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Biomedical subjects

M Thubrikar

Publications and source records attributed to M Thubrikar.

18 recordsLinked to original sources
PubMed1997

Transected thoracic aorta: age-specific differences in incidence and possible reasons.

The objective of this study was to determine the incidence of aortic transaction in relation to age, and to examine possible reasons for the observed differences. Data from the North Carolina Medical Database over a 7-year period were examined for the total number of motor vehicle accident victims and for the subset with aortic rupture, based on age at presentation. Data were then divided into 10-year intervals and the differences analyzed using chi-square analysis. Differences among various age groups were statistically significant (P = 0.0001). The highest rate was in the 21-30-year-old age group and average incidence for all ages was 0.7%. High incidence of aortic transaction in the 21-30-year-old group may be due to an increase in high-risk behaviors in such persons, to an improved survival compared with other age ranges, or to an inherent susceptibility of the aorta at this stage of life. These data have important implications for the diagnosis and treatment of aortic transaction and should be taken into account when developing practice guidelines for its management.

Accidents, Traffic↗
PubMed1991

Modification of low density lipoprotein association with the arterial intima. A possible environment for the antiatherogenic action of beta-blockers.

The accumulation of lipoproteins and lipids associated with the progression of atherosclerotic lesions appears to be a sequential process involving interactions with the intimal extracellular matrix and, subsequently, with cells. Apolipoprotein B-containing lipoproteins appear first to be retained and modified focally by proteoglycans of the extracellular matrix. The association with the extracellular matrix may lead to further modifications. In vitro resident macrophages take up the modified apolipoprotein B lipoproteins via a nonsaturable mechanism that may contribute to their transformation into foam cells characteristic of the atherosclerotic lesion. The affinity of low density lipoprotein (LDL) for arterial proteoglycans in vitro is related to the charge and triglyceride content. Small, triglyceride-poor, cholesterol-rich particles interact more efficiently with proteoglycans and are taken up faster by cultured macrophages than larger triglyceride-rich ones. beta-Blockers increase the relative triglyceride content of circulating LDL, and in vitro this LDL has a lower affinity for arterial proteoglycans. These results suggest that some of the experimental antiatherogenic actions of beta-blockers may be related to a reduced LDL affinity for extracellular intima components associated with the small changes induced by the drugs in the lipoprotein structure. Hypothetically, this reduced affinity could diminish the focal accumulation of LDL in lesion-prone sites.

Adrenergic beta-Antagonists↗
PubMed1982

A method for analysis of bending and shearing deformations in biological tissue.

This paper describes a theoretical and experimental approach to the analysis of the deformations of a thin biological tissue. A biological tissue undergoes complex deformations during which normal and shearing strains are produced. These strains can be very large and yet be within the elastic range of the material. The procedure described is demonstrated for the pericardium used in making bioprosthetic heart valves. It is observed that the pericardium exhibits a directional property in which the shearing deformations occur in one direction but not in the opposite direction. By the application of the proposed method, modes of deformation can be determined and modes of failure predicted.

Animals↗
PubMed1982

Interpretation of aortic root angiography in dogs and in humans.

It has been suggested that aortic root angiography shows the opening of the aortic valve in humans to be triangular. We investigated whether aortic root angiography does indeed permit determination of area and configuration of the aortic valve. Various angiographic views of the aortic valve were recorded in dogs and humans and were compared with similar views obtained photographically of aortic valve models designed to show circular and a triangular orifice. It was observed that the aortic root angiograms were the projection of the entire aortic valve flow conduit in systole. The orifice seen in any angiogram was truly the inside boundary of the dye, composed of the free edge and the line of attachment of the leaflets. This orifice did not represent the true orifice of the valve, and its configuration depended upon the angle of the X-ray and the orientation of the valve. The comparison also revealed that, in dogs and in humans, the opening of the aortic valve is circular, a fact further shown to correlate better with the clinical observations.

Animals↗
PubMed1982

Analysis of the design and dynamics of aortic bioprostheses in vivo.

An understanding of the in vivo design and dynamics of the present bioprosthetic valves should provide the information necessary for an improvement in their efficiency and durability. Three types of commercially available bioprostheses were prepared with radiopaque markers and implanted in the aortic position in calves. One week later, under light general anesthesia, the animals were studied to determine the in vivo design, shape, configuration, and motion of the bioprostheses. This information was then compared to that previously obtained from the natural aortic valve in vivo. The following observations were made: (1) In all three types of bioprostheses, the three leaflets opened and closed simultaneously in less than one thirtieth of a second; (2) there was no detectable flexion of the stent posts in any of the three types of valves; (3) in all of the bioprostheses studied, the greatest flexion occurred along the attachment of the leaflets; (4) the systolic and the diastolic geometry of the three types of valves was completely different for each type of valve, and none duplicated the geometry of the natural aortic valve; (5) the open configuration of the leaflets was different for each type of prosthetic valve and different from the natural aortic valve; (6) the zone of leaflet bending varied in size and extent for each of the valves. It is concluded that the differences between the design of bioprosthetic valves and that of natural valves are probably a major factor in increasing the stresses in bioprostheses. Hence stress failures could be reduced and durability increased by redesigning bioprostheses to duplicate more closely the design of the natural aortic valve.

Aortic Valve↗
PubMed1981

The design of the normal aortic valve.

The design parameters of the natural aortic valve in vivo were not known, which may explain why various bioprosthetic valves have been designed differently. The design of the aortic valve was studied in vivo by placing radiopaque markers in the valve. The marker movement revealed that, during a cardiac cycle, the design parameters of the valve were changing continuously with changing aortic pressure and ventricular geometry. During diastole decreasing radius of the commissures (Rc) and increasing radius of the bases (Rb) caused the leaflets to tilt toward the ventricle, thereby decreasing the bottom surface angle (alpha) and increasing the free-edge angle (phi) of the leaflet. During systole Rc increased, Rb decreased, and interleaflet distance decreased, causing a change in the geometry of the open valve from conical to cylindrical. In middiastole the design parameters were Rb/Rc = 1.2, H/Rc = 1.4, phi = 34 degrees, and alpha = 20 degrees, where H is sinus height. How a significant deviation from the design could compromise the efficiency and longevity of the valve is discussed.

Animals↗
PubMed1980

Stresses of natural versus prosthetic aortic valve leaflets in vivo.

During normal function of the aortic valve, the aortic leaflets undergo not only cyclic loading and unloading but also cyclic reversal of their curvature. The stresses induced in the leaflet due to these variations have been computed using a new concept based on the structure of the leaflet. Membrane stresses have been related to the pressure difference across the leaflet and bending stresses to the leaflet curvature. Total stresses were obtained by adding the two stresses. Total stresses in bioprosthetic and synthetic leaflets also were computed using the same approach. In systole, the natural leaflet is subjected to much lower total stress than a bioprosthetic or a synthetic leaflet. The natural leaflet is not subjected to compressive stresses during the cardiac cycle, whereas bioprosthetic and synthetic leaflets must sustain compressive stresses during systole. The differences in stress patterns of these leaflets indicate that there is a difference in their longevity.

Animals↗
PubMed1980

The elastic modulus of canine aortic valve leaflets in vivo and in vitro.

Aortic valve leaflets undergo extraordinary flexion due to the complete reversal of their curvature during billions of cardiac cycles. The flexion stresses in the leaflet depend on its elastic modulus which we investigated in vivo and in vitro. In six dogs, we placed radiopaque markers on an aortic leaflet. Leaflet length was calculated from the marker positions recorded fluoroscopically. Aortic and ventricular pressures were recorded. Dogs were killed and leaflet stress-strain curves determined in vitro. Leaflet length in vivo decreased 10.4 +/- 4.7% from diastole to systole in each cardiac cycle. Using the law of LaPlace, pressure gradients across the leaflets were converted into the stresses in the leaflets. The leaflets had an initial "elastic phase" of low modulus in systole followed by an "inelastic phase" of high modulus in diastole. The elastic modulus was 2.4 +/- 0.7 x 10(6) dynes/cm2 in systole and 5.2 +/- 1.7 x 10(7) dynes/cm2 in diastole. These results were similar to those obtained in vitro. Since flexion rigidity is proportional to (elastic modulus) x (thickness)3, the lower modulus in systole greatly reduces flexion stresses in the leaflet and increases leaflet longevity. The higher elastic modulus in diastole prevents excessive bulging or prolapse of the leaflet while it is subjected to the diastolic pressure gradient. We conclude that a natural or prosthetic leaflet which is thickened or has a high elastic modulus throughout the cardiac cycle will have a greater flexion stress that could cause early failure.

Animals↗
PubMed1979

The mechanism of opening of the aortic valve.

The mechanism of opening of the aortic valve was investigated in dogs by attaching radiopaque markers to the commissures and the leaflets. Analysis of abnormal cardiac cycles demonstrated that, when the ventricular pressure first equalled the aortic pressure, the intercomissural distances increased 9 percent, and the valve opened with a stellate orifice without forward flow and without a rise in aortic pressure. Further opening of the aortic valve was dependent on forward flow over a narrow range. A new mechanism of aortic valve opening is proposed. This mechanism results in minimal flexion stresses on the leaflets and is important for the longevity of the normal aortic valve. It can occur only if the leaflets arise from an expansile aortic root.

Animals↗