The flow geometry was such that the simple laminar flow theory (SLFT) could be used to model the flow through the region of peak shear stresses with reasonable accuracy. The SLFT predicts that the maximum shear stress depends upon the thickness of the clearance space and the average velocity through that space. These results suggest that the magnitude of the peak shear stress is of the order of 700-1000 Pa for a duration of the order of 0.5-0.4 ms for pressure drops across the valve of 150-300 torr. This suggests that hemolysis is possible for certain unfortunate combinations of clearances and pressure conditions. However, further research is needed before this flow phenomenon can be associated with the reported clinical hemolysis.

A note on the critical flow to initiate closure of pivoting disc mitral valve prostheses.

Newton's second law of motion for rotating bodies and potential flow theory is used to mathematically model the closing process of a pivoting disc prosthetic heart valve in mitral position. The model predicts closure to be dependent upon disc curvature, eccentricity, mass, diameter, density, opening angle and fluid properties. Experiments using two commercially available prostheses are shown to give good correlation with the theory for large opening angles. Divergence between theory and experiment occur at small opening angles because of the limitation of the potential flow assumption.

Estimation of the abrasive wear coefficient in Lillehei-Kaster cardiac valve prostheses.

An approximate hemodynamic theory, which predicts the opening dynamics of the Lillehei-Kaster heart valve, is used in conjunction with an abrasive wear model to predict the wear process on the shields. The hemodynamic theory predicts markedly different opening dynamics between the mitral and aortic positions and is shown to give excellent correlation with the experimental results of other investigations. The abrasive wear model is also shown to give excellent correlation with the experiments of others when the abrasive wear coefficient is taken as k = 6.4 X 10(-6). The theoretical results of this effort and the experimental data from clinical explants of other investigators is used to predict that occluder dislodgement is unlikely in less than 90 yr for either the mitral or aortic positions (for a mean cardiac output of 3.8 l.min-1 and a mean heart rate of 70 beats min-1).

Role of stress concentration in the pathogenesis of cardiac rupture following acute myocardial infarction.

OBJECTIVE: To study the cause of myocardial rupture and its related complications using a computer-assisted model of left ventricular (LV) function following acute myocardial infarction. DESIGN: A model previously described by other authors was modified. The LV was portrayed as a three-layered ellipsoid at end-diastolic volume (135 mL). The aspect ratio of the ellipsoid's semi-minor: semi-major axis was 0.6. The apical middle layer was infarcted with an angle of damage of 40%, the infarcted layer being 81.8% of total LV wall thickness. A one-half symmetry condition generated an axisymmetrical, linear, elastic finite element model with 1056 first-order elements and 1127 nodes. The endocardial surface was subjected to static internal pressure, modelling the instant of end-isovolumetric contraction. Noninfarcted myocardium was assumed to be variably stiffer than infarcted muscle. A simple orthotropic model was used to approximate the directional characteristics of muscle layers. RESULTS: Maximum von Mises stresses were found on the endocardial surface near the centre and edge of the angle of damage, the latter generally observed as the site of myocardial rupture. Static stress concentration factors were computed for the isotropic and orthotropic cases. The directional characteristics of the myocardium appeared to be protective. CONCLUSIONS: A more complete pathogenic model for myocardial rupture following acute infarction is stress concentration --> endocardial tear --> dissecting hemorrhage. Marked deformation is predicted in the endocardial surface within the angle of damage, consistent with the correlation between acute myocardial infarction and LV aneurysm formation.

Design considerations for the omniscience pivoting disc cardiac valve prosthesis.

Parametric studies are conducted in an effort to determine the optimum curvature and eccentricity for concave-convex pivoting disc prosthetic heart valves. Steady-state pressure measurements with prototype valves and laser-Doppler anemometer measurements of the velocity field with a two-dimensional model are presented. These results suggest that an optimum curvature exists and that the minimum eccentricity that promotes full opening should be used. Estimates of the shear stress distribution in the wake of the valve are computed. The maximum shear stress for both the major and minor outflow areas appear to be: 1) fairly evenly distributed, 2) well below the level for damage to the formed elements of blood as they pass through the valve, and 3) above the level required for the formation of a stable thrombus.

A comparative flow study of the Omniscience and the Björk-Shiley cardiac valve prostheses.

The pressure-flow rate characteristics of two new convex-concave pivoting disc prosthetic heart valves are evaluated in vitro. The Omniscience and Björk-Shiley prostheses are chosen since both use curved occluders with similar dynamic function, but with different occluder curvature and eccentricity. The Omniscience prosthesis is shown to have consistently less pressure loss, and therefore less energy loss, over a range of steady state and pulsatile flow rates. The enhanced flow characteristics of the Omniscience valve are attributed to the larger opening angle of the occluder, the method of retaining the occluder, in the valve frame, the smaller curvature of the occluder, the smaller pivot-axis eccentricity, and the smaller profile thickness of the occluder. The valves are also tested in the closed position, with steady state pressure gradients causing leakage or backflow. This data is compared with the pulsatile pressure-flow rate data to gain insight into the nature of regurgitation. The total regurgitation of the pivoting disc valves are thought to be strongly dependent upon both the maximal opening angle of the occluder and the radial clearance between the disc and the valve orifice. Both the Omniscience valve and the Björk-Shiley valve are found to have nearly equal volumes of total regurgitation.

A note on the leakage of the new Omniscience/tm pivoting disk prosthetic heart valve.

Measurements of the steady-state leakage of the new Medical Incorporated Omniscience/tm cardiac valve prosthesis are presented. These results are compared to those obtained from the SJM bi-leaflet cardiac valve/tm. The new Omniscience/tm valve shows significantly less leakage than the bi-leaflet valve. Consideration is given to problems in similitude as well as to the general nature of the flow field.

Estimation of the rotational undamped natural frequency of bileaflet cardiac valve prostheses.

The angular momentum balance is solved numerically for a size 29 mm CarboMedics prosthetic heart valve. The lift force is estimated from potential flow theory, while the drag force is estimated from the lift force and a blunt body empiricism. Buoyancy and gravitational effects are calculated based on the assumption of homogeneous leaflets. Other assumptions include uniform flow, negligible friction at the pivot axis, negligible viscous damping and fluid inertance, and a symmetry flow condition. Oscillations are predicted in the opening dynamics in the range of 2-25 Hz, for flow rates through one-half of the orifice in the range of 0.1-10.0 l/min. The frequency of these oscillations is dependent upon the orientation of the leaflet in relation to the gravitational field and the magnitude of the flow rate. In vivo and in vitro measurements by other investigators demonstrate similar effects of gravity and flow rate, with flutter frequencies of the order of 10-100 Hz. Excitation frequencies, based on vortex shedding, are estimated to be of the order 2-200 Hz, for the range of flow rates of the theoretical model. These results suggest that the natural frequency of this rotational second order system may, in theory, be a contributing factor to the flutter observed in bileaflet cardiac valve prostheses. The clinical significance of this finding is yet to be established.

Identification of capsule-forming Bacillus anthracis spores with the PCR and a novel dual-probe hybridization format.

Anthrax is a fatal infection of humans and livestock that is caused by the gram-positive bacterium Bacillus anthracis. The virulent strains of B. anthracis are encapsulated and toxigenic. In this paper we describe the development of a PCR technique for identifying spores of B. anthracis. Two 20-mer oligonucleotide primers specific for the capB region of 60-MDa plasmid pXO2 were used for amplification. The amplification products were detected by using biotin- and fluorescein-labeled probes in a novel dual-probe hybridization format. Using the combination of PCR amplification and dual-probe hybridization, we detected two copies of the bacterial genome. Because the PCR assay could detect a minimum of 100 unprocessed spores per PCR mixture, we attempted to facilitate the release of DNA by comparing the effect of limited spore germination with the effect of mechanical spore disruption prior to PCR amplification. The two methods were equally effective and allowed us to identify single spores of B. anthracis in PCR mixtures.

Metal wear in Lillehei-Kaster heart valve prostheses.

Ten Lillehei-Kaster heart valve prostheses, in situ for up to 10 years and recovered at surgery or necropsy, were examined by light and scanning electron microscopy. All showed metal wear on the luminal aspect of their struts. The volume of wear related to the duration a prosthesis had been in situ. The worn metal showed distinct, transverse surface corrugations, which became more obvious with time. Aortic prostheses wore more and faster than mitral ones. One strut usually showed more wear than the other, a change likely due to specific manufacturing methods. It is believed that the pattern of wear is caused by a velocity-controlled stick-slip abrasive wear process, resulting from an interaction between the edge of the moving pyrolytic carbon disc, the struts' titanium surface, and the protein coat covering that surface. None of the patients had prosthesis dysfunction attributable to metal wear. Disc escape seems unlikely considering the degree of wear observed after 10 years. Furthermore, the surface corrugations did not appear to cause disc sticking or other problems. However, clinicians might consider monitoring patients who have borne these prostheses for greater than 10 years.

An STS-based radiation hybrid map of the human genome.

We have constructed a physical map of the human genome by using a panel of 83 whole genome radiation hybrids (the Stanford G3 panel) in conjunction with 10,478 sequence-tagged sites (STSs) derived from random genomic DNA sequences, previously mapped genetic markers, and expressed sequences. Of these STSs, 5049 are framework markers that fall into 1766 high-confidence bins. An additional 945 STSs are indistinguishable in their map location from one or more of the framework markers. These 5994 mapped STSs have an average spacing of 500 kb. An additional 4484 STSs are positioned with respect to the framework markers. Comparison of the orders of markers on this map with orders derived from independent meiotic and YAC STS-content maps indicates that the error rate in defining high-confidence bins is < 5%. Analysis of 322 random cDNAs indicates that the map covers the vast majority of the human genome. This STS-based radiation hybrid map of the human genome brings us one step closer to the goal of a physical map containing 30,000 unique ordered landmarks with an average marker spacing of 100 kb.