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Biomedical subjects

D Talbert

Publications and source records attributed to D Talbert.

At least 19 recordsLinked to original sources
PubMed2004

The dynamic placenta: II. Hypothetical model of a fetus driven transplacental water balance mechanism producing low apparent permeability in a highly permeable placenta.

In vitro and isotopic studies in vivo have reported the paradox that the human placenta is highly permeable, water exchanging at 3.6 litres per hour at 35 weeks of gestation, but clinical measurements in vivo show net transfer is minimal, around 2 ml/day. Current theories are based on osmotic pressure balances, but changes in maternofetal hydrostatic pressure change much faster than osmotic factors could respond. An alternative explanation might be that net transfer is not in fact the result of passive mechanisms, but is actively controlled by the fetus itself. The fetus is well equipped to monitor changes in blood volume, such as via sensors in venous and atrial stretch receptors to control ANF and hence urine production. Transplacental water regulation requires modification of transvillus pressures. Placental sub-chorial arteries and veins (of extra-embryonic origin) have different sensitivities from fetal body tissues to some vasoactive substances, and stem villous veins have unusually well developed vascular smooth muscle. It is thus theoretically possible for the fetus to modify subchorial venous resistances, and hence villous capillary pressure with a suitable circulating placental venous constrictive agent. A computer modelling study was undertaken using a fictitious placental venous constricting agent "fictensin", considered to be released by the fetus in proportion to disturbance of vascular volume. The effective placental permeability fell in proportion to the tightness of this fetal control mechanism, suggesting that the apparent placental permeability measured in vivo is a measure of fetal control, not true permeability. However, the range of compensatory pressures that the fetus can produce by this means is limited and failure of such a mechanism could allow flooding or dehydration of the feto-placental unit. This may shed new light on disorders such as polyhydramnios and fetal hydrops.

Animals↗
PubMed2004

The dynamic placenta: I. Hypothetical model of a placental mechanism matching local fetal blood flow to local intervillus oxygen delivery.

The placenta can be severely infarcted and yet return well oxygenated blood in spite of the potential shunt paths produced. Optimisation of oxygen transport by some form of local flow matching has been suggested, either via a direct action of hypoxia on subchorial vessels, or indirectly by syncytiotrophoblastic metabolic products. Using casts of cotyledonal vessels and software modelling, a mechanism of hypoxic fetoplacental vasoconstriction could be demonstrated. A simple previously described passive placental model was extended to include hypoxic sensitive arteries and dependence of syncytio-trophoblastic metabolism on intervillus (maternal) blood oxygen content. Such a mechanism of placental flow matching could maintain fetal pO2 by reducing flow through inadequately oxygenated cotyledons, therefore optimising pO2 at the expense of flow. A further modification stabilising fetal water transfer was required to avoid changes in intervillus oxygen delivery producing changes in fetal water content via placental capillary pressure alterations. Intervillus/villus flow matching is likely in the human placenta and this study suggests probable biologically plausible mechanisms for such a phenomenon.

Casts, Surgical↗
PubMed2004

Alveolar septal collapse in the transitional infant lung: a possible common mechanism in sudden unexpected death in infancy.

Sudden unexpected death in infancy (SUDI) is a category used to represent the largest single group of infant deaths. Although there are several theories, the cause of SUDI remains unknown and the mechanism of co-sleeping associated deaths are also undetermined. We investigate a possible biomechanical mechanism which may be common in SUDI and may provide an explanation for the association of the known risk factors for SUDI such as co-sleeping, prematurity, prone sleeping position, overwrapping, overheating and maternal smoking. The neonatal lung has few, if any, true septa but from about four weeks of age, a period of rapid alveolarisation commences. The developing alveolar walls (septae) have little fibre support against surface tension forces as they grow but are supported by a double layer of capillaries. Until the elastin/collagen supporting network is laid down these nascent septal walls are vulnerable to collapse against sac or duct walls during this transitional period. We hypothesise that such collapse will prevent one side of the septa, and the wall it overlays, from alveolar gas exchange and a functional left-right shunt is formed which may result in hypoxia. Furthermore, lung stretch receptors in bronchi running through or adjacent to collapsed regions will be activated, falsely signalling lung inflation to the brain stem with resultant respiratory inhibition, so precipitating further collapse. The process will continue until lung volume falls below residual capacity, when normal tidal breathing efforts will no longer result in significant air flow, even if stretch receptor signals have not produced complete apnoea. Large inspiratory efforts are then required to break the surface tension seal, which damages capillaries to produce petechial haemorrhages. Many epidemiological risk factors for SUDI could influence such a mechanism, leading to the proposal that Alveolar Septal Collapse in Infancy (ASCI) is a core mechanism via which these factors act.

Airway Obstruction↗
PubMed2003

Transmitted arterio-arterial anastomosis waveforms causing cyclically intermittent absent/reversed end-diastolic umbilical artery flow in monochorionic twins.

OBJECTIVES: To characterize the phenomenon of retrograde transmission of arterio-arterial anastomosis (AAA) interference patterns on umbilical artery (UA) waveform by (a) documenting the periodicity, (b) correlation with in vivo and in vitro demonstration of AAAs and (c) reproducing these patterns by computer modelling. METHODS: Monochorionic twins (MC) twins underwent placental and umbilical Doppler studies. AAAs were sought by pulse wave Doppler of their bi-directional interference pattern and confirmed by postnatal injection studies. The periodicity of transmitted patterns in the UA was determined. Determinants of the transmitted patterns were ascertained by computer modelling of physiological and fetal variables. RESULTS: Among 83 prospectively studied MC twin pregnancies; a transmitted pattern was observed in 6 (7 per cent) patients for 15-114 days. This was found in 20 per cent (6/30) of smaller MC twins discordant for growth restriction but in no appropriately grown twins. It was only observed in association with AAAs validated both in vivo and in ex vivo. Computer modelling demonstrated that this pattern could be reproduced by summating end diastolic flow with a high pulsatility index in the UA in the presence of a large AAA. Consistent with this, MC twins with a transmitted pattern had larger AAAs (median diameter 4.3 mm interquartile range 4.1-5.2) compared to MC twins discordant for intrauterine growth restriction (2.1 mm interquartile range 1.5 to 2.8) (P<0.05) without a transmitted pattern. Perinatal mortality was similar in the fetuses with and without transmitted patterns (0/12 vs. 2/48 P=0.7).

Adult↗
PubMed2002

The role of intraplacental vascular smooth muscle in the dynamic placenta: a conceptual framework for understanding uteroplacental disease.

Although non-innervated, the placenta must continually accommodate changes in uteroplacental pressure (due to spiral artery failure, maternal position, maternal emotional state, etc.), which might otherwise be expected to result in rapid feto-maternal water fluxes in the highly water-permeable human hemochorial placenta. Uteroplacental flow must also be under the same influences, producing temporary, or permanent, regions of poor intervillous flow, yet the reduction of umbilical vein oxygen content that would be expected to be produced by such shunts of feto-placental blood do not occur in the normal fetus. We suggest that there is a local villus tree mechanism matching intravillus flow of fetal blood to local uteroplacental oxygen content. By analogy to ventilation/perfusion (V/Q) matching in the postnatal lung we suggest the term U/Q matching for this mechanism in the placenta. We further suggest that such disturbances in flow matching are compensated for by the fetus, via complementary adjustment of umbilico-chorionic artery and umbilical venous flow resistances, utilizing the differing sensitivities of vascular smooth muscle tissues of embryonic and extra-embryonic origin to vasoactive agents.

Adaptation, Physiological↗
PubMed2001

Twin-to-twin transfusion syndrome results from dynamic asymmetrical reduction in placental anastomoses: a hypothesis.

Although placental vascular anastomoses between the fetoplacental circulations are ubiquitous in monochorionic twin pregnancies, the factors regulating their formation and maintenance are not understood. Increasing evidence implicates asymmetric anastomotic patterns in the aetiology of severe twin-to-twin transfusion syndrome (TTTS). The authors propose that anastomoses between placental circulations in monochorionic twins occur in a random manner at the embryological stage of connection of embryonic and extra-embryonic circulations. Placental expansion is then associated with random disruption of anastomoses and regression of their associated villus districts. TTTS develops as discordant loss of anastomoses results in asymmetrical flow resistance. Pregnancies with fetal growth concordance but discordant nuchal translucency at 10-14 weeks are at increased risk of developing subsequent severe TTTS because these clinical features indicate significant pressure differentials in the presence of a placentoplacental circulation, consistent with the presence of numerous, asymmetric anastomoses. However, since the anastomotic pattern is dynamic in the first half of pregnancy this hypothesis predicts that it will not be possible to devise a clinical test at 12 weeks that will predict with certainty the outcome of monochorionic twin pregnancies in relation to TTTS because this depends on random subsequent events.

Arteriovenous Anastomosis↗
PubMed2001

'Cor placentale': placental intervillus/intravillus blood flow mismatch is the pathophysiological mechanism in severe intrauterine growth restriction due to uteroplacental disease.

The underlying pathophysiology in most cases of severe intrauterine growth restriction and pre-eclampsia is thought to be abnormal and inadequate conversion of the branches of the uterine arteries into low resistance uteroplacental vessels, due to poor extravillous trophoblastic invasion, leading to reduced intervillous blood flow. Since, in most vascular beds the main site of flow resistance is at the level of the small arteries/arterioles rather than the capillary bed itself it is likely that in cases of intrauterine growth restriction due to uteroplacental dysfunction with abnormal fetal umbilical artery flow velocity waveforms, the underlying pathological mechanism is primarily an initial reduction in intervillus flow leading to relative local hypoxia of some villus territories. This results initially in autocrine/paracrine mediated localized stem artery vasoconstriction to minimize intervillus/intravillus flow mismatch which, when widespread, will result in abnormal umbilical artery Doppler waveforms due to the globally increased resistance to fetoplacental flow. Since, a small reduction in vessel radius will result in an exponential increase in flow resistance and reduction in flow, the magnitude of stem vessel constriction need only be small to result in large changes in fetoplacental vascular haemodynamics. Thus, the underlying progressive pathology in this condition may be cardiac failure, secondary to chronic stem vessel vasoconstriction caused by abnormalities in oxygenation of the fetal respiratory system hence the term 'cor placentale' is proposed.

Female↗
PubMed2001

A computer based intervention on the appropriate use of arterial blood gas.

OBJECTIVE: To evaluate impact of a computer-based intervention on arterial blood gas (ABG) usage in an intensive care setting. DESIGN: Retrospectively examined, via mixed group analysis, the effects of the intervention on ABG usage in the intensive care unit (ICU). SUBJECTS: Included all clinicians who placed ABG orders in an ICU using the computerized physician order-entry system, as well as controls in non-order entry units. METHODS: Computer-based intervention presenting ordering clinician with patient s previous ABG values and limiting forward duration of tests ordered. Study spanned 12 weeks, 5 weeks pre-intervention and 7-weeks post-intervention. Of 8 ICUs, intervention implemented in 6, not implemented in 2. Data analyzed using the repeated measure ANOVA. RESULTS: Physicians entered <40% ABG orders. 376 ABGs per week processed pre-intervention, 387 per week post. Results nonsignificant with a p= 0.09. Orders placed declined from 1039 per week, Jan 2000 to 662 per week, April 2001. DISCUSSION: Study did not demonstrate significant change; limited power. Need longer study periods. Impact improved in the future by targeting physician users and tailoring intervention to specific work flow pattern of high utilization units.

Analysis of Variance↗
PubMed1998

Colour Doppler energy insonation of placental vasculature in monochorionic twins: absent arterio-arterial anastomoses in association with twin-to-twin transfusion syndrome.

OBJECTIVE: To determine in vivo whether monochorionic pregnancies complicated by twin-to-twin transfusion syndrome are associated with absence of haemodynamically-compensatory arterio-arterial anastomoses. DESIGN: Forty monochorionic pregnancies were prospectively recruited for an ultrasonographic survey of the chorionic plate using colour Doppler energy. Arterio-arterial anastomoses were identified by their characteristic bidirectional interference pattern on spectral Doppler. Angioarchitecture was confirmed by postnatal injection study. SETTING: Fetal medicine tertiary referral centre in London. MAIN OUTCOME MEASURES: Presence of arterio-arterial anastomoses, development of twin-to-twin transfusion syndrome, survival. RESULTS: Arterio-arterial anastomoses were detected by colour Doppler energy in 21 pregnancies (53%), and there were no false positives. An arterio-arterial anastomosis was more commonly found in unaffected (n=28) compared to pregnancies affected by twin-to-twin transfusion syndrome (n=12), both by colour Doppler energy [20/28 (71%) is 1/12 (8%); delta=63%, 95% CI 40%-86%] and by postnatal injection study [25/28 (89%) vs 3/12 (25%); delta=64%, 95% CI 37%-91%]. In pregnancies in which no arterio-arterial anastomoses were detected, a diagnosis of twin-to-twin transfusion syndrome was made in 58%, and the perinatal loss rate was 40%, compared with one case of twin-to-twin transfusion syndrome (5%) (P < 0.001) and a loss rate of 12% (P=0.005) in pregnancies in which an arterio-arterial anastomosis was detected. CONCLUSION Twin-to-twin transfusion syndrome is associated with an absence of functional arterio-arterial anastomoses in vivo in monochorionic twin pregnancies. This contributes to our understanding of the pathophysiology of twin-to-twin transfusion syndrome and confirms ex vivo studies demonstrating that twin-to-twin transfusion syndrome is associated with a paucity of superficial anastomoses. Prospective studies are indicated to determine the utility of colour Doppler energy for arterio-arterial anastomoses in predicting risk in monochorionic pregnancies.

Arterio-Arterial Fistula↗
PubMed1992

Normal amniotic pressure throughout gestation.

OBJECTIVE: To characterize amniotic pressure (AP) in pregnancies with normal amniotic fluid volume. DESIGN: Observational study, mainly cross-sectional. SETTING: Fetal medicine unit within a tertiary referral hospital. SUBJECTS: Patients undergoing transamniotic invasive procedures in whom amniotic fluid volume was subjectively assessed as normal on ultrasound. Those beyond 16 weeks with a deepest vertical pool on ultrasound less than 3.0 or greater than 8.0 cm were excluded. Overall 194 pregnancies were studied on 232 occasions between 7 and 38 weeks gestation. INTERVENTIONS: Manometry readings referenced to the top of the maternal abdomen were obtained via a fluid-filled line from the needle hub and either connected to a pressure transducer (n = 190) or held vertically against a ruler (n = 42). MAIN OUTCOME MEASURES: AP in mm Hg, AP corrected for gestational age (z scores), semi-quantitative ultrasonic indices of amniotic fluid volume, clinical variables. RESULTS: AP in singleton pregnancies increased with advancing gestation (P less than 0.001), and the sigmoid-shaped regression curve plateaued in the mid-trimester. AP z scores were not influenced by volume-related phenomena such as twin gestation, the deepest vertical pool, or amniotic fluid index, nor by maternal age, parity, gravidity, fetal sex, or subsequent spontaneous preterm delivery. CONCLUSIONS: These findings suggest that AP is not principally determined by intrauterine volume. We speculate that AP, which reflects change in uterine tension as a function of radius, may instead be determined by gestation-specific anatomical and hormonal influences on gravid uterine musculature. A reference range for AP has been constructed for use in amnioinfusion and amnioreduction procedures.

Amniotic Fluid↗
PubMed1992

In-utero sound levels when vibroacoustic stimulation is applied to the maternal abdomen: an assessment of the possibility of cochlea damage in the fetus.

OBJECTIVE: To measure sound pressure level in utero while a vibro acoustic stimulator is applied to maternal abdomen and to calculate whether the estimated effect on fetal cochlea cilia vibration would be hazardous. DESIGN: Prospective descriptive study. SETTING: Labour ward, National University Hospital, Singapore. SUBJECTS: Eight women undergoing induction of labour. INTERVENTION: A hydrophone was introduced via the cervix into the uterus and placed under ultrasound guidance near the fetal ear. Sound pressure was recorded when a vibroacoustic stimulator was applied directly to the maternal abdomen and also when separated by 2 cm of air. MAIN OUTCOME MEASURES: Sound pressure levels in utero. RESULTS: The sound level recorded from the vibrator diaphragm in air was 107 dB at 2 cm and 74 dB at 1 m. The mean sound pressure level in utero was 90.7 dB (range 75-96 dB) when the vibrator was in contact with the abdominal wall and 80.1 dB (range 70-88 dB) when separated by 2 cm of air. CONCLUSIONS: Analysis of factors affecting displacement of cochlear sensing cilia in utero show that, for equal sound pressures, sound intensity and sound vibration are about 4000 times less in amniotic fluid, compared to that produced in air. Further protection is provided by viscous and hydrodynamic features of the ear. The estimated effect on cilia vibration by the mean sound pressure registered in utero, about 90 dB, corresponds to that produced postnatally by an airborne sound registering about 40 dB, which would not be hazardous.

Cochlea↗
PubMed1990

White noise and sleep induction.

We studied two groups of 20 neonates, between 2 and 7 days old, in a randomised trial. Sixteen (80%) fell asleep within five minutes in response to white noise compared with only five (25%) who fell asleep spontaneously in the control group. White noise may help mothers settle difficult babies.

Acoustic Stimulation↗
PubMed1990

When does death occur in an acardiac twin? Ultrasound diagnostic difficulties.

Fetal acardia is a rare abnormality of multiple pregnancies, which is lethal for the affected fetus and can cause death in 50% of normal co-twins. Antenatal recognition with early ultrasound is essential to institute a prospective management to improve the outcome. Our communication outline the difficulties which may be encountered in ultrasound diagnosis. In particular the problem of distinguishing a fetal heart from large pulsating mediastinal vessels, which can be present in these fetuses, and the difficulty of diagnosing death in an acardiac fetus. Our report confirms that the co-twin remains at increased risk of sudden death, even without ultrasound evidence of cardiac failure or biochemical compromise. The finding in this fetus of intravascular fibrin deposits suggests the possibility of acute disseminated intravascular coagulation, not previously reported in association with an acardiac twin.

Death, Sudden↗
PubMed1980

Rehabilitation in spinal cord injury: use of a patient-family group.

A 1-day group meeting for spinal cord injured individuals and their families was initiated approximately 3 years ago with emphasis on providing information and sharing mutual concerns and feelings. From the results of tests administered before and after the program and of evaluation forms filled out by all participants, the Family Group has been found to be a useful adjunct to the rehabilitation program. Family members are effectively and meaningfully involved in the total rehabilitation process with the added benefit of decreasing their feelings of anxiety, helplessness and isolation. Evaluation suggests that the Family Group increases the individual's knowledge and awareness of a disability and its implications. It has introduced important topics of discussion within the family unit and has aided in more open communication among family members surrounding these issues. All of these benefits, combined, have resulted in more effective, realistic and appropriate discharge planning for patients.

Adolescent↗