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Biomedical subjects

T D Clark

Publications and source records attributed to T D Clark.

At least 19 recordsLinked to original sources
PubMed2006

Physiological response to feeding in little penguins.

Specific dynamic action (SDA), the increase in metabolic rate above resting levels that accompanies the processes of digestion and assimilation of food, can form a substantial part of the daily energy budget of free-ranging animals. We measured heart rate (fH) and rate of oxygen consumption (VO2) in 12 little penguins while they digested a meal of sardines in order to determine whether they show specific dynamic action. In contrast to some studies of other penguin species, little penguins showed a substantial SDA, the magnitude of which was proportional to the size of the meal. The energy utilized in SDA was equivalent to 13.4% of the available energy content of the fish. Furthermore, animals such as penguins that forage in a cold environment will probably expend further energy in heating their food to body temperature to facilitate efficient digestion. It is estimated that this additional energy expenditure was equivalent to 1.6%-2.3% of the available energy content of the fish, depending on the time of year and therefore the temperature of the water. Changes in fH during digestion were qualitatively similar to those in VO2, implying that there were no substantial circulatory adjustments during digestion and that the relationship between fH and VO2 in penguins is unaffected by digestive state.

Animals↗
PubMed2006

Predicting rate of oxygen consumption from heart rate while little penguins work, rest and play.

The relationship between heart rate (f(H)) and rate of oxygen consumption (V(.)O2) was investigated under changing conditions of ambient temperature, digestive state and exercise state in the little penguin (Eudyptula minor). Both f(H) and V(.)O2 were recorded simultaneously from 12 little penguins while they each (a) rested and exercised within their reported thermo-neutral zone (TNZ), (b) rested and exercised below their reported TNZ and (c) digested a meal of sardines within their reported TNZ. Contrary to our expectations, we found that minimum V(.)O2 did not vary between the two temperatures used. Comparison with values from the literature suggests that both minimum V(.)O2 and the extent of the TNZ in this species may vary along a latitudinal gradient. Furthermore, while minimum V(.)O2 was unchanged at the lower temperature, minimum f(H) was significantly higher, suggesting a hitherto undescribed cardiac response to lowered ambient temperature in an avian species. This response was maintained when the penguins exercised within and below their apparent TNZ as f(H) was significantly greater in cold conditions for a given level of V(.)O2. Furthermore, both f(H) and V(.)O2 were slightly but significantly elevated for a given walking speed during exercise at the lower temperature. This suggests that the penguins may have been close to their TNZ and that the measures employed to counteract heat loss while at rest may have been compromised during exercise. There was no significant difference in the relationship between f(H) and V(.)O2 while the penguins were inactive ina post-digestive state or inactive and digesting a meal within their TNZ, though both of these relationships were significantly different from that during exercise. This suggests that while digestion has no effect on the f(H)/V(.)O2 relationship, for little penguins at least, it is of little value in deriving a predictive relationship for application to active free-ranging animals.

Animals↗
PubMed2006

Cardiorespiratory physiology and swimming energetics of a high-energy-demand teleost, the yellowtail kingfish (Seriola lalandi).

This study utilizes a swimming respirometer to investigate the effects of exercise and temperature on cardiorespiratory function of an active teleost, the yellowtail kingfish (Seriola lalandi). The standard aerobic metabolic rate (SMR) of S. lalandi (mean body mass 2.1 kg) ranges from 1.55 mg min(-1) kg(-1) at 20 degrees C to 3.31 mg min(-1) kg(-1) at 25 degrees C. This 2.1-fold increase in SMR with temperature is associated with a 1.5-fold increase in heart rate from 77 to 117 beats min(-1), while cardiac stroke volume remains constant at 0.38 ml beat(-1) kg(-1) and the difference in oxygen content between arterial and mixed venous blood [(Ca(O2)-Cv(O2))] increases marginally from 0.06 to 0.08 mg ml(-1). During maximal aerobic exercise (2.3 BL s(-1)) at both temperatures, however, increases in cardiac output are limited to about 1.3-fold, and increases in oxygen consumption rates (up to 10.93 mg min(-1) kg(-1) at 20 degrees C and 13.32 mg min(-1) kg(-1) at 25 degrees C) are mediated primarily through augmentation of (Ca(O2)-Cv(O2)) to 0.29 mg ml(-1) at 20 degrees C and 0.25 mg ml(-1) at 25 degrees C. It seems, therefore, that the heart of S. lalandi routinely works close to its maximum capacity at a given temperature, and changes in aerobic metabolism due to exercise are greatly reliant on high blood oxygen-carrying capacity and (Ca(O2)-Cv(O2)). Gross aerobic cost of transport (GCOT) is 0.06 mg kg(-1) BL(-1) at 20 degrees C and 0.09 mg kg(-1) BL(-1) at 25 degrees C at the optimal swimming velocities (U(opt)) of 1.2 BL s(-1) (opt) and 1.7 BL s(-1), respectively. These values are comparable with those reported for salmon and tuna, implying that the interspecific diversity in locomotor mode (e.g. subcarangiform, carangiform and thunniform) is not concomitant with similar diversity in swimming efficiency. A low GCOT is maintained as swimming velocity increases above U(opt), which may partly result from energy savings associated with the progressive transition from opercular ventilation to ram ventilation.

Analysis of Variance↗
PubMed2005

Signatures of chaoticlike and nonchaoticlike behavior in a nonlinear quantum oscillator through photon detection.

The driven nonlinear Duffing oscillator is a very good, and standard, example of a quantum mechanical system from which classical-like orbits can be recovered from unravelings of the master equation. In order to generate such trajectories in the phase space of this oscillator, in this paper we use the quantum jump unraveling together with a suitable application of the correspondence principle. We analyze the measured readout by considering the power spectra of photon counts produced by the quantum jumps. Here we show that localization of the wave packet from the measurement of the oscillator by the photon detector produces a concomitant structure in the power spectra of the measured output. Furthermore, we demonstrate that this spectral analysis can be used to distinguish between different modes of the underlying dynamics of the oscillator.

Journal Article↗
PubMed2005

Control of multilevel voltage states in a hysteretic superconducting-quantum-interference-device ring-resonator system.

In this paper we study numerical solutions to the quasiclassical equations of motion for a superconducting-quantum-interference device ring-radio frequency (rf) resonator system in the regime where the ring is highly hysteretic. In line with experiment, we show that for a suitable choice of ring circuit parameters the solutions to these equations of motion comprise sets of levels in the rf voltage-current dynamics of the coupled system. We further demonstrate that transitions, both up and down, between these levels can be controlled by voltage pulses applied to the system, thus opening up the possibility of high order (e.g., 10 state), multilevel logic and memory.

Journal Article↗
PubMed2005

A compact electric potential sensor array for the acquisition and reconstruction of the 7-lead electrocardiogram without electrical charge contact with the skin.

Conventional electrocardiogram (ECG) systems make use of separate electrical connections to the arms and legs. These use a 'long baseline' for the voltage reference potential which in the case of precordial ECG leads is provided using a Wilson central terminal (WCT) wiring configuration. The aims of this project were (a) to construct compact, non-invasive surface ECG sensor arrays which would operate without the need for a WCT reference, (b) to obtain high quality precordial ECGs showing fine differences in ECG detail between small adjacent areas of the chest and (c) to reconstruct, from a compact array of four sensors, ECGs which closely match to the conventional 7-lead ECG system, but without the need for multiple wires and long baselines. In this paper, we describe two sensor array configurations which have been constructed using electric potential sensors (EPSs). We show high quality precordial ECGs obtained from small areas of the surface of the chest and show the different angular vectors (leads) in the frontal cardiac plane constructed using signals from the array elements. We suggest that these ECG arrays, which are simple to apply, should prove to be a valuable tool in providing useful information about the state of the heart.

Amplifiers, Electronic↗
PubMed2005

Factorial aerobic scope is independent of temperature and primarily modulated by heart rate in exercising Murray cod (Maccullochella peelii peelii).

Several previous reports, often from studies utilising heavily instrumented animals, have indicated that for teleosts, the increase in cardiac output (Vb) during exercise is mainly the result of an increase in cardiac stroke volume (V(S)) rather than in heart rate (fH). More recently, this contention has been questioned following studies on animals carrying less instrumentation, though the debate continues. In an attempt to shed more light on the situation, we examined the heart rates and oxygen consumption rates (Mo2; normalised to a mass of 1 kg, given as Mo2kg) of six Murray cod (Maccullochella peelii peelii; mean mass+/-SE = 1.81+/-0.14 kg) equipped with implanted fH and body temperature data loggers. Data were determined during exposure to varying temperatures and swimming speeds to encompass the majority of the biological scope of this species. An increase in body temperature (Tb) from 14 degrees C to 29 degrees C resulted in linear increases in Mo2kg (26.67-41.78 micromol min(-1) kg(-1)) and fH (22.3-60.8 beats min(-1)) during routine exercise but a decrease in the oxygen pulse (the amount of oxygen extracted per heartbeat; 1.28-0.74 micromol beat(-1) kg(-1)). During maximum exercise, the factorial increase in Mo2kg was calculated to be 3.7 at all temperatures and was the result of temperature-independent 2.2- and 1.7-fold increases in fH and oxygen pulse, respectively. The constant factorial increases in fH and oxygen pulse suggest that the cardiovascular variables of the Murray cod have temperature-independent maximum gains that contribute to maximal oxygen transport during exercise. At the expense of a larger factorial aerobic scope at an optimal temperature, as has been reported for species of salmon and trout, it is possible that the Murray cod has evolved a lower, but temperature-independent, factorial aerobic scope as an adaptation to the largely fluctuating and unpredictable thermal climate of southeastern Australia.

Animals↗
PubMed2005

Digestive state influences the heart rate hysteresis and rates of heat exchange in the varanid lizard Varanus rosenbergi.

To maximize the period where body temperature (Tb) exceeds ambient temperature (Ta), many reptiles have been reported to regulate heart rate (fH) and peripheral blood flow so that the rate of heat gain in a warming environment occurs more rapidly than the rate of heat loss in a cooling environment. It may be hypothesized that the rate of cooling, particularly at relatively cool Tbs, would be further reduced during postprandial periods when specific dynamic action (SDA) increases endogenous heat production (i.e. the heat increment of feeding). Furthermore, it may also be hypothesized that the increased perfusion of the gastrointestinal organs that occurs during digestion may limit peripheral blood flow and thus compromise the rate of heating. Finally, if the changes in fh are solely for the purpose of thermoregulation, there should be no associated changes in energy demand and, consequently, no hysteresis in the rate of oxygen consumption (V(O2)). To test these hypotheses, seven individual Varanus rosenbergi were heated and cooled between 19 degrees C and 35 degrees C following at least 8 days fasting and then approximately 25 h after consumption of a meal (mean 10% of fasted body mass). For a given Tb between the range of 19-35 degrees C, fh of fasting lizards was higher during heating than during cooling. Postprandial lizards also displayed a hysteresis in fh, although the magnitude was reduced in comparison with that of fasting lizards as a result of a higher fh during cooling in postprandial animals. Both for fasting and postprandial lizards, there was no hysteresis in V(O2) at any Tb throughout the range although, as a result of SDA, postprandial animals displayed a significantly higher V(O2) than fasting animals both during heating and during cooling at Tbs above 24 degrees C. The values of fh during heating at a given Tb were the same for fasting and postprandial animals, which, in combination with a slower rate of heating in postprandial animals, suggests that a prioritization of blood flow to the gastrointestinal organs during digestion is occurring at the expense of higher rates of heating. Additionally, postprandial lizards took longer to cool at Tbs below 23 degrees C, suggesting that the endogenous heat produced during digestion temporarily enhances thermoregulatory ability at lower temperatures, which would presumably assist V. rosenbergi during cooler periods in the natural environment by augmenting temperature-dependent physiological processes.

Animals↗
PubMed2004

Factorial scopes of cardio-metabolic variables remain constant with changes in body temperature in the varanid lizard, Varanus rosenbergi.

The majority of information concerning the cardio-metabolic performance of varanids during exercise is limited to a few species at their preferred body temperature (T(b)) even though, being ectotherms, varanids naturally experience rather large changes in T(b). Although it is well established that absolute aerobic scope declines with decreasing T(b), it is not known whether changes in cardiac output (V(b)) and/or tissue oxygen extraction, (Ca(O2) - Cv(O2)), are in proportion to the rate of oxygen consumption (Vo(2)). To test this, we studied six Rosenberg's goannas (Varanus rosenbergi) while at rest and while maximally exercising on a treadmill both at 25 and 36 degrees C. During maximum exercise both at 25 and 36 degrees C, mass-specific rate of oxygen consumption (Vo(2kg)) increased with an absolute scope of 8.5 ml min(-1) kg(-1) and 15.7 ml min(-1) kg(-1), respectively. Interestingly, the factorial aerobic scope was temperature-independent and remained at 7.0 which, at each T(b), was primarily the result of an increase in V(bkg), governed by approximate twofold increases both in heart rate (f(H)) and cardiac stroke volume (V(Skg)). Both at 25 degrees C and 36 degrees C, the increase in V(bkg) alone was not sufficient to provide all of the additional oxygen required to attain maximal Vo(2kg), as indicated by a decrease in the blood convection requirement V(bkg)/Vo(2kg); hence, there was a compensatory twofold increase in (Ca(O2) - Cv(O2)). Although associated with an increase in hemoglobin-oxygen affinity, a decrease in T(b) did not impair unloading of oxygen at the tissues and act to reduce (Ca(O2) - Cv(O2)); both Ca(O2)) and Cv(O2)) were maintained across T(b). The change in Vo(2kg) with T(b), therefore, is solely reliant on the thermal dependence of V(bkg). Maintaining a high factorial aerobic scope across a range of T(b) confers an advantage in that cooler animals can achieve higher absolute aerobic scopes and presumably improved aerobic performance than would otherwise be achievable.

Aerobiosis↗
PubMed2002

Risk factors and cumulative incidence of anaemia among human immunodeficiency virus-infected children in Uganda.

Anaemia has not been well characterised among HIV-infected children in sub-Saharan Africa. Baseline prevalence and cumulative incidence of anaemia (haemoglobin < 110 g/L) were 91.7% and 100% and, for moderate anaemia (haemoglobin < 90 g/L), were 35.1% and 58.4%, respectively, among 225 HIV-infected children followed from 9 to 36 months of age. Hospitalisation, suspected tuberculosis, malaria and height-for-age Z-score <-2 were significantly associated with moderate anaemia. Moderate anaemia and weight-for-height Z-score <-2 were associated with mortality. Anaemia is common and associated with increased mortality in HIV-infected children.

AIDS-Related Opportunistic Infections↗
PubMed2001

Self-assembly of 10-microm-sized objects into ordered three-dimensional arrays.

This paper describes the self-assembly of small objects--polyhedral metal plates with largest dimensions of 10 to 30 microm--into highly ordered, three-dimensional arrays. The plates were fabricated using photolithography and electrodeposition techniques, and the faces of the plates were functionalized to be hydrophobic or hydrophilic using self-assembled monolayers (SAMs). Self-assembly occurs in water through capillary interactions between thin films of a hydrophobic liquid (a liquid prepolymer adhesive) coated onto the hydrophobic faces of the plates; coalescence of the adhesive films reduces the interfacial free energy of the system and drives self-assembly. By altering the size and surface-patterning of the plates, the external morphologies of the aggregates were varied. Curing the adhesive furnished mechanically stable aggregates that were characterized by scanning electron microscopy (SEM). For assemblies formed by plates partially composed of a sacrificial material, a subsequent etching step furnished fully open, three-dimensional microstructures. This work validates the use of capillary interactions for three-dimensional mesoscale self-assembly in the 10-microm-size regime and opens new avenues for the fabrication of complex, three-dimensional microscructures.

Electroplating↗
PubMed2001

Pinch resonances in a radio-frequency-driven superconducting-quantum-interference-device ring-resonator system.

In this paper we present experimental data on the frequency domain response of a superconducting-quantum-interference-device ring (a Josephson weak link enclosed by a thick superconducting ring) coupled to a radio frequency tank circuit resonator. We show that with the ring weakly hysteretic the resonance line shape of this coupled system can display opposed fold bifurcations that appear to touch (pinch off). We demonstrate that for appropriate circuit parameters these pinch off line shapes exist as solutions of the nonlinear equations of motion for the system.

Journal Article↗
PubMed2001

Iron supplementation during human immunodeficiency virus infection: a double-edged sword?

Although iron supplementation is considered beneficial for groups at risk for anemia, concern has been raised that it could be harmful during human immunodeficiency virus (HIV) infection. Studies suggest: (1) faster HIV disease progression in thalassemia major patients receiving inadequate doses of iron-chelating drug; (2) higher mortality among patients receiving iron supplementation with dapsone compared with aerosolized pentamidine for prophylaxis against Pneumocytis carinii pneumonia; (3) higher iron stores and mortality among patients with haptoglobin Hp 2-2 phenotype; and (4) shorter survival among patients with high bone marrow iron deposition. These studies largely involved men in developed countries. Among HIV-infected pregnant women in Africa with a high prevalence of iron deficiency, no relationship was found between indicators of iron status and HIV disease severity. The available data do not contraindicate the current practice of iron supplementation in developing countries where there is a high prevalence of both HIV infection and iron deficiency.

Anemia, Iron-Deficiency↗
PubMed2001

Stereotactic breast biopsy: a six-year surgical experience.

A retrospective review was done of all stereotactic breast biopsies performed at the Central Baptist Hospital Breast Center from February 1994 through December 1999. A total of 1,080 biopsies were performed in 1,026 patients, all by surgeons working independently. Masses were biopsied in 54% and calcifications in 40%. Eighteen percent of biopsies were malignant. The most common benign diagnosis was fibrocystic disease (72%), followed by fibroadenoma (19%), lymph node (2%), and papilloma (2%). The most common malignant diagnosis was invasive ductal carcinoma (40%) followed by ductal carcinoma in situ (32%) and mixed invasive and in situ ductal carcinoma (19%). A prebiopsy BI-RADS mammographic Category III was associated with a 2% incidence of malignancy; Category IV--17%; Category V--90%. Atypical ductal hyperplasia on stereotactic biopsy was upgraded to a malignant diagnosis after reexcision in 19% of the cases. The false-negative rate was 0.4% (sensitivity 99%) and the complication rate was 3%, mostly related to bleeding. Stereotactic biopsy is a safe and accurate technique for the minimally-invasive diagnosis of abnormal mammograms.

Adult↗
PubMed1999

Practical approach to evidence-based management of caries.

This paper discusses evidence-based management of dental caries with regard to: (1) need to adopt new office methods, (2) potential barriers to change, and (3) possible practical solutions to aid change. The need for classifying individual patients into low-, medium-, and high-risk caries groups is justified from a review of the epidemiological characteristics of caries. In addition, a deficiency is identified in traditional caries recording methods since they are unable to grade the severity and activity of individual lesions. The traditional basis of six-monthly recall examinations for all patients is shown from the literature to have no scientific support. It is suggested a three-twelve month recall interval be used, depending on a patient's risk group classification. Some barriers to change are identified as: (1) the collection of more comprehensive history and clinical caries data, (2) the complexity of evidence-based decision-making, and (3) dentists' difficulty in standardizing decision-making. A new pictorial classification for caries severity and activity is described. A demonstration decision-support system is presented in terms of assisting collection of data, automatic identification of risk factors, patient risk classification, and generation of a suggested treatment plan. Evidence-based management may result in change of professional manpower levels.

Dental Caries↗
PubMed1996

Comparison of three methods to estimate heparin loading dose for cardiopulmonary bypass.

Three available methods used to determine heparin loading dose were studied to determine the most reliable method for reaching a target pre-bypass activated clotting time (ACT) of 510 seconds. One hundred and seven patients were randomly assigned to one of three treatment methods: A) 300 units/kg; B) Hemostasis Management System (HMS); C) RX/DX. Five different lots of heparin were assigned to Groups A and B, and Group C had one heparin lot. Different lots were used to account for possible variations in heparin activity. Post-skin incision ACTs, post-heparin pre-bypass ACTs, and heparin loading doses were compared. The mean and standard deviation of the post-heparin pre-bypass ACTs were used to determine which method was most reliable to obtain a desired ACT. There was no statistical difference between different heparin lots. There was no difference in the post-heparin ACTs for the three methods (A:487 +/- 135 vs. B:474 +/- 105 vs. C:474 +/- 111 sec). There was a statistically significant difference between the standard deviation for the HMS and 300 u/kg standard deviations (p < 0.05). The HMS has the smallest deviation which makes it the most reliable predictor of heparin loading doses to reach a target ACT for cardiopulmonary bypass.

Adult↗
PubMed1993

Effect of dietary vitamin D3 (cholecalciferol) on colon carcinogenesis induced by 1,2-dimethylhydrazine in male Fischer 344 rats.

This study examined the effect of increasing dietary vitamin D on chemically induced colon carcinogenesis. Male Fischer 344 rats were first injected with 1,2-dimethylhydrazine (200 mg/kg) and then fed one of five dietary levels of vitamin D as cholecalciferol (250, 1,000, 2,000, 4,000, and 10,000 IU/kg diet) for nine months. Dietary vitamin D3 had no effect on weight gain. Plasma 25-hydroxyvitamin D3 levels were similar for the 1,000 and 2,000 IU/kg groups but varied in a dose-related manner for the other groups. Vitamin D did not significantly alter the tumor incidence in either the distal or the proximal colon. No significant differences in the labeling index were found in either the proximal or the distal colon. Within the distal colon, the proliferative zone increased in a dose-related manner. Distribution of labeled cells within the crypt compartments was not affected by dietary vitamin D. Bone and serum minerals in general were unaffected by dietary vitamin D. This study shows that, at this level of dietary calcium, vitamin D did not affect 1,2-dimethylhydrazine-induced colon carcinogenesis.

1,2-Dimethylhydrazine↗