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Biomedical subjects

E A Trowbridge

Publications and source records attributed to E A Trowbridge.

At least 37 records · Page 2Linked to original sources
PubMed1988

The mechanics of the anal sphincter complex.

The anal sphincter complex consists of circumferentially arranged muscle fibres, which surround a relatively thick anal lining. This apparatus was modelled mechanistically as two concentric homogeneous isotropic linear elastic cylinders. The inner cylinder (anal lining) was considered to be thick walled, while the outer (the circular muscle) was assumed to be thin walled. The model predicts that the anal sphincter tension varies linearly with luminal diameter. This prediction was confirmed experimentally under normal conditions as well as during external sphincter contraction and internal sphincter relaxation. Under conditions of negligible hoop stress in the anal lining the model also predicts that the intra-luminal pressure falls to zero before the luminal diameter reaches zero. Hence, an autoregulatory mechanism of anal cushion thickening, as the luminal pressure falls to zero, to produce anal closure was proposed. Deficiencies in this autoregulation mechanism may explain anal incontinence and the obstructed defaecation often found in subjects with haemorrhoids.

Adult↗
PubMed1988

A stochastic model of pulmonary platelet production.

Normally, cells reproduce by mitosis. In mammals, a system has evolved that is unique to cell biology. The circulating blood cells called platelets are produced from parent cells called megakaryocytes, but the mechanism of platelet production is not mitosis. At present, both the site and mechanism of production are still debated. This article describes a production mechanism based on a sequence of random binary divisions of the megakaryocyte cytoplasm as it traverses the pulmonary microcirculation. Using the measured megakaryocyte cytoplasmic volumes circulating in the blood of three different mammals, rat, rabbit, and man, a computer simulation of this process is developed. The simulation depends on two separate stochastic processes. The first involves the probability that a division of a cytoplasmic particle of a specified size will occur. The second relates to the relative sizes of the two particles produced by the binary division. These two processes are controlled by three parameters only: a lower threshold L on the platelet volume, below which the probability of a binary division is zero; a parameter lambda which defines the probability of division for volumes larger than this lower threshold; and the standard deviation S of the Gaussian distribution of possible volumes created by the binary division. A minimization scheme is used to establish those values of L, lambda, and S which provide the best fit to the experimental data obtained from the different mammals.

Animals↗
PubMed1988

Human megakaryocyte size varies with anatomical site.

Whole megakaryocyte, megakaryocyte cytoplasmic and nuclear size were evaluated in human bone marrow obtained from posterior iliac crest and rib of subjects undergoing thoracotomy for repair of hiatal hernia. In addition the nuclear DNA content (ploidy) of the cells from each site was measured. Significantly larger mean megakaryocyte (P less than 0.002), and megakaryocyte cytoplasmic volumes (P less than 0.002), and different volume distributions were observed in costal marrow compared to the iliac crest. No change in mean nuclear volume was found. This was paralleled by no significant difference in mean ploidy. This suggests that the variation in megakaryocyte size with anatomical site is determined by the concomitant volume of cell cytoplasm, but not the nuclear volume.

Adult↗
PubMed1988

The mechanism of clavicular fracture. A clinical and biomechanical analysis.

A consecutive series of 150 patients with clavicular fractures is presented. In 81% detailed information regarding the mechanism of the injury was available and, of these, 94% had fractured their clavicle from a direct blow on the shoulder; only 6% had fallen on the outstretched hand. This finding, at variance with commonly held views regarding the mechanism of this injury, was further investigated by biomechanical analysis of the forces involved in clavicular fractures. The biomechanical model supported the clinical findings.

Biomechanical Phenomena↗
PubMed1988

Pericardial heterografts: why do these valves fail?

Eighteen explanted pericardial heterografts were studied (16 standard Ionescu-Shiley, one Hancock, and one Mitroflow). Regurgitation was the reason given for explantation of all the Ionescu-Shiley valves. The other two valves were removed for technical reasons. All the Ionescu-Shiley valves had commissural tears and there was concomitant gross calcification in 10 of the 16 valves. In addition, an apparent increase in cusp area had caused "leaflet sagging". The explanted leaflets were thicker and stiffer than leaflets from an unimplanted valve. These features were confirmed directly with an animal model of subcutaneous implantation. Examination with an electron microscope revealed that these changes in mechanical properties seemed to be linked to fiber separation and infiltration by an amorphous proteinlike matrix. The durability of the glutaraldehyde-fixed pericardium depended on a number of factors. Early and midterm failure appeared to be stress induced. Predisposition to high mechanical stresses near the stent was exacerbated by the changes induced by the host environment. This problem was aggravated further in the Ionescu-Shiley valves by stress concentrations around the hole associated with the holding suture. In the long term, collagen disruption associated with leaflet flexure was followed by secondary calcification at the boundary between the intact and disrupted material.

Aortic Valve↗
PubMed1988

Pulmonary platelet production: a physical analogue of mitosis?

Cellular reduplication is normally achieved by mitosis. In mammals, a system unique to cell biology has evolved that does not utilise this reproductive process. Platelets are derived from the cytoplasm of megakaryocytes but they are not produced by mitosis. At present, both the site and mechanism of platelet production are still debated. This article describes a production mechanism that retains the natural simplicity of binary division. It is predominantly physical. Megakaryocytes leave the bone marrow as whole cells or large cytoplasmic fragments. The cytoplasm of these large circulating cells undergoes a sequence of binary divisions at the dichotomous branches of the pulmonary microcirculation. The lungs trap the majority of megakaryocyte naked nuclei that are removed subsequently by phagocytosis. After the organisation of a microtubular cytoskeleton, the much smaller cytoplasmic fragments leave the pulmonary bed as circulating platelets.

Blood Platelets↗
PubMed1987

The platelet volume distribution: a signature of the prethrombotic state in coronary heart disease?

A coronary care unit population of 175 consecutive patients was studied. A clinical diagnosis of acute myocardial infarction was made in 103 cases (MI group) while 72 had chest pain, ECG indications of myocardial ischaemia, but no other clinical signs of acute myocardial infarction (control group). Based on bivariate Gaussian statistics the MI group could be divided in two groups, with respect to the platelet parameters of count and volume. The majority (65%) had large mean platelet volume and low count, while the remainder were indistinguishable from the control group. Patients with a large mean platelet volume also had significantly different volume distributions when assessed for volume dispersion, asymmetry and convexity. It is argued that this platelet volume distribution provides a signature for the prethrombotic state in ischaemic heart disease. Identification of such a platelet signature in a patient with coronary artery disease may allow appropriate prophylactic action to be taken and reduce the risk of subsequent acute myocardial infarction.

Blood Platelets↗
PubMed1987

Acute and chronic changes in platelet volume and count after cardiopulmonary bypass induced thrombocytopenia in man.

Patients undergoing surgery for coronary artery bypass graft or heart valve replacement had their platelet count and mean volume measured pre-operatively, immediately post-operatively and serially for up to 48 days after the surgical procedure. The mean pre-operative platelet count of 1.95 +/- 0.11 X 10(11)/1 (n = 26) fell significantly to 1.35 +/- 0.09 X 10(11)/1 immediately post-operatively (p less than 0.001) (n = 22), without a significant alteration in the mean platelet volume. The average platelet count rose to a maximum of 5.07 +/- 0.66 X 10(11)/1 between days 14 and 17 after surgery while the average mean platelet volume fell from preparative and post-operative values of 7.25 +/- 0.14 and 7.20 +/- 0.14 fl respectively to a minimum of 6.16 +/- 0.16 fl by day 20. Seven patients were followed for 32 days or longer after the operation. By this time they had achieved steady state thrombopoiesis and their average platelet count was 2.44 +/- 0.33 X 10(11)/1, significantly higher than the pre-operative value (p less than 0.05), while their average mean platelet volume was 6.63 +/- 0.21 fl, significantly lower than before surgery (p less than 0.001). The pre-operative values for the platelet volume and counts of these patients were significantly different from a control group of 32 young males, while the chronic post-operative values were not. These long term changes in platelet volume and count may reflect changes in the thrombopoietic control system secondary to the corrective surgery.

Adult↗
PubMed1987

Pericardial heterograft valves: an assessment of leaflet stresses and their implications for heart valve design.

Bovine pericardium, stabilized with glutaraldehyde, is used widely in the construction of heart valve substitutes, but the design and construction of valve substitutes from this material are empirically based. Collagenous tissue can support tension, but experimental evidence indicates that flexure-induced compressive stresses can lead to fatigue failure. This study uses experimental results obtained from cyclic uniaxial load tests to predict the type and magnitude of operational stresses which occur in pericardial heterograft leaflets. Both Young's modulus and Poisson's ratio varied with uniaxial loading in pericardium, chemically modified free of tension. Leaflet stresses were analysed in using effective incremental representations of these parameters. In leaflets with unrestricted rotation at the point of attachment to the stent, the mid-plane tensions always exceeded the bending stresses, and no zones of leaflet compression were predicted. In contrast, with totally restricted leaflet rotation induced by clamping (possibly between a male and female frame) the bending stresses were greater than the mid-plane tensions at the hinge line and significant compressive stresses were predicted at this site. If elastic boundary conditions were introduced at the stent (possibly by wrapping the stent in pericardium) then the compressive stresses were reduced as the degree of elasticity was increased. Glutaraldehyde fixation of the pericardium under load produced a stiffer material; higher compressive stresses at the stent and significant increases in total stress were predicted for this tissue. The application of elevated pressure loading also increased the compressive and total stresses in the leaflet. Finally, it was shown that bicuspid leaflets were likely to experience higher stresses than tricuspid leaflets. This simple stress analysis should help valve designers of pericardial heterografts to identify those conditions which lead to tissue compression, high total stress, and ultimately material fatigue.

Animals↗
PubMed1987

The extension rate independence of the hysteresis in glutaraldehyde-fixed bovine pericardium.

Glutaraldehyde-fixed bovine pericardium demonstrates both relaxation of stress and hysteresis during uniaxial loading and unloading. These phenomena suggest viscoelastic behaviour of the material. This study uses analysis of variance to test statistically the hypothesis that the chemically modified pericardium possesses a 'fading memory' for the history of previous load procedures and that the hysteresis is extension rate independent.

Aldehydes↗
PubMed1986

Anal cushions.

Explore the source record for details and available documents.

Anal Canal↗
PubMed1986

An analysis of anal sphincter pressure and anal compliance in normal subjects.

To investigate anal sphincter mechanics, anal pressure was measured in 14 normal males and 11 normal females using probes of 0.4 to 3 cm in diameter. Resting pressure profiles on insertion and withdrawal did not differ significantly. Anteroposterior pressure differences could be explained by leverage of rigid probes against the anterior rectal wall. A maximal voluntary squeeze increased pressure throughout the anus, whereas the recto-anal inhibitory reflex resulted in a greater reduction in pressure in the upper part of the anal canal. Resting pressure, squeeze pressure and minimum residual pressure (during rectal distension) rose with increasing anal diameter. Estimated sphincter tension was linearly related to anal diameter and the slope of this relationship was increased by sphincter contraction and reduced by sphincter relaxation. The deviation from linearity of this relationship at low anal diameters may be due to swelling of the anal cushions to maintain anal pressure when muscular tension approaches zero.

Adult↗