Sir Joseph Barcroft: one victorian physiologist's contributions to a half century of discovery

A Multiscale Mathematical Model for Fetal Gas Transport and Regulatory Systems During Second Half of Pregnancy

Fetal asphyxia, a condition resulting from the combined effects of hypoxia and hypercapnia, leads to approximately 900,000 annual deaths worldwide. One cause is umbilical cord compression during labor-induced uterine contractions, disrupting the transport of metabolites to and from the placenta, and resulting in asphyxia. Current fetal well-being assessment relies on monitoring fetal heart rate and uterine contractions as indicators of oxygen delivery to the brain. To enhance our understanding of this complex relationship, this study aims to develop a modular mathematical model including fetal blood gas dynamics, the autonomic nervous system, and cerebral blood flow regulation. The novelty of this study lies in the capability of the model to simulate fetal growth. These submodels are part of a larger multiscale mathematical model describing fetal circulation in the second half of pregnancy. The blood gas model realistically replicates partial oxygen and carbon dioxide pressures in umbilical arteries and veins during healthy fetal development reported in the literature. An in silico experiment is conducted to simulate umbilical cord occlusion and is compared with lamb experiments to verify the realism of the regulation models during fetal growth. Our findings suggest that premature infants are more susceptible to umbilical cord occlusion, exhibiting elevated cerebral perfusion pressure and flow. This modular mathematical model may serve as a valuable tool for testing hypotheses related to the fetal regulatory system.

Is it time to end the use of base deficit for fetal well-being assessment?

Authors have expressed reservations regarding the use of base deficit measured in umbilical artery blood samples to assess fetal well-being during the course of labor and to predict neonatal neurologic morbidity. Despite its integration into clinical practice for more than 50 years, obstetricians and maternal-fetal medicine specialists may not realize that this marker has significant limitations in accurately identifying neonatal metabolic acidosis as a proxy for fetal well-being. In brief, there are 2 large families of base deficit, namely whole blood and extracellular fluid. Both rely on equations that use normal adult acid-base characteristics (pH 7.40 and partial CO2 pressure of 40 mm Hg) that overlook the specificity of the normal in utero acid-base status of pH 7.27 and partial CO2 pressure of 54 mm Hg. In addition, it ignores the unique characteristic of the in utero fetal response to acute hypoxia. The dependence on placental circulation for CO2 elimination may lead to extremely high values (up to 130 to 150 mm Hg) during hypoxic events, a phenomenon that is absent in adults with acute metabolic acidosis who can hyperventilate. The dispute over if to include a correction for high partial CO2 pressure in the bicarbonate estimation, as presented in the Great Trans-Atlantic Debates, remains unresolved. The key constants computed for adult acid-base physiology in the current base deficit algorithms, without accounting for the impact of high partial CO2 pressure or other fetal characteristics of buffering capacity (eg, differences in body water content composition, plasma protein, and hemoglobin attributes), may lead to an overestimation of metabolic acidosis, especially in newborns who are experiencing hypercarbia during the early stages of the hypoxic response. These unrecognized limitations impact the base deficit results and may mislead clinicians on fetal well-being assessments when discussing the management of fetal heart rate monitoring and neonatal outcomes. Based on our arguments, we believe that it is prudent to consider an alternative to base deficit for drawing conclusions regarding fetal well-being during the course of birth management. We propose a marker specifically related to the newborn acid-base physiology--the neonatal eucapnic pH correction. This marker can be added to arterial cord blood gas analysis, and we have described how to interpret it as a marker of neonatal metabolic acidosis.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 4: evolution, thermal adaptation and unsupported theories of thermoregulation

This review is the final contribution to a four-part, historical series on human exercise physiology in thermally stressful conditions. The series opened with reminders of the principles governing heat exchange and an overview of our contemporary understanding of thermoregulation (Part 1). We then reviewed the development of physiological measurements (Part 2) used to reveal the autonomic processes at work during heat and cold stresses. Next, we re-examined thermal-stress tolerance and intolerance, and critiqued the indices of thermal stress and strain (Part 3). Herein, we describe the evolutionary steps that endowed humans with a unique potential to tolerate endurance activity in the heat, and we examine how those attributes can be enhanced during thermal adaptation. The first of our ancestors to qualify as an athlete was Homo erectus, who were hairless, sweating specialists with eccrine sweat glands covering almost their entire body surface. Homo sapiens were skilful behavioural thermoregulators, which preserved their resource-wasteful, autonomic thermoeffectors (shivering and sweating) for more stressful encounters. Following emigration, they regularly experienced heat and cold stress, to which they acclimatised and developed less powerful (habituated) effector responses when those stresses were re-encountered. We critique hypotheses that linked thermoregulatory differences to ancestry. By exploring short-term heat and cold acclimation, we reveal sweat hypersecretion and powerful shivering to be protective, transitional stages en route to more complete thermal adaptation (habituation). To conclude this historical series, we examine some of the concepts and hypotheses of thermoregulation during exercise that did not withstand the tests of time.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 3: Heat and cold tolerance during exercise

In this third installment of our four-part historical series, we evaluate contributions that shaped our understanding of heat and cold stress during occupational and athletic pursuits. Our first topic concerns how we tolerate, and sometimes fail to tolerate, exercise-heat stress. By 1900, physical activity with clothing- and climate-induced evaporative impediments led to an extraordinarily high incidence of heat stroke within the military. Fortunately, deep-body temperatures > 40 °C were not always fatal. Thirty years later, water immersion and patient treatments mimicking sweat evaporation were found to be effective, with the adage of cool first, transport later being adopted. We gradually acquired an understanding of thermoeffector function during heat storage, and learned about challenges to other regulatory mechanisms. In our second topic, we explore cold tolerance and intolerance. By the 1930s, hypothermia was known to reduce cutaneous circulation, particularly at the extremities, conserving body heat. Cold-induced vasodilatation hindered heat conservation, but it was protective. Increased metabolic heat production followed, driven by shivering and non-shivering thermogenesis, even during exercise and work. Physical endurance and shivering could both be compromised by hypoglycaemia. Later, treatments for hypothermia and cold injuries were refined, and the thermal after-drop was explained. In our final topic, we critique the numerous indices developed in attempts to numerically rate hot and cold stresses. The criteria for an effective thermal stress index were established by the 1930s. However, few indices satisfied those requirements, either then or now, and the surviving indices, including the unvalidated Wet-Bulb Globe-Thermometer index, do not fully predict thermal strain.

A fetus in the world: Physiology, epidemiology, and the making of fetal origins of adult disease

Since the late 1980s, the fetal origins of adult disease, from 2003 developmental origins of health and disease (DOHaD), has stimulated significant interest in and an efflorescence of research on the long-term effects of the intrauterine environment. From the start, this field has been interdisciplinary, using experimental animal, clinical and epidemiological tools. As the influence of DOHaD on public health and policy expanded, it has drawn criticism for reducing the complex social and physical world of early life to women's reproductive bodies as drivers of intergenerational ills. This paper explains this narrowing of focus in terms of a formative and consequential exchange between David Barker, the British epidemiologist whose work is credited with establishing the field, and the discipline of fetal physiology. We suggest that fetal physiologists were a crucial constituency of support for Barker's hypothesis about early life origins of disease. Their collaborations with Barker helped secure and sustain the theory amid considerable controversy. The trajectory of DOHaD and its focus on the maternal body can be understood, we argue, as a consequence of this alliance, which brought together two distinct conceptualizations of the intrauterine environment, one from epidemiology and the other from fetal physiology. Along the way, we trace the histories of these conceptualizations, both of which were products of mid-to-late twentieth century British science, and show how Barker's early emphasis on social and economic conditions was superseded by a narrower focus on physiological mechanisms acting upon the autonomous fetus.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 2: physiological measurements

In this, the second of four historical reviews on human thermoregulation during exercise, we examine the research techniques developed by our forebears. We emphasise calorimetry and thermometry, and measurements of vasomotor and sudomotor function. Since its first human use (1899), direct calorimetry has provided the foundation for modern respirometric methods for quantifying metabolic rate, and remains the most precise index of whole-body heat exchange and storage. Its alternative, biophysical modelling, relies upon many, often dubious assumptions. Thermometry, used for >300 y to assess deep-body temperatures, provides only an instantaneous snapshot of the thermal status of tissues in contact with any thermometer. Seemingly unbeknownst to some, thermal time delays at some surrogate sites preclude valid measurements during non-steady state conditions. To assess cutaneous blood flow, immersion plethysmography was introduced (1875), followed by strain-gauge plethysmography (1949) and then laser-Doppler velocimetry (1964). Those techniques allow only local flow measurements, which may not reflect whole-body blood flows. Sudomotor function has been estimated from body-mass losses since the 1600s, but using mass losses to assess evaporation rates requires precise measures of non-evaporated sweat, which are rarely obtained. Hygrometric methods provide data for local sweat rates, but not local evaporation rates, and most local sweat rates cannot be extrapolated to reflect whole-body sweating. The objective of these methodological overviews and critiques is to provide a deeper understanding of how modern measurement techniques were developed, their underlying assumptions, and the strengths and weaknesses of the measurements used for humans exercising and working in thermally challenging conditions.

Update on fetal cardiovascular magnetic resonance and utility in congenital heart disease

Background

Congenital heart disease (CHD) is the most common birth defect, affecting approximately eight per thousand newborns. Between one and two neonates per thousand have congenital cardiac lesions that require immediate post-natal treatment to stabilize the circulation, and the management of these patients in particular has been greatly enhanced by prenatal detection. The antenatal diagnosis of CHD has been made possible through the development of fetal echocardiography, which provides excellent visualization of cardiac anatomy and physiology and is widely available. However, late gestational fetal echocardiographic imaging can be hampered by suboptimal sonographic windows, particularly in the setting of oligohydramnios or adverse maternal body habitus.

Main body

Recent advances in fetal cardiovascular magnetic resonance (CMR) technology now provide a feasible alternative that could be helpful when echocardiography is inconclusive or limited. Fetal CMR has also been used to study fetal circulatory physiology in human fetuses with CHD, providing new insights into how these common anatomical abnormalities impact the distribution of blood flow and oxygen across the fetal circulation. In combination with conventional fetal and neonatal magnetic resonance imaging (MRI) techniques, fetal CMR can be used to explore the relationship between abnormal cardiovascular physiology and fetal development. Similarly, fetal CMR has been successfully applied in large animal models of the human fetal circulation, aiding in the evaluation of experimental interventions aimed at improving in utero development. With the advent of accelerated image acquisition techniques, post-processing approaches to correcting motion artifacts and commercial MRI compatible cardiotocography units for acquiring gated fetal cardiac imaging, an increasing number of CMR methods including angiography, ventricular volumetry, and the quantification of vessel blood flow and oxygen content are now possible.

Conclusion

Fetal CMR has reached an exciting stage whereby it may now be used to enhance the assessment of cardiac morphology and fetal hemodynamics in the setting of prenatal CHD.

The invention of hypoxia

The word "hypoxia" has recently come to the attention of the general public on two occasions, the Nobel Prize in Medicine or Physiology in 2019 and the recent COVID-19 pandemic. In the academic environment, hypoxia is a current topic of research in biology, physiology, and medicine: in October 2020, there were more than 150,000 occurrences of "hypoxia" in the PubMed database. However, the first occurrence is dated to 1945, while the interest for the effects of oxygen lack on the living organisms started in the mid-19th century, when scientists explored high altitude regions and mainly used the terms "anoxia" or "anoxemia." I therefore researched online through multiple databases to look for the first appearance of "hypoxia" and related terms "hypoxemia" and "hypoxybiosis" in scientific literature published in English, German, French, Italian, and Spanish. Viault and Jolyet used "Hypohématose" in 1894, but this term has not been used since. Hypoxybiosis first appeared in 1909 in Germany, then hypoxemia in 1923 in Austria, and hypoxia in 1938 in Holland. It was then exported to the United States where it appeared in 1940 in cardiology and anesthesiology. The clinical distinction between anoxia and hypoxia was clearly defined by Carl Wiggers in 1941. Hypoxia (decrease in oxygen), by essence variable in time and in localization in the body, in contrast with anoxia (absence of oxygen), illustrates the concept of homeodynamics that defines a living organism as a complex system in permanent instability, exposed to environmental and internal perturbations.

Naming Mount Barcroft

Background: The University of California White Mountain Research Center is located on Mount Barcroft, a 13,040-ft (3975-m) peak on the California White Mountain range. This report describes how the peak got its name honoring Sir Joseph Barcroft of Great Britain. Materials and Methods: Several publicly available webpages were the sources for this study. Results: On October 16, 1951, the United States Board on Geographic Names approved "Mount Barcroft" as the name for a peak on the California White Mountain range enabling the building of a facility dedicated to high-altitude research. The process of naming, however, was far from smooth. Objections came from the members of the Sierra Club, editors of a local newspaper, and a few citizens of California delaying the approval process. At least six other names had been proposed, three of which were from a Native American Indian language. Those who opposed the name "Mount Barcroft" argued that Barcroft never visited the United States, let alone the White Mountain region, and there was a paucity of Native American Indian names for geographic features in the United States. Conclusions: Despite oppositions and controversies, however, a much-deserved scientist was duly honored by an agency of the United States federal government.

From Galen to Gross and beyond: a brief history of the enigmatic patent ductus arteriosus

Anatomists since antiquity and pathologists since at least the 17th century had identified the ductus arteriosus (DA) in cadavers and postmortem examinations, respectively. However, healthcare providers for more than a century have struggled to understand the significance of a patent ductus arteriosus (PDA) in patients, debated whether to treat it or not and if so, when and how. Accepted answers depended upon the authoritative position of the person(s) offering recommendations, the cumulative contemporary medical knowledge, and the changing patient population characteristics. The treatment choices were most often based on one's understanding of the balance between the risks and benefits of the chosen treatment. In the current era, with the increasing popularity of transcatheter occlusion of the PDA with relative ease even in extremely premature infants whose survival rates have improved dramatically, a basic question has reemerged-what are the benefits to treating the PDA in any preterm infant. In this brief review, I am providing a chronicle of the evolution of knowledge about the DA, the varying nature of the challenges a patent ductus posed for the caregivers, and the roots of the continued debate concerning the management of the enigmatic PDA.