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Biomedical subjects

M J Thubrikar

Publications and source records attributed to M J Thubrikar.

At least 19 recordsLinked to original sources
PubMed2003

Effect of thrombus on abdominal aortic aneurysm wall dilation and stress.

AIM: Our goal is to understand how a mural thrombus may influence the pressure transmitted to and the dilation experienced by the abdominal aortic aneurysm (AAA) wall. METHODS: Two intact AAAs with mural thrombus were removed from patients and pressurized to 100 mmHg. The pressure was measured using a micro-tip needle transducer inserted in the aneurysm wall and advanced through the thrombus. In 1 patient with AAA, similar measurements were made in vivo. Also, in vitro, in the two aneurysms the dilation as a function of pressure was measured using the markers on the surface before and after the thrombus was removed. RESULTS: Both, in vitro and in vivo, in the presence of the thrombus the pressure transmitted to the aneurysm wall was 91+/-10% of luminal pressure and at 6 mm from the wall it was 96+/-5%. The aneurysm dilated more in the pressure range of 0-40 mmHg (2-8%) than in the range of 40-100 mmHg (0.4-1.8%). Upon removal of the thrombus these dilations increased significantly to 4-15% and 0.9-3.3%, respectively. Overall, the strains (dilation) in the circumferential and longitudinal directions were similar before the thrombus was removed. CONCLUSIONS: Even though the thrombus allows the transmission of luminal pressure to the aneurysm wall, it may prevent the aneurysm from rupture by diminishing the strain on the wall. Consistent with this, a mechanical model of the thrombus proposed is "a thrombus as a fibrous network adherent to the aneurysm wall".

Aged↗
PubMed2002

Mechanical stress as cause of aortic valve disease. Presentation of a new aortic root prosthesis.

We present an overview of studies on the aortic valve and propose that mechanical stress is a main causative factor in the degenerative valvular disease. In the normal aortic valve, the leaflets have a smooth surface, free of wrinkles and creases, throughout the opening process. This smooth leaflet surface during motion is achieved by the "pull and release" movement of the commissures, which occurs because of the compliance of the aortic root. When the aortic root is stiffened, either by artificial means or by the loss of elasticity due to aging, the leaflet dynamics change significantly. The leaflets develop a significant number of creases and wrinkles during the opening process. In the bileaflet valve, the leaflets develop similar creasing and wrinkling during the opening process. This happens mainly due to the less-than-ideal design of the bileaflet valve and in spite of the compliant aortic root. When the aortic valve is spared using a noncompliant tube graft, a similar phenomenon of leaflet creasing occurs. Because the creasing produces high stresses from bending and buckling, it is damaging to the leaflet tissue and can lead to degenerative and calcific valvular disease. Based on these observations a new aortic root prosthesis with compliant sinuses has been designed for the valve sparing operation.

Aging↗
PubMed2001

Wall stress studies of abdominal aortic aneurysm in a clinical model.

To estimate when an abdominal aortic aneurysm (AAA) may rupture, it is necessary to understand the forces responsible for this event. We investigated the wall stresses in an AAA in a clinical model. Using CT scans of the AAA, the diameter and wall thickness were measured and the model of the aneurysm was created. The wall stresses were determined using a finite element analysis in which the aorta was considered isotropic with linear material properties and was loaded with a pressure of 120 mmHg. The AAA was eccentric with a length of 10.5 cm, a diameter of 2.5 to 5.9 cm, and a wall thickness of 1.0 to 2.0 mm. The aneurysm had specific areas of high stress. On the inner surface the highest stress was 0.4 N/mm2 and occurred along two circumferentially oriented belts--one at the bulb and the other just below. The stress was longitudinal at the anterior region of the bulb and circumferential elsewhere, suggesting that a rupture caused by this stress will result in a circumferential tear at the anterior portion of the bulb and a longitudinal tear elsewhere. In the mid-surface the highest stress was 0.37 N/mm2 and occurred at two locations: the posterior region of the bulb and anteriorly just below. The stress was circumferential, suggesting that the rupture caused by this stress will produce a longitudinal tear. The location and orientation of the maximum stress were influenced more by the tethering force than by the wall thickness, luminal pressure, or wall stiffness. In conclusion, the rupture of an AAA is most likely to occur on the inner surface at the bulb. Such analytical approaches could lead to a better understanding of the aneurysm rupture and may be instrumental in planning surgical interventions.

Aortic Aneurysm, Abdominal↗
PubMed2001

A new aortic root prosthesis with compliant sinuses for valve-sparing operations.

BACKGROUND: We designed and tested a novel aortic root prosthesis with compliant sinuses for valve-sparing operations. METHODS: In eight human aortic roots, the aorta was trimmed 2 mm above the leaflet attachment. The aortic portion of the graft was made by scalloping the Dacron tube. Three sinuses were made individually after turning z-folds in the fabric 90 degrees. Three rectangular pieces were cut and purse strings sewn in each to form the sinuses. The graft was sutured to the aortic root and studied in a left heart simulator. The leaflet motion was recorded (500 frames/second), commissural movement was measured with ultrasound, and the shape of the root was determined from a mold. Seven intact aortic roots were also studied. RESULTS: In the aortic graft roots, the valves were competent and leaflets opened rapidly into a circular orifice, not touching the sinus wall. Commissural diameter increased by 22% when pressure increased from 0 to 80 mm Hg, and increased by a further 6.6% when pressure increased to 120 mm Hg. The sinuses had a teardrop shape. CONCLUSIONS: The dynamics of the aortic graft root and the leaflets were comparable to that of the intact aortic root. This prosthesis is being introduced in clinical practice.

Aortic Valve↗
PubMed2001

Stress analysis of the aortic valve with and without the sinuses of valsalva.

BACKGROUND AND AIM OF THE STUDY: In some patients with dilated aortic root, attempts have been made to replace the aorta and preserve the anatomically normal leaflets. The tubular Dacron graft used in the valve-sparing procedure does not have the sinuses of Valsalva, and therefore might adversely affect the valve. The effect of the presence or absence of the sinuses was investigated on stresses in the leaflets. METHODS: Finite element models of the tubular aortic root and the root with the sinuses were constructed. The model dimensions were: aorta radius 13 mm; valve height 18 mm; leaflet thickness 0.5 mm; aorta and sinus wall thickness 1.3 mm. In the root with the sinuses, each leaflet-sinus unit formed a cylindrical geometry. The model was loaded with an internal pressure of 80 mmHg (0.01 N/mm2) and the stress determined. RESULTS: The stress in the leaflet belly was 0.26 N/mm2, and not affected by the geometry and stiffness of the root. In the tubular root there was a stress concentration along the leaflet attachment, and the stress reached 0.65 N/mm2; in contrast, in the sinus root there was no stress concentration and maximum stress was only 0.3 N/mm2. Under pressure, the leaflet-sinus assembly expanded in a uniform manner, whereas in a tubular root large deformations occurred along the leaflet attachment. CONCLUSION: The sinuses of Valsalva play a very important role in minimizing stress in the leaflets; thus, use of the sinus graft is expected to enhance the longevity of the spared valve.

Aortic Valve↗
PubMed2001

Etiology of degenerative disease of the tri-leaflet aortic valve: a simple explanation for a complex problem.

Risk factors for both atherosclerotic aortic wall disease and degenerative disease of the tri-leaflet aortic valve are very similar if not identical. This correlation grows even stronger as the person advances in years. Because of this, it is the prevailing view that sclerosis of the tri-leaflet aortic valve is a disease similar in etiology with sclerosis of the aortic wall. In other words, degenerative aortic valve disease is atherosclerosis of the aortic valve. Our studies challenge these views. The aortic valve is a functional assembly composed of the three cusps, corresponding sinuses, and the sinotubular junction, characterized not only by morphological features but also its functional properties, which together create an environment that is optimal for distribution of diastolic pressure load, and assures proper and timely valve opening and closure. Our experiments also demonstrate that loss of aortic wall compliance at the level of the sinuses leads to significant flexion stress in the aortic leaflets and it is likely to start a chain of events, which begins with mechanical damage to the leaflet microstructure then continues to more evident sclerosis, and finally ends in gross distortion and/or calcification of the cusps. The loss of the "pull-and-release" process may also play a part in disintegration of bioprosthetic valves and in degeneration of native aortic valves encased in non-compliant prostheses.

Adult↗
PubMed2000

Clinical introduction of a novel prosthesis for valve-preserving aortic root reconstruction for annuloaortic ectasia.

OBJECTIVE: Most patients with annuloaortic ectasia are young. They are at risk for complications related to a lifetime of anticoagulation when composite grafts containing mechanical valves are used for reconstruction. The majority of patients have near normal valve cusps. Valve-preserving techniques have been developed to maintain valve function and avoid anticoagulation. The eddy currents occurring within the sinuses of Valsalva in the natural aortic root have been shown to be important in the smooth, gradual, and gentle closure of the valve. Compliance of the sinuses is important in reducing stress in the leaflets. A novel ascending aortic prosthesis with "built in" compliant sinuses (Robicsek-Thubrikar graft) was developed for clinical aortic root replacement. METHODS: Woven Dacron tubes were used to make the prostheses. Three precisely measured square pieces were cut to make the expandable, individual sinuses. Sewing the individual neo-sinuses to a scalloped end of the Dacron tube graft created the neo-sinotubular junction and sinotubular ridge. Five patients with annuloaortic ectasia underwent valve-preserving aortic root reconstruction. RESULTS: All intraoperative transesophageal echocardiographic images after the valve-preserving procedure showed a normal appearing root with 10% radial expansion of each sinus in systole. The space between the cusps and neo-sinus wall in systole was normal. No patient has more than mild aortic regurgitation. CONCLUSIONS: Valve-preserving aortic root reconstruction with a novel Dacron prosthesis with compliant "built in" sinuses re-establishes normal aortic root geometry with near normal valve motion. This may enhance the durability of the valve-preserving operation.

Adult↗
PubMed1999

Role of sinus wall compliance in aortic leaflet function.

Functional morphology of the normal aortic root was studied in vitro using 500 frames/s cinematography. Stiffening of the aortic wall by spraying its exterior with plastic adhesive led to compromised function of the leaflets, a phenomenon that may play a role in valve degeneration as it occurs in old-age fibrosis, atherosclerosis, or in connection with certain surgical procedures.

Aortic Valve↗
PubMed1999

The influence of sizing on the dynamic function of the free-hand implanted porcine aortic homograft: an in vitro study.

BACKGROUND AND AIMS OF THE STUDY: The influence of sizing on the function of a porcine aortic valve after its implantation using the free-hand technique in the subcoronary position was investigated. METHODS: Dynamic function and leaflet configuration of the valve (n = 16) were first analyzed in its natural aortic root in a left heart simulator at 120/80 mmHg pressure and 4 l/min cardiac output. The valve was then implanted in the recipient porcine aortic root and re-studied. Three groups were investigated: group I (n = 4) comprised of 1-2 mm smaller donor aortic valve than the recipient; group II (n = 8) 3-4 mm smaller; and group III (n = 4) 5-7 mm smaller. Orifice area (OA), systolic and diastolic configurations of the leaflets, pattern and timing of leaflet opening and closure, commissural movement, pressure gradient and valvular regurgitation were analyzed. RESULTS: In the intact donor aortic root, average expansion of the aorta at the commissures, for a pressure change from 0 to 80 mmHg, was about 42%. This was reduced significantly in all assemblies. Group I showed a 34% reduction in OA, and excessive leaflet bending; there was no aortic insufficiency (AI) or pressure gradient across the valve. In group III there was a lesser reduction in OA and reduced leaflet bending, but two of four valves had AI. In group II, the reduction in OA was only 13%, there was less leaflet bending, and no AI. CONCLUSIONS: The donor valve 3-4 mm smaller than the recipient seems an optimal match. The current practice of using the same size donor as recipient may be responsible for excessive leaflet bending and may be implicated in early deterioration of the homograft.

Animals↗
PubMed1999

Pressure traps in femoro-popliteal reversed vein grafts. Are valves culprits?

BACKGROUND: Stenosis is a major cause of vein graft failure in peripheral arterial surgery. Our goal is to determine whether vein valves play a role in this process by creating a "pressure trap". METHODS: Seventeen patients with femoro-popliteal reversed saphenous vein grafts were studied intraoperatively. Flow and pressure in the grafts were measured, while the graft outflow was gradually occluded and released for 2-4 seconds. In 3 patients the graft flow was reduced by compressing calf muscles. RESULTS: Patients heart rates were 54-84 BPM, blood pressures 170/80-110/55 mm Hg, and normal graft flow was 40-180 ml/min. In 12 patients with competent vein valves, at reduced flow (<30 m/min) the valves opened and closed in each cardiac cycle. At each closure the pressure was "trapped" distal to the valve producing diastolic hypertension. Also the flow was stagnant for a considerable portion of the cardiac cycle. Maximum diastolic pressure gradient across the valve ranged from 35 to 60 mm Hg and the level of pressure trapped was inversely proportional to the graft flow. CONCLUSIONS: In patients in whom reversed vein grafts with competent valves are placed in the femoro-popliteal positions a "pressure-trap" develops in the distal segment. This segmental hypertension combined with the flow stagnation could play an important role in the graft thickening and stenosis.

Anastomosis, Surgical↗
PubMed1995

Pressure-induced arterial wall stress and atherosclerosis.

We present the hypothesis that high wall stress and accompanying stretch, particularly that caused by arterial pressure, are the primary factors responsible for the topography of atherosclerotic lesions. In our view the pattern in the localization of atherosclerotic lesions indicates that the artery behaves as both a pressure vessel and a conduit of blood flow. The phenomenon of "stress concentration" in the artery wall is described and the area of pressure-induced high stress is related to the sites of atherosclerotic plaques. Data are presented indicating that reduction of pressure-induced stress may lead to absence of atherosclerotic changes. The proposed mechanism explains the prevalence of atherosclerotic lesions at the ostia of major arterial branches, at the aortic bifurcation, at the carotid bifurcation, and in the descending thoracic aorta, and also explains the absence of atherosclerosis in the intramyocardial coronary arteries and in the intraosseal portions of the vertebral vessels and why a reduction in heart rate, blood pressure, or wall stress by external support reduces the occurrence of atherosclerosis. The effect of wall stress and stretch on atherosclerosis could be mediated by the endothelial cells, the smooth muscle cells, and the penetration of low-density lipoproteins. The comprehensive presentation made in this article could lead to a better understanding of atherosclerosis, its treatment, and its prevention.

Arteries↗
PubMed1995

Pressure-induced mechanical stress in the carotid artery bifurcation: a possible correlation to atherosclerosis.

A possible correlation between regions of high intramural wall stress and the development of atherosclerotic lesions in the carotid artery bifurcation is investigated. The bifurcation geometry is determined through in vivo studies, as well as the analysis of cadaver specimens. Having compiled accurate geometric data, two representative finite element models were created in order to determine the areas of localized stress concentrations that occur in the bifurcation. The artery is assumed isotropic and is mechanically loaded with an incremental pressure of 40 mmHg. A highly localized stress concentration of approximately 9 to 14 times the proximal circumferential wall stress occurs at the point of bifurcation. A lower stress concentration of approximately 3 to 4 times the proximal circumferential stress occurs over a large area of the sinus bulb. Acknowledging that these two regions of the carotid bifurcation are highly susceptible to atherosclerotic lesions, it appears possible that a correlation between wall stress and atherosclerosis may exist.

Angiography↗
PubMed1994

Conservative operation in the management of annular dilatation and ascending aortic aneurysm.

Ascending aortic aneurysm associated with annular dilatation is generally handled by resection and replacement of the aortic root. Such an extensive operation may be avoided by a much simpler surgical intervention, ie, aortoplasty, valve replacement, and aortic wall reinforcement. Such an operation may be performed in cases where the shape of the aneurysm is a relatively regular "pear" or "teardrop" shape and is not extremely large. It also may be applied in cases of aortic stenosis with post-stenotic dilatation. The procedure requires barely more operative time and effort than a "simple" aortic valve replacement.

Aortic Aneurysm↗
PubMed1994

Hemodynamic considerations regarding the mechanism and prevention of aortic dissection.

We describe the hemodynamic principles that we postulate are responsible for the development of aortic dissection. Two types are differentiated: the "hypertensive" and the "Marfan" types. The former is characterized by moderate aortic dilatation and considerable blood pressure elevation, the latter by considerable aortic dilatation only. The role of wall stress in the development of the intimal tear and the propagation of the process of dissection are discussed. We recommend that patients who are at high risk for developing aortic dissection--those with Marfan's syndrome and dilated aortas--should undergo aortoplasty and wall reinforcement as a simple, safe, and effective way to prevent aortic dissection.

Aortic Dissection↗
PubMed1994

The freedom from atherosclerosis of intramyocardial coronary arteries: reduction of mural stress--a key factor.

The phenomenon that coronary arteries which course entirely intramyocardially remain immune to atherosclerosis was investigated in 250 patients undergoing coronary bypass surgery. In 26 the circumflex artery gave off major branches whose distal portions ran entirely intramyocardially, which required coronary bypass grafting because of proximal disease. Of these, 24 had no atherosclerosis in the intramyocardial segment, one had a calcified plaque, and one had a recanalized thrombus. The cause of this phenomenon was studied further in 14 canine hearts by measuring intramyocardial pressure. The intramyocardial pressure in the subepicardial region was as high as 63% of the left ventricular cavity pressure and in the subendocardial region it was even greater than the left ventricular cavity pressure, which indicates that the pressure gradient across the artery wall is significantly reduced, and sometimes even eliminated. It is therefore concluded that the freedom from atherosclerosis of the intramyocardial coronary arteries is due to the lack of mural stress, i.e. the lower or absent transmural pressure gradient.

Animals↗
PubMed1994

Pressure trap created by vein valve closure and its role in graft stenosis.

Saphenous vein graft stenosis has become the leading cause of reoperation in coronary bypass operations. We investigated the role of vein valves in vein graft stenosis by studying 14 human saphenous veins placed in a simulator of the left side of the heart in parallel with the arterial system. The vein had a variable resistance and a capacitance simulating the distal vascular bed. The pressures at the proximal and distal ends of the vein and the venous flow were measured while the following were changed: venous flow 200 to 0 ml/min, aortic pressure 150/120 to 80/60 mm Hg, cardiac output 3 to 5 L/min, and compliance of distal vascular bed 0 to 1 ml of air. The pressures at both ends of the vein were the same when venous flow rate was greater than 60 ml/min and the vein valve remained open. As the venous flow decreased the valve began to open and close in each cardiac cycle. The flow rate at which the valve began to close ranged from 30 to 10 ml/min. When the valve closed, it trapped the pressure in the segment distal to the valve. Because the segmental hypertension is expected to accelerate atherosclerotic changes, the pressure trap created by closure of the vein valve could be an important cause of vein graft stenosis.

Arteriosclerosis↗
PubMed1993

High speed cine-radiographic study of aortic valve leaflet motion.

The leaflets of the aortic valve move extremely rapidly during opening and closing. To analyse this movement, radiopaque markers were placed on the aortic valves of four dogs during cardiopulmonary bypass. One to five months later the dogs were studied using x-ray, and the marker motion was recorded on cine-film at 500 frames/second. Simultaneous aortic pressure varied from 70/30 to 188/152 mmHg and heart rate from 33 to 150 bpm. Analysis of 19 systolic periods and 2500 cine-frames indicated that leaflet motion occurred in four phases: 1--rapid movement during initial opening, 2--a little movement near maximal excursion, 3--a slow movement during early closing and 4--a rapid movement to complete closure. Phases 1 and 4 averaged 17.6 and 16.5 msec respectively, and were independent of heart rate, whereas the length of phases 2 and 3 varied with heart rate. The leaflets moved 82% of their maximum excursion during rapid opening and 53% during rapid closure. Leaflet curvature reversed in both the radial and the circumferential directions during opening. The leaflet profile in the radial direction changed as follows: during rapid opening, the belly of the leaflet moved outward, first causing the leaflet to straighten and then to conform to the profile of blood flow. During closure the belly of the leaflet moved centrally, causing the leaflet to curve toward the ventricle. The leaflet profile in the circumferential direction indicated that during opening the midpoint of the leaflet leads the motion towards the aortic wall.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗