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Biomedical subjects

S W Smye

Publications and source records attributed to S W Smye.

At least 19 recordsLinked to original sources
PubMed2002

A mathematical model of post-canalization thrombolysis.

During the initial phase of lysis of an occlusive thrombus using lytic agents such as tissue plasminogen activator, blood flow through the centre of the clot is established (the process of recanalization). Following canalization, the clot remains on the vessel wall and further lysis is required. This paper develops a multi-species mathematical model to describe the bulk chemical reactions in the bloodstream and the convective and diffusive transport of chemical species to and from the clot surface in conditions following canalization. For the steady state case, the model indicates that the process of clot lysis following initial recanalization is dominated by surface chemical reactions and the bulk reactions play little role in the lytic process. Lytic rate is dependent on the clot geometry and flow conditions. The rate of clot dissolution is greatest at the upstream end of the clot and decreases steadily downstream due to lytic agent being removed from the flowing blood as it binds to the clot surface. This model may be further developed and used to simulate and compare different lytic regimes.

Animals↗
PubMed2002

Total body water measurement using bioelectrical impedance analysis, isotope dilution and total body potassium: a scoring system to facilitate intercomparison.

OBJECTIVES: (1) To develop a scale that is useful in evaluating the accuracy of multifrequency bioelectrical impedance analysis (MF-BIA) in the assessment of body water volumes against the accepted gold standard measurements based on isotope-dilution and total body potassium (TBK). (2) To perform a pilot test of the scale. DESIGN: A scale was developed to evaluate the accuracy of MF-BIA in the assessment of body water volumes. Questions were obtained from reading the scientific literature and discussions involving the four authors. Three of these and two additional independent readers pre-tested the scale. A weighting was identified for each question and a pilot test with a sample of 10 articles (different to those used for the questionnaire performance) was conducted. A further validation was carried out with a second set of 20 articles and two additional independent readers. RESULTS: The kappa statistic expressing the level of agreement between pairs of the first three authors using this scale with 10 articles, was 0.3, 0.4 and 0.6 after the first attempt. A second evaluation after specific changes improved the agreement to 0.8, 0.6 and 0.8. The mean score for 10 articles was 252+/-36 points from a total score of 400 (63+/-9%). The evaluation with the second set of 20 articles resulted in a kappa of 0.7 from two pairs of authors. The evaluation with two additional reviewers resulted in a kappa=0.7. CONCLUSION: A tool has been developed to assess the accuracy of the MF-BIA technique and to identify methodological components, plan future studies and critically evaluate data in this area. It is likely that this tool may also be used to assess the accuracy of single frequency studies.

Body Water↗
PubMed2002

Temperature rise during reamed tibial nailing.

The current study determined the temperature rise during reamed tibial intramedullary nailing in vivo. Eighteen adult patients were studied. The tibial medullary canal diameter ranged from 8 to 11 mm and was reamed to at least 1.5 mm above the required nail diameter with AO reamers. Reaming of the medullary cavity ranged from 9 to 12 mm before nail insertion. Intraoperative monitoring of the heat produced during reaming of the medullary cavity was done by inserting two platinum resistance thermometer probes into the cortical bone at the short isthmic segment of the tibial shaft. The probes were connected to a data logger, and temperature readings were taken every 5 seconds during each reaming procedure. The mean tibial temperature before initiation of reaming was 35.6 degrees C (standard deviation, +/- 0.5 degrees), and peak temperatures recorded were from 36.1 degrees C to 51.6 degrees C. A direct correlation was observed between temperature elevation and amount of reaming. With reaming above 10 mm, tibias with a canal diameter of 8 mm showed a statistically higher temperature rise compared with tibias with a canal diameter of 9, 10, or 11 mm. No patients had intraoperative or postoperative complications related to skin or bone thermal necrosis, and bony healing progressed uneventfully. The small amount of reaming required to insert a nail into a normal 9-, 10-, or 11- mm tibial canal does not seem to produce a clinical problem. Reaming smaller canals (8 mm) to a larger size may induce a significant heating effect.

Adolescent↗
PubMed2002

Friction burns within the tibia during reaming. Are they affected by the use of a tourniquet?

We have carried out a prospective, randomised trial to measure the rise of temperature during reaming of the tibia before intramedullary nailing. We studied 34 patients with a mean age of 35.1 years (18 to 63) and mean injury severity score of 10 (9 to 13). The patients were randomised into two groups: group 1 included 18 patients whose procedure was undertaken without a tourniquet and group 2, 16 patients in whom a tourniquet was used. The temperature in the bone was measured directly by two thermocouples inserted into the cortical bone near the isthmus of the tibial diaphysis. Reaming was carried out to at least 1.5 mm above the required diameter of the nail. Blood loss was assessed by recording the preoperative and postoperative haemoglobin (Hb) level. The minimum clinical follow-up was six months. In group 1 (no tourniquet), the mean Hb dropped 2.8 g/dl from 14.3 +/- 1.02 g/dl to 11.5 +/- 1.04 g/dl (p = 0.0001), whereas with the tourniquet, the mean decrease was 1.3 g/dl from 14 +/- 1 g/dl to 12.7 +/- 1.3 g/dl (p = 0.007). This difference was not statistically significant. The mean initial tibial temperature was 35.6 degrees C (SD 0.6) and rose with reaming to levels between 36.3 degrees C and 51.6 degrees C. The highest temperatures were obtained with the largest reamers (11 and 12 mm, p = 0.0001) and the most rapid rise with the smallest diameters of medullary canal (8 or 9 mm). The rise of temperature was transient (20 s). We were unable to identify any effect of the use of a tourniquet on the temperature achieved. Reamed intramedullary tibial nailing induces a transient elevation of temperature which is directly related to the amount of reaming.

Adolescent↗
PubMed2001

A mathematical comparison of two models of the electrical properties of biological tissues.

The purpose of this paper is to compare two models of the electrical properties of tissue, which may be used to relate the effective conductivity to the volume fraction f of cells in the tissue. Both models assume that tissue comprises spherical cells, which behave electrically as dipoles. The first model, developed by Hanai, describes the tissue as a concentrated suspension of weakly conducting spheres in a conducting medium, with each sphere experiencing a uniform mean field. The second approach, developed by Chiew and Glandt, explicitly describes the effect of a random but statistically homogeneous cell structure on the average field and magnitude of the dipole interaction. The two analyses are identical to first order in f, but differ in the way in which the interactions between the dipoles are accounted for. The model developed by Chiew and Glandt appears to offer a more robust theoretical framework for describing the electrical properties of tissue. The comparison aims to contribute to an improved understanding of the relationship between the electrical properties and spatial structure of tissue.

Electric Conductivity↗
PubMed2001

The interaction between Terahertz radiation and biological tissue.

Terahertz (THz) radiation occupies that region of the electromagnetic (EM) spectrum between approximately 0.3 and 20 THz. Recent advances in methods of producing THz radiation have stimulated interest in studying the interaction between radiation and biological molecules and tissue. Given that the photon energies associated with this region of the spectrum are 2.0 x 10(-22) to 1.3 x 10(-20) J, an analysis of the interactions requires an understanding of the permittivity and conductivity of the medium (which describe the bulk motions of the molecules) and the possible transitions between the molecular energy levels. This paper reviews current understanding of the interactions between THz radiation and biological molecules, cells and tissues. At frequencies below approximately 6 THz. the interaction may be understood as a classical EM wave interaction (using the parameters of permittivity and conductivity), whereas at higher frequencies. transitions between different molecular vibrational and rotational energy levels become increasingly important and are more readily understood using a quantum-mechanical framework. The latter is of particular interest in using THz to probe transitions between different vibrational modes of deoxyribonucleic acid. Much additional experimental work is required in order to fully understand the interactions between THz radiation and biological molecules and tissue.

Amino Acids↗
PubMed2001

Measurement of intercompartmental fluid shifts during haemodialysis in children.

Seven children (age range 12-19 years, post-dialysis weights 23-43 kg) were studied during 20 haemodialysis sessions. Impedance between wrist and ankle (on the non-fistula side) was recorded using the Xitron 4000B analyser. A 2 ml sample of blood was taken for total protein and haematocrit from the arterial line at the start of dialysis. At approximately 20 minute intervals during dialysis, the time and volume of ultrafiltrate removed were recorded, and a simultaneous measurement of whole body impedance made over 25 logarithmically spaced frequencies in the range 5-500 kHz. A 2 ml sample of blood was also taken, from which serum protein and haematocrit were calculated. Hypotensive episodes occurred during four haemodialysis sessions. The percentage change in extracellular fluid (ECF) volume was calculated, at each sample time for each session, using the impedance measurements and ultrafiltration measurements (denoted delta Vi and delta U respectively). Changes in the intravascular volume were estimated using measurements of haematocrit and serum protein (and denoted delta Vh and delta Vp respectively). Least-squares regression gave delta Vi = 3.77 delta Vh, 1.33 delta Vp and 0.39 delta U, and r2 = 0.72, 0.94 and 0.95 respectively (p < 0.0001 in each case) for the 16 dialysis sessions without hypotensive episodes. Similar analysis of four dialysis sessions with hypotensive episodes gave similar relationships with correlation coefficients 0.64, 0.92 and 0.94. These relationships may not be accounted for by the anthropometric terms alone in the impedance equations. Impedance measurements also detected the addition of 300 ml isotonic saline given at the onset of each of the four hypovolaemic episodes. The regression equations support the following hypothesis: during haemodialysis, ultrafiltrate is removed from the intravascular volume but is replenished by fluid from the interstitial volume. The reduction in ECF volume measured by impedance (where the ECF comprises the intravascular and interstitial volumes) delta Vi is therefore greater than delta Vh and delta Vp, which only measure intravascular volume, but less than delta U since the ECF is replenished by fluid from the interstitial space. That delta Vh is greater than delta Vp may be due to protein loss during dialysis. The results suggest that whole body impedance measurements reflect changing body water distribution during dialysis in children.

Adolescent↗
PubMed2000

Standardized limb abduction for bioimpedance measurements using position restraints.

Bioelectrical impedance analysis (BIA) measurements are widely used in the assessment of body hydration. However, there are many variables which can influence the impedance values obtained and their individual significance is not entirely clear. One such variable is limb position and it is possible to standardize this either by invoking subject cooperation or by the use of some form of mechanical constraint. This study evaluates these two approaches. BIA measurements were made on five healthy male volunteers both with the positioners and without. Additional variables including room temperature, prandial status, exercise, alcohol intake, bed surface for the test, and bladder status were controlled. The mean percentage difference between impedance values with and without positioners was (-0.37 +/- 0.69)%. The mean difference between occasions with and without mechanical fixing was (2.02 +/- 2.5)% and (2.58 +/- 3)% respectively. None of these differences is statistically significant. It was concluded that reproducibility of BIA measurements obtained with active subject cooperation in limb position is not significantly improved if mechanical positioners are used. This may not apply where subject cooperation is poor e.g. in very sick or very young people or in repeated measurements in different days.

Adult↗
PubMed1998

Impedance index measurements of in vitro PTFE end-to-side anastomoses: effect of angle and Miller cuff.

OBJECTIVE: To measure non-invasively the impedance index of a range of geometries of PTFE end-to-side anastomoses (ESA). DESIGN: In vitro experiments using a custom-built flow rig. SUBJECTS: Anastomoses constructed at each angle. (15 degrees, 30 degrees, 45 degrees, 60 degrees, 90 degrees, 120 degrees) for both standard and cuffed ESA. RESULTS: The impedance index of each ESA increased non-linearly with flow rate. The impedance index at a given flow rate was reduced by decreasing the anastomotic angle and further reduced by the addition of a Miller cuff. CONCLUSIONS: The reduction in impedance index achieved with a Miller cuff may help to explain the improved patency rates of femoropopliteal grafts incorporating a cuff.

Anastomosis, Surgical↗
PubMed1998

Sources of error in intra-arterial pressure measurements across a stenosis.

OBJECTIVE: To investigate potential errors associated with different techniques of intra-arterial pressure measurement at angiography. MATERIALS AND METHODS: An experimental model of an arterial stenosis was developed. Experiments were performed to assess the relevance of catheter position, catheter direction and catheter type on the recorded intraluminal pressure. Transstenotic pressure gradients were recorded with and without angiographic catheters crossing the stenosis. RESULTS: At physiological flow rates angiographic catheter type does not influence the recorded pressure. At high flow rates through tight stenoses there is a significant catheter-related difference in recorded pressure adjacent to a stenosis. Downstream pressures may be altered by up to 85 mmHg when standard angiographic catheters are placed across a stenosis. CONCLUSION: The different techniques employed to measure pressure differences across a model stenosis may introduce significant errors up to 85 mmHg. Care must be taken when pressure measurements alone are used to interpret the clinical significance of a stenosis. In low flow conditions there may not be a detectable pressure gradient across a 95% stenosis.

Angiography↗
PubMed1998

Extracellular fluid volume determined by bioelectric impedance and serum albumin in CAPD patients.

AIM: To investigate the relationship between serum albumin and extracellular fluid volume, as measured by multifrequency bioelectrical impedance, in stable patients treated by CAPD. METHOD: Fifty-nine stable CAPD patients were assessed. Serum albumin (bromocresol green) and CRP, age, dialysate to plasma (D/P) creatinine ratio, normalized protein catabolic rate (nPCR), daily urine and peritoneal protein losses, and extracellular fluid volume (Vecf) were measured in each patient. Vecf was calculated as a percentage of actual body weight (Vecf% ABW), of lean body mass derived from anthropometry (Vecf% LBM) and of total body water (Vecf% Vtbw). Comparisons between those with a normal serum albumin (> or = 37 g/l) and those with a low serum albumin (< 37 g/l) were made by Mann-Whitney U test. Correlations with serum albumin were sought by Pearson's test. RESULTS: The D/P creatinine ratio, daily peritoneal and urine protein losses, and extracellular fluid volume (Vecf% LBM and Vecf% Vtbw) were all significantly greater in patients with serum albumin < 37 g/l as compared to those > or = 37 g/l; P < 0.05. Age, CRP, and nPCR were not different. Serum albumin was negatively correlated with Vecf% LBM, r = -0.25; P = 0.05, Vecf% Vtbw, r = -0.39; P = 0.002, and daily urinary albumin loss, r = -0.25, P = 0.06. CONCLUSION: Hypoalbuminaemia is partly dependent on subclinical overhydration in CAPD patients. Serum albumin is negatively correlated with increased extracellular fluid volume and the proportion of Vecf to Vtbw is increased in hypoalbuminaemic patients. Multifrequency bioelectrical impedance is able to identify these abnormalities.

Adult↗
PubMed1998

Simulating the effect of exercise on urea clearance in hemodialysis.

A two-compartment model of urea kinetics during hemodialysis is used to predict the effect of exercise on hemodialysis dose. It is assumed that the two compartments represent tissues that are perfused by low and high blood flows (initially 1.1 L/min and 3.8 L/min). The effect of changing the distribution of flows between the compartments, emulating the effect of exercise, is simulated using the model equations for a range of dialyzer clearances. Compartmental volumes are assumed constant (33.4 L and 8.6 L for low- and high-flow compartments, respectively). The analysis identifies muscle perfusion as a rate-limiting factor during the later stages of hemodialysis and illustrates the benefit of exercise during this phase in increasing dialysis efficiency. The model suggests that the postdialysis rebound in the blood urea concentration is eliminated by increasing flow to the low-flow compartment from 1.1 L/min to 7.1 L/min and sustaining this for at least 30 min of a 150-min dialysis session, independent of the dialyzer clearance. Additional exercise will not increase the dialysis dose. Experimental studies are required to confirm the analysis.

Computer Simulation↗
PubMed1997

The application of neural networks in predicting the outcome of in-vitro fertilization.

Infertility affects one in six couples at some time in their lives, with 48% of these couples requiring assisted conception techniques in order to achieve a pregnancy. Whilst the overall clinical pregnancy rate per embryo transfer is 23%, this varies widely between clinics. The Human Fertilisation and Embryology Authority has attempted to analyse the results of all units, with weighting of different factors affecting assisted conception, and the published data have invariably led to comparisons between units. However, statistical models need to be developed to eliminate bias for valid comparisons. Neural networks offer a novel approach to pattern recognition. In some instances neural networks can identify a wider range of associations than other statistical techniques due in part to their ability to recognize highly non-linear associations. It was hoped that a neural network approach may be able to predict success for individual couples about to undergo in-vitro fertilization (IVF) treatment. A neural network was constructed using the variables of age, number of eggs recovered, number of embryos transferred and whether there was embryo freezing. Overall the network managed to achieve an accuracy of 59%.

Adult↗
PubMed1997

Assessment of nutritional status in CAPD patients: serum albumin is not a useful measure.

INTRODUCTION: In CAPD patients serum albumin is frequently used as an index of nutritional status, although it is recognized that hypoalbuminaemia may be caused by many factors. We have further examined the relationship between serum albumin and nutrition. METHODS: Nutritional status was assessed by biochemistry, anthropometry, mid-arm muscle circumference, muscle strength (hand grip and back), and lean body mass (from anthropometry, creatinine kinetics and bioimpedance) in a group of 76 stable CAPD patients. Correlations between biochemical and nutritional parameters were sought and data were compared between patient groups defined by serum albumin (> or = 37 vs < 37 g/l on two occasions 2 months apart) and separately according to subjective global assessment score (normal nutrition, A vs mild to moderate, B, and severe, C, malnutrition). RESULTS: In patients with a low SGA score, actual body weight, body mass index, mid-arm muscle circumference, lean body mass, subscapular skinfold thickness, hand grip strength (males and females) and iliac and triceps skinfold thicknesses and back strength (females only) were all significantly less than in patients with a normal SGA score. In contrast, none of these variables differed in either gender when patients were compared according to serum albumin. Serum albumin was correlated with serum creatinine (r = 0.45, P = 0.01), daily urine protein excretion (r = -0.42, P = 0.02) and uncorrected weekly creatinine clearance (r = -0.39) in females, but not with any index of body composition in either gender. CONCLUSION: Whilst SGA identified a patient group with significantly abnormal body mass, muscle mass and muscle strength, serum albumin did not. Serum albumin is not a useful marker of malnutrition in stable patients on CAPD.

Adolescent↗
PubMed1996

Total body water measurement in renal insufficiency.

Total body water was measured in 15 children with renal insufficiency (glomerular filtration rate < 25 ml/min per 1.73 m2) using deuterium oxide dilution. Total body water was also measured using bioelectrical impedance and skinfold anthropometry in the same 15 children. There was a linear correlation (r = 0.98, P < 0.01) between total body water measured by deuterium and height2/impedance. The 95% confidence limits for estimates of total body water were -1.5 to 0.9 for impedance and 0.65-3.16 l for skinfold anthropometry when compared with deuterium dilution. Bioelectrical impedance estimation of total body water is better than the current existing non-invasive method of skinfold anthropometry.

Adolescent↗